Oil and Gas Terms Beginning with “C”

A type of pump commonly used in the handling and mixing of oilfield fluids. The rotary motion of a profiled impeller in combination with a shaped pump housing or volute applies centrifugal force to discharge fluids from the pump. Centrifugal pumps generally operate most efficiently in high-volume, low-output-pressure conditions. Unlike a positive-displacement pump, the flow from centrifugal pumps can be controlled easily, even allowing flow to be completely closed off using valves on the pump discharge manifold while the pump is running. This pump is known as a "centrifugal pump."

In multichannel seismicacquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveoutprocessing is necessary to reduce smearing, or inappropriate mixing, of the data.

Quantity of positively charged ions (cations) that a claymineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3.

Abbreviation for "Comprehensive Environmental Response, Compensation and Liability Act" of 1980. CERCLA is an expansion of RCRA, "Resources Conservation and Recovery Act" of 1976. These acts of the US Congress outline responsibilities of operators for transportation, storage, treatment or disposal of regulated "hazardous substances," which include certain oilfield materials.

The acronym for cold heavy oil production with sand.

The value of the separation between two adjacent contours. A net payisopachmap might have a contour interval of 10 feet [3 m], whereas a structurecontour map might have a contour interval of 1000 feet [300 m]. Contour intervals are chosen according to the map scale and the amount and distribution of control points.

A drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC's effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity ("CMC-Lo Vis") or high-viscosity ("CMC-Hi Vis") grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material.Reference:Hughes TL, Jones TG and Houwen OW: "The Chemical Characterization of CMC and Its Relationship to Drilling-MudRheology and Fluid Loss," SPE Drilling & Completion 8, no. 3 (September 1993): 157-164.

A cellulose polymer that contains anionic carboxymethyl and nonionic hydroxyethyl groups added by ether linkages to the OHs on the cellulose backbone. This polymer has seen limited use in drilling mud, but more use in brines and completion fluids.

In multichannel seismicacquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms.

A natural starch derivative. CMS is used primarily for fluid-loss control in drilling muds, drill-in, completion and workover fluids. It is slightly anionic and can be affected by hardness and other electrolytes in a mud. CMS is similar to CMC (carboxymethylcellulose) in method of manufacture and many of its uses. The linear and branched starch polymers in natural starch react with monochloroacetic acid in alkaline solution, adding carboxymethyl groups at the OH positions by an ether linkage. By adding the carboxymethyl groups, the starch becomes more resistant to thermal degradation and bacterial attack.

An enhanced oil recovery method in which carbon dioxide (CO2) is injected into a reservoir to increase production by reducing oil viscosity and providing miscible or partially miscible displacement of the oil.

The amount of oxygen needed to oxidize reactive chemicals in a water system, typically determined by a standardized test procedure. COD is used to estimate the amount of a pollutant in an effluent. Compare to biochemical oxygen demand, BOD.

(noun) Abbreviation for Combination of Forward Combustion and Waterflooding. An enhanced oil recovery technique in which air is injected into a reservoir to initiate in-situ combustion of a portion of the crude oil, followed by water injection behind the combustion front to scavenge additional heat and displace oil toward production wells, improving overall sweep efficiency beyond either method alone.

In a nuclear magnetic resonance (NMR) measurement, referring to the cycle of radio frequency pulses designed by Carr, Purcell, Meiboom and Gill to produce pulse echoes and counteract dephasing due to magnetic field inhomogeneities. In the CPMG sequence, an initial radio frequency pulse is applied long enough to tip the protons into a plane perpendicular to the static magnetic field (the 90o pulse). Initially the protons precess in unison, producing a large signal in the antenna, but then quickly dephase due to the inhomogeneities. Another pulse is applied, long enough to reverse their direction of precession (the 180o pulse), and causing them to come back in phase again after a short time. Being in phase, they produce another strong signal called an echo. They quickly dephase again but can be rephased by another 180o pulse. Rephasing is repeated many times, while measuring the magnitude of each echo. This magnitude decreases with time due to molecular relaxation mechanisms surface, bulk and diffusion. One measurement typically may comprise many hundreds of echoes, while the time between each echo (the echo spacing) is of the order of 1 ms or less.Carr HY and Purcell EM: ?Effects of Diffusion on Free Precession in Nuclear Magnetic Resonance Experiments,? Physical Review 94, no. 3 (1954): 630-638.Meiboom S and Gill D: ?Modified Spin-Echo Method for Measuring Nuclear Relaxation Times,? The Review of Scientific Instruments 29, no. 8 (1958): 688-691.

Abbreviation for cyclic steamstimulation. Better known as cyclic steam injection.

A long, continuous length of pipe wound on a spool. The pipe is straightened prior to pushing into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Depending on the pipe diameter (1 in. to 4 1/2 in.) and the spool size, coiled tubing can range from 2,000 ft to 15,000 ft [610 to 4,570 m] or greater length.

Abbreviation for "Clean Water Act," a law passed by the US Congress to control the discharge of contaminants, particularly oil, into the waters of the US.

A fastening device to secure or hold together two ropes.

A mast assembled horizontally at ground level and pinned onto the substructure with hydraulic cylinders for raising.

The process of gathering drilled formation cuttings coming over the shale shakers from the wellbore for geological examination.

A special hand tool that consists of a handle and chain that wraps around tubulars to apply rotational force.

The name given by API to the electrohygrometer method for testing oil mud and cuttings samples for water-phase activity, aw.

An assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. Christmas trees are available in a wide range of sizes and configurations, such as low- or high-pressure capacity and single- or multiple-completion capacity.

A gamma ray interaction in which the gamma ray collides with an electron, transferring part of its energy to the electron, while itself being scattered at a reduced energy. Compton scattering occurs with high probability at intermediate gamma ray energies, between 75 keV and 10 MeV in sedimentary formations. When a beam of gamma rays traverses a material, the total reduction due to Compton scattering depends on the electron density of the material the higher the density, the larger the reduction. This is the basis for the density log. Compton scattering is also an important mechanism in gamma ray detectors.

Generally the first string of casing in a well, preventing surface formation collapse.

A sub that allows different sizes and types of drill string components to be joined together.

A device mounted near the drawworks drum to keep the driller from inadvertently raising the travelling block into the crown block.

The spherical diameter corresponding to the ellipsoidal volume distribution of the screen opening sizes as measured by image analysis techniques. Named after Al Cutt of Amoco who developed the technique. Not to be confused with cut point.Reference:Cutt AR: "Shaker Screen Characterization Through Image Analysis," paper SPE 22570, presented at the 66th SPE Annual Technical Conference and Exhibition, Dallas, Texas, USA, October 6-9, 1991.

The cable on which wirelinelogging tools are lowered into the well and through which signals from the measurements are passed. The cable consists of a central section with conductors surrounded by a metal, load-bearing armor.

An electromechanical device used to connect an electrical tool string to a logging cable, electrical wireline or coiled tubing string equipped with an electrical conductor. It provides attachments to both the mechanical armor wires (which give logging cable its tensile strength) and the outer mechanical housing of a logging tool, usually by means of threads. This connection to the logging tool results in a good electrical path from the electrical conductors of the logging cable to the electrical contacts of the logging tool, and shields this electrical path from contact with conductive fluids, such as certain drilling muds. The basic requirements of most cable heads include providing reliable electrical and mechanical connectivity between the running string and tool string. Another attribute of cable heads is that they serve as a "weak link," so that if a logging tool becomes irretrievably stuck in a well, the operator may intentionally pull in excess of the breaking strength of the logging cable head, causing the cable to pull out of the cable head in a controlled fashion.

A method of drilling whereby an impact tool or bit, suspended in the well from a steel cable, is dropped repeatedly on the bottom of the hole to crush the rock. The tool is usually fitted with some sort of cuttingsbasket to trap the cuttings along the side of the tool. After a few impacts on the bottom of the hole, the cable is reeled in and the cuttings basket emptied, or a bailer is used to remove cuttings from the well. The tool is reeled back to the bottom of the hole and the process repeated. Due to the increasing time required to retrieve and deploy the bit as the well is deepened, the cable-tool method is limited to shallow depths. Though largely obsolete, cable-tool operations are still used to drill holes for explosive charge placement (such as for acquisition of surface seismic data) and water wells.

[CaCO3]The crystalline form of calcium carbonate and chief constituent of limestone and chalk. Calcite reacts readily with dilute hydrochloric acid [HCl], so the presence of calcite can be tested by simply placing a drop of acid on a rock specimen.

A compound of formula CaBr2 used in conjunction with calcium chloride [CaCl3] in completion operations to make solids-free brines with densities in the range 11.5 to 14.5 ppg.

A compound with formula CaCO3 that occurs naturally as limestone. Ground and sized calcium carbonate is used to increase mud density to about 12 lbm/gal [1.44 kg/m3], and is preferable to barite because it is acid-soluble and can be dissolved with hydrochloric acid to clean up production zones. Its primary use today is as a bridging material in drill-in, completion and workover fluids. Sized calcium carbonate particles, along with polymers, control fluid loss in brines or drill-in, completion and workover fluids. Insoluble calcium carbonate is the precipitated byproduct of mud treatments used for removal of either Ca+2 or CO3-2 by addition of the other ion.

A temporary plug formulated with graded granules or flakes of calcium carbonate that are generally circulated into place as a slurry and allowed to settle out. Calcium carbonate plugs commonly are used to isolate lower production zones, either to enable a column of well control fluid to be placed, or to provide some protection for a lower zone while treating upper zones. Because of their high reaction rate with hydrochloric acid, calcium carbonate plugs are easily removed using common acidizing materials and equipment.

A highly soluble calcium salt of formula CaCl2 used to make drilling and workover fluids or brines with a density range from 8.33 to 11.6 lbm/gal [1.39 g/cm3] at saturation. CaCl2 can be blended with other brines, including sodium chloride [NaCl], calcium bromide [CaBr2] and zinc bromide [ZnBr2]. Emulsification of CaCl2brine as the internal phase of oil-base or synthetic-base mud is an important use because the brine provides osmotic wellbore stability while drilling water-sensitive shale zones.

A contamination problem caused by Ca+2 ions, usually occurring in fresh water, seawater and other low-salinity and low-hardness mud systems. Soluble calcium comes into a mud from various sources: gypsum- or anhydrite-bearing strata, unset cement and hardness ions in make-up water or from an influx of formation water. Ca+2 can flocculate colloidal clays and precipitate large anionic polymers that contain carboxylate groups, such as an acrylate polymer. On the other hand, some mud types tolerate calcium, in which case calcium is not considered a contaminant.

A chemical with formula Ca(OH)2, commonly called slaked lime. Lime is used in lime muds and as a treatment to remove carbonate ions. It is used as a stabilizing ingredient in oil- and synthetic-base mud, essential to formation of fatty-acid soap emulsifiers. It is an alkaline material that can be carried in excess to neutralizehydrogen sulfide [H2S] and carbon dioxide [CO2].

A class of water-base drilling fluid that utilize dissolved Ca+2 as a component. Examples are lime mud, gyp mud and calcium chloride [CaCl2] mud. The latter is rarely used, but is based on solutions of CaCl2 that, in high concentration, can impart density up to 11.6 lbm/gal (1.39 g/cm3) and has been touted as providing shale inhibition.

A calcium soap of naphthenic acids in crude oil. Naphthenates are formed through interaction of naphthenic acids in crude oil with metal ions such as calcium and sodium. Insoluble in either the oil or water phase, and with a density between that of oil and water, naphthenates tend to accumulate at the oil/water interface and act as surfactants to help stabilize emulsions. Naphthenates can also be deposited as solids in pipelines, and can cause flow-assurance problems.

A chemical with formula CaO, commonly called quick lime or hot lime. When hydrated with one mole of water, it forms slaked lime, Ca(OH)2. Quick lime is used in preference to slaked lime at oil mud mixing plants because it generates heat when it becomes slaked with water and therefore speeds up emulsification by the reaction to form calcium fatty-acid soap.

The chemical CaSO4, which occurs naturally as the mineral anhydrite. Gypsum is the dihydrate mineral form, CaSO4·2H2O. Anhydrite and gypsum (commonly called gyp) are found in the subsurface and drilling even small stringers of these minerals can upset a freshwater or seawater mud. Gyp muds, lime muds and oil muds tolerate these salts best. CaSO4 is used as a mud treatment when no pH increase is needed to remove carbonate ion contamination in freshwater and seawater muds. (Lime increases pH when added for this purpose.) Gypsum and lime treatments are often used together to keep pH in the proper range. The test for determining the dissolved and undissolved calcium sulfate in a gyp mud requires two titrations with the strong EDTA reagent and Calver II® indicator when performed to API standards. It also requires a retort analysis for water content in the mud in order to calculate CaSO4 content, lbm/bbl.

A quantitative analytical procedure for water-mud filtrate and for calcium in an oil mud.

The process of adjusting a measurement to a standard, so that copies of the same type of logging tool or laboratory instrument will read the same. The tool or instrument is placed in the presence of a calibrator or calibrating environment, for example, a source of gamma rays for a gamma ray tool, or the air, far from the ground, for an induction tool. Calibration coefficients, typically a gain and an offset, are calculated so that the tool or instrument reads correctly in the calibrator. The coefficients are then applied during subsequent measurements.The term master calibration is used for the regular, as for example quarterly, calibration of a logging tool in the workshop. For most wireline tools, a secondary calibrator is adjusted during the master calibration and taken to the wellsite so that a wellsite calibration can be done just prior to the logging job. Some tools, such as the gamma ray, are calibrated only at the wellsite. For most measurements-while-drilling tools, the environment requires that the calibration be performed at the workshop and only a verification made at the wellsite.For some measurements, there is a primary worldwide standard against which calibrators are calibrated, as for example, the radioactive formations at the University of Houston used to define gamma ray API units.

A deposit of sodium nitrate that is mined and used for fertilizer in parts of South America.

A representation of the measured diameter of a borehole along its depth. Caliper logs are usually measured mechanically, with only a few using sonic devices. The tools measure diameter at a specific chord across the well. Since wellbores are usually irregular (rugose), it is important to have a tool that measures diameter at several different locations simultaneously. Such a tool is called a multifinger caliper. Drilling engineers or rigsite personnel use caliper measurement as a qualitative indication of both the condition of the wellbore and the degree to which the mud system has maintained hole stability. Caliper data are integrated to determine the volume of the openhole, which is then used in planning cementing operations.

The device used in early logging to record logging measurements on photographic film. The camera consisted of a light shining on galvanometers, which reflected the light to produce a trace on one or more films. The galvanometers deflected according to the log measurement to give the log reading. The films were turned by the depth wheel, which gave the depth axis of the log.

A small, electrically activated explosive charge that detonates a larger charge. Caps, also called seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation.

A relatively impermeablerock, commonly shale, anhydrite or salt, that forms a barrier or seal above and around reservoir rock so that fluids cannot migrate beyond the reservoir. It is often found atop a salt dome. The permeability of a cap rock capable of retaining fluids through geologic time is ~ 10-6-10-8 darcies.

To regain control of a blowout well by installing and closing a valve on the wellhead.

An in siturecord of the capability of the fluid passing through a sensor to store electrical charge. Since water has a high dielectric constant, and hence capacitance, it can be distinguished from oil or gas. The capacitance, or fluid capacitance log, can therefore identify water and be scaled in terms of water holdup. However, the relation between capacitance and holdup depends strongly on whether the water is the continuous phase, complicating quantitative evaluation.The log was introduced in the 1960s with the so-called holdup meter. It was mainly used in three-phase flow, or when fluid-density measurements were insufficiently sensitive to water at low holdup, or with heavy oils. Since the late 1980s, other holdup measurements have been preferred.

(noun) An instrument that measures the dielectric constant or capacitance of a fluid mixture flowing through a pipe or conduit, used in production logging and surface facilities to determine the water cut or oil-water ratio of a multiphase production stream.

A dimensionless group used in analysis of fluid flow that characterizes the ratio of viscous forces to surface or interfacial tension forces. It is usually denoted NC in the oil field and Ca in chemical engineering. For a flowing liquid, if NC >>1, then viscous forces dominate over interfacial forces; however if NC

The relationship describing the capillary pressure required to obtain a given nonwetting phase saturation in a rock. Rocks have a distribution of pore throat sizes, so as more pressure is applied to the nonwetting phase, increasingly smaller pore openings are invaded. The capillary pressure curve is important for understanding saturation distribution in the reservoir and affects imbibition and multiphase fluid flow through the rock.

A type of static filtration test for water-base drilling fluid that measures the filtration rate (time for free water to pass between two electrodes) using filter paper as the medium. It is used primarily to indicate filter-cakepermeability, but data from the test have been used to study how clays and shales react in filter cakes and how brines of various types affect clays in a filter cake.

An instrument for measuring the viscosity of a fluid by passing the fluid at a known pressure gradient or velocity through a length of tubing of known diameter. The viscosity of base oils for oil muds, which are Newtonian fluids, is measured using a glass capillary tube in a thermostatic bath, when performed according to API procedures.

A type of static filtration test for water-base drilling fluid that measures the filtration rate (time for free water to pass between two electrodes) using filter paper as the medium. It is used primarily to indicate filter-cake permeability, but data from the test have been used to study how clays and shales react in filter cakes and how brines of various types affect clays in a filter cake.

A type of positive gravityanomaly that results from the presence of a dense cap rock overlying a relatively low-density salt dome.

An exposed gun system used primarily in wireline operations. This gun system has shaped charges that are housed in individual pressure-tight capsules mounted on a metal strip, which is lowered into the well. Each pressure-tight capsule, along with the entire string, is thus exposed to well fluids.

A test performed by the mudlogger or wellsite geologist, used to calculate sample lag. The lag period can be measured as a function of time or pump strokes. Acetylene is commonly used as a tracer gas for this purpose. This gas is generated by calcium carbide, a man-made product that reacts with water. Usually, a small paper packet containing calcium carbide is inserted into the drillstring when the kelly is unscrewed from the pipe to make a connection, and the time is noted, along with the pump-stroke count on the mud pump. Once the connection is made and drilling resumes, the packet is pumped downhole with the drilling fluid. Along the way, the drilling fluid breaks down the paper and reacts with the calcium carbide. The resulting acetylene gas circulates with the drilling fluid until it reaches the surface, where it is detected at the gas trap, causing a rapid increase or spike in gas readings. The time and pump-stroke count are again noted, and the cuttings sample lag interval is calculated.

The density of carbon in oil. This density affects the interpretation of the carbon-oxygen log. The term may also be used for the density of carbon in other materials.

The compound with the formula CO2. An odorless gas, carbon dioxide [CO2] is widely distributed in nature and is a minor component of air. It is highly soluble in water and oil, especially under pressure. In water, it occurs as carbonic acid, a weak acid that can donate one or two hydrogen ions in neutralization reactions that produce bicarbonate [HCO3-] and carbonate [CO3-2] salts or ions. CO2, being an acid in water, reacts instantly with NaOH or KOH in an alkalinewater mud, forming carbonate and bicarbonate ions. Similarly, it reacts with Ca(OH)2 (lime) to form insoluble calcium carbonate and water.

The deterioration of metal components resulting from contact with a gas or solution containing carbon dioxide.

A group of minerals found mostly in limestone and dolostone that includes aragonite, calcite and dolomite. Calcite is the most abundant and important of the carbonate minerals.

An anion with formula CO3-2. Carbonate chemistry involves a pH-dependent equilibrium between H2O, H+, OH-, CO2, HCO3- and CO3-2. At low pH, carbon dioxide [CO2] dominates. As pH rises from acidic toward neutral, HCO3- ions dominate. As pH rises above neutral, CO3-2 ions dominate. If no component is lost from the system (such as CO2 gas evolving), changing pH up and down continually reverses the relative proportion of the carbonate species. Carbonates play several important roles in water mud chemistry. One role is the corrosion of metals by acidic CO2. A second is the formation of calcium carbonate [CaCO3] scale on surfaces by carbonate and calcium ion reactions. Another role is in the chemistry of deflocculated mud, where bicarbonate ions prevent attachment of deflocculants such as lignosulfonate, onto clay edge charges.

A common type of mineral deposit that is often found on wellbore tubulars and components as the saturation of produced water is affected by changing temperature and pressure conditions in the production conduit. Carbonate scales have a high dissolution rate in common oilfield acids and generally can be effectively removed using acid or chemical treatments. Scale inhibition techniques also may be used to prevent scale formation. In the majority of cases, scale prevention is simpler and more cost-effective than attempting a cure.

An analytical procedure to determine the concentration of carbonate species using the Garrett Gas Train (GGT) when performed to API specifications. A water mudfiltrate sample is put into the GGT. N2 or N2O is the carrier gas. A CO2Drdger tube is used to measure the total carbonates released as CO2 when sulfuric acid is added to the chamber containing the sample. Total carbonates are measured by the amount of CO2 evolved in the test.Reference:Garrett RL: "A New Field Method for the Quantitative Determination of Carbonates in Water-Base Drilling Fluids," Journal of Petroleum Technology 30, no. 7 (July 1978): 860-868.

A cellulose polymer that contains anionic carboxymethyl and nonionic hydroxyethyl groups added by ether linkages to the OHs on the cellulose backbone. This polymer has seen limited use in drilling mud, but more use in brines and completion fluids.

A natural starch derivative. CMS is used primarily for fluid-loss control in drilling muds, drill-in, completion and workover fluids. It is slightly anionic and can be affected by hardness and other electrolytes in a mud. CMS is similar to CMC (carboxymethylcellulose) in method of manufacture and many of its uses. The linear and branched starch polymers in natural starch react with monochloroacetic acid in alkaline solution, adding carboxymethyl groups at the OH positions by an ether linkage. By adding the carboxymethyl groups, the starch becomes more resistant to thermal degradation and bacterial attack.

A cellulose polymer that contains anionic carboxymethyl and nonionic hydroxyethyl groups added by ether linkages to the OHs on the cellulose backbone. This polymer has seen limited use in drilling mud, but more use in brines and completion fluids.

A drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC's effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity ("CMC-Lo Vis") or high-viscosity ("CMC-Hi Vis") grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material.Reference:Hughes TL, Jones TG and Houwen OW: "The Chemical Characterization of CMC and Its Relationship to Drilling-Mud Rheology and Fluid Loss," SPE Drilling & Completion 8, no. 3 (September 1993): 157-164.

A working interest generally paid in consideration for work related to the prospect. This interest is paid, or carried, for the drilling and or completion costs as specified in the contract between the parties, by another working interest owner typically until casing point is reached, or through the tanks, meaning through completion of the well, as agreed upon contractually.

A fluid that is used to transport materials into or out of the wellbore. Carrier fluids typically are designed according to three main criteria: the ability to efficiently transport the necessary material (such as pack sand during a gravel pack), the ability to separate or release the materials at the correct time or place, and compatibility with other wellbore fluids while being nondamaging to exposed formations.

A perforating gun, consisting of a loading tube and shaped charges. The shaped charges are housed inside a metal tube or pipe known as a carrier. The carrier protects the charges against well fluids.

The ability of a circulating drilling fluid to transport rock fragments out of a wellbore. Carrying capacity is an essential function of a drilling fluid, synonymous with hole-cleaning capacity and cuttings lifting. Carrying capacity is determined principally by the annular velocity, hole angle and flow profile of the drilling fluid, but is also affected by mud weight, cuttings size and pipe position and movement.

A phenomenon in which free liquid leaves with the gas phase at the top of a separator. Carryover can indicate high liquid level, damage of the separator or plugged liquid valves at the bottom of the separator.

The section of a wirelinelogging tool that contains the telemetry, the electronics and power supplies for the measurement, as distinct from the sonde that contains the measurement sensors. Strictly speaking, the term refers to the package of electronic hardware inside a steel housing, but it is also used to refer to the complete assembly including housing.

A wellbore lined with a string of casing or liner. Although the term can apply to any hole section, it is often used to describe techniques and practices applied after a casing or liner has been set across the reservoir zone, such as cased-hole logging or cased-hole testing.

Steel pipe cemented in place during the construction process to stabilize the wellbore. The casing forms a major structural component of the wellbore and serves several important functions: preventing the formation wall from caving into the wellbore, isolating the different formations to prevent the flow or crossflow of formation fluid, and providing a means of maintaining control of formation fluids and pressure as the well is drilled. The casing string provides a means of securing surface pressure control equipment and downhole production equipment, such as the drilling blowout preventer (BOP) or production packer. Casing is available in a range of sizes and material grades.

A wellhead component or a profile formed in wellhead equipment in which the casing hanger is located when a casing string has been installed. The casing bowl incorporates features to secure and seal the upper end of the casing string and frequently provides a port to enable communication with the annulus.

The theoretical internal pressure differential at which a joint of casing will fail. The casing burst pressure value is a key consideration in many well-control and contingency operations and is a major factor in the well design process.

A mechanical device that keeps casing from contacting the wellbore wall. A continuous 360-degree annular space around casing allows cement to completely seal the casing to the borehole wall. There are two distinct classes of centralizers. The older and more common is a simple, low-cost bow-spring design. Since the bow springs are slightly larger than the wellbore, they can provide complete centralization in vertical or slightly deviated wells. However, they do not support the weight of the casing very well in deviated wellbores. The second type is a rigid blade design. This type is rugged and works well even in deviated wellbores, but since the centralizers are smaller than the wellbore, they will not provide as good centralization as bow-spring type centralizers in vertical wells. Rigid-blade casing centralizers are slightly more expensive and can cause trouble downhole if the wellbore is not in excellent condition.

The threaded collar used to connect two joints of casing. The resulting connection must provide adequate mechanical strength to enable the casing string to be run and cemented in place. The casing collar must also provide sufficient hydraulic isolation under the design conditions determined by internal and external pressure conditions and fluid characteristics.

A downhole tool used to confirm or correlate treatment depth using known reference points on the casing string. The casing collar locator is an electric logging tool that detects the magnetic anomaly caused by the relatively high mass of the casing collar. A signal is transmitted to surface equipment that provides a screen display and printed log enabling the output to be correlated with previous logs and known casing features such as pup joints installed for correlation purposes.

A log provided by a casing collar locator tool that generally incorporates a gamma ray log to correlate the relative position of casing string features, such as the location of a pup joint, with the reservoir or formation of interest.

A completion configuration in which a productioncasing string is set across the reservoir interval and perforated to allow communication between the formation and wellbore. The casing performs several functions, including supporting the surrounding formation under production conditions, enabling control of fluid production through selective perforation and allowing subsequent or remedial isolation by packers, plugs or special treatments.

A short length of pipe used to connect two joints of casing. A casing coupling has internal threads (female threadform) machined to match the external threads (male threadform) of the long joints of casing. The two joints of casing are threaded into opposite ends of the casing coupling.

A system of identifying and categorizing the strength of casing materials. Since most oilfield casing is of approximately the same chemistry (typically steel), and differs only in the heat treatment applied, the grading system provides for standardized strengths of casing to be manufactured and used in wellbores. The first part of the nomenclature, a letter, refers to the tensile strength. The second part of the designation, a number, refers to the minimum yield strength of the metal (after heat treatment) at 1000 psi [6895 KPa]. For example, the casing grade J-55 has minimum yield strength of 55,000 psi [379,211 KPa]. The casing grade P-110 designates a higher strength pipe with minimum yield strength of 110,000 psi [758,422 KPa]. The appropriate casing grade for any application typically is based on pressure and corrosion requirements. Since the well designer is concerned about the pipe yielding under various loading conditions, the casing grade is the number that is used in most calculations. High-strength casing materials are more expensive, so a casing string may incorporate two or more casing grades to optimize costs while maintaining adequate mechanical performance over the length of the string. It is also important to note that, in general, the higher the yield strength, the more susceptible the casing is to sulfide stress cracking (H2S-induced cracking). Therefore, if H2S is anticipated, the well designer may not be able to use tubulars with strength as high as he or she would like.

A perforating gun assembly designed to be used in a wellbore before the productiontubulars or completion equipment have been installed, thus allowing access for a larger diameter gun assembly. Casing guns are typically 3- to 5-in. In diameter and carry up to four perforating charges per foot.

The subassembly of a wellhead that supports the casing string when it is run into the wellbore. The casing hanger provides a means of ensuring that the string is correctly located and generally incorporates a sealing device or system to isolate the casing annulus from upper wellhead components.

A generic term used to describe equipment attached to, and run with, the casing string. Commonly used casing hardware includes guide or float shoes, float or landing collars, centralizers, scratchers and cement baskets. More specialized casing hardware may include stage-cementing collars, differential fill-up equipment and other specialized equipment to help achieve successful placement and cementation of the casing string.

A length of steel pipe, generally around 40 ft [13 m] long with a threaded connection at each end. Casing joints are assembled to form a casing string of the correct length and specification for the wellbore in which it is installed.

A downhole assembly or tool system used in the remedial repair of casing damage, corrosion or leaks. Casing patches are most frequently used as short- to medium-term repairs that enable production to be resumed until a major workover operation is scheduled. In some cases, such as in depleted wells nearing the end of viable production, a casing patch may be the only economic means of safely returning the well to production.

The location, or depth, at which drilling an interval of a particular diameter hole ceases, so that casing of a given size can be run and cemented. Establishing correct casing points is important in the design of the drilling fluid program. The casing point may be a predetermined depth, or it may be selected onsite by a pressure hunt team, selected onsite according to geological observations or dictated by problems in the openhole section. In many cases, weak or underpressure zones must be protected by casing to enable mud weight adjustments that control unstable formations or overpressure zones deeper in the wellbore.

A term used in well-control operations, typically during the drilling or workover phases of a well, to describe the pressure in the drillpipe or tubing annulus.

Movement applied to the casing string during the cementing operation to help in removal of drilling fluid and efficient placement of the cementslurry.

A heavy-duty downhole tool used to restore the internal diameter of collapsed or buckled casing. Casing rollers generally are configured with an incremental series of rollers that act to gradually form the damaged casing to the desired size. Depending on the degree of damage and the requirement for wellbore access below the site of damage, the nominal diameter of the casing roller and repaired wellbore may be significantly less than the nominal drift diameter of the original casing string.

A downhole tool incorporating a blade assembly that is used to remove scale and debris from the internal surface of a casing string. Generally run on tubing or drillpipe, casing scrapers are routinely used during workover operations to ensure that the wellbore is clean before reinstalling the completion string.

A short assembly, typically manufactured from a heavy steel collar and profiled cement interior, that is screwed to the bottom of a casing string. The rounded profile helps guide the casing string past any ledges or obstructions that would prevent the string from being correctly located in the wellbore.

A pressure test applied to the formation directly below a casing shoe. The test is generally conducted soon after drilling resumes after an intermediate casing string has been set. The purpose of the test is to determine the maximum pressures that may be safely applied without the risk of formation breakdown. The results of the test are used to design the mud program for the subsequent hole section and to set safe limits on casing shut-in or choke pressures for well-control purposes.

A wellhead component used in flanged wellhead assemblies to secure the upper end of a casing string. Casing spools or bowls are available in a wide range of sizes and pressure ratings and are selected to suit the specific conditions.

An assembled length of steel pipe configured to suit a specific wellbore. The sections of pipe are connected and lowered into a wellbore, then cemented in place. The pipe joints are typically approximately 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Long casing strings may require higher strength materials on the upper portion of the string to withstand the string load. Lower portions of the string may be assembled with casing of a greater wall thickness to withstand the extreme pressures likely at depth.Casing is run to protect or isolate formations adjacent to the wellbore. The following are the most common reasons for running casing in a well:1) protect fresh-water aquifers (surface casing)2) provide strength for installation of wellhead equipment, including BOPs3) provide pressure integrity so that wellhead equipment, including BOPs, may be closed4) seal off leaky or fractured formations into which drilling fluids are lost 5) seal off low-strength formations so that higher strength (and generally higher pressure) formations may be penetrated safely6) seal off high-pressure zones so that lower pressure formations may be drilled with lower drilling fluid densities7) seal off troublesome formations, such as flowing salt8) comply with regulatory requirements (usually related to one of the factors listed above).

A short crossoverjoint used between two sizes or specifications of casing. A circulating swage is an adapter that enables a temporary circulating line to be rigged to the top of the casing string, allowing circulation of fluids to help properly locate the casing string.

A general term used to describe a drillstem test (DST) performed in cased hole.

The threadform found on casing joints. In addition to providing mechanical or structural strength, the casing thread must be compatible with the pressures and fluids associated with the application. Some advanced threadforms incorporate a gas seal.

A valve installed in the wellhead assembly to provide access to the casingannulus of non-producing casings.

An in situ record of casing thickness and integrity, to determine whether and to what extent the casing has undergone corrosion. The term refers to an individual measurement, or a combination of measurements using acoustic, electrical and mechanical techniques, to evaluate the casing thickness and other parameters. The log is usually presented with the basic measurements and an estimate of metal loss. It was first introduced in the early 1960s. Today the terms casing-evaluation log and pipe-inspection log are used synonymously.

An in situ log of the electrical potential on the inner wall of a casing. The log is used to identify intervals that are susceptible to corrosion. A negative slope in the profile indicates a zone in which current is leaving the casing and therefore acting as an anode. Such zones are susceptible to corrosion. The log was first introduced in the early 1960s. Modern logs are recorded with the tool stationary, and measure the potential difference and casing resistance between several pairs of sensors pushed against the casing wall, and between sensors and surface.The log is usually represented with casing resistance and casing axial current. Sharp increases in casing resistance can indicate corroded zones or even holes in the casing. Decreasing axial current with depth indicates a corroding region.

The adapter between the first casing string and either the BOP stack (during drilling) or the wellhead (after completion). This adapter may be threaded or welded onto the casing, and may have a flanged or clamped connection to match the BOP stack or wellhead.

A relatively thin cable used with other equipment to move small rig and drillstring components and to provide tension on the tongs for tightening or loosening threaded connections.

Pertaining to a type of metamorphicrock with shearing and granulation of minerals caused by high mechanical stress during faulting or dynamic metamorphism, typically during episodes of plate tectonic activity.

The physical and chemical alteration of sediments and pore fluids at temperatures and pressures higher than those of diagenesis. Catagenesis involves heating in the range of 50° to 150°C [122° to 302°F]. At these temperatures, chemical bonds break down in kerogen and clays within shale, generating liquid hydrocarbons. At the high end of this temperature range, secondary cracking of oil molecules can generate gas molecules.

A clutched spool connected to the drawworks power system used to tension chains, cables and softline rope.

A technique used to minimize the rate of corrosion of a structure. Cathodic protection does not eliminate corrosion, it transfers corrosion from the structure under protection to a known location where artificial anodes (plates or metal bars) are placed and could be replaced easily. Cathodic protection is used for floating vessels, platforms, storage tanks and pipelines The cathodic protection principle is based on the electrochemical nature of the corrosion phenomena; the anodic area corrodes (current is discharged) and the cathodic area does not corrode (current is received). Cathodic protection overrides the naturally occurring anodic areas inside a structure, thus turning the structure under protection completely cathodic, which means it receives current from the surrounding electrolyte (for example, soils, water) and does not corrode. Cathodic protection is achieved by passing enough direct current electricity from an external source (a more powerful anode), which could be a galvanic anode or an impressed current anode.

(noun) A positively charged ion formed when an atom loses one or more electrons. In drilling fluid chemistry, cations such as sodium (Na⁺), calcium (Ca²⁺), and potassium (K⁺) interact with clay minerals to influence hydration, swelling, dispersion, and flocculation behaviour, significantly affecting mud properties and shale stability.

Quantity of positively charged ions (cations) that a claymineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3.

Quantity of positively charged ions (cations) that a claymineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3.

A relatively thin cable used with other equipment to move small rig and drillstring components and to provide tension on the tongs for tightening or loosening threaded connections.

A long, rectangular platform about 3 ft [0.9 m] high, usually made of steel and located perpendicular to the vee-door at the bottom of the slide. This platform is used as a staging area for rig and drillstring tools, components that are about to be picked up and run, or components that have been run and are being laid down. A catwalk is also the functionally similar staging area, especially on offshore drilling rigs, that may not be a separate or raised structure.

A test used to determine if a barite sample contains caustic-soluble sulfide or carbonate minerals.Reference:Binder GG, Carlton LA and Garrett RL: "Evaluating Barite as a Source of Soluble Carbonate and Sulfide Contamination in Drilling Fluids," Journal of Petroleum Technology 33, no. 12 (December 1981): 2371-2376.Garrett RL: "Quality Requirements for Industrial Minerals Used in Drilling Fluids," Mining Engineering 39, no. 11 (November 1987): 1011-1016.

The common name for potassium hydroxide [KOH]. Caustic potash is used in potassium-based water muds to increase pH and alkalinity and to help maintain the K+ ion concentration. As the name implies, it is highly caustic and gives off heat when dissolved in water. Caustic potash is hazardous to use without proper training and equipment.

The common name for sodium hydroxide [NaOH]. Caustic soda is used in most water-base muds to increase and maintain pH and alkalinity. It is a hazardous material to handle because it is very caustic and gives off heat when dissolved in water. Proper training and equipment are needed to handle it safely.

The effect of a sharp change in the borehole diameter, such as that caused by a cave or rugose hole, on an induction log. In smooth boreholes of constant diameter, the effect of the borehole is well understood and can be corrected for. However, a sharp increase in diameter over a small depth interval can induce signals on one coil in the array and not in others. This signal is not handled by the normal borehole correction and may result in a spike on the log. The spike usually is significant only when the resistivity is high and the contrast between formation and borehole resistivity is very large. The spike also depends on the design of the array or the processing.

Pieces of rock that came from the wellbore but that were not removed directly by the action of the drill bit. Cavings can be splinters, shards, chunks and various shapes of rock, usually spalling from shale sections that have become unstable. The shape of the caving can indicate why the rock failure occurred. The term is typically used in the plural form.

An implosion produced by locally low pressure, such as the collapse of a gas bubble in liquid (the energy of which is used as the source of seismic energy from air guns).

A dug-out area, possibly lined with wood, cement or very large diameter (6 ft [1.8 m]) thin-wall pipe, located below the rig. The cellar serves as a cavity in which the casing spool and casinghead reside. The depth of the cellar is such that the master valve of the Christmas tree are easy to reach from ground level. On smaller rigs, the cellar also serves as the place where the lower part of the BOP stack resides, which reduces the rig height necessary to clear the BOP stack on the top. Prior to setting surface casing, the cellar also takes mud returns from the well, which are pumped back to the surface mud equipment.

A generic term used to describe Portland cement used in oil- and gas-well applications. In its simplest form, cement powder is ground from kiln-fired limestone and clay. However, modern oilfield cements are precise blends of quality assured materials to achieve consistent and predictable performance. Cement sets as the water in the slurry reacts chemically with the active ingredients, the most significant of which is tricalcium silicate reacting to create calcium silicate hydrate. Cement additives are used to control the setting process of the cement slurry and enhance the performance of the set cement. The API has developed a classification system for oilwell cement specification and performance.

A chemical additive mixed with cementslurry to reduce the time required for the set cement to develop sufficient compressive strength to enable drilling operations to continue. Accelerators are generally used in near-surface applications in which the temperature is relatively low.

Chemicals and materials added to a cementslurry to modify the characteristics of the slurry or set cement. Cement additives may be broadly categorized as accelerators, retarders, fluid-loss additives, dispersants, extenders, weighting agents, lost circulation additives and special additives designed for specific operating conditions. Cement additives are commonly available in powder or liquid form, enabling some flexibility in how the cement slurry is prepared.

A log that uses the variations in amplitude of an acousticsignal traveling down the casing wall between a transmitter and receiver to determine the quality of cement bond on the exterior casing wall. The fundamental principle is that the acoustic signal will be more attenuated in the presence of cement than if the casing were uncemented. The measurement is largely qualitative, as there is no indication of azimuthal cement variations such as channeling, and as it is sensitive to the effect of a microannulus.

A chemical additive that reduces the cementslurryviscosity to improve fluid flow characteristics. Adequately dispersed cement slurries exhibit improved fluid-loss control, can displace drilling fluid more efficiently and be successfully mixed and pumped at higher densities.

A chemical additive or inert material used to decrease the density or increase the yield of a cementslurry. The slurry yield is typically expressed in cubic feet of slurry per sack of cement. Increasing the yield reduces the cost per volume of cement slurry, while reducing the slurry density reduces the hydrostatic pressure of the cement column, enabling weak zones to be successfully cemented and isolated.

A device fitted to the top joint of a casing string to hold a cement plug before it is pumped down the casing during the cementing operation. In most operations, a bottom plug is launched before the spacer or cement slurry. The top plug is released from the cement head after the spacer fluid. Most cement heads can hold both the top and bottom plugs. A manifold incorporated into the cement head assembly allows connection of a fluid circulation line.

A balanced plug of cementslurry placed in the wellbore. Cement plugs are used for a variety of applications including hydraulic isolation, provision of a secure platform, and in window-milling operations for sidetracking a new wellbore.

An isolation tool set in the casing or liner that enables treatments to be applied to a lower interval while providing isolation from the annulus above. Cement retainers are typically used in cement squeeze or similar remedial treatments. A specially profiled probe, known as a stinger, is attached to the bottom of the tubing string to engage in the retainer during operation. When the stinger is removed, the valve assembly isolates the wellbore below the cement retainer.

A chemical agent used to increase the thickening time of cementslurries to enable proper placement. The need for cement retardation increases with depth due to the greater time required to complete the cementing operation and the effect of increased temperature on the cement-setting process.

A remedial cementing operation designed to force cement into leak paths in wellbore tubulars. The required squeeze pressure is achieved by carefully controlling pump pressure. Squeeze cementing operations may be performed to repair poor primary cement jobs, isolate perforations or repair damaged casing or liner.

The process of precipitation of cement between mineral or rock grains and forming solid clastic sedimentary rock, one phase of lithification.

The exponent of porosity, m, in the relation of formation factor, F, to porosity, phi. In the Archie equation, F = 1 / phim, H. Guyod termed m the cementation exponent because m was observed to be higher in cemented rock. The more general term is porosity exponent.Reference:Guyod H: Fundamental Data for the Interpretation of Electric Logs, The Oil Weekly 115, no. 38 (October 30, 1944): 21-27.

The colloquial term for the crew member in charge of a specialized cementing crew and trucks.

To prepare and pump cement into place in a wellbore. Cementing operations may be undertaken to seal the annulus after a casing string has been run, to seal a lost circulation zone, to set a plug in an existing well from which to push off with directional tools or to plug a well so that it may be abandoned. Before cementing operations commence, engineers determine the volume of cement (commonly with the help of a caliper log) to be placed in the wellbore and the physical properties of both the slurry and the set cement needed, including density and viscosity. A cementing crew uses special mixers and pumps to displace drilling fluids and place cement in the wellbore.

A rubber plug used to separate the cementslurry from other fluids, reducing contamination and maintaining predictable slurry performance. Two types of cementing plug are typically used on a cementing operation. The bottom plug is launched ahead of the cement slurry to minimize contamination by fluids inside the casing prior to cementing. A diaphragm in the plug body ruptures to allow the cement slurry to pass through after the plug reaches the landing collar. The top plug has a solid body that provides positive indication of contact with the landing collar and bottom plug through an increase in pump pressure.

A unit of measurement for viscosity equivalent to one-hundredth of a poise and symbolized by cp. Viscosity is the ratio of shear stress to shear rate, giving the traditional unit of dyne-sec/cm2 for Poise. In metric (SI) units, one cp is one millipascal-second.

A device that helps to maintain the logging tool in the center of the borehole. On wireline tools such devices typically have three or more flexible bow springs. They may be mounted on the outside surface of the logging tool or else mounted in-line, between two cartridges or sondes. Some measurements, such as acoustic logs, respond better when the tool is centralized, while others, including induction logs, are better when eccentralized.

A type of pump commonly used in the handling and mixing of oilfield fluids. The rotary motion of a profiled impeller in combination with a shaped pump housing or volute applies centrifugal force to discharge fluids from the pump. Centrifugal pumps generally operate most efficiently in high-volume, low-output-pressure conditions. Unlike a positive-displacement pump, the flow from centrifugal pumps can be controlled easily, even allowing flow to be completely closed off using valves on the pump discharge manifold while the pump is running. This pump is sometimes known as a "C pump."

A rapidly rotating flywheel on a vertical axle to whose rim is attached a series of tubes at one end, the other end being free to tilt upwards and outwards. At high speeds, the centrifugal force in the tubes is far greater than gravity. The centrifuge is used to expel fluids from core samples, either to clean them or to desaturate them for measurements of irreducible water saturation, resistivity index or nuclear magnetic resonance properties. It can be used at multiple speeds to obtain a capillary pressure curve. Centrifuges are also used in studies of clay mineralogy, where micron-scale fractions are extracted from suspension and subjected to analysis by X-ray diffraction (XRD).

A salt of cesium hydroxide and acetic acid, with formula CH3COO-Cs+, used to make high-density completion fluids. It has neutral to alkalinepH in water solutions and has better temperature stability than cesium formate.

A neutral to slightly alkalinesalt of cesium hydroxide and formic acid having the formula HCOO-Cs+. It is extremely soluble in water. An 82 wt.% cesium formate solution has a density of 2.4 g/cm3 [19.9 lbm/gal]. It has shown favorable health, safety and environmental (HSE) characteristics in laboratory tests and has applications as a drill-in, completion or workover fluid. Cesium formate can be mixed with less expensive potassium formate to make clear brine mixtures with a density range from 1.8 to 2.4 g/cm3. Formates have temperature stability in the range of 375°F [190°C], depending on the duration of exposure to such a temperature.

A type of pipe wrench used for hand-tightening various threaded connections around the rigsite. It consists of a handle, a set of gripping die teeth, a length of flat chain and a hooking slot where the chain may be adjusted to fit the pipe.

A porousmarinelimestone composed of fine-grained remains of microorganisms with calcite shells, coccolithophores, such as the White Cliffs of Dover (UK). The Austin Chalk of the US Gulf coast is a prolific, fractured oil reservoir that spurred widespread horizontal drilling activity.

A device to carry data from a receiver to a recorder, such as from a group of geophones. Simultaneous recording of 500 to 2000 channels is common during 3D seismic acquisition, and 120 to 240 channels during onshore 2D seismic acquisition.

An type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal.

The condition in which cement flows in a channel only on some sides of the casing or boreholeannulus, and thus does not provide adequate hydraulic isolation in all radial azimuths. The channel frequently manifests itself as an intermediate amplitudesignal on a cement bond log. Pulse-echo tools are able to detect a channel because they measure the cement bond at different azimuths.

A distinguishing feature of a waveform in a seismicevent, such as shape, frequency, phase or continuity.

A formation interval that has become overpressured by the injection of drilling or treatment fluids.

A mechanical device that permits fluid to flow or pressure to act in one direction only. Check valves are used in a variety of oil and gas industry applications as control or safety devices. Check valve designs are tailored to specific fluid types and operating conditions. Some designs are less tolerant of debris, while others may obstruct the bore of the conduit or tubing in which the check valve is fitted.

A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wavevelocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore.

To combine a metal ion and a complexing agent to form a ring structure.

A chemical added to an acid to stabilize iron. The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron [Fe+3] or ferrous iron [Fe+2]. If the iron is not controlled, it will precipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide [FeS], which will damage the formation.Chelating agents associate with iron [Fe+3 or Fe+2] to form soluble complexes. Citric acid, acetic acid and EDTA are effective chelating agents and can be used at temperatures up to 400oF [204oC].

An equilibrium reaction between a metal ion and a complexing agent. Chelation reactions are characterized by the formation of more than one bond between the metal and a molecule of the complexing agent. Chelation results in the formation of a ring structure incorporating the metal ion. In the oil field, chelation is often used to enhance stimulation treatments and to clean surface facilities.

A 20- to 50-gallon [3.2- to 7.9 m3] container for liquid mud additives, usually located above the suction pit on a drilling rig. The chemical barrel is used to slowly dispense various types of liquids into the active mud system. It has traditionally been used to add caustic (NaOH or KOH) solution at a slow and steady rate in order to maintain a uniform pH throughout a circulating mud system. Adding caustic solution is an especially risky operation and the proper design and use of the chemical barrel for safety is vitally important. Derrickmen must be informed of the dangers, proper protective clothing and safety rules to follow when using the chemical barrel.

A downhole tool run on wireline to sever tubing at a predetermined point when the tubing string becomes stuck. When activated, the chemical cutter use a small explosive charge to forcefully direct high-pressure jets of highly corrosive material in a circumferential pattern against the tubular wall. The nearly instantaneous massive corrosion of the surrounding tubing wall creates a relatively even cut with minimal distortion of the tubing, aiding subsequent fishing operations.

Use of a chemical agent to achieve diversion during matrix stimulation or similar injected treatments.

A chemical agent used in stimulation treatments to ensure uniform injection over the area to be treated. Chemical diverters function by creating a temporary blocking effect that is safely cleaned up following the treatment, enabling enhanced productivity throughout the treated interval.In matrix acidizing of injection wells, benzoic acid is used as a chemical diverter, while oil-soluble resins are employed in production wells. Both compounds are slightly soluble or inert in the acidic medium [HCl], but after functioning as diverters, they dissolve with water injection or oil production, respectively. Stable, viscous foams generated in the rockmatrix are also considered to be chemical diverters.

A general term for injection processes that use special chemical solutions. Micellar, alkaline and soap-like substances are used to reduce surface tension between oil and water in the reservoir, whereas polymers such as polyacrylamide or polysaccharide are employed to improve sweep efficiency. The chemical solutions are pumped through specially distributed injection wells to mobilize oil left behind after primary or secondary recovery. Chemical flooding is a major component of enhanced oil recovery processes and can be subdivided into micellar-polymer flooding and alkaline flooding.The general procedure of a chemical flooding includes a preflush (low-salinity water), a chemical solution (micellar or alkaline), a mobility buffer and, finally, a driving fluid (water), which displaces the chemicals and the resulting oil bank to production wells. The preflush and the mobility buffer are optional fluids.

A general term for injection processes that use special chemical solutions to improve oil recovery, remove formation damage, clean blocked perforations or formation layers, reduce or inhibitcorrosion, upgrade crude oil, or address crude oil flow-assurance issues. Injection can be administered continuously, in batches, in injection wells, or at times in production wells.

An encapsulated radioactive material that emits neutrons for use in neutron porosity measurements. The most common source relies on alpha-beryllium reactions in a 241Am-Be mixture. Beryllium releases a neutron of approximately 4 MeV when struck by an alpha particle. The americium is the source of alpha particles. 253Californium fission is an intense source of 2.3 MeV neutrons but is used only in special applications due to its short half-life of 2.65 years and special licensing requirements.

The amount of oxygen needed to oxidize reactive chemicals in a water system, typically determined by a standardized test procedure. COD is used to estimate the amount of a pollutant in an effluent. Compare to biochemical oxygen demand, BOD.

1. n. [EOR]The change in the Gibbs free energy of a system when an infinitesimally small amount of a component is added under constant pressure and temperature while keeping the mass of the other components of the system unchanged. Concentration variation within a system tends to drive a particle along a gradient from higher to lower chemical potential. Chemical potential can also be defined in terms of Helmholtz free energy under conditions of constant volume and temperature.Chemical potential equation:

A fluid, generally water-based, to thin and disperse mud in preparation for cementing. The chemical wash is pumped ahead of the cementslurry to help ensure effective mud removal and efficient cement placement. Other specialized chemical washes may be used in the remedial treatment of scales or paraffin deposits in productiontubulars.

A technique in which a slug of material is introduced into the flowstream of a producing well to determine the flow rate of one or more of the fluids. The marker has specific properties, such as high neutron capturecross section, that allow it to be detected by sensors of a productionlogging tool. Some markers are specifically designed to be soluble in only one fluid phase, so that they can be used to produce a phase-velocity log. The term refers to nonradioactive markers, in contrast to the more traditional radioactive markers, or tracers.

A sedimentaryrock and a variety of quartz made of extremely fine-grained, or cryptocrystalline, silica, also called chalcedony. The silica might be of organic origin, such as from the internal structures of sponges called spicules, or inorganic origin, such as precipitation from solution. The latter results in the formation of flint. Chert can form beds, but is more common as nodules in carbonate rocks.

A titration procedure standardized by the API to quantitatively determine Cl- (chloride ion) concentration by using silver nitrate as titrant with potassium chromate as the endpoint indicator.

[(Mg,Al,Fe) 12(Si,Al) 8O20(OH) 16]A platy, pale green mineral of the mica group of sheet silicates, also considered to be a type of clay mineral, found in sedimentary and low-grade metamorphic rocks. Chlorite is a common authigenic mineral lining the pores of sandstones. In some cases, the presence of authigenic chlorite on sand grains can inhibit the growth of pore-filling cements during diagenesis and preserve pore space for occupation by hydrocarbons.

A device incorporating an orifice that is used to control fluid flow rate or downstream system pressure. Chokes are available in several configurations for both fixed and adjustable modes of operation. Adjustable chokes enable the fluid flow and pressure parameters to be changed to suit process or production requirements. Fixed chokes do not provide this flexibility, although they are more resistant to erosion under prolonged operation or production of abrasive fluids.

A high-pressure pipe leading from an outlet on the BOP stack to the backpressurechoke and associated manifold. During well-control operations, the fluid under pressure in the wellbore flows out of the well through the choke line to the choke, reducing the fluid pressure to atmospheric pressure. In floating offshore operations, the choke and kill lines exit the subsea BOP stack and then run along the outside of the drilling riser to the surface. The volumetric and frictional effects of these long choke and kill lines must be considered to control the well properly.

A manifold assembly incorporating chokes, valves and pressure sensors used to provide control of flow back or treatment fluids.

A type of salt in which chromium atoms are in the plus-6 valence state, such as potassium chromate, K2CrO4. Chromium compounds of various types have been used in lignite and lignosulfonate and other mud additives to enhance thermal stability. Since the late 1970s, they are prohibited in muds to be discarded offshore and in other environmentally sensitive areas of the US.

Pertaining to a mud additive (usually lignosulfonate or lignite) that does not contain any chromium compounds.

A lignite that has been treated (admixed or reacted with) chromic or chromate salt, such as potassium or sodium chromate or dichromate or chromic chloride. Also, chrome lignite may contain added (sometimes reacted) KOH or NaOH. Chromed mud additives have largely been eliminated from usage in the US because of environmental concerns. Chrome lignite was more temperature-stable than plain lignite in clay-based water muds.

A lignosulfonate that has been treated by mixing or reacting into the molecular structure some form of chromium (either chromate or chromic salt). Although still used today in less environmentally sensitive areas, it has been replaced by iron or calcium lignosulfonates. Ferro-chrome lignosulfonate is a popular type of deflocculant that contains iron and chromium salts.

Tubing manufactured from an alloy containing a high proportion of chrome, typically greater than 13%. Chrome tubing is classified as a corrosion-resistant alloy (CRA) and is used where the wellbore conditions or reservoir fluid create a corrosive environment that conventional tubing cannot safely withstand. Wells that produce hydrogen sulfide, and similar corrosive fluids, typically require chrome tubing.

Pertaining to a mud additive (usually lignosulfonate or lignite) that does not contain any chromium compounds.

A salt of chromium in which chromium atoms are in the plus-3 valence state, such as chromic chloride, CrCl3. Chromic compounds are considered less harmful to the environment than chromates (plus-6 valence) because they are at a low oxidation state and not highly reactive.

A graphic display, with geologic time along the vertical axis and distance along the horizontal axis, to demonstrate the relative ages and geographic extent of strata or stratigraphic units in a given area, also known as a Wheeler diagram. In addition, information from seismic data, well logs and rock samples, and biostratigraphic and lithostratigraphic information can be shown within each chronostratigraphic unit. A chronostratigraphic chart can concisely illustrate sequence stratigraphic interpretations.

The study of the ages of strata. The comparison, or correlation, of separated strata can include study of their relative or absolute ages.

A multiphaseflow regime in near-vertical pipes in which large, irregular slugs of gas move up the center of the pipe, usually carrying droplets of oil or water with them. Most of the remaining oil or water flows up along the pipe walls. Unlike slug flow, neither phase is continuous. The gas slugs are relatively unstable, and take on large, elongated shapes. Churn flow occurs at relatively high gas velocity and is similar to froth flow. As the gas velocity increases, it changes into annular flow.

A technique for marine seismic acquisition around salt domes or other circular features in which the acquisition vessel travels in a spiral path above the feature. Circle shooting can also be performed to increase efficiency. Rather than acquiring a line of data and turning the seismic vessel around to acquire another line, the vessel can travel in a pattern of offset circles and collect data continuously.

To pump fluid through the whole active fluid system, including the borehole and all the surface tanks that constitute the primary system.

To pump the drilling fluid until a sample from the bottom of the hole reaches the surface. This is commonly performed when drilling has ceased so that the wellsite geologist may collect a cuttings sample from the formation being drilled, or when the driller suspects that a small amount of gas has entered the wellbore. Thus, by circulating out, the gas bubble is eased out of the wellbore safely.

(noun) The continuous flow of drilling fluid from the surface mud system, down through the drillstring, out through the bit nozzles, up the annulus carrying cuttings, and back to the surface for solids removal and reconditioning. Circulation is essential for hole cleaning, cooling the bit, and maintaining wellbore stability during drilling operations.

A completion component, generally included in the lower assembly near the packer, that allows communication between the tubing and annulus. Circulation devices enable the circulation of fluids for well control or kickoff purposes.

The loss of drilling fluid to a formation, usually caused when the hydrostatic headpressure of the column of drilling fluid exceeds the formation pressure. This loss of fluid may be loosely classified as seepage losses, partial losses or catastrophic losses, each of which is handled differently depending on the risk to the rig and personnel and the economics of the drilling fluid and each possible solution.

A downhole tool typically used with motors or assemblies that restrict the allowable fluid-circulation rates. When operated, the circulation sub allows a higher circulation rate to be established by opening a path to the annulus in the top section of the tool string. This is especially useful in applications such as drilling in slim-diameter wells, where a higher circulation rate may be necessary to effect good cuttings transport and hole cleaning before the string is retrieved.

The complete, circuitous path that the drilling fluid travels. Starting at the main rig pumps, major components include surface piping, the standpipe, the kelly hose (rotary), the kelly, the drillpipe, drill collars, bit nozzles, the various annular geometries of the openhole and casing strings, the bell nipple, the flowline, the mud-cleaning equipment, the mud tanks, the centrifugal precharge pumps and, finally, the positive displacement main rig pumps.

A downhole device that enables circulation through the tubing string and associated annulus. As a completion accessory, a circulation valve is included to circulate fluid for well kill or kickoff. Circulation valves typically are operated by slickline tools and are generally capable of several opening and closing cycles before requiring service.

An organic acid, properly called 2-Hydroxy-1,2,3-propanetricarboxylic acid, with formula C6H8O7. Citric acid is used to reduce the pH of drilling fluids and hence for treatment of cement contamination. It also acts as a polymer stabilizer.

A conventional method of mapping reservoir parameters in two dimensions, x and y. The resulting map set usually includes the top and bottom structure map derived from seismic and well data and that are used to generate thickness maps, in addition to maps of other geological and petrophysical parameters produced by standard interpolation techniques. These techniques are appropriate for describing reservoirs that are reasonably continuous and not too heterogeneous. They are usually much faster than full 3D techniques or geostatistical methods, but may be inaccurate when applied to description of complex, heterogeneous strata.

Sediment consisting of broken fragments derived from preexisting rocks and transported elsewhere and redeposited before forming another rock. Examples of common clastic sedimentary rocks include siliciclastic rocks such as conglomerate, sandstone, siltstone and shale. Carbonate rocks can also be broken and reworked to form clastic sedimentary rocks.

A large family of complex minerals containing the elements magnesium, aluminum, silicon and oxygen (magnesium, aluminum silicates) combined in a sheet-like structure. Clays are mined from surface pits as relatively pure deposits and used for bricks, pottery, foundry molds and in drilling fluids among other uses. Clays, as claystones, shales and intermixed with sands and sandstones make up the largest percentage of minerals drilled while exploring for oil and gas. Sodium bentonite is a useful additive for increasing the density of drilling muds, but other clay types are considered contaminants to be avoided and removed. Individual clay platelets can be viewed only with an electron microscope. Crystal structures are also determined by X-ray diffraction (XRD). The atomic structure of the clay group of layered silicate minerals varies from two-layer to three-layer or four-layer (mixed-layer) structures. One of the structural layers is a plane of silicon dioxide tetrahedra (silicon at the center and oxygen at all four corners of the tetrahedron). The other structural layer is a plane of aluminum hydroxide octahedra (aluminum at the center and hydroxides at all six corners). The tetrahedral and octahedral layers fit one on top of the other, with oxygen atoms being shared as oxide and hydroxide groups.

A class of polymers added to a drilling-grade claymineral during grinding, or added directly into a clay-based mud system, to enhance the clay's rheological performance. In concept, clay-extender polymers achieve the type of rheology needed for fast drilling with fewer colloidal solids and lower viscosity at high shear rate (at the bit). This is the concept of a "low-solids, nondispersed mud" system. Extenders are usually long-chain anionic or nonionic polymers that link clay platelets together in large networks. Anionic polymers are highly effective but can be precipitated by hardness ions. Nonionic polymers are less effective but also much less sensitive to hardness ions. Excessively long, linear polymers may break up under mechanical shearing. Either by precipitation or breakup, extender polymers can quickly become ineffective if poorly chosen and used improperly. A drilling-grade clay that has no extender is that which meets the standard for API nontreated bentonite. API bentonite and OCMA-grade API bentonite usually contain extender polymers.

A chemical additive used in stimulation treatments to prevent the migration or swelling of clay particles in reaction to water-base fluid. Without adequate protection, some water-base fluids can affect the electrical charge of naturally occurring clay platelets in the formation. Modifying the charge causes the platelets to swell or migrate in the flowing fluid and, once these are dispersed, it is likely that some clay plugging of the formation matrix will occur. Clay stabilizers act to retain the clay platelets in position by controlling the charge and electrolytic characteristics of the treatment fluid.

A type of damage in which formationpermeability is reduced because of the alteration of clay equilibrium.Clay swelling occurs when water-base filtrates from drilling, completion, workover or stimulation fluids enter the formation. Clay swelling can be caused by ion exchange or changes in salinity. However, only clays that are directly contacted by the fluid moving in the rock will react; these include authigenic clays, some detrital clays on the pore boundaries and unprotected clay cement. The nature of the reaction depends on the structure of the clays and their chemical state at the moment of contact.The most common swelling clays are smectite and smectite mixtures that create an almost impermeable barrier for fluid flow when they are located in the larger pores of a reservoir rock. In some cases, brines such as potassium chloride [KCl] are used in completion or workover operations to avoid clay swelling.

An all-inclusive term to describe various progressive interactions between clay minerals and water. In the dry state, clay packets exist in face-to-face stacks like a deck of playing cards, but clay packets begin to change when exposed to water. Five descriptive terms describe the progressive interactions that can occur in a clay-water system, such as a water mud.1) Hydration occurs as clay packets absorb water and swell.2) Dispersion (or disaggregation) causes clay platelets to break apart and disperse into the water due to loss of attractive forces as water forces the platelets farther apart.3) Flocculation begins when mechanical shearing stops and platelets previously dispersed come together due to the attractive force of surface charges on the platelets.4) Deflocculation, the opposite effect, occurs by addition of chemical deflocculant to flocculated mud; the positive edge charges are covered and attraction forces are greatly reduced.5) Aggregation, a result of ionic or thermal conditions, alters the hydrational layer around clay platelets, removes the deflocculant from positive edge charges and allows platelets to assume a face-to-face structure.

Water within the clay lattice or near the surface within the electrical double layer. This water does not move when fluid is flowed through the rock. In the normal definition used by a log analyst, clay-bound water is not part of the effective porosity and is the difference between total porosity and effective porosity. Clay-bound water is understood to include the interlayer water, although the contribution of the latter to the electrical properties of the clay may be different from the water in the electrical double layer.In the dual-water and the Hill-Shirley-Klein models, the volume of clay-bound water is related to the cation-exchange capacity per unit volume, Qv, by expressions that depend on the salinity and temperature of the electrolyte in which the clay is immersed. Direct measurement of the clay-bound water volume in the laboratory is difficult.

An all-inclusive term to describe various progressive interactions between clay minerals and water. In the dry state, clay packets exist in face-to-face stacks like a deck of playing cards, but clay packets begin to change when exposed to water. Five descriptive terms describe the progressive interactions that can occur in a clay-water system, such as a water mud.1) Hydration occurs as clay packets absorb water and swell.2) Dispersion (or disaggregation) causes clay platelets to break apart and disperse into the water due to loss of attractive forces as water forces the platelets farther apart.3) Flocculation begins when mechanical shearing stops and platelets previously dispersed come together due to the attractive force of surface charges on the platelets.4) Deflocculation, the opposite effect, occurs by addition of chemical deflocculant to flocculated mud; the positive edge charges are covered and attraction forces are greatly reduced.5) Aggregation, a result of ionic or thermal conditions, alters the hydrational layer around clay platelets, removes the deflocculant from positive edge charges and allows platelets to assume a face-to-face structure.

Pertaining to a sedimentary rock, such as sandstone or limestone, that contains only minimal amounts of clay minerals. Clean reservoir rocks typically have better porosity and permeability than dirty rocks whose pores are clogged with fine clay particles. Clean and dirty are qualitative, descriptive terms.

The removal of wellbore-fill material, such as sand, scale or organic materials, and other debris from the wellbore. Many reservoirs produce some sand or fines that may not be carried to surface in the produced fluid. Accumulations of fill material may eventually increase in concentration within the lower wellbore, possibly restricting production. Cleanouts using coiled tubing, snubbing or hydraulic workover techniques are performed routinely.

A period of controlled production, generally following a stimulation treatment, during which time treatment fluids return from the reservoir formation. Depending on the treatment, the cleanup period can be relatively short and uncomplicated. However, following more complex treatments such as gravel pack or hydraulic fracturing, the cleanup process should be conducted carefully to avoid jeopardizing the long-term efficiency of the treatment.

Drilling operations using a water-base drilling fluid that contains few solids. Clear-water drilling is done in "hard rocks" in which density and fluid loss are not critical. Rapid drilling rate is the incentive for using a solids-free mud. Fluid returned to the surface must be screened and processed by hydrocyclones and centrifuges to remove larger solids. Colloidal solids can be agglomerated by adding polymers and removing the aggregates. Polymers such as acrylates, acrylamides and partially-hydrolyzed polyacrylamides are used. They are added at the flowline as mud exits the well or added in pits downstream from the flowline.

Drilling operations using a water-base drilling fluid that contains few solids. Clear-water drilling is done in "hard rocks" in which density and fluid loss are not critical. Rapid drilling rate is the incentive for using a solids-free mud. Fluid returned to the surface must be screened and processed by hydrocyclones and centrifuges to remove larger solids. Colloidal solids can be agglomerated by adding polymers and removing the aggregates. Polymers such as acrylates, acrylamides and partially-hydrolyzed polyacrylamides are used. They are added at the flowline as mud exits the well or added in pits downstream from the flowline.

To close a valve to stop or isolate fluid flow. The term is most commonly applied to "closing-in the well," meaning isolation of the wellbore by closing the master valve.

A mud and solids-control system in which the only discarded waste is moist, drilled-up rock materials. Such systems are used for drilling wells in environmentally sensitive areas. No reserve-mud pit is used in a truly closed mud system. Mud is continually processed primarily by mechanical means, such as screening, hydrocycloning and centrifuging to remove solids initially. A second stage to remove colloidal solids is by wastewater cleanup techniques.

A type of drillstem testing conducted with the drillstring in the hole and the surface valve closed to create a closed chamber of known volume into which the reservoir fluid can flow. The drillstring is sometimes filled with nitrogen at a relatively low pressure prior to testing. When the well begins to flow, the nitrogen or air is compressed and the volume of fluid inflow can be calculated as a function of time by monitoring the surface pressure in the drillstring. The bottomhole valve is closed to halt flow when the surface pressure reaches a value calculated prior to testing. This ensures that a precisely known amount of production has taken place.

A well with a valve closed to halt production. Wells are often closed in for a period of time to allow stabilization prior to beginning a drawdown-buildup test sequence.

A generic term given to the hydraulic power pack and accumulators used to control the blowout preventers on a drilling or workoverrig.

The area, or areal closure, included in the lowest closing contour of a trap. Measurements of both the areal closure and the distance from the apex to the lowest closing contour are typically incorporated in calculations of the estimated hydrocarbon content of a trap.

An analysis parameter used in hydraulic fracture design to indicate the pressure at which the fracture effectively closes without proppant in place.

The temperature at which wax crystals first start to form in a crude oil. Wax appearance temperature (WAT) and wax precipitation temperature (WPT) are other synonyms.

The act of determining clusters from data sets.

Mathematical techniques for summarizing large amounts of multidimensional data into groups. The two most popular techniques are: hierarchicalk-means.The hierarchical system calculates as many clusters as there are data points and displays their relative closeness by means of a dendogram. This system is preferred when there are few data points but the user wishes to see the dendogram to chose an appropriate number of clusters for analysis. Principal Component Analysis (PCA) is a form of hierarchical cluster analysis.The k-means system requires the user to choose the number of cluster to be determined. The computation scatters the centers of the clusters among the data and then moves them until they are "gravitationally bound" to the larger groups of data and no longer move. The points determined in this way represent the central points of the clusters. This technique is very fast and appropriate for very large data sets. It is most commonly used in electrofacies calculations.Cluster analysis is often used to provide electrofacies from wireline data where each curve is set to be a dimension.

A carbon-rich sedimentaryrock that forms from the remains of plants deposited as peat in swampy environments. Burial and increase in temperature bring about physical and chemical changes called coalification. Because of the organic origin of coal, it cannot be classified as a mineral. The main types of coal, anthracite, bituminous coal and lignite, can be distinguished by their hardness and energy content, which are affected by their organic content as well as their conditions of formation. Natural gas associated with coal, called coal gas or coalbed methane, can be produced economically from coal beds in some areas. In some basins coals form source rocks.

The process of droplet growth as small drops merge together when they come in contact. If this occurs repeatedly, a continuous liquid phase forms. Through this phenomenon, emulsions break and form two distinct liquid phases that tend to separate. In oil-base mud, the water phase is dispersed as small droplets, with oil as the continuous (external) phase. A stable oil mud will remain dispersed under normal drilling conditions because when droplets contact each other, they do not coalesce due to the strong emulsifier film around each droplet. However, when the emulsion film around each droplet becomes weakened, droplets will begin to coalesce. If not corrected, this can lead to total emulsion breakdown with solids becoming water-wetted.

Referring in the strict sense (API Bulletin 13C) to any particle larger than 2000 microns.

Any thin material, liquid or powder, which, applied over a structure, forms a continuous film to protect against corrosion.Corrosion coatings should possess flexibility, resistance against impact and moisture, good adhesion and cohesion, and chemical resistance to the exposure conditions (such as temperature, hydrogen sulfide).Organic coatings such as polyethylenes (plastic) are normally used for external protection of pipelines while asphalt and coal tar enamels are used to protect buried pipes or undersides of oilfield tanks. Inorganic coating such as zinc-silicate is used to protect drilling and production platforms above the splash zone and nickel phosphate coating is used to protect packer body parts.

A void in the pipe coating. Coating flaws are detected by either mechanical or visual inspections and must be repaired to avoid significant corrosion problems.A coating flaw is also called a holiday.

The characteristics of the trace used to display a log. The most common codings are solid, long-dashed, short-dashed and dotted. The trace can also have a different line weight or thickness, from light to heavy.

The process of generating two or more forms of energy from a single energy source. For example, in a heavy oilfield, turbines are often used to generate electricity while their waste heat is removed to generate steam. Other alternatives exist, with turbines being run by burning gas or crude oil. Alternatively, the primary heat source can be used to generate steam directly at extremely high pressure and temperature, with the steam being run through a turbine to generate electricity before the steam is distributed to injection wells.

A quantitative assessment of the similarity of three or more functions, also called semblance.

A technique for removing noise and emphasizing coherent events from multiple channels of seismic data.

(noun) A seismic attribute display that measures the trace-to-trace similarity of seismic waveforms across a survey area. Areas of low coherence indicate discontinuities such as faults, fractures, stratigraphic edges, or channel boundaries, making coherence maps a valuable tool for structural and stratigraphic interpretation.

A map that displays the degree of correlation between wells as a vector that points from one well to another, the length of the vector being related to the degree of correlation from a correlogram. These maps are used in automatic correlation of well logs across a field and indicate where formations are continuous or are terminated.Reference:Poelchau HS: Coherence Mapping - An Automated Approach to Display Goodness-of-Correlation Between Wells in a Field, Mathematical Geology 19, no. 8 (1987): 833-850.

Pertaining to seismic events that show continuity from trace to trace. Seismic processing to enhance recognition of coherent events and emphasize discontinuities such as faults and stratigraphic changes has gained popularity since the mid-1990s.

Undesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples.

A technique for acquiring full-azimuthmarineseismic data. This technique uses a vessel equipped with source arrays and streamers to shoot and record seismic data; however, unlike conventional surveys acquired in a series of parallel straight lines, coil shooting surveys are acquired as the vessel steams in a series of overlapping, continuously linked circles, or coils. The circular shooting geometry acquires a full range of offset data across every azimuth to sample the subsurface geology in all directions. The resulting full azimuth (FAZ) data are used to image complex geology, such as highly faulted strata, basalt, carbonate reefs and subsalt formations.

A generic term relating to the use of a coiled tubing string and associated equipment. As a well-intervention method, coiled tubing techniques offer several key benefits over alternative well-intervention technologies. The ability to work safely under live well conditions, with a continuous string, enables fluids to be pumped at any time regardless of the position or direction of travel. This is a significant advantage in many applications. Installing an electrical conductor or hydraulic conduit further enhances the capability of a coiled tubing string and enables relatively complex intervention techniques to be applied safely.

A completion that utilizes coiled tubing as the production conduit, or as a means of conveying and installing completion equipment or components. Since the coiled tubing string is continuous, problems associated with connections are avoided. Also, the pressure-control equipment used on coiled tubing operations enables work to be safely conducted on live wells.

The downhole device used to connect the tool string to the coiled tubing string. Several types of devices with varying principles of operation are commonly used. The primary requirement is provision of an adequate mechanical connection capable of withstanding the necessary tensile and compressive forces, while ensuring efficient hydraulic isolation of the connection between the tool string and the coiled tubing string.

The use of coiled tubing with downhole mud motors to turn the bit to deepen a wellbore. Coiled tubing drilling operations proceed quickly compared to using a jointed pipe drilling rig because connection time is eliminated during tripping. Coiled tubing drilling is economical in several applications, such as drilling slimmer wells, areas where a small rig footprint is essential, reentering wells and drilling underbalanced.

A continuous length of low-alloy carbon-steel tubing that can be spooled on a reel for transport, then deployed into a wellbore for the placement of fluids or manipulation of tools during workover and well-intervention operations. The process of spooling and straightening a coiled tubing string imparts a high degree of fatigue to the tube material. Therefore, a coiled tubing string should be regarded as a consumable product with a finite service life. Predicting and managing the factors that affect the safe working life of a coiled tubing string are key components of the string-management system necessary for ensuring safe and efficient coiled tubing operations.

The package of equipment required to run a coiled tubing operation. Four basic components are required: the coiled tubing reel to store and transport the coiled tubing string, the injector head to provide the tractive effort to run and retrieve the coiled tubing string, the control cabin from which the equipment operator controls and monitors the operation, and the power pack that generates the necessary hydraulic and pneumatic power required by the other components. The dimensions and capacities of the coiled tubing unit components determine the size and length of coiled tubing string that can be used on the unit. Pressure-control equipment is incorporated into the equipment to provide the necessary control of well pressure fluid during normal operating conditions and contingency situations requiring emergency control.

An insoluble organic deposit that has low hydrogen content. Coke, also known as pyrobitumen, is formed by thermal cracking and distillation during in-situ combustion.

A form of kriging that involves multiple variables. For example, well data may be used to generate one semivariogram, and three-dimensional seismic data used to generate another. Both semivariograms, along with a cross-variogram model, can then be used to generate a cokriged map.

A non-thermal primary process for producing heavy oil, also called CHOPS. In this method, continuous production of sand improves the recovery of heavy oil from the reservoir. There is both a theoretical basis and physical evidence that, in many cases, wormholes are formed in the unconsolidated sand reservoir, thereby increasing oil productivity. In most cases, an artificial lift system is used to lift the oil with sand.

Nonthermal primary methods of heavy oilproduction, which include technologies such as production with horizontal wells, multilaterals, CHOPS, water or gas injection.

The pressure at which a tube, or vessel, will catastrophically deform as a result of differential pressure acting from outside to inside of the vessel or tube. The collapse-pressure rating of perfectly round tubing is relatively high. However, when the tubing is even slightly oval, the differential pressure at which the tube will collapse may be significantly reduced. This is an important factor in determining the operating limits of coiled tubing strings since the action of spooling the string tends to induce some ovality.

A threaded coupling used to join two lengths of pipe such as production tubing, casing or liner. The type of thread and style of collar varies with the specifications and manufacturer of the tubing.

A downhole tool or logging device used to detect and track (log) casing or tubing collars across a zone of interest, typically for correlation purposes. Most collar locators detect the magnetic anomaly created by the mass of the steel collar and transmit a signal to surface-display and depth correlation equipment.

A type of lock designed to be set in the recess of a tubing collar. Collar locks are compatible only with conventional thread connections where a space exists between the two tubing joints. Premium tubing grades have flush internal surfaces with no space to enable setting of the retaining dogs.

A log showing the depth or relative position of casing or tubing collars that is used to correlate depth for depth-sensitive applications such as perforating or isolation treatments. Indications are provided by a collar locator tool and correlations are made with previous baseline logs, such as the gamma ray log, or the casing or tubing running tally prepared during the installation process.

The electrical device used on the axle of a spool or reel to provide electrical continuity between the rotating reel core and the stationary reel chassis. When using a coiled tubing string equipped with an electrical conductor, such as required during coiled tubing logging operations, a collector is fitted to the reel axle to allow connection of the surface data-acquisition equipment.

An interaction of lithospheric plates that can result in the formation of mountain belts and subduction zones. The collision of two plates of continental lithosphere, known as an A-type collision, can produce high mountains as rocks are folded, faulted and uplifted to accommodate the converging plates, as observed in the Alps and the Himalayas. B-type collisions, in which oceanic lithospheric plates collide with continental lithospheric plates, typically produce a subduction zone where the relatively denser oceanic plate descends below the relatively lighter continental plate, as seen on the Pacific coast of South America.

A finely divided, solid material, which when dispersed in a liquid medium, scatters a light beam and does not settle by gravity; such particles are usually less than 2 microns in diameter. Some drilling fluid materials become colloidal when used in a mud, such as bentonite clay, starch particles and many polymers. Oil muds contain colloidal emulsion droplets, organophilic clays and fatty-acid soap micelles.

Solid particles of size less than 2 microns equivalent spherical diameter, also identified as clay by definitions in International Standards Organization ISO/CD 13501, par. 3.1.17. Because of extremely small size, these solids largely defy direct removal by mechanical devices that rely on screening or gravitational forces. Their removal is aided by chemical aggregation prior to gravity separation or filtration.

A type of blowout preventer (BOP) in which each ram set combines two conventional ram functions, such as blind/shear and pipe/slip. The principal advantage of the combi-BOP is the reduced height required for rig up of the required ram functions.

To remove the drillstring from the wellbore.

A rate, or production volume, sufficient to satisfy project economics.

A wellbore completed in two or more reservoir zones that are not in hydraulic communication in the reservoir. Backflow (often incorrectly referred to as crossflow) is common during rate cutbacks and buildup tests on these types of completions. Analysis of buildup data acquired from a commingled completion can be difficult or impossible.

A term used to describe the flow pattern where two or more fluid phases may be present in a relatively even distribution. The flow rate and conduit geometry may cause an apparent mixing of the phases. However, if the flow characteristics are changed through flow rate or conduit geometry, fluid separation may occur. Fine solids also may be entrained in a commingled flow. Commingled flow may also describe the production of fluid from two or more separate zones through a single conduit.

In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveoutprocessing is necessary to reduce smearing, or inappropriate mixing, of the data.

In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms.

Method of seismic reflection surveying and processing that exploits the redundancy of multiple fold to enhance data quality by reducing noise. During acquisition, an energy source is supplied to a number of shotpoints simultaneously. Once data have been recorded, the energy source is moved farther down the line of acquisition, but enough overlap is left that some of the reflection points are re-recorded with a different source-to-receiver offset. Multiple shotpoints that share a source-receiver midpoint are stacked. The number of times that a common midpoint is recorded is the fold of the data.

In multichannel seismic acquisition, the common midpoint on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver that is shared by numerous locations if the reflector is flat-lying. Like common depth point, this term is commonly misused, because in the case of dipping layers, common reflection points do not exist.

Pertaining to traces that have the same offset, or distance between source and receiver.

Pertaining to traces that have a different source but share a receiver.

The combining of smaller federal tracts of land to total the acreage required by the US Bureau of Land Management and/or state regulations to form a legal spacing and proration unit.

The physical process by which sediments are consolidated, resulting in the reduction of pore space as grains are packed closer together. As layers of sediment accumulate, the ever increasing overburdenpressure during burial causes compaction of the sediments, loss of pore fluids and formation of rock as grains are welded or cemented together.

A change made to porosity measurements, such as those from sonic logs, to compensate for the lack of compaction, or the predicted loss of pore space as sediments are buried by overburden. Compaction corrections are commonly performed in uncompacted sediments.

The representative of the oil company or operator on a drilling location. For land operations, the company man is responsible for operational issues on the location, including the safety and efficiency of the project. Even administrative managers are expected to respond to the direction of the company man when they are on the rigsite. Offshore, depending on the regulatory requirements, there may be an offshore installation manager, who supervises the company man on safety and vessel integrity issues, but not on operational issues.

The productive segment of an oil or gas field that is not in fluid communication with the remainder of the field. Productive compartments may become isolated at the time of accumulation by depositional processes or become isolated after deposition and burial by diagenesis or by structural changes, such as faulting.

The geological segmentation of once continuous reservoirs into isolated compartments. Reservoirs that have become compartmentalized require different approaches to interpretation and production than continuous reservoirs. The degree of compartmentalization may vary as a consequence of production.

In matrix stimulation, a characteristic of rock that indicates formationpermeability is not reduced when treating fluids and their additives contact the formation minerals or fluids inside the reservoir.Compatibility is especially important in sandstone treatments, in which potentially damaging reactions may occur. The treatment fluid should remove existing damage without creating additional damage, such as precipitates or emulsions, through interactions with the formation rock or fluids.

Scales for different logs that are chosen so that the logs will overlay in certain conditions. For example, a sandstone-compatible scale may have the neutron log scaled from 0.45 to -0.15 vol/vol and the density from 1.9 to 2.9 g/cm3. Then, in a pure quartz sandstone filled with fresh water, the two logs will overlay as the porosity varies.

A neutron porosity log in which the effects of the borehole environment are minimized by using two detectors. In the most common technique, the two source-detector spacings are chosen so that the ratio of the two count rates is relatively independent of the borehole environment. This ratio is then calibrated in terms of porosity in a known formation and borehole environment typically with the tool placed against the side of an 8-in. [20-cm] borehole in a limestone block, both filled with fresh water at surface temperature and pressure. The response is also determined at different porosities and in sandstones, dolomites and other borehole environments. Correction factors are developed to convert the measured log to the standard conditions.The source and detectors are not azimuthally focused. Wireline tools are run eccentralized against the borehole wall. Since the neutrons emitted into the mud are strongly attenuated, the resulting log is effectively focused into the formation. Measurements-while-drilling tools will normally be unfocused since they are centralized unless the borehole is overgauge.The vertical resolution is about 2 ft [0.6 m], but can be improved by alpha processing.

A density log that has been corrected for the effect of mud and mudcake by using two or more detectors at different spacings from the source. The shorter the spacing, the shallower the depth of investigation and the larger the effect of the mudcake. Thus, a short spaced detector, which is very sensitive to the mudcake, can be used to correct a long-spaced detector, which is only slightly sensitive to it.In a typical two-detector compensation scheme, the density measured by the longest spacing detector is corrected by an amount, delta rho, which is a function of the difference between long- and short-spacing densities. The correction is found to depend on the difference between formation and mudcake density multiplied by mudcake thickness. Although there are three unknowns, simple functions are reliable for moderate corrections. Experimental results are often presented in the form of a spine and ribs plot. There are other schemes using, for example, more detectors. Dual detector density logs were introduced in the mid 1960s.

Describes a bed that maintains its original thickness during deformation. Often pertains to relatively brittle, solid strata that deform by faulting, fracturing or folding, rather than flowing under stress. Incompetent beds are more ductile and tend to flow under stress, so their bed thickness changes more readily during deformation.

To perform activities in the final stages of well construction to prepare a well for production. The well is completed once zones of interest have been identified.

A generic term used to describe the events and equipment necessary to bring a wellbore into production once drilling operations have been concluded, including but not limited to the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil or gas well. Completion quality can significantly affect production from shale reservoirs.

A solids-free liquid used to "complete" an oil or gas well. This fluid is placed in the well to facilitate final operations prior to initiation of production, such as setting screens production liners, packers, downhole valves or shooting perforations into the producing zone. The fluid is meant to control a well should downhole hardware fail, without damaging the producing formation or completion components. Completion fluids are typically brines (chlorides, bromides and formates), but in theory could be any fluid of proper density and flow characteristics. The fluid should be chemically compatible with the reservoirformation and fluids, and is typically filtered to a high degree to avoid introducing solids to the near-wellbore area. Seldom is a regular drilling fluid suitable for completion operations due to its solids content, pH and ionic composition. Drill-in fluids can, in some cases, be suitable for both purposes.

An indicator used to determine the effect that key completion components have on the production efficiency of a well. If one or more of the well-completion components create a localized pressure drop, the effect may be a reduction in the production capability of the well. Such conditions are evident as completion skin.

A mathematical method to determine seismic attributes, including reflection strength and instantaneous frequency, by using the Hilbert transform, a special form of the Fourier transform, and the quadrature trace, or the component of the signal that is 90 degrees out of phase.

A single log created by splicing together two logs of the same type run at different times in the well; or by splicing two different types of log run at the same time. For example, it is common practice to splice all the basic logs run over different depth intervals in a well to obtain a single composite record.

The flow of different fluids such as oil, gas or water, in a single production stream.

Any of a variety of analytical techniques carried out to determine the composition of a crude oil by breaking it down into basic chemical components. The hydrocarbon components are usually identified by carbon number fractions: C1, C2, C3, etc. up to Cn, where the limiting carbon number, n, is defined by the particular analytical technique. These analytical techniques include, but are not limited to, gas or liquid chromatography, cryogenic and flash distillations, true boiling-point distillations, structural fluid characterizations such as polynuclear aromatic hydrocarbon analysis, SARA analysis, sonic testing and other crude oil assay methods. Other nonhydrocarbon components can also be identified, such as nitrogen, heavy metals, sulfur and salts.

(noun) A measure of the relative volume change of a fluid or porous medium in response to a change in pressure, expressed as the fractional change in volume per unit change in pressure. In reservoir engineering, compressibility values for oil, water, gas, and rock are critical parameters in material balance calculations and well test analysis.

The ratio of the volume of an engines cylinder at the beginning of the compression to its volume at the end of the compression process. For example, a cylinder with a volume of 20 cubic inches before compression and 1 cubic inch as its final volume after compression has a compression ratio of 20:1.

A type of downhole packer that is activated or set by applying compressive force to the packer assembly. In most cases, this is achieved with set-down weight from the running string, which is controlled by the driller or operator observing the weight indicator on the rig or coiled tubing unit.

A device that raises the pressure of air or natural gas. A compressor normally uses positive displacement to compress the gas to higher pressures so that the gas can flow into pipelines and other facilities.

A facility consisting of many compressors, auxiliary treatment equipment and pipeline installations to pump natural gas under pressure over long distances. A compressor plant is also called a compressor station. Several compressor stations can be used to repressurize gas in large interstate gas pipelines or to link offshore gas fields to their final terminals.

A technique for imaging a core by scanning it with a highly focused source of X-rays and recording the attenuated X-rays on the other side. The source and detector are rotated and moved along the core. The measurements are combined mathematically to give a full core image.

Having the same center, such as when the casing and the wellbore have a common center point and, therefore, a uniform annular dimension.

The deformation of rock layers in which the thickness of each layer, measured perpendicular to initial undeformed layering, is maintained after the rock layers have been folded.

A hypothetical model characterizing strata, generally strata deposited in one or a related set of environments. Conceptual models usually incorporate rules about possible geometries and successions of facies that can be included in a geological scenario. These often provide limitations to the interpretation of a given reservoir. Conceptual models commonly incorporate sequence stratigraphic concepts such as facies tracts, unconformities, flooding surfaces, erosional surfaces and parasequences. Conceptual models are often used in conjunction with geostatistical and classical technologies for reservoir characterization.

The geographic area in which the government allows a company to operate.

A natural gas liquid with a low vapor pressure compared with natural gasoline and liquefied petroleum gas. Condensate is mainly composed of propane, butane, pentane and heavier hydrocarbon fractions. The condensate is not only generated into the reservoir, it is also formed when liquid drops out, or condenses, from a gas stream in pipelines or surface facilities.

Hydrocarbons that are in the gaseous phase at reservoir conditions but condense into liquid as they travel up the wellbore and reach separator conditions. Condensate liquids are sometimes called distillate.

The ratio of the volume of liquid produced to the volume of gas produced.

In sequence stratigraphy, a section of fine-grained sedimentary rocks that accumulated slowly, thereby representing a considerable span of time by only a thin layer. In condensed sections, fossils and organic, phosphatic and glauconitic material tend to be concentrated compared with rapidly deposited sections that contain few fossils. Condensed sections are most commonly deposited during transgressions. In such cases they are associated with "maximum flooding surfaces" and form important sequence stratigraphic markers.

A gasflood process in which an injection gas enriched with components of intermediate molecular weight, for example butane, is injected into a reservoir to achieve multiple-contact miscibility. Upon contact with the oil, intermediate molecular-weight hydrocarbons transfer from the injected gas phase into the oil phase, a process in which those components are said to condense into the oil.Formation of miscibility may require several contacts between fresh enriched gas and the oil containing condensed components. If the reservoir oil becomes sufficiently enriched with these components that miscibility results between the injection gas and the enriched oil, then the enriched gas and oil have multiple-contact miscibility. A backward multiple-contact test is a laboratory evaluation of a condensing drive process. In the field, both forward- and backward-contact processes can occur during a given gasflood.

A geostatistical tool that yields a quantitative measure of the error in a map. It is performed when multiple maps have been created using kriging or cokriging and where each map has similar mean and variance as the control points, has approximately the same semivariogram, and approximately honors the control points. If guide data are used, the average of the conditional simulation images is the kriging with external drift (KED) solution.In general, conditional simulation maps contain more detail than maps produced by kriging or KED, but require much more effort to produce.

The reciprocal of resistance in a direct current circuit, measured in siemens (formerly mhos). In an alternating current circuit, conductance is the resistance divided by the square of impedance, also measured in siemens.

A situation in which the resistivity of the flushed zone is less than the resistivity of the undisturbed zone. Such a setting generally favors the use of electrode resistivity devices (laterologs, ring resistivity), which respond to resistivity, rather than induction and propagation resistivity devices, which respond to conductivity.

A model, or set of equations, for the resistivity response of formations with conductive minerals, such as shaly sands. The model is used to analyze core data and to calculate water saturation from resistivity and other logs. The conductive rock matrix model (CRMM) was proposed by W. Givens. The model treats the rock as two components in parallel: a conductive pore network with fluid that is free to move, and the remainder of the rock, which may have conductive minerals or immobile but conductive water. The model is not concerned with the origin of this conductivity, but gives it a resistivity, Rm. The two components are in parallel as follows: 1 / Rt = 1 / Rp + 1 / Rmwhere Rp is the resistance of the free-fluid pore network and can be expressed in terms of the porosity and formation water resistivity by the Archie equation. The model was developed from core data, and can explain the observed variations of the porosity exponent with porosity and the saturation exponent with water saturation in shaly sands. For log analysis Rm needs to be related to parameters that can be measured by logs.Reference:Givens WW: Formation Factor, Resistivity Index and Related Equations Based upon a Conductive rock Matrix Model (CRMM), Transactions of the SPWLA 27th Annual Logging Symposium, Houston, Texas, USA, June 9-13, 1986, paper P.

The ability of a material to conduct electricity. It is the inverse of resistivity and is measured in siemens per meter (S/m) or mho/m. The conductivity is a property of the material, whereas the conductance also depends on the volume measured. The two are related by a system constant, which in simple cases is the length between the measurement electrodes divided by the area. In the most general case, the conductivity is the current density divided by the electric field and depends on the frequency of the applied signal.

A technique for estimating the cation-exchange capacity of a sample by measuring the conductivity of the sample during titration. The technique includes crushing a core sample and mixing it for some time in a solution like barium acetate, during which all the cation-exchange sites are replaced by barium (Ba++) ions. The solution is then titrated with another solution, such as MgSO4, while observing the change in conductivity as the magnesium (Mg++) ions replace the Ba++ ions. For several reasons, but mainly because the sample must be crushed, the measured cation-exchange capacity may differ from that which affects the in situ electrical properties of the rock.

A short string of large-diameter casing set to support the surface formations. The conductor pipe is typically set soon after drilling has commenced since the unconsolidated shallow formations can quickly wash out or cave in. Where loose surface soil exists, the conductor pipe may be driven into place before the drilling commences.

The nature of the contact between strata deposited in continuous succession.

A bedding surface separating younger from older strata, along which there is no evidence of subaerial or submarine erosion or of nondeposition, and along which there is no evidence of a significant hiatus. Unconformities (sequence boundaries) and flooding surfaces (parasequence boundaries) pass laterally into correlative conformities, or correlative surfaces.

A projection of data from the apex of a cone in a three-dimensional plot onto a surface at the base of the cone. This projection often is performed in log analysis to remove a dimension and see what a data point would read in the absence of that dimension. For example, removal of shale effects in a plot of neutron, density and gamma ray data helps determine the mineralogy of a sample where the apex of the cone would represent the shale point in the plot.The M-N plot is a plot in which the fluid has been removed by conical projection from the neutron, density and sonic data to provide a porosity-independent plot that can be used to determine lithology.

The change in oil-water contact or gas-oil contact profiles as a result of drawdown pressures during production. Coning occurs in vertical or slightly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability.

Water trapped in the pores of a rock during formation of the rock. The chemistry of connate water can change in composition throughout the history of the rock. Connate water can be dense and saline compared with seawater. Formation water, or interstitial water, in contrast, is simply water found in the pore spaces of a rock, and might not have been present when the rock was formed. Connate water is also described as fossil water.

The act of adding a joint or stand of drillpipe to the top of the drillstring, also described as "making a connection."

A brief influx of gas that is introduced into the drilling fluid when a pipe connection is made. Before making a connection, the driller stops the mud pumps, thereby allowing gas to enter the wellbore at depth. Gas may also be drawn into the wellbore by minor swabbing effects resulting from short movements of the drillstring that occur during the connection. Connection gas usually occurs after one lag interval following the connection. On a mud log, it will appear as a short peak above background levels. This peak often appears at 30-foot intervals, depending on the lengths of drillpipe being connected as the well is drilled.

A rheological property of matter related to the cohesion of the individual particles of a given material, its ability to deform and its resistance to flow. The consistency of cementslurries is determined by thickening time tests in accordance with API Recommended Practice 10B and is expressed in Bearden units of consistency (Bc).

A laboratory device used to determine the thickening time of cementslurries under simulated downhole pressure and temperature conditions.

Pertaining to sediments that have been compacted and cemented to the degree that they become coherent, relatively solid rock. Typical consequences of consolidation include an increase in density and acoustic velocity, and a decrease in porosity.

A laboratory test usually performed as part of a routine PVT analysis that measures the change in volume of a reservoir fluid as a function of pressure. This change is determined by measuring the total volume of a sample of reservoir fluid at various pressures above and below the saturation pressure. The pressure-dependent volumes are normalized to the volume of the sample at the saturation pressure.

A flow rate that does not change appreciably during a test period. Flow rates are never truly constant, but changes of only a few percent do not affect the results of well-test analysis appreciably if the rate is averaged over the flow period.

The angle of intersection of the interface between two fluids at a solid surface. The angle is measured from the solid surface through the aqueous phase, or in an oil and gas test through the oil phase. The contact angle displays hysteresis based on direction of motion of the interface. Surface roughness affects the equilibrium contact angle, so measurements are typically made on smooth, flat surfaces.A contact-angle test uses carefully captured and preserved samples of reservoir oil to determine the wetting preference. A droplet of the crude oil is suspended between two parallel plates of quartz or calcite inside a simulated formation water bath at reservoir temperature and sometimes at reservoir pressure. By periodically displacing one of the plates sideways, a contact angle is determined at the side of the droplet where water is forcing the oil from the solid. A small angle indicates water-wetting preference, while a large angle indicates oil-wetting. Angles near 90 degrees are intermediate-wetting. Different minerals display different wetting preferences, although most are more likely to be water-wet.

The elapsed time required for a specific fluid to pass a designated depth or point in the annulus during pumping operations. Contact time is normally used as a design criterion for mud removal in turbulent flow.

A chemical or fluid that alters the performance of an engineered slurry or treatment fluid. Some remedial cementing treatments require unpredictable volumes of cement slurry to achieve the desired results. When excess slurry is left in the wellbore, it may not be possible to remove the excess slurry by conventional means, such as reverse circulation, before the slurry thickens and becomes immovable. Mixing the contaminant with the slurry in the correct proportions increases the thickening time of the slurry, allowing it to be safely removed from the wellbore.

Gas that is introduced into the drilling mud from a source other than the formation. Contamination gas normally evolves as a by-product of oil-base mud systems and those using volatile additives such as diesel fuel or other lubricants.

A key component of the operational planning process that takes account of reasonably foreseeable events that may prevent completion of normal operations. The formal plans and procedures for any operation should include normal operating procedures, contingency plans and emergency responses.

Material balance expressed in a differential equation.

An artificial-lift method in which the gas-lift system is operated on a continuous basis to sustain liquid production at an efficient rate.

A type of areally extensive reservoir that contains hydrocarbon throughout, rather than containing a water contact or being significantly affected by a water column or a defined structuralclosure. The areal extent of a continuous reservoir, such as a shale reservoir, can be as large as the extent of the sedimentary basin in which the shale was deposited.

(noun) A line drawn on a map connecting points of equal value for a given property, such as depth, thickness, pressure, porosity, or hydrocarbon saturation. Contour maps are fundamental tools in subsurface mapping for visualising the geometry of geological structures, reservoir properties, and fluid contacts.

The value of the separation between two adjacent contours. A net pay isopachmap might have a contour interval of 10 feet [3 m], whereas a structurecontour map might have a contour interval of 1000 feet [300 m]. Contour intervals are chosen according to the map scale and the amount and distribution of control points.

A map displaying lines that include points of equal value and separate points of higher value from points of lower value. Common types of contour maps include topographic contour maps, which show the elevation of the Earth's surface; structure contour maps, which show the elevation or depth of a formation; and gross or net sand or pay maps, which show variations in the thickness of a stratigraphic unit, also called isopachs.

The depth in a drilling well at which the drilling contractor receives a lump-sum payment for reaching a particular milestone. The contract depth is specified in a legal agreement between the operator, who pays for the well, and the drilling contractor, who owns and operates the drilling rig. Contract depth may be the final or total depth (TD) of the well, an intermediate point in the well or another milestone, such as running well-logging tools to the bottom of the hole. In the case of an intermediate contract depth, the work to deepen the well would likely be done on a day rate basis, or a "time and materials" contract.

A small-diameter hydraulic line used to operate downhole completion equipment such as the surface controlled subsurface safety valve (SCSSV). Most systems operated by control line operate on a fail-safe basis. In this mode, the control line remains pressurized at all times. Any leak or failure results in loss of control line pressure, acting to close the safety valve and render the well safe.

(noun) A directional survey method in which measurements of wellbore inclination and azimuth are taken at predetermined time intervals or depth stations during drilling operations to monitor the trajectory of the wellbore and ensure it conforms to the planned directional well path.

The density- and heat-driven cycling, transfer or circulation of energy through which material initially warms up and becomes relatively less dense, then rises, cools and becomes relatively more dense, and finally sinks. As a consequence of convection, material can turn over repeatedly in a convection cell. Within the Earth, radiogenic heating results in convection appearing in the mantle and might drive plate tectonic motions. Convection also occurs in the ocean waters and in the Earth's atmosphere.

A term that, in the past, referred to a mud containing bentonite clay, water, caustic soda and perhaps barite (as needed for density) usually with lignite or lignosulfonate present. Modern drilling does not necessarily recognize this as a conventional mud because polymer muds, special drill-in fluids and synthetic-base muds are now in common use. There may not be a "conventional mud" today.

A reservoir in which buoyant forces keep hydrocarbons in place below a sealing caprock. Reservoir and fluid characteristics of conventional reservoirs typically permit oil or natural gas to flow readily into wellbores. The term is used to make a distinction from shale and other unconventional reservoirs, in which gas might be distributed throughout the reservoir at the basinscale, and in which buoyant forces or the influence of a water column on the location of hydrocarbons within the reservoir are not significant.

The effect of performing computations using a planar surface instead of the curved surface of survey measurements. A convergence correction accommodates the change from rectangular coordinates to latitude and longitude.

A seismic wave that changes from a P-wave to an S-wave, or vice versa, when it encounters an interface.

A written contract between a grantor and grantee, used to transfer title or rights to real estate or property. Typical conveyances include oil, gas and mineral leases; assignments; deeds and rights of way.

A mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Convolution also can use surface rates to transform wellhead pressures to an interpretable form. Convolution assumes a particular model for the pressure-transient response, usually infinite-acting radial flow. This operation is similar to what is done to account for the flow history in rigorous pressure-transient analysis.

A polymer that is formed from two or more different structural units.

A compound, CuCO3, that was used as a sulfide scavenger for water-base muds. However, it was found to be corrosive due to spontaneous plating of metallic copper onto metal surfaces, causing pitting corrosion; it has largely been replaced by zinc compounds.Reference:Perricone AC and Chesser BG: "Corrosive Aspects of Copper Carbonate in Drilling Fluids," Oil & Gas Journal 68, no. 37 (September 14, 1970): 82-85.

A cylindrical sample of geologicformation, usually reservoirrock, taken during or after drilling a well. Cores can be full-diameter cores (that is, they are nearly as large in diameter as the drill bit) taken at the time of drilling the zone, or sidewall cores (generally less than 1 in. [2.5 cm] in diameter) taken after a hole has been drilled. Cores samples are used for many studies, some of which relate to drilling fluids and damage done by them.

Laboratory study of a sample of a geologicformation, usually reservoirrock, taken during or after drilling a well. Economic and efficient oil and gas production is highly dependent on understanding key properties of reservoir rock, such as porosity, permeability, and wettability. Geoscientists have developed a variety of approaches, including log and core analysis techniques, to measure these properties. Core analysis is especially important in shale reservoirs because of the vertical and lateralheterogeneity of the rocks. Core analysis can include evaluation of rock properties and anisotropy; organic matter content, maturity, and type; fluid content; fluid sensitivity; and geomechanical properties. This information can be used to calibrate log and seismic measurements and to help in well and completion design, well placement, and other aspects of reservoir production.

A log obtained in the laboratory by moving the core past a gamma ray detector. The log can be of the total gamma ray in API units, or of the spectral response in weight concentrations of thorium, uranium and potassium. The main purpose is to correlate the depth of each section of core with the depth of a log.

An image of the external or internal features of a core. External images are photographs taken under natural or ultraviolet light; natural light highlights lithology and sedimentary structures, while ultraviolet light causes hydrocarbon zones to fluoresce. Internal images are obtained using X-rays or nuclear magnetic resonance (NMR).X-ray techniques measure the attenuation of X-rays passing through the core. The attenuation depends mainly on the density. Hence the image reflects density and lithology changes, internal bedding planes, fractures and nodules. These techniques include, in increasing resolution, fluoroscopy, X-radiography and computed tomography.Most NMR images measure the quantity and relaxation time of hydrogen, and therefore give information on fluid distribution. Some NMR techniques examine carbon, sodium and phosphorous.

A plug, or sample, taken from a conventional core for analysis. Core plugs are typically 1 in. to 1 1/2 in. [2.5 to 3.8 cm] in diameter and 1 in. to 2 in. [5 cm] long. Core plugs are ordinarily cut perpendicular to the axis of the core or parallel to the axis, called horizontal and vertical plugs, respectively, when cut from a vertical wellbore. The terms horizontal and vertical are often applied for cores cut from a deviated or horizontal wellbore, even though they are not strictly correct unless core orientation is measured and plugs are cut to the true in-situ orientation.

Laboratory analyses performed on formationcore samples as part of a stimulation-treatment design process. Tests such as the formation flow potential, fracture orientation and fluid compatibility tests are commonly run in preparation for stimulation treatments.

A laboratory test in which a fluid or combination of fluids is injected into a sample of rock. Objectives include measurement of permeability, relative permeability, saturation change, formation damage caused by the fluid injection, or interactions between the fluid and the rock. The core material often comes from an oil reservoir, but some tests use outcrop rock. The fluid in place at the start of the test is typically either a simulated formationbrine, oil (either crude oil or refined oil), or a combination of brine and oil. Injected fluids may include crude oil, simulated reservoir brine, refined fluids, drilling mud filtrate, acids, foam or other chemicals used in the oil field. Depending on the purpose of the test, conditions may be either ambient temperature and low confining pressure or high temperature and pressure of a subject reservoir. Pressures and flow rates at both ends of the core are measured, and the core can also be investigated using other measurements such as nuclear magnetic resonance (NMR) during the test. A coreflood is typically used to determine the optimum development option for an oil reservoir and often helps evaluate the effect of injecting fluids specially designed to improve or enhance oil recovery.

A specially designed fluid that is used for cutting cores with a core barrel and core bit. Sometimes the fluid used is the drilling mud, but if cores are for special studies, the coring fluid must be carefully designed to avoid damaging or altering the rock sample, such as a bland coring fluid.

The result of certain drilling conditions that cause the borehole to take the shape of a corkscrew. Most logging tools are much longer than the wavelength of the corkscrew, and therefore see it as a change in standoff or a change in hole size. For this reason, the corkscrew is often observed as a wave on the caliper log. A corkscrew hole affects measurements sensitive to standoff, such as induction and neutron porosity, and may affect pad tools, if they cannot follow the changes.

A gamma ray log from which the uranium contribution has been subtracted. In some rocks, and in particular in carbonate rocks, the contribution from uranium can be large and erratic, and can cause the carbonate to be mistaken for a shale. The carbonate gamma ray is then a better indicator of shaliness.

A procedure for correcting pressure measurements in a reservoir to a common datum level. This is not required for calculating kh, permeability thickness, and s, skin effect, but is required for determining average reservoir pressures or for any comparison of pressures in one area of the reservoir to those in another area. The correction is done by determining the average pressure for a given well test in which the pressure gauge is at a known level, and then adding or subtracting the calculated weight of the column of reservoir fluid in pounds per square inch from the difference in elevation between the pressure gauge and the datum level.

A positive relationship between data samples that implies a connection or a relationship between them.

A log run for the purpose of correlating between wells. The most common logs used for this purpose are the gamma ray, the resistivity and the acoustic log; the most common depth scales are 1/500 and 1/1000, or 2 in./100 ft [5 cm/30 m] and 1 in./100 ft [2.5 cm/30 m].

A graphical representation of the degree of agreement between segments of curves being correlated between different wells. The degree of lag (required shift), the amplitude of the peaks and the shapes of the peaks are parameters used to calculate the match in a correlogram.

The loss of metal due to chemical or electrochemical reactions, which could eventually destroy a structure.Corrosion can occur anywhere in the production system, either at bottomhole or in surface lines and equipment. The corrosion rate will vary with time depending on the particular conditions of the oil field, such as the amount of water produced, secondary recovery operations and pressure variations.

The measures used to prevent or considerably reduce the effects of corrosion. Corrosion can occur anywhere in the production system, either at bottomhole or in surface lines and equipment. Some practices for corrosion control involve: cathodic protection, chemical inhibition, chemical control (removal of dissolved gases such as hydrogen sulfide, carbon dioxide and oxygen), oxygen scavenging, pH adjustment, deposition control (for example, scales) and coatings. One of the most difficult environments for corrosion control is high bottomhole temperatures, such as 400 to 500oF [200 to 260oC].The corrosion rate will vary with time depending on the particular conditions of the oil field, such as the amount of water produced, secondary recovery operations and pressure variations. Therefore, corrosion control is a continuous process in oil and gas production operations.

A specimen of test material to be used in a corrosion test, usually a metal strip or ring shaped to fit into a testing cell or between joints of drillpipe. Rings, or coupons, are weighed before and after exposure, and weight loss is measured. They are also examined for pits and cracks. Corrosion products are analyzed to define the type of corrosion reaction.

A type of corrosion in which the metal components of a structure fail due to cyclic stresses applied in a corrosive environment, such as salt water. Consequently, a break in the metal will occur at stresses considerably lower than the tensile strength of the material.Corrosion fatigue is the main cause of sucker-rod and drillstring failures.

In matrix treatments, a chemical added to acid that adsorbs on the pipe surface to form a protective film. This decreases the destructive reaction of acid with metals. The inhibitor does not completely stop the corrosion reaction, but it eliminates more than 99% of the metal losses that would occur if the inhibitor were not present. The inhibitor has little or no effect on the reaction rate of acid with limestone, dolomite or acid-soluble minerals.Specific corrosion inhibitors are environmentally compatible, effective in hydrogen sulfide [H2S] environments, effective on high chrome steel, and effective on special steel alloys, such as coiled tubing. These inhibitors may be used at temperatures approaching 500oF [260oC].

The weight loss of a corrosioncoupon after exposure to a corrosive environment, expressed as mils (thousandths of an inch) per year penetration. Corrosion rate is calculated assuming uniform corrosion over the entire surface of the coupon.mpy = (weight loss in grams) * (22,300)/(Adt)mpy = corrosion rate (mils per year penetration)A = area of coupon (sq. in.)d = metal density of coupon (g/cm3)t = time of exposure in corrosive environment (days).It is important to note that the calculated values using this formula are not representative in cases of severe pitting. Therefore, a complete report, including a visual inspection, is required to determine either the type of attack or the appropriate corrosion control program.Corrosion rate is also known as corrosion ratio.

A specially formulated material used for completion components in wells likely to present corrosion problems. Corrosion-resistant alloys can be formulated for a wide range of aggressive wellbore conditions. However, cost generally determines the viability of any particular completion design. Alloys with a high chrome content are commonly used for tubing strings.

A chemical used in small quantities to improve the effectiveness of a primary solvent in a chemical process.

A portion of produced oil that the operator applies on an annual basis to recover defined costs specified by a production sharing contract.

A chemical added to a process to enhance the effectiveness of a surfactant. In the oil industry, cosurfactants are often used to increase the oil-solubilizing capacity of microemulsion surfactant systems. An example of such a cosurfactant is a long-chain alcohol. Pure surfactants often organize well at a liquid/liquid boundary, which leads to relatively stiff interfaces or even liquid-crystal phases. To achieve ultralow interfacial tension for enhanced oil recovery applications, a cosurfactant is added to disturb this organization at the liquid/liquid interface. Cosurfactants can also be used to fine-tune the formulation phase behavior, for example, by expanding the temperature or salinity range of microemulsion formation.

Part of rod pumping unit. The counterbalance weight is installed on the end of the walking beam, opposite to the end over the well, and counterbalances the weight of the sucker rods and the fluid being pumped.

The lifting device on a snubbing unit used to pick up and lay down the tool string and running-string tubulars.

An electrical or mechanical device that joins parts of systems and can affect the interaction of, or energy transfer between, parts of systems. Electrical couplings promote the passage of certain signals but prevent the passage of others, such as an alternating current coupling that excludes direct current.

(noun) A small metal sample of known weight, dimensions, and metallurgy that is exposed to a corrosive environment within a pipeline, vessel, or wellbore for a specified period to measure the rate of corrosion by comparing its mass loss or surface condition before and after exposure.

The process of splitting a large heavy hydrocarbon molecule into smaller, lighter components. The process involves very high temperature and pressure and can involve a chemical catalyst to improve the process efficiency.

A stable area of continental crust that has not undergone much plate tectonic or orogenic activity for a long period. A craton includes a crystalline basement of commonly Precambrian rock called a shield, and a platform in which flat-lying or nearly flat-lying sediments or sedimentary rock surround the shield. A commonly cited example of a craton is the Canadian Shield.

The separation of phases of an emulsion with the lighter phase on top and denser phase on bottom. When oil muds are stagnant, the less dense oil phase rises and the denser aqueous phase settles. This behavior is not necessarily related to emulsion weakness, nor does it portend breaking, as does coalescence.

The highest point of a wave, mountain or geologic structure.

The change in oil-water or gas-oil contact profiles as a result of drawdown pressures during production. Cresting occurs in horizontal or highly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability.

The angle of incidence according to Snell's law at which a refracted wave travels along the interface between two media. It can be quantified mathematically as follows:

The minimum damping that will prevent or stop oscillation in the shortest amount of time, typically associated with oscillatory systems like geophones.

In sand control operations, the maximum production rate below which the production of solids along with the produced fluid is uniform. When the flow rate exceeds this threshold, the production of sand and fines increases significantly. Sand-production control is important to avoid formationdamage, possible casing collapse and deterioration of surface equipment due to drag forces.

The gas flow rate equivalent to the speed of sound in that fluid. Exceeding this limit during gas production accelerates corrosion in the pipelines.

A near-wellbore area where injected fluids such as acids can restore original permeability. Most of the reservoir pressure drop during production occurs in this near-wellbore part of the reservoir.

The time of maximum depth of burial of a hydrocarbon source rock. The critical moment is the time of highest probability of entrapment and preservation of hydrocarbons in a petroleum system-after traps form and hydrocarbons migrate into a reservoir and accumulate-and marks the beginning of preservation in a viable petroleum system.

The minimum rate required to achieve turbulent flow.

A reflection, typically at a large angle, that occurs when the angle of incidence and the angle of reflection of a wave are equal to the critical angle.

Antiquated term for a deviated wellbore, usually used to describe a well deviated accidentally during the drilling process.

Describing a waveform or a log that has been recorded by a set of dipole receivers oriented orthogonally (or 900 out of line) with a dipole transmitter. In soniclogging, cross-dipole flexural modes are used to determine shearanisotropy together with in-line flexural modes. The data are processed using the Alford rotation.

Constant of proportionality relating the fraction of incident particles that undergo an interaction to the thickness and number of target atoms within a material, and the incident flux. It is a measure of the probability of an interaction. The microscopic cross section has units of area per interacting atom. The macroscopic cross section, which is the product of the microscopic cross section and the number of particles per unit volume, has units of inverse length. Cross sections for most reactions are determined experimentally and depend on the type of interaction, the material and the energy of the incident particle.

The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero.

A device for measuring fluid velocity in a production well. The device measures the transit time of a disturbance between two sensors separated by a fixed distance. The technology applies to multiphase flow, in which the disturbance is caused, for example, by the passage of a bubble of gas over each sensor. In practice, there will be many bubbles of gas, so it is necessary to record both sensor signals over a time window and compare, or correlate, them. The two signals will correlate best after shifting one of them by a time corresponding to the average transit time of the bubbles. Different sensors may be used, for example a measure of electrical capacitance as in a holdupmeter.The crosscorrelationflowmeter gives the velocity of the disturbance. Since this is caused by just one of the phases, it produces a type of phase velocity log.

A condition that exists when two production zones with dissimilar pressure characteristics are allowed to communicate during production. Reservoir fluid from the high-pressure zone will flow preferentially to the low-pressure zone rather than up the production conduit unless the production parameters are closely controlled.

A seismic line within a 3D survey perpendicular to the direction in which the data were acquired.

A compound, typically a metallic salt, mixed with a base-gel fluid, such as a guar-gel system, to create a viscous gel used in some stimulation or pipeline cleaning treatments. The crosslinker reacts with the multiple-strand polymer to couple the molecules, creating a fluid of high, but closely controlled, viscosity. Treatments using crosslinkers should take account of the conditions needed to break the gel structure to ensure satisfactory cleanup and disposal.

A short subassembly used to enable two components with different thread types or sizes to be connected.

A specialized tool, frequently used in gravel-pack operations, that enables the circulation of the treatment fluid (slurry) from the internal flow path of the tool string into the annulus area to be packed. The returned carrier fluid enters the internal flow path at the base of the tool before crossing over to the annulus above the packer assembly, isolating the annulus.

A two-dimensional plot with one variable scaled in the vertical (Y) direction and the other in the horizontal (X) axis. The scales are usually linear but may be other functions, such as logarithmic. Additional dimensions may be represented by using color or symbols on the data points. These plots are common tools in the interpretation of petrophysical and engineering data.

The porosity obtained by plotting two porosity logs against each other, normally density and neutron porosity. The computation assumes a particular fluid, usually fresh water, and particular response equations. The result is largely independent of lithology and is often a more reliable estimate of porosity than a single porosity log. It is often displayed as a quicklook log.

A technique for measuring formationresistivity between two or more wells. This technique measures the signal between an electromagnetic induction transmitter in one well, and a receiverarray located in another well. The transmitting antenna broadcasts a continuous sinusoidal signal at programmable frequencies. Tomographic processing creates a map of resistivity of the area between the wells. Measurements acquired by this technique have a greater depth of investigation than conventional logging tools and are sensitive to fluid content. Crosswell electromagnetic induction surveys fill an intermediate role between high-resolution well logs and lower-resolution surface measurements. Asset managers utilize crosswell electromagnetic surveys for a variety of applications, such as monitoring sweep efficiency, identifying bypassed pay, planning infill drilling locations and improving the effectiveness of reservoir simulations.

A crosswell seismic technique that incorporates reflection traveltimes and direct traveltimes into a tomographic inversion algorithm to produce images of seismic velocity between wells.

A survey technique that measures the seismicsignal transmitted from a source, located in one well, to a receiverarray in a neighboring well. The resulting data are processed to create a reflectionimage or to map the acoustic velocity or other properties (velocities of P- and S-waves, for example) of the area between wells. Placement of the source and receiver array in adjacent wells not only enables the formation between wells to be surveyed, it also avoids seismic signal propagation through attenuative near-surface formations. Another advantage is that it places the source and receiver near the reservoir zone of interest, thereby obtaining better resolution than is possible with conventional surface seismic surveys. This technique is often used for high-resolution reservoir characterization when surface seismic or vertical seismic profile (VSP) data lack resolution, or for time-lapse monitoring of fluid movements in the reservoir.

A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiverarray placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismicreflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements.

The fixed set of pulleys (called sheaves) located at the top of the derrick or mast, over which the drilling line is threaded. The companion blocks to these pulleys are the traveling blocks. By using two sets of blocks in this fashion, great mechanical advantage is gained, enabling the use of relatively small drilling line (3/4 to 1 1/2 in. diameter steel cable) to hoist loads many times heavier than the cable could support as a single strand.

(noun) A valve installed at the top of a Christmas tree or wellhead assembly that provides the uppermost point of closure on the vertical bore of the tree, used to isolate the well during wireline operations, intervention activities, or as a secondary barrier.

A general term for unrefined petroleum or liquid petroleum.

The rubblized or damaged zone surrounding a perforation tunnel where the action of the perforating charge or bullet has altered the formation structure and permeability. Although it is generally damaging to production, the severity or extent of the crushed zone depend greatly on the characteristics of the formation, the perforating charge and the underbalance or overbalance conditions at time of perforating. Measures to reduce the effect of the crushed zone include underbalanced perforating in which the crushed zone and perforating debris are flushed from the perforating tunnel by the reservoir fluid as soon as the perforation is created. Where overbalanced perforating techniques are used, it may be necessary to acidize the crushed zone to achieve maximum productivity from the perforated interval.

The thin, outermost shell of the Earth that is typically 5 km to 75 km thick [3 to 46 miles]. The continental crust comprises rocks similar in composition to granite and basalt (i.e., quartz, feldspar, biotite, amphibole and pyroxene) whereas the composition of oceanic crust is basaltic (pyroxene and feldspar). The crust overlies the more dense rock of the mantle, which consists of rocks composed of minerals like pyroxene and olivine, and the iron and nickel core of the Earth. The Mohorovicic discontinuity abruptly separates the crust from the mantle; the velocity of compressional waves is significantly higher below the discontinuity. The crust, mantle and core of the Earth are distinguished from the lithosphere and asthenosphere on the basis of their composition and not their mechanical behavior.

The temperature at which crystals will appear in a brine solution of a given density as it cools. In preparing oilfield brines, the crystallization temperature can be used to indicate the maximum saturation (density) achievable for a brine solution at a given temperature.

The arrangement in space of uniform spheres (atoms and molecules in mineral crystals, or grains in clastic sedimentary rocks) that results in a cubic material structure. Cubic packing is mechanically unstable, but it is the most porous packing arrangement, with about 47% porosity in the ideal situation. Most sediments are not uniform spheres of the same size, nor can they be arranged in a cubic structure naturally, so most sediments have much less than 47% porosity.

A local geophysical anomaly generated by a man-made feature, such as electrical and communications wires, steel beams and tanks and railroad tracks.

Undesirable energy, or noise, generated by human activity, such as automobile traffic that interferes with seismic surveying, or electrical power lines or the steel in pipelines that can adversely affect electromagnetic methods.

The total amount of oil and gas recovered from a reservoir as of a particular time in the life of the field. Cumulative production can be referenced to a well, a field, or a basin.

The aging of cement under specific temperature and pressure conditions.

The presentation on hard copy of log data from a single measurement versus depth. The term is also used to refer to the log data themselves, as a synonym for a single log.

The generation of a theoretical equation to define a given data set. In contrast, curve matching involves the comparison of well-understood data to a data set of interest.

The graphical comparison of well-understood data sets, called type curves, to another data set. If a certain type curve closely corresponds to a data set, then an interpretation of similarity can be made, although, as Sheriff (1991) points out, there might be other type curves that also match the data of interest. Curve matching differs from curve fitting in that curve fitting involves theoretical models rather than actual examples.

A fluid column (usually water or nitrogen) put in the drillstem to provide the desired backpressure at the start of a drillstem test. The cushion usually serves to limit the differential pressure across the test string and packer to avoid flow below the bubblepoint pressure (in which case water is the usual cushion) or to enable a depleted reservoir to flow (nitrogen is the likely cushion).

The fraction of the total flow rate produced from a well that is due to a particular fluid, for example the water cut in the case of water. The cut is normally quoted at standard surface conditions.

A crude oil that contains water, normally in the form of an emulsion. The emulsion must be treated inside heaters using chemicals, which will break the mixture into its individual components (water and crude oil).

The particle size that has a specified chance of being removed by an item of solids control equipment. Most commonly, D10, D50 and D90 cut points are specified corresponding to 10, 50 and 90% chances of removal, respectively. Taken together, they approximate the separation curve. If the percent is not specified, it is normally taken to be the D50 value. For example, if the D50 of a shakerscreen is 100 microns, then a particle of this size has an equal chance of being removed or staying in the system. Larger particles are more likely to be removed and smaller ones more likely to be retained in the underflow.

A method for recovering wirelinestuck in a wellbore. In cut-and-thread operations, the wireline is gripped securely with a special tool and cut at the surface. The cut end is threaded through a stand of drillpipe. While the pipe hangs in the wellbore, the wireline is threaded through another stand of drillpipe, which is screwed onto the stand in the wellbore. The process is repeated until the stuck wireline is recovered. This technique, while dangerous and time-consuming, is known to improve greatly the chances of full recovery of the wireline and the tool at its end in the shortest overall time compared with trying to grab the wireline in the openhole with fishing tools.

Rock pieces dislodged by the drill bit as it cuts rock in the hole. Cuttings are distinct from cavings, rock debris that spalls as a result of wellbore instability. Visual inspection of rock at the shale shaker usually distinguishes cuttings from cavings.

A condensate (liquid hydrocarbon) produced at surface from cycle gas.

A gas that is compressed and injected back to the reservoir. In gas-condensate reservoirs, after the liquids or condensate are recovered at the surface, the residue gas (dry gas) is returned to the reservoir to maintain pressure. This prevents retrograde condensation, which will form unrecoverable liquid hydrocarbons in the reservoir.

An anomalously high transit time in a log, such as a continuous velocity log, observable as a spike on the log, commonly caused by the presence of fractures, gas, unconsolidated formations, aerated drilling mud and enlarged boreholes.

The elapsed time for mud to circulate from the suction pit, down the wellbore and back to surface. Cycle time allows the mud engineer to catch "in" and "out" samples that accurately represent the same element of mud in a circulating system. Cycle time is calculated from the estimated hole volume and pump rate and can be checked by using tracers such as carbide or rice granules.

A method of thermal recovery in which a well is injected with steam and then subsequently put back on production. A cyclic steam-injection process includes three stages. The first stage is injection, during which a slug of steam is introduced into the reservoir. The second stage, or soak phase, requires that the well be shut in for several days to allow uniform heat distribution to thin the oil. Finally, during the third stage, the thinned oil is produced through the same well. The cycle is repeated as long as oil production is profitable. Cyclic steam injection is used extensively in heavy-oil reservoirs, tar sands, and in some cases to improve injectivity prior to steamflood or in situ combustion operations.Cyclic steam injection is also called steam soak or the huff `n puff (slang) method.

An oilfield installation used when producing from a gas-condensatereservoir. In a cycling plant, the liquids are extracted from the natural gas and then the remaining dry gas is compressed and returned to the producing formation to maintain reservoir pressure. This process increases the ultimate recovery of liquids.

A succession of strata deposited during a single cycle of deposition. These sedimentary successions usually occur repeatedly, one above the other. The two main varieties are the cyclic units that are symmetrical cyclothems, and the rhythmic units that are asymmetrical cyclothems. Cyclic groupings of cyclothems are called megacyclothems, and cyclic groupings of megacyclothems are called hypercyclothems. Cyclothems are thought to be due to natural cycles, such as changes in sea levels related to changes in the volume of polar ice caps.

The barrel of the sucker rod pump. The plunger travels up and down in the cylinder. The plunger and the barrel operate as a piston mechanism to lift reservoir fluids into the subsurface pump. A cylinder is also known as a pump barrel.