Oil and Gas Terms Beginning with “A”

Two large support legs that when standing form a steel A-frame structure that supports the derrick; provides support frames for raising/lowering the derrick.

A budgetary document, usually prepared by the operator, to list estimated expenses of drilling a well to a specified depth, casing point or geological objective, and then either completing or abandoning the well. Such expenses may include excavation and surface site preparation, the daily rental rate of a drilling rig, costs of fuel, drillpipe, bits, casing, cement and logging, and coring and testing of the well, among others. This estimate of expenses is provided to partners for approval prior to commencement of drilling or subsequent operations. Failure to approve an authority for expenditure (AFE) may result in delay or cancellation of the proposed drilling project or subsequent operation.

A system to control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output, automatically. AGC is commonly used in seismicprocessing to improve visibility of late-arriving events in which attenuation or wavefrontdivergence has caused amplitude decay.

A system to control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output, automatically. AGC is commonly used in seismicprocessing to improve visibility of late-arriving events in which attenuation or wavefrontdivergence has caused amplitude decay.

A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds.Reference:Perricone AC, Enright DP and Lucas JM: "Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Base Muds," SPE Drilling Engineering 1, no. 5 (October 1986): 358-364.

Abbreviation for absolute open flow.

Abbreviation for absolute open flow potential.

Abbreviation for American Petroleum Institute, a trade association founded in 1919 with offices in Washington, DC, USA. The API is sponsored by the oil and gas industry and is recognized worldwide. Among its long-term endeavors is the development of standardized testing procedures for drilling equipment, drilling fluids and cements, called API Recommended Practices ("RPs"). The API licenses the use of its monogram (logo), monitors supplier quality assurance methods and sets minimum standards for materials used in drilling and completion operations, called API Specifications ("Specs"). The API works in conjunction with the International Organization of Standards (ISO).Note: "API Publications, Programs and Services Catalogue" can be ordered from the API in electronic form at: http://www.api.org.Reference:Recommended Practice on the Rheology and Hydraulics of Oil-Well Drilling Fluids, 3rd ed. Washington, DC, USA: American Petroleum Institute, 1995.Recommended Practice Standard Procedure for Laboratory Testing of Drilling Fluids, 5th ed. Washington, DC, USA: American Petroleum Institute, 1995.

The industry standard document that specifies requirements for API well cements and specification-testing methods.

One of several classes of cement manufactured to the specifications of the American Petroleum Institute (API) Specification 10A. Classes of API cement are A, B, C, D, E, F, G and H.

A specific gravity scale developed by the American Petroleum Institute (API) for measuring the relative density of various petroleum liquids, expressed in degrees. API gravity is gradated in degrees on a hydrometer instrument and was designed so that most values would fall between 10° and 70° API gravity. The arbitrary formula used to obtain this effect is: API gravity = (141.5/SG at 60°F) - 131.5, where SG is the specific gravity of the fluid.

The unit of radioactivity used for natural gamma ray logs. This unit is based on an artificially radioactive concrete block at the University of Houston, Texas, USA, that is defined to have a radioactivity of 200 American Petroleum Institute (API) units. This was chosen because it was considered to be twice the radioactivity of a typical shale. The formation is the primary standard for calibrating gamma ray logs. However, even when properly calibrated, different gamma ray tools will not necessarily have identical readings downhole because their detectors can have different spectral sensitivities. They will read the same only if the downhole formation contains the same proportions of thorium, potassium and uranium as the Houston standard. For example, logging while drilling (LWD) tools have thicker housings than wireline tools, causing a different spectral response to the three sources of radioactivity, and therefore a different total gamma ray reading in some formations.The nuclear well logcalibration facility at the University of Houston, known as the API pits, was opened in 1959 for the calibration of natural gamma ray and neutron logs. A facility for calibrating natural gamma ray spectroscopy logs was added later.

The amount of mixing water specified in API Specification 10A for specification testing of cement to meet API requirements. This amount is not intended to be a guide for mix water requirements in field applications.

The designation of a standard developed by ASTM International. Until 2001, ASTM was an acronym for the American Society for Testing and Materials, but the organization changed its name to ASTM International to reflect its global scope as a forum for development of international voluntary consensus standardsSome API procedures for drilling fluids are similar to ASTM procedures.

The viscosity of a fluid measured at a given shear rate at a fixed temperature. In order for a viscosity measurement to be meaningful, the shear rate must be stated or defined.

Variation in seismicreflectionamplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation ("fizz water"). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments.

Equipment in mud systems that suspends solids and maintains homogeneous mixture throughout the system using mechanical impellers driven by an explosion-proof motor coupled to a gear box.

A machine that takes air, compresses it, and stores it in a tank for distribution to air-driven rig equipment.

A compressed air powered winch-like device, usually mounted on the rig floor, used to lift heavy objects or tubulars to the rig floor. Also called a tugger or a winch.

A processing technique to project formation shear data recorded in any two orthogonal directions into the fast and slow shear directions in the presence of shear-wave anisotropy. In the sonic logging application, a dipole transmitter excites a flexural mode that is recorded at one set of receivers that is in-line with the dipole and other receivers that are 900 out of line (the cross-dipole component). A similar recording is made of the wave from a second dipole transmitter, mounted orthogonally to the first. The flexural-wave velocity is closely related to the formation shear velocity, particularly at low frequencies and in hard formations. Using all four waveforms, the Alford rotation is used to determine the speed and direction of the fast and the slow shear wave.Reference: Shear Data in the Presence of Azimuthal Anisotropy: Dilley, Texas, Expanded Abstracts, 56th SEG Annual International Meeting and Exposition, Houston, Texas, USA, November 2-6, 1986, Paper S9.6

A particular relation proposed by G.E. Archie between the formation factor (F) and porosity (phi), in which F = 1 / phim, where the porosity exponent, m, is a constant for a particular formation or type of rock. In the original work, Archie proposed that m lay between 1.8 and 2.0 for consolidated sandstones, and close to 1.3 for loosely consolidated sandstones. m was named the cementation exponent shortly afterwards. This relation is also known as the Archie II equation.

A rock whose petrophysical properties are well described by the Archie equation with constant values for the porosity exponent and the saturation exponent. Such rocks typically have very little clay, a regular pore structure and high-salinity water. The term often is used to describe a rock that is petrophysically simple.

The value a in the relation of formation factor (F) to porosity (phi): F = a / phim. The value a is derived empirically from best fits of measured values of F and phi on a group of rock samples. It has no clear physical significance, although it has been related to grain shape and tortuosity. In the saturation equation, it always occurs associated with the water resistivity as (a * Rw).It is sometimes claimed that a must be 1 since at phi = 1, F must be 1. However, a material with phi = 1 is not a rock: a is essentially an empirical factor for rocks and as such can take any value. A wide range of values has been found, from 0.5 to 5.

The costs associated with abandoning a well or production facility. Such costs are specified in the authority for expenditure (AFE), and typically cover the plugging of wells; removal of well equipment, production tanks and associated installations; and surface remediation.

A term to indicate features in seismic data other than reflections, including events such as diffractions, multiples, refractions and surface waves. Although the term suggests that such events are not common, they often occur in seismic data.

Reservoir pore fluid pressure that is not similar to normal saltwater gradient pressure. The term is usually associated with higher than normal pressure, increased complexity for the well designer and an increased risk of well control problems. Pressure gradients in excess of around 10 pounds per gallon equivalent fluid density (0.52 psi/foot of depth) are considered abnormal. Gradients below normal are commonly called subnormal.

A laboratory test to evaluate drilling-grade weighting material for potential abrasiveness. The test measures weight loss of a specially shaped, stainless-steel mixer blade after 20 minutes at 11,000 rpm running in a laboratory-prepared mud sample. Abrasiveness is quantified by the rate of weight loss, reported in units of mg/min. Mineral hardness, particle size and shape are the main parameters that affect abrasiveness of weighting materials. Some crystalline forms of hematite grind to a higher percentage of large particles than do other forms and are therefore more abrasive. Hematites are harder than barites, grind courser and are more abrasive. Thus, a hematite that is proposed as a weighting material for mud is typically a candidate for abrasion testing.

A wellbore treatment in which a fluid laden with solid particles is used to remove deposits from the surface of wellbore tubulars and completion components. The treatment fluid is pumped at high pressure through a downhole tool equipped with nozzles that direct a jet, or jets, of fluid onto the target area. Most tool designs use a controlled rotary motion to ensure complete circumferential treatment of internal surfaces. Abrasive jetting techniques can also be used to cut completion or wellbore components. For this application, highly abrasive particles, such as sand, are carried in a fluid and jetted at the target area over an extended period to erode the tubular.

The measurement of age in years. The determination of the absolute age of rocks, minerals and fossils, in years before the present, is the basis for the field of geochronology. The measurement of the decay of radioactive isotopes, especially uranium, strontium, rubidium, argon and carbon, has allowed geologists to more precisely determine the age of rock formations. Tree rings and seasonal sedimentary deposits called varves can be counted to determine absolute age. Although the term implies otherwise, "absolute" ages typically have some amount of potential error and are inexact. Relative age, in contrast, is the determination of whether a given material is younger or older than other surrounding material on the basis of stratigraphic and structural relationships, such as superposition, or by interpretation of fossil content.

A type of high-specification fluid filter frequently used to remove small solid particles from workover or treatment fluids that may be injected into, or placed adjacent to, the reservoirformation. In using absolute filters, all particles larger than the micron rating of the filter element in use will be removed from the treated fluid.

The maximum flow rate a well could theoretically deliver with zero pressure at the middle of the perforations. The term is commonly abbreviated as AOFP or OFP.

The measurement of the permeability, or ability to flow or transmit fluids through a rock, conducted when a single fluid, or phase, is present in the rock. The symbol most commonly used for permeability is k, which is measured in units of darcies or millidarcies.

The measurement of pressure relative to the pressure in a vacuum, equal to the sum of the pressure shown on a pressure gauge and atmospheric pressure.

The volume a solid occupies or displaces when added to water divided by its weight, or the volume per unit mass. In the oil field, absolute volume is typically given in units of gallons per pound (gal/lbm) or cubic meters per kilogram (m3/kg).

An algorithm used in numerical simulation along the boundary of a computational domain to absorb all energy incident upon that boundary and to suppress reflection artifacts.

The ratio of absorbed incident energy to the total energy to which a body is exposed.

The property of some liquids or solids to soak up water or other fluids. The natural gas dehydration process uses glycols (liquids) that absorb the water vapor to finally obtain dehydrated gas. In the same way, light oil, also called absorption oil, is used to remove the heavier liquid hydrocarbons from a wet gas stream to obtain dry gas.

The range of wavelengths of energy that can be absorbed by a given substance.

A light liquid hydrocarbon used to absorb or remove the heavier liquid hydrocarbons from a wet gas stream. Absorption oil is also called wash oil.

Pertaining to the depositional environment of the deepest area of the ocean basins, the abyss. The depositional energy is low, the abyssal plain is flat and nearly horizontal, and fine-grained sediments are deposited slowly by waning turbidity currents or from suspension in the water. The water is thousands of meters deep (> 2000 m) [6520 ft], so the water is cold and sunlight is minimal.

A downhole tool used in conjunction with a jar to store energy that is suddenly released when the jar is activated. The energy provides an impact force that operates associated downhole tools or, in a contingency role, helps release a tool string that has become stuck. Depending on the operating mode, the energy in tension or compression can be stored by means of a mechanical spring or a compressible fluid such as nitrogen gas. Accelerators should be selected on the basis of their compatibility with the jar to be used.

A device used during surveying to measure the acceleration of a ship or aircraft, or to detect ground acceleration in boreholes or on the Earth's surface produced by acoustic vibrations.

Sequence stratigraphic term for the amount of space available for sediment accumulation. Dominant influences on the amount of accommodation, or accommodation space, include subsidence and eustasy.

The mechanism by which partially hydrated cuttings stick to parts of the bottomhole assembly and accumulate as a compacted, layered deposit.

An occurrence of trapped hydrocarbons, an oil field.

A device used in a hydraulic system to store energy or, in some applications, dampen pressure fluctuations. Energy is stored by compressing a precharged gas bladder with hydraulic fluid from the operating or charging system. Depending on the fluid volume and precharge pressure of the accumulator, a limited amount of hydraulic energy is then available independent of any other power source. Well pressure-control systems typically incorporate sufficient accumulator capacity to enable the blowout preventer to be operated with all other power shut down.

The closeness of the agreement between the result of the measurement and the conventional true value of the quantity. Accuracy should not be confused with precision. (ISO)Core measurements have well-defined calibration techniques and standards. Logging measurements are characterized during tool design and construction, and calibrated regularly to some standard. The quoted accuracy of a log then depends on the initial characterization, the reproducibility of the standard, and the stability of the measurement between calibrations and under downhole conditions. The actual accuracy also depends on the equipment performing and being operated to specification.

(noun) A weak organic acid with the chemical formula CH₃COOH, used in oilfield operations as a low-corrosivity stimulation fluid for dissolving carbonate scale and calcium carbonate formations, as a pH buffer in drilling fluids, and as a solvent in certain completion and workover procedures where a less aggressive acid treatment is required.

A generic term used to describe a treatment fluid typically comprising hydrochloric acid and a blend of acid additives. Acid treatments are commonly designed to include a range of acid types or blends, such as acetic, formic, hydrochloric, hydrofluoric and fluroboric acids. Applications for the various acid types or blends are based on the reaction characteristics of the prepared treatment fluid.

The change in a pulsed neutron capture measurement produced by acidizing a carbonateformation. Acidizing tends to increase the porosity as well as leave chlorides in the formation, thereby increasing the capture cross section. Both of these results affect the formation thermal decay time and must be taken into account in the interpretation.

A hydraulic fracturing treatment performed in carbonate formations to etch the open faces of induced fractures using a hydrochloric acid treatment. When the treatment is complete and the fracture closes, the etched surface provides a high-conductivity path from the reservoir to the wellbore.

A gas that can form acidic solutions when mixed with water. The most common acid gases are hydrogen sulfide [H2S] and carbon dioxide [CO2] gases. Both gases cause corrosion; hydrogen sulfide is extremely poisonous. Hydrogen sulfide and carbon dioxide gases are obtained after a sweetening process applied to a sour gas.

A chemical additive used to protect wellbore components and treatment equipment from the corrosive action of an acid. The type and concentration of acid inhibitors are determined by the type of metal to be protected and the specific wellbore conditions, such as temperature and the length of exposure time anticipated during the treatment. To ensure efficient protection, the inhibitor should be consistently blended throughout the treatment fluid.

A measure of the amount of acidic components present in a crude oil. This measurement is the mass of potassium hydroxide (KOH) in milligrams titrated into a one-gram sample of oil—such as stock-tank oil—that is required reach a neutral pH of 7. The test is performed under ASTM Standard D664.

The rubber-lined vessel used to transport raw or concentrated acid to the wellsite. Some acid additives attack or degrade rubber. Consequently, acid treatment fluids are not generally mixed or transported in acid tanks, but are instead mixed in special batch tanks or continuously mixed as the treatment is pumped.

A wellbore acid treatment designed to remove scale or similar deposits from perforations and well-completion components. Acid-wash treatments generally do not include injection of treatment fluid into the reservoirformation.

The pumping of acid into the wellbore to remove near-well formation damage and other damaging substances. This procedure commonly enhances production by increasing the effective well radius. When performed at pressures above the pressure required to fracture the formation, the procedure is often referred to as acid fracturing.

In geophysics, acoustic refers specifically to P-waves in the absence of S-waves (i.e., in fluids, which do not support S-waves, or in cases in which S-waves in solids are ignored).

The portion of the Earth below which strata cannot be imaged with seismic data, or the deepest relatively continuous reflector. Acoustic basement, in some regions, coincides with economic basement and geologic basement, or that portion of the Earth that does not comprise sedimentary rocks.

An obsolete piece of equipment that converts acoustic signals from analog to electrical form and back. A common use of an acoustic coupler was to provide an interface between a telephone and an early type of computer modem.

A type of elastic wave produced by deformation or brittle failure of material and characterized by relatively high frequency.

The product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient.

A seismicreflectivity section, or a 2D or 3D seismic section, that has been inverted for acoustic impedance. Sonic and density logs can be used to calibrate acoustic impedance sections.

A record of some acoustic property of the formation or borehole. The term is sometimes used to refer specifically to the sonic log, in the sense of the formation compressional slowness. However, it may also refer to any other sonic measurement, for example shear, flexural and Stoneley slownesses or amplitudes, or to ultrasonic measurements such as the borehole televiewer and other pulse-echo devices, and even to noise logs.

A situation in which acoustic energy that propagates in one direction is confined in the other two directions as, for example, a mode confined to an interface between two different materials or within the borehole. The Stoneley wave, tube wave and flexural mode have important applications in formation evaluation, while most of the others, such as the Rayleigh wave and the various guided borehole modes (normal mode, leaky mode and hybrid mode), are considered interference that must be filtered out. In y slow formations, leaky modes can help determine formation compressional slowness.

A method of calculating the position of marine seismic equipment. Range measurements are made whereby distance is equal to acoustic signal traveltime from transmitter to hydrophone multiplied by the speed of sound in water. When sufficient acoustic ranges with a proper geometricdistribution are collected, location coordinates x, y and z of the marine seismic equipment can be computed by the method of trilateration (measuring the lengths of the sides of overlapping triangles). Acoustic positioning is commonly used in towed streamer and ocean-bottom cableseismic acquisition modes.

A device for transforming electrical energy into sound, or vice versa. In sonic logging applications, acoustic transducers are usually made of piezoelectric ceramic or magnetostrictive materials, and may be used as either receivers or transmitters in a frequency range between about 1 and 30 kHz. The transducers are excited as either monopoles, emitting or receiving sound in all directions, or dipoles, emitting or receiving in one plane. In ultrasonic logging applications, acoustic transducers are made of piezoelectric ceramic materials, and often are used in alternating transmitter/receiver (pulse-echo) mode, in a frequency range from a few hundred kilohertz to a few megahertz.

The quality of a medium whose acoustic impedance is constant throughout, such that it contains no seismic reflections. An example of an acoustically transparent medium is water.

(noun) A mechanical pressure disturbance that propagates through a medium as a compressional (P-wave) or shear (S-wave) oscillation. In petroleum geoscience, acoustic waves are generated by seismic sources and logging tools to image subsurface structures, measure formation properties, and evaluate cement bond quality.

The generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate, and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing.

The log that is actually recorded while taking the measurements. It is distinct from a playback, which is produced later on from digital data.

A linear copolymer of acrylate (anionic) and acrylamide (nonionic) monomers, also called partially-hydrolyzed polyacrylamide (PHPA). The ratio of acrylic acid to acrylamide groups on the polymer chain can be varied in manufacturing, as can molecular weight. Another variable is the base used to neutralize the acrylic acid groups, usually NaOH or KOH, or sometimes NH4OH. A concentration of approximately 10 to 30% acrylate groups provides optimal anionic characteristics for most drilling applications. High-molecular weight PHPA is used as a shale-stabilizing polymer in PHPA mud systems. It is also used as clay extender, either dry-mixed into clay or added at the rig to a low-bentonitemud. PHPA can also be used to flocculatecolloidal solids during clear-water drilling and for wastewater cleanup. Low molecular-weight PHPA is a clay deflocculant.

A linear, nonionic polymer made of acrylamide monomers, CH2=CHCONH2 . High molecular-weight polyacrylamides are used as selective flocculants in clear-water drilling, low-solids muds and wastewater cleanup. Polymers made of smaller molecules are used as clay deflocculants in water muds, which can contain hardness ions. Polyacrylamides are not nearly as sensitive to salinity and hardness as the anionic polyacrylates (SPA). Also, being nonionic, they are not as powerful for flocculation or deflocculation applications. Acrylamide polymers are, however, susceptible to hydrolysis and release ammonia under hot, alkaline conditions.

A linear copolymer of acrylate (anionic) and acrylamide (nonionic) monomers, also called partially-hydrolyzed polyacrylamide (PHPA). The ratio of acrylic acid to acrylamide groups on the polymer chain can be varied in manufacturing, as can molecular weight. Another variable is the base used to neutralize the acrylic acid groups, usually NaOH or KOH, or sometimes NH4OH. A concentration of approximately 10 to 30% acrylate groups provides optimal anionic characteristics for most drilling applications. High-molecular weight PHPA is used as a shale-stabilizing polymer in PHPA mud systems. It is also used as clay extender, either dry-mixed into clay or added at the rig to a low-bentonite mud. PHPA can also be used to flocculate colloidal solids during clear-water drilling and for wastewater cleanup. Low molecular-weight PHPA is a clay deflocculant.

A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds.Reference:Perricone AC, Enright DP and Lucas JM: "Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Base Muds," SPE Drilling Engineering 1, no. 5 (October 1986): 358-364.

A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds.Reference:Perricone AC, Enright DP and Lucas JM: "Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Base Muds," SPE Drilling Engineering 1, no. 5 (October 1986): 358-364.

Linear, anionicpolymer made from the monomer acrylic acid, CH2=CHCOO- H+. The acrylic acid groups are evenly spaced along the chain. Acrylic acid polymer neutralized with NaOH is sodium polyacrylate (SPA). Polyacrylates are best utilized in soft water with low salinity to achieve the best dispersion and full chain elongation. Even low concentrations of hardness ions, for example, Ca+2, precipitate polyacrylates. Low molecular-weight polyacrylates are used as clay deflocculants. High molecular weight polymers are used for fluid-loss control and as a clay extender. As an extender, SPA is added to bentonite at the grinding plant. It is also used at the rig in low-solids mud. Divalent cations can negate its benefits as a clay extender. SPA is highly efficient when used to flocculate colloids in native-solids muds, clear-water muds and wastewater cleanup. The polymer chain links together colloidal solids that can be removed by gravity settling in shallow pits or by applying hydrocyclone, centrifuge or filtration techniques.

A log of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. The carbon-oxygen log, elemental capture spectroscopy log, pulsed neutron spectroscopy log, aluminum activation log and oxygen activation log are all examples of activation logs. However, the term is most commonly used to refer to the aluminum and oxygen activation logs, the latter also being known as a water-flow log.

A boundary of colliding lithospheric plates. The present subduction zones of the Pacific Rim, the older mountains of the Alps, and the Himalayas represent active margins.

The escaping tendency, or vapor pressure, of water molecules in an aqueous solution compared with that of pure water, typically abbreviated aw. Activity is expressed mathematically as the ratio of two vapor pressures: aw = p/po, where p is vapor pressure of the solution and po is vapor pressure of pure water. The ratio ranges from near 0 to 1.0 and corresponds to percent relative humidity (% RH) of air in equilibrium with the aqueous solution. For pure water, aw = po/po = 1.00 and RH = 100%. By increasing the concentration of salt (or other solutes) in the solution, aw decreases, because vapor pressure of the solution decreases. However, aw never reaches zero. Known-activity, saturated-salt solutions are used to calibrate RH meters. Measuring RH of air above an oil mud is a simple way to measure the activity (salinity) of its water phase. Adjusting the salinity of the water phase is a way to control movement of water into or out of shales that are being drilled with an oil mud. Chenevert related aw in oil mud to RH above the mud sample and devised a practical test using an electrohygrometer to measure RH, called the "Chenevert Method."

A property of semivariogram models. Any linear combination of admissible models with positive coefficients can be nested or added together. Generally, single models are used for modeling experimental semivariograms that are close in shape to one of the basic admissible models, or for the approximate fitting of complex structural functions. Nested models are used to better fit complex structural functions.Reference:Olea RA: "Fundamentals of Semivariogram Estimation, Modeling, and Usage," in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994.

In a system with two immiscible fluids in contact with a solid, the difference in the two fluid-solid surface tensions. In thermodynamic equilibrium this difference is equivalent as a result of the Young-Laplace equation to the product of the interfacial tension between the two fluids and the cosine of the contact angle at the fluid/fluid/solid interface. As the combination of these two individual interfacial terms, adhesion tension is a useful measure of the wetting character of a petroleumreservoir's pore system.

A valve, located on or near the Christmas tree that is used to control the production of fluid from a well. Opening or closing the variable valve influences the rate and pressure at which production fluids progress through the pipeline or process facilities. The adjustable choke is commonly linked to an automated control system to enable the production parameters of individual wells to be closely controlled.

The approximated flow time used for a well-test analysis when the flow rate varies before or during the test period. It is calculated as t = cumulative well production since the last extended shut-in period divided by the flow rate just before a well is shut in for a buildup test.

The gas accumulated on the surface of a solid material, such as a grain of a reservoirrock, or more particularly the organic particles in a shale reservoir. Measurement of adsorbed gas and interstitial gas, which is the gas contained in pore spaces, allows calculation of gas in place in a reservoir.

The property of some solids and liquids to attract a liquid or a gas to their surfaces. Some solids, such as activated charcoal or silica gel, are used as surfaces of adhesion to gather liquid hydrocarbons from a natural gas stream. To complete the process, the solids are treated with steam to recover the liquid hydrocarbons.

A series of techniques that use geostatistical methods to determine fluid and contaminant flow in the subsurface. These techniques are used primarily to study contamination in groundwater in environmental studies.Reference:McKenna SA and Poeter EP: "Simulating Geological Uncertainty with Imprecise Data for Groundwater Flow and Advective Transport Modeling," in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994.

The surface or near-surface, unconsolidated sedimentary layer that has been subject to weathering and whose pores are air-filled instead of liquid-filled. An aerated layer typically has a low seismic velocity.

Referring to a condition or a situation or a living creature, such as a bacteria, in which oxygen is required to sustain life.

Measurements of the Earth's magnetic field gathered from aircraft. Magnetometers towed by an airplane or helicopter can measure the intensity of the Earth's magnetic field. The differences between actual measurements and theoretical values indicate anomalies in the magnetic field, which in turn represent changes in rock type or in thickness of rock units.

The flow associated with wellbore storage following a surface shut-in. When a well is first shut in at the surface, flow from the formation into the bottom of the wellbore continues unabated until compression of the fluids in the wellbore causes the downhole pressure to rise. If the wellbore fluid is highly compressible and the well rate is low, the afterflow period can be long. Conversely, high-rate wells producing little gas have negligible afterflow periods.

The accumulation of stratigraphic sequences by deposition that stacks beds atop each other, building upwards during periods of balance between sediment supply and accommodation.

The formation of groups or clusters of particles (aggregates) in a fluid. In water or in water-base drilling fluid, clay particles form aggregates in a dehydrated, face-to-face configuration. This occurs after a massive influx of hardness ions into freshwater mud or during changeover to a lime mud or gyp mud. Aggregation results in drastic reductions in plastic viscosity, yield point and gel strength. It is part of wastewater cleanup and water clarification. Alum or polymers cause colloidal particles to aggregate, allowing easier separation.

A drilling technique whereby gases (typically compressed air or nitrogen) are used to cool the drill bit and lift cuttings out of the wellbore, instead of the more conventional use of liquids. The advantages of air drilling are that it is usually much faster than drilling with liquids and it may eliminate lost circulation problems. The disadvantages are the inability to control the influx of formation fluid into the wellbore and the destabilization of the borehole wall in the absence of the wellbore pressure typically provided by liquids.

A source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles.

A method of seismic acquisition using charges detonated in the air or on poles above the ground as the source. Air shooting is also called the Poulter method after American geophysicist Thomas Poulter.

A sound wave that travels through the air at approximately 330 m/s and can be generated and recorded during seismic surveying. Air waves are a type of coherent noise.

A filter, or a set of limits used to eliminate unwanted portions of the spectra of the seismic data, to remove frequencies that might cause aliasing during the process of sampling an analogsignal during acquisition or when the sample rate of digital data is being decreased during seismic processing.

The distortion of frequency introduced by inadequately sampling a signal, which results in ambiguity between signal and noise. Aliasing can be avoided by sampling at least twice the highest frequency of the waveform or by filtering frequencies above the Nyquist frequency, the highest frequency that can be defined accurately by that sampling interval.

A telescopic surveying device used to construct surface topographic and geologic maps in the field. The alidade is mounted on a plane table, which has a sheet of paper on which to draw the map, and an object or location is sighted through the alidade. The edge of the alidade is aligned in the azimuthal direction of the object or location. The vertical angle from which elevation of the location can be calculated is measured using the calibrated arc of the alidade.

One of a group of organic compounds of carbon (C) and hydrogen (H) in which the carbon atoms have linear, branched chain (open), or both types of structures. Aliphatics, as they are informally called, can be divided into paraffinic (saturated) and olefinic (unsaturated) chain types. The simplest aliphatic, paraffinic hydrocarbon is methane, CH4. The simplest aliphatic, olefinic hydrocarbon is ethylene, C2H6. In drilling fluids, particularly oil-base muds, the amounts and types of hydrocarbon in the mud can be an important parameter in overall performance of the mud.

Pertaining to an aqueous solution, such as a water-base drilling fluid, which has more hydroxyl ions (OH-) than hydrogen ions (H+) and pH greater than 7.

An enhanced oil recovery technique in which an alkaline chemical such as sodium hydroxide, sodium orthosilicate or sodium carbonate is injected during polymer flooding or waterflooding operations. The alkaline chemical reacts with certain types of oils, forming surfactants inside the reservoir. Eventually, the surfactants reduce the interfacial tension between oil and water and trigger an increase in oil production.Alkaline flooding is not recommended for carbonate reservoirs because of the abundance of calcium: the mixture between the alkaline chemical and the calcium ions can produce hydroxide precipitation that may damage the formation.Alkaline flooding is also known as caustic flooding.

A chemical enhanced oil recovery flood that uses two sources of surfactant and a polymer. Alkaline chemicals such as sodium carbonate react with acidic oil components in situ to create petroleumsoap, which is one of the surfactants. A synthetic surfactant is injected simultaneously with the alkali. A water-soluble polymer is also injected, both in mixture with the alkali and surfactant and as a slug following the mixture, to increase the viscosity of the injectant, thereby improving mobility control of the flood fronts.

A chemical property of an aqueous system that implies that there are more hydroxyl ions (OH-) in the system, or a potential to produce more hydroxyl ions, than there are hydrogen ions (H+), or potential to produce hydrogen ions.

A measure of the total amount of hydroxyl ions in a solution as determined by titration with standardized acid. This test is a well-known water-analysis procedure to estimate hydroxyl, carbonate ion and bicarbonate ion concentrations. There are two pH endpoints, P and M, in this titration, corresponding to phenolphthalein and methyl orange indicators. The "P" endpoint is at pH 8.3 and the "M" endpoint is at pH 4.3. Each is reported in units of cm3 acid/cm3 sample. For water samples and very simple mud filtrates, P and M data indicate OH-, HCO3- and CO3-2 concentrations, but an alkalinity test is unreliable for analyzing complex mud filtrates. The API has established standards for conducting alkalinity tests.

A rock mass formed somewhere other than its present location, which was transported by fault movements, large-scale gravity sliding, or similar processes.

Pertaining to materials, particularly rock masses, that formed somewhere other than their present location, and were transported by fault movements, large-scale gravity sliding, or similar processes. Autochthonous material, in contrast, formed in its present location. Landslides can result in large masses of allochthonous rock, which typically can be distinguished from autochthonous rocks on the basis of their difference in composition. Faults and folds can also separate allochthons from autochthons.

Pertaining to minerals or rock fragments that formed in one location but were transported to another location and deposited. Clastic sediments in a rock such as sandstone are allogenic, or formed elsewhere.

Pertaining to the subaerial (as opposed to submarine) environment, action and products of a stream or river on its floodplain, usually consisting of detrital clastic sediments, and distinct from subaqueous deposition such as in lakes or oceans and lower energy fluvial deposition. Sediments deposited in an alluvial environment can be subject to high depositional energy, such as fast-moving flood waters, and may be poorly sorted or chaotic.

Material deposited in an alluvial environment, typically detrital sediments that are poorly sorted.

A technique for combining a measurement that has a high accuracy but low precision with another measurement of the same quantity that has a high precision but low accuracy in order to produce a result that is better than either alone. Alpha processing is used to improve the vertical resolution of neutron porosity and other dual-detector nuclear logs. The detector near the source has better precision than the far detector in the sense that it responds more precisely to vertical changes. However, the near detector is less accurate because it is more affected by the borehole environment. Alpha processing mathematically superimposes the rapid changes of the near detector on the slowly changing but accurate far detector to produce an accurate log with high vertical resolution.The technique is also used to improve results from the carbon-oxygen log and other pulsed neutron spectroscopy measurements. Two methods are used to determine the carbon/oxygen ratio. The windows method counts the number of gamma rays within energy windows placed at the main peaks for carbon and oxygen. This method has good statistical precision but poor accuracy, as gamma rays from other elements contaminate these windows. The other method, spectral stripping, compares the total spectrum against standards for many elements, inverting the spectrum to obtain the yield for each element. This method is more accurate but has less statistical precision. Averaging over a number of measurements, alpha processing adjusts the windows result with the more accurate spectral stripping in order to obtain a precise and accurate result.

A near-wellbore formation zone, a few inches thick, whose acoustic velocity has been affected by impregnation with drilling fluids, stress relief, or both. The acoustic velocity of the rock in the immediate vicinity of the borehole wall can be much slower than that in the virgin formation. To measure the formation velocity, it may be necessary to use a soniclogging tool that has a greater spacing between transmitter and receiver array (about 10 to 15 ft [3 to 4.5 m]) than the standard sonic tool (about 3 to 5 ft [0.9 to 1.5 m]). The altered zone may also give rise to different acoustic modes, for example the hybrid mode or a second Stoneley wave.

A series of double salts of aluminum sulfate and potassium sulfate with the formula Al2(SO4)3·K2SO4·nH2O. Alum is used as a colloidal flocculant in wastewater cleanup.

A wireline log of the concentration by weight of aluminum in the formation, based on the principle of neutron activation. Aluminum (27Al) can be activated by capturing relatively low-energy neutrons from a chemical source to produce the isotope 28Al, which decays with a half-life of 2.3 minutes and emits a relatively easily detected 1.78 MeV gamma ray. A natural gamma ray spectrometer will detect this gamma ray along with the other natural gamma rays. If the natural gamma spectrum has been measured before activation, it can be subtracted from the spectrum after activation to give an estimate of Al content.Al is a relatively direct indicator of the volume of clay, since clay minerals are alumino-silicates.

The salt of aluminum hydroxide and stearic acid (saturated C-18 fatty acid) with the formula Al(O2C18H35)3. It is a grease-like solid. When mixed with oil (for example, diesel oil) and the mixture sprayed onto the surface of a foamy water mud, it helps the gas bubbles break out of the mud.

The temperature at a point or area expressed as an average of the surrounding areas or materials. Ambient surface temperature is generally given to be 70 to 80oF [21 to 27oC]-an average of daily and seasonal variations.

A group of organic chemicals with the general formula RCO-NH2 formed from reactions of ammonia (NH3) and a carboxylic acid, RCOO-H+. "R" groups range from hydrogen to various linear and ring structures. Amides and polyamides are emulsifiers and surfactants, many of which are made from fatty acids.

A group of organic chemicals that are analogs of ammonia (NH3), in which either one, two or three hydrogen atoms of ammonia are replaced by organic radicals. General formulas are: (1) primary amines, RNH2, (2) secondary amines, R1R2NH, (3) tertiary amines, R1R2R3N and quaternary amines, R1R2R3 R4N+X (where X represents an anion). Amines are organic bases (mildly alkaline) and react with acids to form nitrogenous, organic salts. Amines made from fatty acids are emulsifiers and oil-wetting agents for oilfield chemicals.

The amount of displacement of a seismicwavelet measured from peak to trough.

An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator.

Variation in seismicreflectionamplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation ("fizz water"). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments.

Pertaining to systems, reactions or life processes of species, such as bacteria, in which atmospheric oxygen is not present or not required for survival.

An example used for comparison. In oil and gas exploration, geoscientists and engineers compare new prospects and fields with fields and surface exposures thought to be similar in depositional environment and reservoircharacter to guide predictions. Wide variations in shale reservoirs create doubt about the utility of analog comparisons.

The acute angle at which a wavefront impinges upon an interface, such as a seismicwave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell's law describes the relationship between the angle of incidence and the angle of refraction of a wave.

The acute angle at which a raypath impinges upon a line normal to an interface, such as a seismicwave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell's law describes the relationship between the angle of incidence and the angle of refraction of a wave.

The variation of seismic velocity in different directions.

A surface that separates younger strata from eroded, dipping, older strata and represents a gap in the geologic record.

The anhydrous mineral form of calcium sulfate, CaSO4. (Gypsum, CaSO4·2H2O, is the hydrated form.) The presence of anhydrite or gypsum in rock will influence the type of mud selected for drilling the rock because when CaSO4 dissolves in a water mud, Ca+2 and SO4-2 ions are formed. Although both ions are detrimental to freshwater mud properties, Ca+2 is the more harmful of the two.

A test to evaluate base oils that are used in oil mud. The test indicates if an oil is likely to damage elastomers (rubber compounds) that come in contact with the oil. The aniline point is called the "aniline point temperature," which is the lowest temperature (°F or °C) at which equal volumes of aniline (C6H5NH2) and the oil form a single phase. The aniline point (AP) correlates roughly with the amount and type of aromatic hydrocarbons in an oil sample. A low AP is indicative of higher aromatics, while a high AP is indicative of lower aromatics content. Diesel oil with AP below 120°F [49°C] is probably risky to use in oil-base mud. The API has developed test procedures that are the standard for the industry.

A negatively charged ion. Clay surfaces, groups on polymer chains, colloids and other materials have distinct, negatively charged areas or ions. Anionic characteristics affect performance of additives and contaminants in drilling fluids, especially water muds, in which clays and polymers are used extensively.

A formation with directionally dependent properties. The most common directionally dependent properties are permeability and stress. Most formations have vertical to horizontal permeability anisotropy with vertical permeability being much less (often an order of magnitude less) than horizontal permeability. Bedding plane permeability anisotropy is common in the presence of natural fractures. Stress anisotropy is frequently greatest between overburden stress and horizontal stress in the bedding plane. Bedding plane stress contrasts are common in tectonically active regions. Permeability anisotropy can sometimes be related to stress anisotropy.

(noun) The directional dependence of a physical property, such that measurements yield different values when taken along different axes. In petroleum geoscience, anisotropy in permeability, seismic velocity, and electrical resistivity arises from sedimentary layering, fracture orientation, and mineral alignment, and significantly affects fluid flow, seismic imaging, and log interpretation.

A device that uses Pitot tubes to measure the gas flow rate within a pipeline. The gas volume is calculated from the difference between the flowing pressure and the static pressure of the gas.

A large valve used to control wellbore fluids. In this type of valve, the sealing element resembles a large rubber doughnut that is mechanically squeezed inward to seal on either pipe (drill collar, drillpipe, casing, or tubing) or the openhole. The ability to seal on a variety of pipe sizes is one advantage the annular blowout preventer has over the ram blowout preventer. Most blowout preventer (BOP) stacks contain at least one annular BOP at the top of the BOP stack, and one or more ram-type preventers below. While not considered as reliable in sealing over the openhole as around tubulars, the elastomeric sealing doughnut is required by API specifications to seal adequately over the openhole as part of its certification process.

A multiphase flow regime in which the lighter fluid flows in the center of the pipe, and the heavier fluid is contained in a thin film on the pipe wall. The lighter fluid may be a mist or an emulsion. Annular flow occurs at high velocities of the lighter fluid, and is observed in both vertical and horizontal wells. As the velocity increases, the film may disappear, leading to mist flow or emulsion flow. When the interface between the fluids is irregular, the term wavy annular flow may be used.

A flow of formation gas in the annulus between a casing string and the borehole wall. Annular gas flows occur when there is insufficient hydrostatic pressure to restrain the gas. They can occur in uncemented intervals and even in cemented sections if the cement bond is poor. After cementing, as the cement begins to harden, a gel-like structure forms that effectively supports the solid material in the cement slurry. However, during this initial gelling period, the cement has no appreciable strength. Hence, with the solid (weighting) material now supported by the gel structure, the effective density of the slurry that the reservoir experiences falls rather suddenly to the density of the mix water of the cement, which is usually fresh water, whose density is 8.34 lbm/gal, or a gradient of 0.434 psi/ft of vertical column height. Various chemical additives have been developed to reduce annular gas flow.

Fluid pressure in the annulus between tubing and casing or between two strings of casing.

Production of formation fluid through the casing-tubing annulus.

The space surrounding one cylindrical object placed inside another, such as the space surrounding a tubular object placed in a wellbore.

The linear velocity of a fluid passing through an annular space. The term critical annular velocity is often used to describe the flow rate or velocity at which entrained solids will be efficiently transported by the annular fluid. If the fluid velocity falls below the critical rate, there will be a risk of particles settling, forming beds or bridges that may obstruct the wellbore.

(noun) The plural form of annulus. In well construction, annuli refers to the multiple concentric annular spaces formed between successive strings of casing, or between the innermost casing or tubing and the wellbore wall. Monitoring and managing pressures in each annulus is critical for well integrity and safety.

The space between two concentric pipe strings, such as between the production tubing and casing in a well. The term may also refer to the space between a pipe string and the borehole wall in an openhole completion or openhole drillstem test (DST).

A protective device to prevent electrolytic corrosion. Anodes (often made of Mg or Al metal) are sacrificed intentionally to protect a steel system, such as a buried pipeline or offshore platform.

An entity or property that differs from what is typical or expected, or which differs from that predicted by a theoreticalmodel. May be the measurement of the difference between an observed or measured value and the expected values of a physical property. Anomalies can be of great interest in hydrocarbon and mineral exploration because they often indicate hydrocarbon and mineral prospects and accumulations, such as geologic structures like folds and faults. Geochemical anomalies at the surface of the Earth can also indicate an accumulation of hydrocarbons at depth. Geophysical anomalies, such as amplitude anomalies in seismic data and magnetic anomalies in the Earth's crust, can also be associated with hydrocarbon accumulations.

The condition of an environment in which free oxygen is lacking or absent.

A type of structural hydrocarbon trap whose closure is controlled by the presence of an anticline.

An arch-shaped fold in rock in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core.

A chemical additive used to prevent the formation of foam during the preparation of a treatment fluid or slurries at surface. Excess foam created during the mixing process may cause handling and pumping difficulties and may interfere with the performance or quality control of the mixed fluid. Antifoam agents may also be used to break foams returned from the wellbore, following a treatment, in preparation for disposal of the fluids.

The effect on a laterolog whereby the current lines are no longer properly focused but spread out at a certain distance into the formation. The effect occurs opposite a high-resistivitybed with low-resistivity shoulders. The result is that laterolog devices, in particular deep devices, tend to read too low and have less depth of investigation. Shoulder bed correction charts correct for these effects in certain well-defined situations, such as no invasion in horizontal beds with vertical wells.

A minor, secondary fault, usually one of a set, whose sense of displacement is opposite to its associated major and synthetic faults. Antithetic-synthetic fault sets are typical in areas of normal faulting.

A drill bit, usually polycrystalline diamond compact bit (PDC) type, designed such that the individual cutting elements on the bit create a net imbalance force. This imbalance force pushes the bit against the side of the borehole, which in turn creates a stable rotating condition that resists backwards whirling, wobbling and downhole vibration. Antiwhirl bits allow faster rates of penetration, yet achieve longer bit life than more conventional bits, which are not dynamically biased to run smoothly, are inherently unstable, are vibration-prone and thus have shorter lives. No bit is whirl-proof, however.

A mechanism to limit the affects of measurements on a device or system. In seismic data acquisition, the length of the spread has the effect of an aperture.

In seismic data, the ratio of the velocity determined from normal moveout (i.e., primarily a horizontal measurement) to velocity measured vertically in a vertical seismic profile or similar survey. Apparent anisotropy is of particular importance when migrating long-offset seismic data and analyzing AVO data accurately. The normal moveout velocity involves the horizontal component of the velocity field, which affects sources and receivers that are offset, but the horizontal velocity field is not involved in velocity calculations from vertically measured time-depth pairs.

The angle that a plane makes with the horizontal measured in any randomly oriented section rather than perpendicular to strike.

A calculation of the properties of the solid fraction of a rock from the combination of two logs. For example, by combining the density and neutron porosity measurements, it is possible to compute an apparent matrix density; by combining neutron porosity and sonic measurement, it is possible to compute an apparent matrix traveltime. The computations assume a particular fluid, usually fresh water, and particular response equations. The results are often displayed as quicklook logs for lithology identification. The word matrix is used here in the formation evaluation sense of the term rather than the geological one.

In geophysics, the speed of a wavefront in a certain direction, typically measured along a line of receivers and symbolized by va. Apparent velocity and velocity are related by the cosine of the angle at which the wavefront approaches the receivers:

The viscosity of a fluid measured at the shear rate specified by API. In the Bingham plastic rheology model, apparent viscosity (AV) is one-half of the dial reading at 600 rpm (1022 sec-1 shear rate) using a direct-indicating, rotational viscometer. For example, a 600-rpm dial reading is 50 and the AV is 50/2, or 25 cp.

The wavelength measured by receivers when a wave approaches at an angle. The relationship between true and apparent wavelength can be shown mathematically as follows:

The phase of petroleum operations that immediately follows successful exploratory drilling. During appraisal, delineation wells might be drilled to determine the size of the oil or gas field and how to develop it most efficiently.

Any water-bearing formation encountered while drilling. Drillers often are concerned about aquifers and are required to take special precautions in the design and execution of the well plan to protect fresh water aquifers from contamination by wellbore fluids. Water in aquifers can flow into the wellbore, contaminate drilling fluids and cause well control problems.

The calculated flow area provided by perforations across a specific zone of interest. The resulting value is used to calculate pressure drops and fluid-flow performance.

In a reservoirwaterflood or other fluid injection using a well pattern, the fraction of the pattern area from which reservoir fluid is displaced by the injected phase at the time of breakthrough. Parameters such as formationdip angle and dip azimuth, presence of fractures, mobility ratio, injection pattern and directional permeability affect areal displacement efficiency (EA).

Describing sandy-textured rock or sediment. Arenaceous does not necessarily imply silica-rich, but rather particles of sand size, 0.625 to 2 mm, according to the Udden-Wentworth scale.

Describing rocks or sediments containing particles that are silt- or clay-sized, less than 0.625 mm in size. Most have a high clay-mineral content, and many contain a sufficient percentage of organic material to be considered a source rock for hydrocarbon.

A mathematical method of finding a central value for a group of data. It is most often referred to as the average but also as the mean. The arithmetic mean is the sum of all the observed values divided by the number of observations.

The metal strands on the outside of a wireline logging cable. Typical cables have two layers of metal strands, one wound clockwise and the other counterclockwise. The armor gives the cable its strength. It is used as the current return in some electrical measurements.

One of two quantitative analysis procedures for measuring aromatic content of base oils for use in oil mud as proscribed by the API. Results of the two aromatic content tests may differ because of the way the two gas-chromatography techniques separate and identify aromatics. The Institute of Petroleum (IP) instruments are more widely available than the American Society for Testing and Materials (ASTM) instruments, and the Institute of Petroleum method measures trace levels better than the more complex ASTM method.

A type of compound containing hydrogen and carbon atoms arranged in a symmetrical 6-carbon ring structure with single (C-C) and double (C=C) bonds alternating around the ring. Rings are single, multiple or fused and can have other chemical groups attached in place of hydrogen. Benzene, C6H6 is the simplest single-ring aromatic, napthalene, C10H8, the simplest fused-ring aromatic and toluene is the simplest aromatic, having an alkyl side chain, C6H5-CH3. Xylene, a common oilfield chemical, has two methyl side chains, C6H4-(CH3)2. Aniline is the simplest aromatic amine, C6H5-NH2 and is used in the aniline point test. Aromatic hydrocarbons in oils used to prepare oil-base mud can damage elastomers and increase the toxicity of the fluid. The aniline point test is used for screening oils to infer aromatic content.

In computing, code written to access data in more than one dimension according to a name and subscripts that correspond to each dimension.

An induction tool or log that consists of several mutually balanced arrays whose signals are recorded separately and combined in software to produce the response desired. Typically, there is one transmitter and five to ten pairs of receivers and bucking coils that are balanced to remove direct coupling. The signals are combined in a wide variety of ways to produce the responses desired, as for example, deep-reading, high vertical resolution or some combination of both. There are trade-offs in any response. For example, a deep-reading log typically will not have high vertical resolution. If it does, it will be more sensitive to the invasion condition and cave effect.

An electrode device with multiple current electrodes configured in several different ways to produce several different responses. A typical array consists of a central electrode emitting survey current, with multiple guard electrodes above and below it. Current is sent between different guard electrodes to achieve greater or less focusing. The greater the focusing, the greater the depth of investigation. About five basic measurements are obtained in this way. This hardware focusing may be further improved by software focusing, in which the signals from the basic measurements are superimposed mathematically to ensure proper focusing in a wide range of conditions.

A resistivity recorded by a measurements-while-drilling propagation tool consisting of an array of transmitters and receivers whose signals are recorded separately and combined by software to produce the response desired. In a typical design, five transmitters emit a signal, and the phase shift and attenuation between two receivers are recorded. The phase shifts and attenuations are combined in different ways to produce borehole-compensated logs with different depths of investigation and radial resolution.

A type of acousticlogging tool that uses a large number of receivers, typically 4 to 12. Modern acoustic logging tools are designed to measure not only the compressional wave but also the shear and other acoustic waves generated by the transmitter. The separation and identification of these waves are facilitated by the use of an array of receivers placed about 6 in. [15 cm] apart, which is close enough to avoid aliasing but far enough to sample a significant moveout in the wave. The waveforms at each receiver are recorded and processed by signalprocessing techniques, such as slowness-time coherence, to measure the velocities of the different waves.

An event or appearance of seismic data as a reflection, refraction, diffraction or other similar feature, or the time at which seismic data appear. An event in a seismic section can represent a geologic interface.

The elapsed time between the release of seismic energy from a source and its arrival at the receiver.

The study of ideas that enable computers to do the things that make people seem intelligent. The term is commonly abbreviated as A.I.Many computer programs written for use in the oil field utilize "rule based" approaches to provide expert systems. The rules are taken from an expert working in the field and are written in a way that attempts to reproduce the knowledge and approaches used by that expert to solve a range of real problems. Most such programs are limited to specific areas such as dipmeter interpretation, electrofacies determination, reservoir characterization, blowout prevention, drilling fluid selection, etc. Sometimes expert systems are written in computer languages that easily handle "rules" such as LISP, but once fully tested are usually translated to BASIC, C or FORTRAN to be compiled into efficient applications or programs.

Any system that adds energy to the fluid column in a wellbore with the objective of initiating and improving production from the well. Artificial-lift systems use a range of operating principles, including rod pumping, gas lift and electric submersible pump.

The number of BTUs in a cubic foot of natural gas. The natural gas heat energy (BTU) will depend mainly on its water content at the delivered pressure and temperature conditions.

The number of BTUs in a cubic foot of natural gas. The natural gas heat energy (BTU) will depend mainly on its water content at the delivered pressure and temperature conditions.

A solid or nearly solid form of bitumen that can melt upon heating and contains impurities such as nitrogen, oxygen and sulfur. Asphalt forms naturally when the light components or volatiles of petroleum have been removed or evaporated.

The minimum concentration of solvent injected into a reservoir oil at a given test pressure and temperature that causes asphaltene particles to precipitate from the oil.

As pressure decreases, the pressure at a given test temperature that first causes asphaltene to precipitate from a reservoir fluid.

The flocculation of asphaltene particles from reservoir fluid. The precipitation is typically measured at specific conditions of temperature and pressure, such as at reservoir or flowline conditions.

Organic materials consisting of aromatic and naphthenic ring compounds containing nitrogen, sulfur and oxygen molecules. The asphaltene fraction of crude is defined as the organic part of the oil that is not soluble in straight-chain solvents such as pentane or heptane.Asphaltenes exist as a colloidal suspension stabilized by resin molecules (aromatic ring systems) in the oil. The stability of asphaltic dispersions depends on the ratio of resin to asphaltene molecules. The determination of the quantity of resin is important in estimating the potential damage created by asphaltenes.Asphaltene precipitates as a result of pressure drop, shear (turbulent flow), acids, solution carbon dioxide [CO2], injected condensate, mixing of incompatible crude oils or other conditions or materials that break the stability of the asphaltic dispersion. For example, in matrix acidizing, iron ions in solution favor the precipitation of asphaltene deposits.

Petroleum with a high content of naphthenic compounds, such as asphaltenes. Asphaltic crude is also known as naphthene-based crude oil when the paraffin wax content is low.

A group of high-viscosity or solid hydrocarbons obtained from naturally occurring deposits or from the residue of petroleum refining, commonly used as additives for oil-base and water-base muds. Molten asphalt can be further processed by heating and passing air through the melt to oxidize and polymerize its components. Cooled, air-blown asphalt is glassy and can be ground. It has a high softening point and polar sites that offer emulsion-stabilizing qualities and affinity for clays and shales.

The sale, transfer or conveyance of all or a fraction of ownership interest or rights owned in real estate or other such property. The term is commonly used in the oil and gas business to convey working interest, leases, royalty, overriding royalty interest and net profits interest.

The relatively plastic layer of the upper mantle of the Earth on which the tectonic plates of the lithosphere move. The asthenosphere is approximately 200 km [124 miles] thick and, owing to its depth below the Earth's surface, warm (~ 1400 oC) [2640 oF] but not molten. Here the mantle deforms by plastic flow in response to applied pressures above 100 MPa [14,500 psi]. This zone is considered coincidental, at least below oceanic crust, with the low-velocity zone of the upper mantle.

Corrosion (oxidization) resulting from exposure of susceptible materials to oxygen and moisture. Atmospheric corrosion is generally associated with surface storage conditions, or with upper wellbore annuli that may not be fluid-filled.

A needle-like clay mineral composed of magnesium-aluminum silicate. Major deposits occur naturally in Georgia, USA. Attapulgite and sepiolite have similar structures and both can be used in saltwater mud to provide low-shear rate viscosity for lifting cuttings out of the annulus and for barite suspension. Attapulgite and sepiolite are sometimes called "salt gel." Attapulgite has no capability to control the filtration properties of the mud. For use as an oil mud additive, the clay is coated with quaternary amine, which makes it oil-dispersible and provides gel structure but does not improve the filter cake, unlike organophilicbentonite clay.

The removal of undesirable features, such as multiple events, from seismic data.

The reduction in amplitude of an electromagnetic wave passing through the formation, usually measured in decibels/meter, dB/m. The term is used in particular with reference to the propagation resistivity log and the electromagnetic propagation log.

The ability of a formation to resist electrical conduction, as derived from the reduction in amplitude of the electromagnetic wave generated in a propagation resistivity measurement. At the frequencies used and within the range of measurement, the attenuation depends almost solely on the resistivity, so that the former can be transformed to the latter with a simple algorithm. The transform also depends on transmitter/receiver spacings and tool design. For a 2-MHz measurement, a typical measurement range is 0.2 to 50 ohm-m. Above 50 ohm-m, the dependence of attenuation on resistivity is too small to measure accurately.

The orientation of a planar or linear feature in three-dimensional space. Planar features that are not horizontal, such as tilted strata, are described by their strike, or the azimuth of the intersection of the plane with a horizontal surface, and the dip, or the magnitude of its inclination from a horizontal reference. The trend and plunge of linear features, such as the axis of a fold, describe the azimuth of the line and its deviation from horizontal.

A measurable property of seismic data, such as amplitude, dip, frequency, phase and polarity. Attributes can be measured at one instant in time or over a time window, and may be measured on a single trace, on a set of traces or on a surface interpreted from seismic data. Attribute analysis includes assessment of various reservoir parameters, including ahydrocarbon indicator, by techniques such as amplitude variation with offset (AVO) analysis.

A technique for recording sound at different positions in the borehole to generate a noise log. The measurement technique uses a microphone to record signals in the audible range approximately 20 to 20,000 Hz . In some circumstances, the frequency of the signal can be related to the source of noise and the flow regime, while the amplitude of the signal can be related to the flow rate. The useful signal lies approximately between 100 and 5000 Hz, with lower frequencies generally representing background and mechanical noise. The measurement may record the total signal over all frequencies, the signal at a single frequency, or consist of a set of measurements over different frequency ranges.

In plate tectonics, a failed rift arm. At the junctions of tectonic plates, three intersecting lithospheric plates typically are separated by "arms." Arms might be areas of rifting, convergence or transform faults (similar to a strike-slip fault). The arm along which the motion that spreads the plates apart ceases is termed the failed arm, or aulacogen. Spreading or rifting along the other arms of the triple junction can form new oceanic basins, whereas the aulacogen can become a sediment-filled graben.

Pertaining to minerals or materials that grow in place with a rock, rather than having been transported and deposited. These include quartz, chlorite and other pore-filling minerals or cements that grow during diagenesis. Evaporite minerals are authigenic, or formed in situ.

A budgetary document, usually prepared by the operator, to list estimated expenses of drilling a well to a specified depth, casing point or geological objective, and then either completing or abandoning the well. Such expenses may include excavation and surface site preparation, the daily rental rate of a drilling rig, costs of fuel, drillpipe, bits, casing, cement and logging, and coring and testing of the well, among others. This estimate of expenses is provided to partners for approval prior to commencement of drilling or subsequent operations. Failure to approve an authority for expenditure (AFE) may result in delay or cancellation of the proposed drilling project or subsequent operation.

Materials, especially rock masses, that formed in their present location and have not been transported. Fault surfaces can separate indigenous rocks from allochthonous rocks, although some allochthonous rocks are clearly delineated by their differing composition.

The comparison of a waveform to itself. Autocorrelation is useful in the identification of multiples or other regularly repeating signals, and in designing deconvolution filters to suppress them.

A system to control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output, automatically. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefrontdivergence has caused amplitude decay.

To use computer software to pick a particular reflection or attribute in seismic data automatically. Autotracking can speed interpretation of three-dimensional seismic data, but must be checked for errors, especially in areas of faulting and stratigraphic changes.

A volumetric average of the pressure exerted by the fluids inside the reservoir at a specific depletion stage. Average reservoir pressure can be measured only when the well is shut in.

In geophysics, the depth divided by the traveltime of a wave to that depth. Average velocity is commonly calculated by assuming a vertical path, parallel layers and straight raypaths, conditions that are quite idealized compared to those actually found in the Earth.

The force acting along the axis of an object. In wellbore tubulars, axial loading is typically expressed as tension or compression and may result from applied conditions such as set-down-weight, or be induced by operating conditions or variations such as changes in temperature that cause expansion or contraction of components.

In folded rocks, the imaginary surface bisecting the limbs of the fold. The axial surface is called the axial plane when the fold is symmetrical and the lines defined by the points of maximum curvature of each folded layer, or hinge lines, are coplanar.

The direction in which a deviated or horizontal well is drilled relative to magnetic north. Most horizontal wells in shale reservoirs are drilled in the direction of the minimum horizontal stress. This allows for the creation of multiple hydraulic fractures that are normal to the wellbore.

A type of logging while drilling density log in which the density is measured at different azimuths around the drill collar. The density measurement is focused, so that when the collar rotates, the measurement sees different azimuths around the borehole. An average density can be calculated by summing all the azimuthal data. Alternatively, the data can be summed over different segments, for example in four quadrants, to give an azimuthal density in four directions. When the hole is overgauge, certain quadrants will be firmly pressed against the borehole wall, while others may have a significant standoff and too high a delta rho. The good quadrants can then be chosen for formation evaluation.

A type of electrode device that is able to measure resistivity in different directions around the sonde. In most laterologs, the electrodes are cylinders that average the resistivity azimuthally around the sonde. In azimuthal laterologs, the electrode is segmented radially in several portions, each of which responds to the resistivity in the direction it is facing.

An angle that characterizes the ability of an azimuthal logging measurement to resolve changes in different directions around the tool; alternatively, the smallest angle for which a significant change can be detected.