Oil and Gas Terms Beginning with “L”

A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. LAOs and other synthetic fluids are used in synthetic-base drilling fluids and other applications in which refined oils might otherwise be used if not for HSE concerns. LAOs have a linear structure with a double bond (olefin) at the end of the chain, making them more biodegradable than other olefins. LAOs can be catalytically reacted to move the double bond toward the center of the chain length to convert them to isomerized olefins, IOs.

The lethal concentration of a substance, reported in ppm, that kills 50% of a population of test organisms, such as mysid shrimp, in a standard, controlled laboratory bioassay test. In offshore drilling operations, the LC50 number is used to determine whether waste mud or cuttings can be discharged into the water. The larger the LC50 ppm number from the test, the less toxic the sample is to the organism. For example, if LC50 number is 1,000,000 ppm, the sample is presumably nontoxic according to the test protocol.

Solid material intentionally introduced into a mud system to reduce and eventually prevent the flow of drilling fluid into a weak, fractured or vugularformation. This material is generally fibrous or plate-like in nature, as suppliers attempt to design slurries that will efficiently bridge over and seal loss zones. In addition, popular lost circulation materials are low-cost waste products from the food processing or chemical manufacturing industries. Examples of lost circulation material include ground peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials.

A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentoniteclay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3.

Abbreviation for liquefied natural gas.

Abbreviation for liquefied natural gas carrier, which is a sea vessel used to transport liquefied petroleum gas (LPG).

A test to determine the strength or fracturepressure of the open formation, usually conducted immediately after drilling below a new casingshoe. During the test, the well is shut in and fluid is pumped into the wellbore to gradually increase the pressure that the formation experiences. At some pressure, fluid will enter the formation, or leak off, either moving through permeable paths in the rock or by creating a space by fracturing the rock. The results of the leakoff test dictate the maximum pressure or mud weight that may be applied to the well during drilling operations. To maintain a small safety factor to permit safe well control operations, the maximum operating pressure is usually slightly below the leakoff test result.

(noun) Abbreviation for Liquefied Petroleum Gas. A mixture of light hydrocarbon gases, primarily propane (C₃H₈) and butane (C₄H₁₀), that are compressed and stored as liquids at moderate pressure for use as fuel, petrochemical feedstock, and oilfield applications including well stimulation and enhanced oil recovery.

A highly anionicpolymer used to deflocculateclay-based muds. Lignosulfonate is a byproduct of the sulfite method for manufacturing paper from wood pulp. Sometimes it is called sulfonated lignin. Lignosulfonate is a complex mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule. LS became a popular deflocculant in the late 1950s as a replacement for quebracho. Chromium and iron compounds were admixed to get stability benefits at higher temperature. These were called chrome lignosulfonates (CLS) and ferro-chrome lignosulfonates (FCLS). There is less chrome is in most CLS than in the past (now about 2.5 to 3 %), and chrome-free products are available.

The measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly. LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible.Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs.

One of two elastic constants named for French mathematician Gabriel Lame (1795 to 1870). The first, the shear modulus, can be expressed as:The other Lame constant is the bulk modulus less two-thirds of the shear modulus:

A partial differential equation that governs potential fields (in regions where there are no sources) and is equivalent, in three dimensions, to the inverse square law of gravitational or electrical attraction. In Cartesian coordinates, the Laplace equation equates the sum of the second partial (spatial) derivatives of the field to zero. (When a source is present, this sum is equal to the strength of the source and the resulting equation is called Poisson's equation). The differential equation is named for French mathematician Pierre-Simon de Laplace (1749 to 1827), and applies to electrical, gravity and magnetic fields.

To attach elevators to the upper section of drillpipe to pull it out of or run into the hole.

To pull drill pipe from the hole and lay it down on the catwalk.

A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation.

A ball valve installed at one end of the kelly that can be used to stop mud draining from the kelly during a connection.

Pertaining to an environment of deposition in lakes, or an area having lakes. Because deposition of sediment in lakes can occur slowly and in relatively calm conditions, organic-rich source rocks can form in lacustrine environments.

The distance between the static measure point and the dynamic measure point of a logging measurement. For nuclear logs and any others that must be recorded over a significant time period, there is a difference between the measure point with the tool stationary and moving. If the tool is moving during this period, the effective center of measurement will be a certain distance from the point at which the measurement started. This distance is the lag. The lag depends on the logging speed and the sampling interval.

Any gas deliberately introduced into the mud system to help a mudlogger or wellsite geologist track the amount of time or the number of mud pump strokes it takes to circulate mud from the kelly downhole through the drillstring to the bit, and back uphole to the gas trap at the shale shaker. This interval is used to calculate the lag period.

The time taken for cuttings to reach the surface. The term is also used in place of cycle time.

A type of streamlined flow for single-phase fluids in which the fluid moves in parallel layers, or laminae. The layers flow smoothly over each other with instabilities being dampened by the viscosity. Laminar flow occurs in straight pipes when the Reynolds number is below a critical value, corresponding to a low production rate. Above this value, the flow is turbulent. For laminar flow in straight pipes, the velocity profile across the pipe is parabolic, increasing from zero at the wall of the pipe to a maximum at the center equal to twice the mean velocity.

A particular model, or equation, for deriving the water saturation from resistivity and other logs. The model assumes a laminar shale distribution and considers the total resistivity to be the sum in parallel of the sand and shale laminae.

A fine layer (~ 1 mm thick) in strata, also called a lamina, common in fine-grained sedimentary rocks such as shale, siltstone and fine sandstone. A sedimentary bed comprises multiple laminations, or laminae.

A component installed near the bottom of the casing string on which the cement plugs land during the primary cementing operation. The internal components of the landing collar are generally fabricated from plastics, cement and other drillable materials.

(noun) A short section of internally machined tubing with a precisely profiled bore, installed at specific locations in a completion string to provide a seating and locking surface for flow-control devices such as plugs, chokes, and gauges that are set and retrieved by wireline or slickline.

A technique for analyzing the grain-size distribution of a core sample. A cleaned, disaggregated sample is dispersed in a carrier fluid. The grains cause diffraction of a laser beam directed through the fluid. The angle of scattering is inversely proportional to the particle size, while the intensity of scattering is proportional to the number of particles. Laser diffraction also may be referred to as laser sieve analysis.

The depth of the last reliable reading of a log. For the normal bottom-to-top survey, the last reliable reading often occurs just before the logging tool enters the casing. With several logging tools in a tool string, the last readings will be at different depths, depending on the measure point of each measurement.

A surface detection system used to ensure that all tubing-conveyed perforating guns have fired, from the top shot to the bottom shot.

The portion of the pressure transient occurring after radial flow. Analysis of the late-time transient data provides characterization of outer boundaries such as faults or fluid contacts. This portion of the data appears only in transient tests of sufficient duration.

Referring to a type of conventional electrical log in which the current-emitting and the current-return electrodes (A and B) are placed close together on the sonde, with the measure electrode (M) several feet away and the measure return (N) far away. This arrangement is sensitive to the potential gradient between A and B. The spacing is defined by the distance from M to the midpoint between A and B. The most common spacing is 18 ft, 8 in. [5.7 m]. The lateral gives a sharper response to a bed boundary than a normal but also introduces several artifacts that can give misleading results.

A colloidal suspension or emulsion of specific organic materials. Certain latices may be used as cement additives. Latex is used to provide gas-migration control, improve durability and improve bonding. It also offers excellent fluid-loss control. Latex additives also impart some acid resistance to cement.

A slab of reservoirrock bounded above and below by another layer in vertical hydraulic communication.

A method of seismic inversion whereby the effects of rock layers having different seismic characteristics are removed from layers below.

A highly simplified description of a geological scenario. Although sometimes used for "quick and dirty" simulation models, this description is often not appropriate for detailed or accurate work. Generally, layer-cake geometry is an oversimplification of actual structure and stratigraphy of a reservoir. It assumes that the reservoir comprises a stack of conformable layers.

An advanced testing technique using a combination of transient-rate and pressure measurements and stabilized flow profiles to determine permeability and skin for each of several layers commingled in a well. The technique requires a series of flow-rate changes, with at least one flow-rate change for each layer to be characterized.

A commonly used (but strictly speaking, incorrect) version of lbm/bbl.

The abbreviation for concentration in US oilfield units, pound per barrel. One lbm/bbl is the equivalent of one pound of additive in 42 US gallons of mud. The "m" is used to denote mass to avoid possible confusion with pounds force (denoted by "lbf"). Lbm/bbl is sometimes written as PPB, but must not be confused with parts per billion. In SI units, the conversion factor is one pound per barrel equals 2.85 kilograms per cubic meter. For example, 10 lbm/bbl = 28.5 kg/m3.

A test to detect hydrogen sulfide in a fluid by discoloration of a paper moistened with the lead acetate solution. It is important to determine the presence and amount of hydrogen sulfide because this gas is extremely poisonous, highly flammable, explosive and corrosive.

(noun) The first cement slurry pumped during a primary cementing operation, designed with lower density and extended volume to fill the upper portion of the casing-borehole annulus. Lead cement is typically lighter than the tail cement that follows and may contain extenders such as bentonite or hollow microspheres to reduce cost and hydrostatic pressure.

The determination of the location of a leak in a pipeline. In onshore operations, this can be done by external detection or by using material balance leak-detection systems. In offshore operations, the task is more difficult because of the lack of inlet flow-rate measurements and the considerable solubility of natural gas in seawater at high pressures and low temperatures (seafloor level).In deepwater operations with multiphase flow, indications of a leak may not be present at the ocean surface or it could be considerably displaced from the site of origination. In these circumstances, an energy-balance technique based on the changes in frictional losses through the pipeline is a powerful tool.

The magnitude of pressure exerted on a formation that causes fluid to be forced into the formation. The fluid may be flowing into the pore spaces of the rock or into cracks opened and propagated into the formation by the fluid pressure. This term is normally associated with a test to determine the strength of the rock, commonly called a pressure integrity test (PIT) or a leakoff test (LOT). During the test, a real-time plot of injected fluid versus fluid pressure is plotted. The initial stable portion of this plot for most wellbores is a straight line, within the limits of the measurements. The leakoff is the point of permanent deflection from that straight portion. The well designer must then either adjust plans for the well to this leakoff pressure, or if the design is sufficiently conservative, proceed as planned.

The magnitude of pressure exerted on a formation that causes fluid to be forced into the formation. The fluid may be flowing into the pore spaces of the rock or into cracks opened and propagated into the formation by the fluid pressure. This term is normally associated with a test to determine the strength of the rock, commonly called a pressure integrity test (PIT) or a leakoff test (LOT). During the test, a real-time plot of injected fluid versus fluid pressure is plotted. The initial stable portion of this plot for most wellbores is a straight line, within the limits of the measurements. The leakoff is the point of permanent deflection from that straight portion. The well designer must then either adjust plans for the well to this leakoff pressure, or if the design is sufficiently conservative, proceed as planned.

A test to determine the strength or fracturepressure of the open formation, usually conducted immediately after drilling below a new casing shoe. During the test, the well is shut in and fluid is pumped into the wellbore to gradually increase the pressure that the formation experiences. At some pressure, fluid will enter the formation, or leak off, either moving through permeable paths in the rock or by creating a space by fracturing the rock. The results of the leakoff test dictate the maximum pressure or mud weight that may be applied to the well during drilling operations. To maintain a small safety factor to permit safe well control operations, the maximum operating pressure is usually slightly below the leakoff test result.

A type of acoustic energy that propagates in one direction while being confined in the other two directions, in this case by the borehole wall. Leaky modes can be considered as multiply reflected and constructively interfering waves propagating in the borehole. Each time a compressional wave hits the borehole wall, part of the energy is reflected into the borehole, while the rest is converted to compressional or shear energy that radiates into the formation, hence the term 'leaky'. Leaky modes are dispersive, starting at a certain cutoff frequency with the formation compressional velocity and increasing towards the borehole fluid velocity at high frequency. In slow formations, where no head wave is generated because the borehole fluid is faster than the formation compressional wave, the low-frequency end of the leaky mode can be used to determine formation compressional velocity.The term 'hybrid mode' is used to describe a form of leaky mode that is associated with an altered zone.

Residual gas, mainly methane and ethane, that remains after the heavier hydrocarbons have been condensed in the wellhead. When the lean gas is liquefied, it is called liquefied natural gas (LNG).

A gas condensate with low condensate formation in the reservoir (when the bottomhole pressure is reduced below the dewpoint pressure).

In a glycoldehydrator, glycol that has been boiled and no longer contains any water. When the glycol is lean, it can be pumped back to the absorber for reuse.

Liquid hydrocarbon utilized to remove heavier components from the gas stream in a gas processing plant.

The contract that conveys the rights to explore and produce from the owner of the mineral rights (lessor) to a tenant (lessee), usually for a fee and with a specified duration. A lease usually includes a provision for sharing production.

The fastest route that a seismic ray can travel between two points, generally dictated by Fermat's principle.

A percentage share of production, or the value derived from production, which is granted to the lessor in the oil and gas lease, and which is free of the costs of drilling and producing.

The assembly on a coiled tubing reel that guides the tubing string onto the drum. Accurate spooling is necessary to avoid damaging the tubing and to ensure that the entire string can be run and retrieved without jamming. The levelwind functions automatically, although it incorporates a manual override to facilitate minor corrections.

An occasion when a governmental body offers exploration acreage for leasing by exploration and production companies, typically in return for a fee and a performance or work obligation, such as acquisition of seismic data or drilling a well. Exploration licenses are initially of limited duration (about 5 years) after which there might be a requirement to return half or more of the licensed acreage to the state. If hydrocarbons are discovered, a separate production license or production-sharing agreement is usually drawn up before development can proceed.

The period of time during which economically sustainable production levels may be expected from a well. The anticipated well life and the characteristics of the reservoir fluid are the two main factors in specifying the completion system components.

A lifting device used when performing coiled tubing operations from a semisubmersiblerig or drillship. The coiled tubing injector and pressure-control equipment are positioned within the lifting frame, which is attached to the flow head and running string and supported by the traveling blocks. This configuration enables the heave-compensation system of the rig to counteract the vessel motion.

A short drillstring component that is temporarily connected to the top of a tool assembly that is to be lifted vertically, such as when running or retrieving a tool string. The external profile on the upper section of the lifting sub is similar to that of the completion tubing, enabling the rig elevators to lift the assembled tool string safely.

Crude oil that has a high APIgravity, usually more than 40o.

Hydrocarbons with low molecular weight such as methane, ethane, propane and butane.

An operating condition during a snubbing operation in which the wellheadpressure and buoyancy forces are greater than the force resulting from the weight of the pipe or tubing string. In the light-pipe condition the string will be ejected from the wellbore if the gripping force of the slips is lost.

The component of a tree that is extracted in the paper-manufacturing process and used as an additive in drilling fluids. Specifically, lignin is a highly polymerized, amorphous material that makes up the middle lamella of woody fibers and cements the fibers together. Methoxy groups are abundant on the lignin structure, giving lignin many reactive sites and promoting its water solubility. In paper manufacturing, lignin is dissolved from wood chips. In the sulfite paper process, the liquor byproduct contains wood sugars and lignosulfonate. The wood sugars are removed and the lignosulfonate is used as a claydeflocculant. In the kraft paper process, lignin is solubilized by caustic soda. Kraft lignin must be further reacted to make a functional drilling-fluid additive.

The mineral leonardite, similar to brown coal. Lignite is found in surface deposits worldwide. Lignite is mined and put into piles where it can oxidize in the air before it is dried, ground and bagged for use in drilling fluids. The humic acid content of lignite, which varies widely, controls its solubility. The soluble and colloidal lignite components both help in fluid-loss control. Soluble components serve as clay deflocculants and improve filter cake quality. Colloidal lignite helps plug off the permeable parts of filter cake. When straight lignite is added to a mud, caustic soda is also needed to make it dissolve. Precaustisized lignite is available, which contains NaOH or KOH already mixed. Adding chromium salts improves high-temperature performance, but their use is limited by HSE concerns. Organophilic lignite is a straight lignite that has been treated with quaternary amine compounds to make it oil dispersible in oil- and synthetic-base muds.

A highly anionic polymer used to deflocculateclay-based muds. Lignosulfonate is a byproduct of the sulfite method for manufacturing paper from wood pulp. Sometimes it is called sulfonated lignin. Lignosulfonate is a complex mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule. LS became a popular deflocculant in the late 1950s as a replacement for quebracho. Chromium and iron compounds were admixed to get stability benefits at higher temperature. These were called chrome lignosulfonates (CLS) and ferro-chrome lignosulfonates (FCLS). There is less chrome is in most CLS than in the past (now about 2.5 to 3 %), and chrome-free products are available.

A type of water-base mud that is saturated with lime, Ca(OH)2, and has excess, undissolved lime solids maintained in reserve. Lime muds are classified according to excess lime content: (1) low-lime, 0.5 to 2 lbm/bbl, (2) medium-lime, 2 to 4 lbm/bbl and (3) high-lime, over 4 lbm/bbl. All lime muds have pH in the range of 12, and the filtrate is saturated with lime. Fluid-loss additives include starch, HP-starch, carboxymethylcellulose (CMC) or polyanionic cellulose (PAC). Prehydrated bentonite can improve the fluid loss and rheology of a lime mud. A maltodextrin in lime muds has been used as a claydeflocculant, a shale stabilizer and to increase calcium solubility. KCl in lime muds has been another innovation for successful drilling of hydratable shales. The ability to carry very high mud alkalinity (as excess lime) to neutralizeacid gases is one reason lime muds are used. H2S zones can be drilled with more safety and copious amounts of CO2 can be neutralized by a large excess of lime.

A carbonatesedimentaryrock predominantly composed of calcite of organic, chemical or detrital origin. Minor amounts of dolomite, chert and clay are common in limestones. Chalk is a form of fine-grained limestone.

A transform from raw log data chosen so that a log recorded in these units will give the correct porosity of the formation, providing the matrix is pure calcite and the pores are filled with fresh water. The unit, which may be in vol/vol or p.u., is most commonly used for neutron porosity logs but may also be used for density and acoustic logs. The definition is strictly true only if all borehole and other environmental corrections have been applied.

Display ranges chosen for the density and neutron porosity logs such that the two curves will overlay at all porosity values providing the matrix is pure calcite and the pores are filled with fresh water. The most common overlay spans two tracks, with the density reading from 1.95 to 2.95 g/cm3, and the neutron in limestone porosity units from 0.45 to −0.15 vol/vol.

A completion with only a portion of the productive interval open to flow, either by design or as a result of damage. Limited-entry completions in vertical wells are designed to avoid unwanted fluid production, such as gas from an overlying gas cap or water from an underlying aquifer. The effects of limited entry may be seen in perforated and gravel-packed wells when some of the perforations fail to clean up. This is also called a partial completion.The limited entry also results from partial penetration, which occurs when the productive formation is only partly drilled. This partial penetration represents a near-well flow restriction that results in a positive skin effect in a well-test analysis.

An injection pattern in which the injection wells are located in a straight line parallel to the production wells. In a line drive pattern, the injected fluid, which is normally water, steam or gas, creates a nearly linear frontal movement. A line drive pattern is also called direct line drive.

The solution to differential equations treating the well as a vertical line through a porous medium. The solution is nearly identical to the finite-wellbore solution. At very early times, there is a notable difference in the solutions, but the differences disappear soon after a typical well is opened to flow or shut in for a buildup test, and in practice the differences are masked by wellbore storage.

A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. LAOs and other synthetic fluids are used in synthetic-base drilling fluids and other applications in which refined oils might otherwise be used if not for HSE concerns. LAOs have a linear structure with a double bond (olefin) at the end of the chain, making them more biodegradable than other olefins. LAOs can be catalytically reacted to move the double bond toward the center of the chain length to convert them to isomerized olefins, IOs.

A flow regime characterized by parallel flow lines in the reservoir. This results from flow to a fracture or a long horizontal well, or from flow in an elongated reservoir, such as a fluvialchannel, or as a formation bounded by parallel faults. Linear flow is recognized as a +1/2 slope in the pressure derivative on the log-log diagnostic plot. Its presence enables determination of the fracture half-length or the channel or reservoir width, if permeability can be determined independently.

Any string of casing in which the top does not extend to the surface but instead is suspended from inside the previous casing string. Many conventional well designs include a production liner set across the reservoir interval. This reduces the cost of completing the well and allows some flexibility in the design of the completion in the upper wellbore, such as when the fluid characteristics make it beneficial to increase the diameter of the conduit and components.

A device used to attach or hang liners from the internal wall of a previous casing string. Liner hangers are available in a range of sizes and specifications to suit a variety of completion conditions.

Pertaining to an attraction for oil by a surface of a material or a molecule. This term is applied to the oil-wetting behavior of treatment chemicals for oil muds. Lipophilic oil-mud additives are required because most minerals drilled and additives such as barite are naturally hydrophilic and must be rendered lipophilic.

Natural gas, mainly methane and ethane, which has been liquefied at cryogenic temperatures. This process occurs at an extremely low temperature and a pressure near the atmospheric pressure. When a gas pipeline is not available to transport gas to a marketplace, such as in a jungle or certain remote regions offshore, the gas may be chilled and converted to liquefied natural gas (a liquid) to transport and sell it. The term is commonly abbreviated as LNG.

A sea vessel used to transport liquefied petroleum gas (LPG). The term is commonly abbreviated as LNGC

Gas mainly composed of propane and butane, which has been liquefied at low temperatures and moderate pressures. The gas is obtainable from refinery gases or after the cracking process of crude oil.Liquefied petroleum gas is also called bottle gas. At atmospheric pressure, it is easily converted into gas and can be used industrially or domestically. The term is commonly abbreviated as LPG.

A material used in a liquid form to modify the properties of cement for use in oil- or gas-well cementing.

A phenomenon encountered during dry forward combustion in which an oil zone around the production well cannot be pushed forward by the heated oil. The fluid located in this zone is still at the original reservoir temperature. Therefore, the fluid is still highly viscous and normally not mobile.

A hygroscopic liquid used to remove water and water vapor from a gas stream. Some liquid desiccants are glycols (diethylene, triethylene and tetraethylene), which are substances that can be regenerated. Regeneration means that the water absorbed by these substances can be separated from them. Some liquid desiccants, such as methanol or ethylene, cannot be regenerated.

Liquid compounds such as propanes, butanes, pentanes and heavier products extracted from the gas flowstream.

The depth at which the first liquid is found in a well.

A technique for measuring the pore volume of a core sample from the difference in its weight when dry and when saturated with a liquid. A clean, dry sample is weighed and then evacuated for several hours in a vacuum chamber, flushing with CO2 to remove remaining air if necessary. A de-aerated liquid is introduced into the chamber and pressured to ensure complete saturation. The saturated sample is then weighed again. The difference in weight divided by the density of the liquid is the connected, or effective, pore volume.It is also common to measure the weight of the sample when immersed in the liquid. The grain and bulk volume can then be calculated as in the buoyancy method.

The electromagnetic force generated by a boundary between solutions of high salinity and low salinity. In a permeableformation, a liquid-junction potential is generated between the invaded zone and the undisturbed zone when the mudfiltrate and the formation water have different salinities. This potential is one component of the electrochemical potential, from which the spontaneous potential log is derived. The other, much larger component is the membrane potential at a shale boundary. The liquid-junction potential is reduced if there is clay in the permeable formation, since this generates another, local membrane potential with the opposite polarity to the liquid-junction potential.

The process by which unconsolidated sediments become sedimentaryrock. Sediments typically are derived from preexisting rocks by weathering, transported and redeposited, and then buried and compacted by overlying sediments. Cementation causes the sediments to harden, or lithify, into rock.

A mappable subdivision of a stratigraphic unit that can be distinguished by its facies or lithology-the texture, mineralogy, grain size, and the depositional environment that produced it.

The surface that separates rock bodies of different lithologies, or rock types. A contact can be conformable or unconformable depending upon the types of rock, their relative ages and their attitudes. A fault surface can also serve as a contact.

The macroscopic nature of the mineral content, grain size, texture and color of rocks.

The brittle outer layer of the Earth that includes the crust and uppermost mantle. It is made up of six major and several minor tectonic plates that move around on the softer asthenosphere. The lithosphere of the oceans tends to be thinner (in some oceanic areas, less than 50 km [30 miles] thick) and more dense than that of the continents (more than 120 km [70 miles] thick in places like the Himalayas) because of isostasy. The movement of the plates of the lithosphere results in convergence, or collisions, that can form mountain belts and subduction zones, and divergence of the plates and the creation of new crust as material wells up from below separating plates. The lithosphere and asthenosphere are distinguished from the crust, mantle and core of the Earth on the basis of their mechanical behavior and not their composition.

The pressure of the weight of overburden, or overlying rock, on a formation; also called geostatic pressure.

A seismic inversion technique that attempts to describe lithology of individual rock layers and evaluate properties and distribution of pore fluids through analysis of variation of reflected seismic amplitude with offset.

The study and correlation of strata to elucidate Earth history on the basis of their lithology, or the nature of the well log response, mineral content, grain size, texture and color of rocks.

Pertaining to an environment of deposition affected by tides, the area between high tide and low tide. Given the variation of tides and land forms from place to place, geologists describe littoral zones locally according to the fauna capable of surviving periodic exposure and submersion.

A term used to describe a cementslurry that remains liquid but is still capable of thickening or setting to become an unmovable solid mass. Some remedial operations treat the excess live cement slurry with a contaminant to extend the thickening time and allow its safe removal from the wellbore.

Oil containing dissolved gas in solution that may be released from solution at surface conditions. Live oil must be handled and pumped under closely controlled conditions to avoid the risk of explosion or fire.

The sensor component in a weight-indicator system that detects the tensional or compressional forces being imparted to the running string at surface. Load cells are hydraulically or electronically operated and are connected to the weight-indicator display system on the equipment operator's console.

Oil pumped into a wellbore in preparation for, or as part of, a treatment. Some treatments, such as hydraulic fracturing, involve pumping large volumes of fluid. Using load oil, often produced and processed from adjacent wells in the field, reduces the cost of fluids and can enhance the cleanup process when the treatment is complete.

The amount of local personnel, material and services that working interest owners are required to employ when drilling and operating a well, as specified under the terms of a concession agreement.

The fraction of a particular fluid measured in the vicinity of a small probe in a production well. The small, or local, probes respond digitally to the type of fluid in front of them, indicating gas, oil or water depending on the type of probe. The local holdup of oil, for example, is determined by the percentage of time the probe spends in front of oil.

A small sensor, part of a productionlogging tool, which determines the type of fluid in its vicinity as it moves up and down a production well. Typically there are four or more sensors, or probes, held on arms to measure the four quadrants of the well cross-section. The probes may be electrical, to distinguish hydrocarbon from water; optical, mainly to distinguish gas from liquid, but also oil from water; or dielectric, mainly to distinguish water from hydrocarbon, but also, with less resolution, oil from gas. They can detect bubbles that are larger than about 1 mm diameter. Their response is essentially digital, indicating either one fluid or the other, so that the percentage of time that they see a fluid is a direct measure of its holdup. The rate of change between the two fluids is known as the bubble count.The results can be averaged to give the mean holdup and bubble count, or converted into an image, showing the holdup or bubble count at different locations across the well at different depths. The image is particularly useful in highly deviated or horizontal wells where different flow regimes may be found in different quadrants.

A downhole device, run and retrieved on slickline, that is placed and anchored within the tubing string to provide a setting point for flow-control equipment such as valves, chokes and plugs. The three main types of lock use different means of locating and securing: a slip lock locates and anchors anywhere within the correct size of tubing; the collar lock locates in the space within tubing collars; and the nipple lock locates within completion nipple profiles.

(noun) A tubing-mounted receptacle with an internal locking profile that accepts and secures a flow-control device (such as a gas lift valve, chemical injection valve, or dummy valve) delivered and retrieved by wireline. The lock mandrel provides a gas-tight seal and mechanical anchor for the installed device.

A condition that may occur when a coiled tubing string is run into a horizontal or highly deviated wellbore. Lock-up occurs when the frictional force encountered by the string running on the wellbore tubular reaches a critical point. Although more tubing may be injected into the wellbore, the end of the tool string cannot be moved farther into the wellbore.

Associated with the information from a log. For example, a log print is a paper print on which log data have been recorded.

The average value of a set of measurements, calculated by taking the logarithms of the measurements, finding the arithmetic average of the logarithms and then taking the antilogarithm of the average.

(noun) The practice of acquiring continuous measurements of formation and borehole properties as a function of depth using specialised instruments conveyed on wireline, drillpipe, coiled tubing, or permanently installed fibre optic cables. Logging provides essential data for formation evaluation, reservoir characterisation, and well integrity assessment.

An operation in which a logging tool is lowered into a borehole and then retrieved from the hole while recording measurements. The term is used in three different ways. First, the term refers to logging operations performed at different times during the drilling of a well. For example, Run 3 would be the third time logs had been recorded in that well. Second, the term refers to the number of times a particular log has been run in the well. Third, the term refers to different runs performed during the same logging operation. For example, resistivity and nuclear logs may be combined in one tool string and recorded during the first run, while acoustic and nuclear magnetic resonance logs may be recorded during the second run.

The downhole hardware needed to make a log. The term is often shortened to simply "tool." Measurements-while-drilling (MWD) logging tools, in some cases known as logging while drilling (LWD) tools, are drill collars into which the necessary sensors and electronics have been built.Wireline logging tools are typically cylinders from 1.5 to 5 in. [3.8 to 12.7 cm] in diameter. Since the total length is more than can be conveniently handled in one piece, the logging tool is divided into different sections that are assembled at the wellsite. These sections consist of cartridges and sondes. Different measurements can be combined to make up a tool string. The total length of a tool string may range from 10 to 100 ft [3 to 30 m] or more. Flexible joints are added in long tool strings to ease passage in the borehole, and to allow different sections to be centralized or eccentralized. If the total length is very long, it is often preferable to make two or more logging runs with shorter tool strings.

The cabin that contains the surface hardware needed to make wirelinelogging measurements. The logging unit contains at the minimum the surface instrumentation, a winch, a depth recording system and a data recorder. The surface instrumentation controls the logging tool, processes the data received and records the results digitally and on hard copy. The winch lowers and raises the cable in the well. A depth wheel drives the depth recording system. The data recorder includes a digital recorder and a printer.

The measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly. LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible.Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs.

A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing.

A sonic tool with a longer transmitter-to-receiverspacing (generally 10 to 15 ft) than a standard sonic tool. The rock near the borehole is sometimes altered by drilling fluids, stress relief, or both, causing a thin zone whose velocity is lower than that of the true formation. With standard spacings, the wave traveling through the altered zone may arrive first at the receiver, since this zone is closer to both transmitter and receiver. The increased spacing permits the wave traveling through the true formation to arrive first and be measured. The depth of investigation varies with slowness and transmitter-receiver spacing but is of the order of 2 to 3 ft. An increased transmitter-to-receiver spacing also allows better separation of waveforms relating to different acoustic waves, such as compressional, shear and Stoneley arrivals.

A plot of the longitudinal component of the dip vector computed from a dipmeter. Longitudinal plots are used in the SCAT (Statistical Curvature Analysis Technique) method of interpreting dipmeter data for geologicalstructure. They are especially useful in doubly plunging dip situations.

During a nuclear magnetic resonance measurement, the loss of energy by hydrogen atoms in a rock as they align themselves with the static magnetic field. The atoms behave like spinning bar magnets so that when a static magnetic field is applied, they initially precess about the field. Then, through interactions with nuclei and electrons, they lose energy, or relax, and align themselves with the magnetic field. The relaxation of the hydrogen atoms does not occur immediately but grows exponentially with a time constant T1. There are two mechanisms for longitudinal relaxation, surface relaxation and bulk relaxation.

An emulsion with large and widely distributed droplets. A loose emulsion can be easy to break.

A lack of mud returning to the surface after being pumped down a well. Lost circulation occurs when the drill bit encounters natural fissures, fractures or caverns, and mud flows into the newly available space. Lost circulation may also be caused by applying more mud pressure (that is, drilling overbalanced) on the formation than it is strong enough to withstand, thereby opening up a fracture into which mud flows.

Solid material intentionally introduced into a mud system to reduce and eventually prevent the flow of drilling fluid into a weak, fractured or vugularformation. This material is generally fibrous or plate-like in nature, as suppliers attempt to design slurries that will efficiently bridge over and seal loss zones. In addition, popular lost circulation materials are low-cost waste products from the food processing or chemical manufacturing industries. Examples of lost circulation material include ground peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials.

The collective term for substances added to drilling fluids when drilling fluids are being lost to the formations downhole. Commonly used lost-circulation materials include are fibrous (cedar bark, shredded cane stalks, mineral fiber and hair), flaky (mica flakes and pieces of plastic or cellophane sheeting) or granular (ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls). Laymen have likened lost-circulation materials to the "fix-a-flat" materials for repair of automobile tires.

An oil mud designed and maintained with a minimum of colloid-sized solids, typically by omitting fatty-acid soap and lime, and minimizing organophilic clays and fluid-loss additives. Low-colloid oil mud, also called a relaxed filtrate oil mud, increases drilling rate. A disadvantage is that filtercake formed on sands is not tight, can quickly become very thick, and can cause pipe to stick by differential pressure.

A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentoniteclay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3.

A mud that has fewer solids than conventional clay-based muds of the same density and similar use. Low-solids mud design and maintenance is accomplished primarily by substituting one or more polymers for the ordinary bentonite clay. Viscosity can be obtained either entirely by polymers or by using a premium quality (nontreated) bentonite along with the appropriate extender polymer. Together, these give rheology comparable to that of a higher concentration of ordinary bentonite. Polyanionic cellulose (PAC) may be needed for fluid-loss control. XC polymer can be effective for cuttings carrying. By combining premium bentonite and the right extender polymer, PAC and XC polymer, solids can be kept low, if solids control is required. This concept applies best to low-density muds, below about 13 lbm/gal, but has some validity in all muds .

Native clays that are generally unsuitable for use in a clay-based drilling mud. Low-yield clays are considered to be drill solids, although they may give high values for bentonite-equivalent in a mud according to the methylene blue test.

An oil mud designed and maintained with a minimum of colloid-sized solids, typically by omitting fatty-acid soap and lime, and minimizing organophilic clays and fluid-loss additives. Low-colloid oil mud, also called a relaxed filtrate oil mud, increases drilling rate. A disadvantage is that filter cake formed on sands is not tight, can quickly become very thick, and can cause pipe to stick by differential pressure.

A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentonite clay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3.

A test to measure static filtration behavior of water mud at ambient (room) temperature and 100-psi differential pressure, usually performed according to specifications set by API, using a static filter press. The filter medium is filter paper with 7.1 sq. in. filtering area. A half-size cell is sometimes used, in which case the filtrate volume is doubled.

An enhanced oil recovery method that uses water with a low concentration of dissolved salts as a flooding medium. The sources of low-salinity water are typically rivers, lakes or aquifers associated with meteoric water.

A mud that has fewer solids than conventional clay-based muds of the same density and similar use. Low-solids mud design and maintenance is accomplished primarily by substituting one or more polymers for the ordinary bentonite clay. Viscosity can be obtained either entirely by polymers or by using a premium quality (nontreated) bentonite along with the appropriate extender polymer. Together, these give rheology comparable to that of a higher concentration of ordinary bentonite. Polyanionic cellulose (PAC) may be needed for fluid-loss control. XC polymer can be effective for cuttings carrying. By combining premium bentonite and the right extender polymer, PAC and XC polymer, solids can be kept low, if solids control is required. This concept applies best to low-density muds, below about 13 lbm/gal, but has some validity in all muds .

A low-solids mud in which there is no claydeflocculant chemical.

Antonym: high-specific-gravity solids

Native clays that are generally unsuitable for use in a clay-based drilling mud. Low-yield clays are considered to be drill solids, although they may give high values for bentonite-equivalent in a mud according to the methylene blue test.

A systems tract overlying a sequence boundary and overlain by a transgressive surface. Characterized by a progradational to aggradationalparasequence set, this systems tract commonly includes a basin-floor fan, a slope fan and a lowstand wedge. It is often abbreviated as LST.

A mud additive for lowering torque (rotary friction) and drag (axial friction) in the wellbore and to lubricate bit bearings if not sealed. Lubricants may be solids, such as plastic beads, glass beads, nut hulls and graphite, or liquids, such as oils, synthetic fluids, glycols, modified vegetable oils, fatty-acid soaps and surfactants.

A term initially applied to the assembly of pressure-control equipment used on slickline operations to house the tool string in preparation for running into the well or for retrieval of the tool string on completion of the operation. The lubricator is assembled from sections of heavy-wall tube generally constructed with integral seals and connections. Lubricator sections are routinely used on the assembly of pressure-control equipment for other well-intervention operations such as coiled tubing.

A measure of the degree of lubrication.

A descriptive term for the strong affinity that a solid material (usually a colloid) has for the liquid in which the solid is dispersed. For example, clay is a lyophilic colloid in water.

A descriptive term for the lack of affinity (or repulsion) that a solid material has for the liquid in which the solid is dispersed. For example clay is lyophobic to oil.