Oil and Gas Terms Beginning with “G”

An instrument used for quantitative analyses of sulfides and carbonates. Specific test methods have been published by API. The oil-mud procedure analyzes active sulfides and uses whole mud samples, whereas the water-base drilling fluid procedure tests filtrate. The GGT unit is a clear, plasticblock (2.5 in. x 4 in. x 6 in.) that contains three interconnected chambers. A carrier gas is used to flow an inert gas through the chambers. The sample is placed in chamber #1 and is acidified to release sulfides as H2S and carbonates as CO2. The appropriate Drdger tube is used to measure the effluent gas that is evolved from the sample. The device is named after Bob Garrett, who invented it while at Exxon Production Research.Reference:Garrett RL: "A New Field Method for the Quantitative Determination of Sulfides in Water-Based Drilling Fluids," Journal of Petroleum Technology 29, no. 9 (September 1977): 1195-1201.Garrett RL: "A New Field Method for the Quantitative Determination of Carbonates in Water-Base Drilling Fluids," Journal of Petroleum Technology 30, no. 7 (July 1978): 860-868.Garrett RL, Carlton LA and Denekas MO: "Methods for Field Monitoring of Oil-Based Drilling Fluids for Hydrogen Sulfide and Water Intrusions," SPE Drilling Engineering 3, no.3 (September 1988): 296-302.

Abbreviation for gas/liquid ratio, the ratio of produced gas to produced liquids (oil and water).

Abbreviation for gas/oil ratio, the ratio of produced gas to produced oil.

A system of numerous Earth-orbiting satellites that can be used to determine the location (latitude, longitude and elevation) of a receiver or station on the Earth within about 2 m [6 ft]. Fixed receivers on Earth can be used to determine the relative motions of fault blocks and lithospheric plates. Hand-held receivers can be used for producing accurate geologic maps, acquiring navigation data for 3D seismic surveys, and positioning wells in the field.

A record of the onset and duration of the multitude of episodes of reversal of the Earth's magnetic polarity, or geomagnetic polarity reversals. The GPTS was developed by thorough study of rocks from around the world, during which it was observed that rocks from specific time periods contained magnetic minerals whose orientation was opposite to that of the current magnetic field. By comparing the patterns of magnetic reversals with those of rocks of known age, the approximate ages of rocks can be established. This is particularly useful for basalts of the oceanic crust, which record the Earth's magnetic field as they solidify from molten lava symmetrically about the midocean ridges. The time scale has been accurately extended back to the Upper Jurassic, the age of oldest existing oceanic crust.

An abbreviation for gamma ray, usually with reference to the gamma ray log.

The unit of acceleration commonly used in gravity surveying. 1 Gal (1000 milliGal or 106 microGal) = 1 cm/sec/sec.

An instrument used for quantitative analyses of sulfides and carbonates. Specific test methods have been published by API. The oil-mud procedure analyzes active sulfides and uses whole mud samples, whereas the water-base drilling fluid procedure tests filtrate. The GGT unit is a clear, plastic block (2.5 in. x 4 in. x 6 in.) that contains three interconnected chambers. A carrier gas is used to flow an inert gas through the chambers. The sample is placed in chamber #1 and is acidified to release sulfides as H2S and carbonates as CO2. The appropriate Drdger tube is used to measure the effluent gas that is evolved from the sample. The device is named after Bob Garrett, who invented it while at Exxon Production Research.Reference:Garrett RL: "A New Field Method for the Quantitative Determination of Sulfides in Water-Based Drilling Fluids," Journal of Petroleum Technology 29, no. 9 (September 1977): 1195-1201.Garrett RL: "A New Field Method for the Quantitative Determination of Carbonates in Water-Base Drilling Fluids," Journal of Petroleum Technology 30, no. 7 (July 1978): 860-868.Garrett RL, Carlton LA and Denekas MO: "Methods for Field Monitoring of Oil-Based Drilling Fluids for Hydrogen Sulfide and Water Intrusions," SPE Drilling Engineering 3, no.3 (September 1988): 296-302.

An algorithm built on a Markov-type hypothesis, whereby collocated secondary information is assumed to screen out secondary data from farther away. The method allows the direct cosimulation of several interdependent variables, integrating several different sources of soft information.Reference:Almeida AS and Frykman P: "Geostatistical Modeling of ChalkReservoir Properties in the Dan Field, Danish North Sea," in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994.

A parametric approach to stochastic imaging or simulation of a reservoir. Simpler than the indicator (nonparametric) approach, Gaussian techniques include a normal score transform of the data to produce a new variable that is, by construction, univariate and normally distributed.

The ringing near a discontinuity in a signal that is caused by incomplete Fourier synthesis, or missing frequencies.

A system of numerous Earth-orbiting satellites that can be used to determine the location (latitude, longitude and elevation) of a receiver or station on the Earth within about 2 m [6 ft]. Fixed receivers on Earth can be used to determine the relative motions of fault blocks and lithospheric plates. Hand-held receivers can be used for producing accurate geologic maps, acquiring navigation data for 3D seismic surveys, and positioning wells in the field.

The change in the amplitude of an electrical signal from the original input to the amplified output.

The mineral form of lead sulfide, PbS, and the most common ore for lead smelting. In drilling muds, powdered galena has been used, although rarely, to construct extremely high density (kill-weight) muds by taking advantage of its 7.5 g/cm3 density.

Materials used to provide cathodic protection. Galvanic anodes are made of metals such as zinc, magnesium or aluminum, which corrode more easily than the structure, thus developing enough electric current flow through the electrolyte (such as soils or water).Galvanic anodes, also called sacrificial anodes, are commonly used when the current required for cathodic protection is small.

(noun) An instrument that measures fluid density by directing a beam of gamma radiation through a pipe or vessel and detecting the attenuated signal on the opposite side. The degree of attenuation is proportional to the density of the fluid, enabling determination of phase fractions in multiphase flow.

Phenomena resulting from the transfer of energy from a gamma ray to matter, usually to an electron. The probability of a specific interaction occurring depends on the atomic number of the material and the energy of the gamma ray. In formation evaluation, there are two types of interactions of interest: the photoelectric effect, which indicates lithology, and Compton scattering, which depends on formation density. The third type of interaction, pair production, occurs at energies above those used for logging.

A log of the total natural radioactivity, measured in API units. The measurement can be made in both openhole and through casing. The depth of investigation is a few inches, so that the log normally measures the flushed zone.Shales and clays are responsible for most natural radioactivity, so the gamma ray log often is a good indicator of such rocks. However, other rocks are also radioactive, notably some carbonates and feldspar-rich rocks. The log is also used for correlation between wells, for depth correlation between open and cased hole, and for depth correlation between logging runs. The gamma ray log was the first nuclear well log and was introduced in the late 1930s.

A measurement that uses a source of gamma rays and a detector of gamma rays. The term is synonymous with density log, and is just an older name.

A perforated tubular attached to the subsurface sucker-rod pump that controls the entrance of gas. Since it is the only way for formation fluid to enter the pump, its use increases the efficiency of the subsurface sucker-rod pump. It also helps to prevent the phenomenon called gas lock. A gas anchor is similar to a bottomhole gas separator.

A simple separator vessel used to remove free or entrained gas from fluids circulated in the wellbore, such as mud used during drilling operations. The gas buster typically comprises a vessel containing a series of baffles with a liquid exit on the bottom and a gas-vent line at the top of the vessel.

The gas that accumulates in the upper portions of a reservoir where the pressure, temperature and fluid characteristics are conducive to free gas. The energy provided by the expansion of the gas cap provides the primary drive mechanism for oil recovery in such circumstances.

A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the expansion of gas either in the gas cap or inside the oil phase.

A subsurface leakage of gas from a poorly sealed hydrocarbonaccumulation. The gas can cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs.

Hydrocarbon liquid dissolved in saturated natural gas that comes out of solution when the pressure drops below the dewpoint.

(noun) A production phenomenon in which gas from a gas cap migrates downward into the oil-producing perforations due to the pressure drawdown created by production, forming a cone-shaped intrusion of gas around the wellbore that reduces oil production rate and increases the gas-oil ratio.

Drilling fluid whose bulk, unpressurized density is reduced as a small volume of gas displaces an equivalent volume of liquid. The derrickman periodically measures mud density and communicates the results to the rig team via an intercom. He usually reports something like "9.6 heavy," "10.4," or "13.2 light," indicating more than 9.6 pounds per gallon, 10.4 pounds per gallon, or less than 13.2 pounds per gallon, respectively. Each tenth of a pound per gallon is referred to as a "point" of mud weight. Note that for this low-accuracy measurement, no direct mention of gas cut is made. A gas cut is inferred only if the mud returning to the surface is significantly less dense than it should be. In the case of the mud logger's measurement, "units" of gas (having virtually no absolute meaning) are reported. For the mud logger's measurement, a direct indication of combustible gases is made, with no direct correlation to mud weight.

The factor z, defined by the real (nonideal) gas equation of state, pV = znRT. Values for z have been correlated for mixtures of reservoir gases over pressure and temperature ranges encountered in well tests.

A primary recovery mechanism for oil wells containing dissolved and free gas, whereby the energy of the expanding gas is used to drive the oil from the reservoirformation into the wellbore.

Gas volume at reservoir conditions divided by gas volume at standard conditions. This factor is used to convert surface measured volumes to reservoir conditions, just as oil formation volume factors are used to convert surface measured oil volumes to reservoir volumes.

Hydrocarbon gas density expressed as the ratio of the molecular weight of the gas to the molecular weight of air.

A crystalline solid consisting of water with gas molecules in an ice-like cage structure. The general term for this type of solid is clathrate. Water molecules form a lattice structure into which many types of gas molecules can fit. Most gases, except hydrogen and helium, can form hydrates. C1 to nC5 hydrocarbons, H2S and CO2 readily form hydrates at low temperature and high pressure. Heavier hydrocarbons may also enter the structure but do not form hydrates by themselves. Gas-cut muds can form hydrates in deepwater drilling operations, plugging BOP lines, risers and subsea wellheads, causing a well-control risk. Gas hydrates are thermodynamically suppressed by adding antifreeze materials such as salts or glycols. A common practice is to use 20 to 23 wt.% NaCl. Nucleation and growth of hydrates can be dynamically inhibited by certain polymers or surfactants. Gas hydrates are found in nature, on the bottom of cold seas and in arctic permafrost regions. Drilling into these can be hazardous, but they offer another source of hydrocarbons for future exploitation.Reference:Ebeltoft H, Yousif M and Soergaard E: "Hydrate Control During Deep-water Drilling: Overview and New Drilling Fluids Formulations," paper SPE 38567, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 5-8, 1997.

Gas that is dissolved in a liquid, such as water or oil.

A reservoir maintenance or secondary recovery method that uses injected gas to supplement the pressure in an oil reservoir or field. In most cases, a field will incorporate a planned distribution of gas-injection wells to maintain reservoir pressure and effect an efficient sweep of recoverable liquids.

A phenomenon that occurs when gas enters the subsurface sucker-rod pump. After the downstroke begins, the compressed gas reaches the pressure needed to open the traveling valve before the traveling valve reaches liquid. The traveling valve opens slowly, without the drastic load change experienced in fluid pound. It does not cause premature equipment failure, but can indicate poor pump efficiency. A bottomhole separator or a gas anchor can correct gas interference.

An artificial-lift method in which gas is injected into the production tubing to reduce the hydrostatic pressure of the fluid column. The resulting reduction in bottomhole pressure allows the reservoir liquids to enter the wellbore at a higher flow rate. The injection gas is typically conveyed down the tubing-casingannulus and enters the production train through a series of gas-lift valves. The gas-lift valve position, operating pressures and gas injection rate are determined by specific well conditions.

A condition sometimes encountered in a pumping well when dissolved gas, released from solution during the upstroke of the plunger, appears as free gas between the valves. On the downstroke, pressure inside a barrel completely filled with gas may never reach the pressure needed to open the traveling valve. In the upstroke, the pressure inside the barrel never decreases enough for the standing valve to open and allow liquid to enter the pump. Thus no fluid enters or leaves the pump, and the pump is locked. It does not cause equipment failure, but with a nonfunctional pump, the pumping system is useless.A decrease in pumping rate is accompanied by an increase of bottomhole pressure (or fluid level in the annulus). In many cases of gas lock, this increase in bottomhole pressure can exceed the pressure in the barrel and liquid can enter through the standing valve. After a few strokes, enough liquid enters the pump that the gas lock in broken, and the pump functions normally.

A generic term referring to all possible routes for annular gas entry and propagation through and around the cement sheath. Gas migration is also known as annular gas flow.

An installation that processes natural gas to recover natural gas liquids (condensate, natural gasoline and liquefied petroleum gas) and sometimes other substances such as sulfur. A gas processing plant is also known as a natural gas processing plant.

A poroussandlayer or sand body charged with natural gas.

A device used to separate entrained gas from production liquids. Surface processing facilities generally use gas separators to render the liquids safe for further processing or disposal. Gas-separation equipment is also used in downhole applications, such as the protection of pumping equipment against gas lock by separating and redirecting free gas at the pump suction or inlet.

Gas that rises to the surface, usually detected because it reduces the density of the drilling mud. Gas detectors, which the mud logger monitors, measure combustible gases (methane, ethane, butane and others). The mud logger reports total gas, individual gas components, or both, on the mud log. In extreme cases, gas visibly bubbles out of the mud as it returns to the surface. Because the mud does not circulate to the surface for a considerable time, sometimes lagging several hours after a formation is drilled, a gas show may be representative of what happened in the wellbore hours (or many feet) prior to the current total depth of the well.

A producing well with natural gas as the primary commercial product. Most gas wells frequently produce some condensate (natural gas liquids such as propane and butane) and occasionally produce some water.

Relating to a formation or interval containing gas, either dissolved in the formation fluid or as free gas. The term is occasionally used to describe wellbore fluids containing dissolved gas.

A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the expansion of gas either in the gas cap or inside the oil phase.

A mud that has gas (air or natural gas) bubbles in it. Defoamer chemicals added to the mud or a mechanical vacuum pump degasser can liberate the trapped gas. When weighed with a regular mud balance, a gas-cut mud has lower density than a mud not cut by gas. Accurate density of gas-cut mud can be measured using a pressurized mud balance.

A record of the fraction of gas present at different depths in the borehole. Although several techniques may be used for this purpose, the term usually refers to logs based on one of two principles. In the first, four or more optical probes are used to detect the passage of gas bubbles at different points across the borehole. As with other local probes, holdup is determined by the fraction of time the probe detects gas. In the second technique, a 57Co (cobalt) source emits low-energy gamma rays that undergo backscattering and photoelectric absorption in the borehole fluid before being counted in a detector. The number of counts is related to the fluid density, and can be calibrated in terms of gas holdup.The first technique produces an image of gas holdup along and around the borehole, while the second technique produces a log of the average holdup along the well.

A gas-lift system component that is assembled with the production tubing string to provide a means of locating gas-lift valves. The position or depth of the gas lift valves is crucial to the efficient operation of the entire system. Consequently, proper assembly of the gas lift mandrels within the completiontubulars is essential. A port in the gas-lift mandrel provides communication between the lift-gas supply in the tubing annulus and the production-tubing conduit.

A valve used in a gas-lift system to control the flow of lift gas into the production tubing conduit. The gas-lift valve is located in the gas-lift mandrel, which also provides communication with the lift gas supply in the tubing annulus. Operation of the gas lift valve is determined by preset opening and closing pressures in the tubing or annulus, depending on the specific application.

A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly oil occurs. Gas and oil are miscible, so the contact between gas and oil is transitional, forming a zone containing a mix of gas and oil.

The interface between the gas and oil phases present in a reservoirformation. During the production of a well, the GOC may move, resulting in undesirable production conditions such as a high proportion of gas that may be too much for surface processing facilities. Monitoring the gas-oil and oil-water contacts is a key element of good reservoir management practices.

The quality of a source rock that makes it more likely to generate gas than oil. The nature of the organic matter or kerogen in source rocks varies from coaly, plant-like material commonly found in terrestrial source rocks to algal or other marine material that makes up marine source rocks. Terrestrial source rocks are commonly gas-prone.

The volume of gas (at standard conditions) liberated from a volume of oil (at standard conditions).

A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly water occurs. Gas and water are somewhat miscible, so the contact between gas and water is not necessarily sharp and there is typically a transition zone between 100% gas and 100% water in reservoirs.

The calculated or measured rate a gas well will produce for a given bottomhole or wellheadpressure.

The ratio of produced gas to produced liquids (oil and water), often abbreviated GLR.

The ratio of produced gas to produced oil, commonly abbreviated GOR.

The gas produced or separated at surface conditions from the full well stream produced from a natural gas reservoir.

The liquids separated at surface conditions from the full well stream produced from a natural gas reservoir.

A type of valve that incorporates a sliding gate to block fluid flow. The design of the valve operating and sealing systems typically requires that gate valves should be operated either fully open or fully closed.

A display of seismic traces that share an acquisitionparameter, such as a common midpoint gather, which contains traces having a common midpoint.

The pipes used to transport oil and gas from a field to the main pipeline in the area.

The flowline network and process facilities that transport and control the flow of oil or gas from the wells to a main storage facility, processing plant or shipping point. A gathering system includes pumps, headers, separators, emulsion treaters, tanks, regulators, compressors, dehydrators, valves and associated equipment.There are two types of gathering systems, radial and trunk line. The radial type brings all the flowlines to a central header, while the trunk-line type uses several remote headers to collect fluid. The latter is mainly used in large fields.The gathering system is also called the collecting system or gathering facility.

A wellbore that is essentially the same diameter as the bit that was used to drill it. It is common to find well-consolidated sandstones and carbonate rocks that remain gauge after being drilled. For clays, it is common for the hole to slowly enlarge with the passing of time, especially if water-base muds are being used. Bit gauges, rings of defined circumference, are slipped around drill bits to detect and measure wear, which reduces the circumference of the bit during drilling.

The measured pressure within a system in which the pressure gauge reads 0 psi at nominal atmospheric pressure.

A precisely machined test device, typically fabricated from steel or similar durable material, having a specified internal or external diameter. The gauge ring is used to confirm the dimensional compatibility of tools and equipment that must pass through restrictions of a certain diameter.

A small tank with accurate volume markings used to measure flow into or out of a well. Treatments that require accurate volume tracking of fluids, such as squeezecementing, generally use a gauge tank to measure fluid volumes.

A name used to refer to one of the gel-strength numbers.

The shear stress measured at low shear rate after a mud has set quiescently for a period of time (10 seconds and 10 minutes in the standard API procedure, although measurements after 30 minutes or 16 hours may also be made).

The process of a mud becoming "gelled-up" or developing high gel strength.

A mud that is excessively viscous, having high gel strengths and high yield point. A gelled-up mud may not be pumpable without exceeding limits on pump pressure. Often caused by excessive solids content, especially colloidal solids, or, in the case of oil or synthetic muds, by low temperature.

A mud that is excessively viscous, having high gel strengths and high yield point. A gelled-up mud may not be pumpable without exceeding limits on pump pressure. Often caused by excessive solids content, especially colloidal solids, or, in the case of oil or synthetic muds, by low temperature.

Jargon referring to the two gel-strength values for a mud. The 10-second and 10-minute "gels," often written as one number over the other. For example, 6/16 means 6 lb/100 ft2 and is 10-second gel, and 16 lb/100 ft2 is the 10-minute gel.

The formation of hydrocarbons from a source rock as bitumen forms from kerogen and accumulates as oil or gas. Generation depends on three main factors: the presence of organic matter rich enough to yield hydrocarbons, adequate temperature, and sufficient time to bring the source rock to maturity. Pressure and the presence of bacteria and catalysts also affect generation. Generation is a critical phase in the development of a petroleum system.

A log of elemental concentrations from which the geochemistry of the formation may be derived. Several logs provide information on elemental weight concentrations: natural gamma ray spectroscopy, elemental capture spectroscopy or pulsed neutron spectroscopy and aluminum activation. The combination of some or all of their outputs is known as a geochemical log, since it provides information on most of the principal elements found in sedimentary rocks. Pulsed neutron spectroscopy provides relative elemental yields, whereas absolute concentrations are needed for quantitative results. Absolute concentrations can be derived by calibration to core or by using a model such as the oxide-closure model. The absolute elemental concentrations can then be converted into mineral concentrations using a model that defines what minerals are present. The first complete geochemical logs were run in the mid 1980s.

The study of the chemistry of the Earth and within solid bodies of the solar system, including the distribution, circulation and abundance of elements (and their ions and isotopes), molecules, minerals, rocks and fluids. For geochemists in the petroleum industry, source rock geochemistry is a major focus. Geochemical techniques can determine whether a given source rock is rich enough in organic matter to generate hydrocarbons, whether the source rock has generated hydrocarbons, and whether a particular oil sample was generated by a given source rock.

The study of the relative or absolute age of rocks, minerals and fossils. Absolute age is the measurement of age in years, but "absolute" ages typically have some amount of error and are inexact. Relative age, in contrast, is the approximate age of rocks, fossils or minerals made by determining the age of the material relative to other surrounding material.

A map showing the type and spatial distribution of rocks at the surface of the Earth. Rock formations are color-coded and symbols for geological structures are annotated, so age relationships are evident. Topographic contours and cultural features can also appear on geologic maps.

A chronological chart of the stages and ages of events in the history of the Earth, from its initial formation to present, that has been constructed on the basis of the rock record. As is the typical natural position of rocks, the oldest event is at the bottom of the chart and the youngest is at the top. Both absolute and relative ages of rocks and fossils supplement interpretations from rocks. The vastness of geologic time and the slowness of geological processes are difficult to capture in a simple chart.

A scientist trained in the study of the Earth. In the petroleum industry, geologists perform a wide variety of functions, but typically generate prospects and interpret data such as maps, well logs, outcrops, cuttings, core samples and seismic data.

The study of the Earth-its history, structure, composition, life forms and the processes that continue to change it.

The periodic switching of the magnetic north and south poles of the Earth throughout time, probably as a result of movement of fluid within the Earth's core. The onset and duration of the many episodes of reversed polarity have been documented by examining the polarity of magnetic minerals within rocks of different ages from around the world, particularly in basalts or igneous rocks of the oceanic crust. Oceanic basalts record the Earth's magnetic field as they solidify from molten lava symmetrically on each side of the midoceanic ridges. These data have been compiled to create a time scale known as the geomagnetic polarity time scale (GPTS). In the oil field, borehole recordings allow direct correlation to GPTS and well-to-well correlations.

A record of the onset and duration of the multitude of episodes of reversal of the Earth's magnetic polarity, or geomagnetic polarity reversals. The GPTS was developed by thorough study of rocks from around the world, during which it was observed that rocks from specific time periods contained magnetic minerals whose orientation was opposite to that of the current magnetic field. By comparing the patterns of magnetic reversals with those of rocks of known age, the approximate ages of rocks can be established. This is particularly useful for basalts of the oceanic crust, which record the Earth's magnetic field as they solidify from molten lava symmetrically about the midocean ridges. The time scale has been accurately extended back to the Upper Jurassic, the age of oldest existing oceanic crust.

The geologic specialty that deals with understanding how rocks, stresses, pressures, and temperatures interact. This understanding is used to solve oilfield problems, such as optimizing hydraulic fracturing treatments of shale reservoirs. Geomechanics specialists typically work with experts in geophysics, geology, petrophysics, reservoir engineering, drilling engineering, and rock physics to solve geomechanical problems and address production challenges in shale reservoirs.

Pertaining to variation of the survey geometry while maintaining the frequency of electromagnetic surveying. In contrast, parametric pertains to keeping frequency the same while varying the geometry.

The response of a logging measurement as a function of distance from the tool. The geometrical factor can be radial, reflecting the response perpendicular to the tool; vertical, reflecting the response along the tool axis; or two-dimensional, reflecting both. It can also be a differential geometrical factor, which is the contribution to the signal at a particular distance; or integrated, which is the sum of all signals from the tool to a particular distance. The term geometrical factor was introduced for induction logging since it gave a convenient method for computing the reading in a heterogeneous environment. For example, in an invaded formation the log reading, Clog, can be written as:Clog = Gi * Cxo + (1 - Gi) * Ctwhere Gi is the geometrical factor for a diameter of invasion Di, and Cxo and Ct are the conductivities of the invaded zone and the undisturbed zone. A true geometrical factor depends only on the geometry of the volume concerned, which in practice is only true for induction logs at zero conductivity. However, the term has come to be used for other cases and for other measurements even though the geometrical factor depends significantly on formation properties. The correct term for these cases is the pseudogeometrical factor.

A device used in surface seismicacquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction. Three mutually orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones, unlike geophones, detect changes in pressure rather than motion.

The distance between geophones or the centers of groups of geophones.

A scientist trained in the study of the physics of the Earth, particularly its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. In the petroleum industry, geophysicists perform a variety of functions, chiefly the processing and interpretation of seismic data and generation of subsurface maps on the basis of seismic data. Such interpretations enhance understanding of subsurface geology.

The study of the physics of the Earth, especially its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. Geophysics plays a critical role in the petroleum industry because geophysical data are used by exploration and development personnel to make predictions about the presence, nature and size of subsurface hydrocarbon accumulations.

The pressure within the Earth, or formation pressure. The common oilfield usage, however, is to indicate anomalous subsurface pore pressure that is higher or lower than the normal, predicted hydrostatic pressure for a given depth, or the pressure exerted per unit area by a column of fresh water from sea level to a given depth. Abnormally low pore pressure might occur in areas where fluids have been drained, such as a depleted hydrocarbonreservoir. Abnormally high pore pressure might occur in areas where burial of water-filled sediments by an impermeablesediment such as clay was so rapid that fluids could not escape and the pore pressure increased with deeper burial.

The change in porepressure per unit depth, typically in units of pounds per square inch per foot (psi/ft) or kilopascals per meter (kPa/m). The geopressure gradient might be described as high or low if it deviates from the normal hydrostatic pressure gradient of 0.433 psi/ft [9.8 kPa/m].

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

Techniques that are applied to reservoir characterization using various statistical approaches to estimate the geological characteristics of formations at a distance from known points, such as within wellbores. These techniques include the use of semivariograms, kriging and multivariate analysis.

The process of generating models of the subsurface using geostatistical methods. The methods derive a detailed model using only sparse data (such as logs and cores from a few wells in a large area) and a knowledge of the depositional systems and structural settings to estimate the subsurface characteristics between the wells. These methods were originally developed in the mining industry, where boreholes are generally much closer, and later were adapted for use in the oil and gas industry. Geostatistical models are used for reservoir simulation and reservoir management.

The study of samples of data from a complete data set (or population) to attempt to estimate the behavior of the population. Typically, geostatistics is applied during the creation of high-resolution subsurface models of mineral deposits or oil reservoirs. The data are available only from well cores and logs (high vertical resolution but laterally infrequent data samples) and from surface seismic (low vertical resolution) data. Geostatistically derived reservoir models, when successful, are used in reservoir simulations and for reservoir management.

The intentional directional control of a well based on the results of downhole geologicallogging measurements rather than three-dimensional targets in space, usually to keep a directional wellbore within a pay zone. In mature areas, geosteering may be used to keep a wellbore in a particular section of a reservoir to minimize gas or water breakthrough and maximize economic production from the well.

The natural increase of temperature with depth in the earth. Temperature gradients vary widely over the earth, sometimes increasing dramatically around volcanic areas. It is particularly important for drilling fluids engineers to know the geothermal gradient in an area when they are designing a deep well. The downhole temperature can be calculated by adding the surface temperature to the product of the depth and the geothermal gradient.

A short-path multiple, or a spurious reflection that occurs when seismic energy initially reverberates upward from the shallow subsurface and then is reflected downward, such as at the base of weathering or between sources and receivers and the sea surface.

A trademarked product of American Gilsonite Co.

A lifting device, similar in function to a crane jib, that is used in a number of oilfield applications, such as for handling tubulars on a snubbing unit, tool strings on a slickline unit or on a winch truck, and for general lifting at the wellsite.

Pertaining to the environment of deposition by glaciers.

[(K,Na,Ca) 1.2-2.0(Fe+3,Al,Fe+2,Mg)4(Si7-7.6Al1-0.4O20)(OH)47nH20]A green silicatemineral found in sedimentary rocks and formed on continental shelves characterized by slow sedimentation and organic matter, such as fecal pellets, present in an oxidizing environment. In sufficient quantity, it can form a sandy, green deposit such as the Cretaceous greensands of the US and UK.

A series of alcohols with general formula C2nH4n+2On+1. The simplest member is ethylene glycol C2H6O2, widely used as antifreeze. Glycols may be used in drilling fluids as gas hydrate inhibitors.

In a glycoldehydrator unit, the cylinder composed of various perforated trays in which wet gas and glycol are put in contact.

A unit used to remove minute water particles from natural gas if dehydration was not attained using separators. A glycoldehydrator unit is usually composed of an absorber and a reboiler.The wet gas enters at the bottom of the absorber. As the wet gas percolates upward, it releases its water into the glycol solution and dry gas is obtained at the top of the absorber.When the glycol solution becomes saturated with water, the glycol solution is pumped through a reboiler, also called a reconcentrator, which boils the glycol-water mixture and separates the glycol from the water. After separation, the glycol can return to the absorber to contact additional wet gas.

A diagram that displays multivariate data. A good choice of glyph design can aid the recognition of complex similarities or distinctions in a set of data. A well-known example of an oilfield glyph is the Stiff diagram, which shows the patterns of cations and anions in fluids.Reference:Stiff HA Jr: The Interpretation of Chemical Water Analysis by Means of Patterns, Journal of Petroleum Technology 3, no. 10 (1951): 15-16.

The assembly mounted on a coiled tubing injector head that guides the tubing string as it passes through an arc from the reel into a vertical alignment with the injector-head chains and wellbore. The radius of the guide arch is generally designed to be as large as practicable since the plastic deformation created in the coiled tubing string induces material fatigue in the tube.

The assembly mounted on a coiled tubing injector head that guides the tubing string as it passes through an arc from the reel into a vertical alignment with the injector-head chains and wellbore. The radius of the guide arch is generally designed to be as large as practicable since the plastic deformation created in the coiled tubing string induces material fatigue in the tube.

The total amount of revenue that a host government receives from production. This amount can include taxes, royalties and government participation.

A relatively low-standing fault block bounded by opposing normal faults. Graben (used as both singular and plural) can form in areas of rifting or extension, where normal faults are the most common type of fault. Between graben are relatively high-standing blocks called horsts. A half-graben is a downdropped block bounded by a normal fault on only one side.

A device for measuring the average density of the fluid at different depths in a completed production or injection well to produce a fluid-density log. Knowing the density of the individual phases allows their holdups to be determined, directly in the case of biphasic flow, and in combination with other measurements for triphasic flow. Introduced in the late 1950s, the gradiomanometer measures the pressure difference between two pressure sensors, placed approximately 2 ft [0.6 m] apart. The pressure difference reflects the average fluid density across the well within that depth interval.The resolution is high, around 0.005 g/cm3, but the accuracy can be affected by a friction effect, a kinetic effect and well deviation. The effect of deviation can be corrected, but the sensitivity to holdup is reduced as the deviation increases until it is zero in a horizontal well.Note: Gradiomanometer is a mark of Schlumberger. It is now a commonly accepted term for a certain tool that measures differential pressure.

The density of the grains in a formation or core sample. As used in log and core analysis, the term 'grain' refers to all the solid material in the rock, since, when interpreting the measurements, no effort is made to distinguish grains from other solid material. The grain density of core samples is calculated from the measured dry weight divided by the grain volume. In logs, grain density is calculated from the density log, using an estimate of porosity and knowledge of the fluid content.

A coarse-grained, plutonic or intrusive igneousrock of felsic composition having large crystals of quartz, feldspar and mica. In the oil field, "granite" is sometimes used incorrectly to indicate any type of hard rock.

A type of lost-circulation material that is chunky in shape and prepared in a range of particle sizes. Granular LCM is added to mud and placed downhole to help retard the loss of mud into fractures or highly permeable formations. Ideally, granular LCM should be insoluble and inert to the mud system in which it is used. Examples are ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the loss zone to seal the formation into which circulation is lost.

A type of lost-circulation material that is chunky in shape and prepared in a range of particle sizes. Granular LCM is added to mud and placed downhole to help retard the loss of mud into fractures or highly permeable formations. Ideally, granular LCM should be insoluble and inert to the mud system in which it is used. Examples are ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the loss zone to seal the formation into which circulation is lost.

A generic name given to tools that engage on the outer surface of a tubing string or tool assembly, generally for fishing purposes.

A sand-control method used to prevent production of formation sand. In gravel pack operations, a steel screen is placed in the wellbore and the surrounding annulus packed with prepared gravel of a specific size designed to prevent the passage of formation sand. The primary objective is to stabilize the formation while causing minimal impairment to well productivity.

A perforating-gun system containing big-hole or gravel-pack charges. A gravel-pack gun usually has a large outside diameter to minimize standoff between the gun and casing. It is loaded to achieve a high shot density while producing large holes in the casing.

A record of the quality of a gravel pack and the quantity of solid particles it contains. The traditional logging technique uses a type of nuclear fluid densimeter, with a gamma ray source and a single detector. The number of gamma rays reaching the detector is inversely proportional to the gravel-pack density. The count rate is used qualitatively, being scaled in each well between zones with 100% pack and zero pack, or else compared before and after a repair to the pack.The density measurement is not entirely independent of the formation, and is not effective when the density of the particles and the completion fluid are similar. In these cases, a neutron activation or neutron-porosity measurement may be used. Other techniques include a neutron porosity log and tracer measurements. Radioactive tracers may be coated on the outside of the particles or else included within the particles. They can be detected by gamma ray logs, or if a variety of tracers is used they can be tracked with a multiple-isotope log.

A metal filter assembly used to support and retain the sand placed during gravel pack operations. A range of sizes and screen configurations is available to suit the characteristics of the wellbore, production fluid and the formation sand.

A device used to measure the acceleration due to gravity, or, more specifically, variations in the gravitational field between two or more points.

The measurement of gravity or the study of its variations.

The Earth's gravitational field, or the attractive force produced by the mass of the Earth. Variations in the gravitational field can be used to map changes in the density of formations in the Earth. Gravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects.

The difference between the actual value of gravity measured at a location and the value predicted by a particular Earth model. Gravity anomalies are usually determined by adjusting the known value of (absolute) gravity at a reference station by Bouguer, free-air or other corrections and subtracting the final predicted value from the measurement. (A different description is that the various corrections are subtracted from the data to reduce it to the reference level. Both interpretations are valid provided it is remembered that the resulting gravity anomaly can be caused by density anomalies-i.e., differences in density between Earth and the theoretical model-that can lie anywhere either above or below the reference level.)

The least common primary recovery mechanism in which the force of gravity pushes hydrocarbons out of the reservoir, into the wellbore and up to surface.Gravity force is always present in the reservoir, but its effect is greater in thick gas-condensate reservoirs and in shallow, highly permeable, steeply dipping reservoirs.

A phenomenon of multiphase flow in a reservoir in which a less dense fluid flows preferentially on the top of a reservoir unit and a more dense fluid flows at the bottom. For example, in a steamflood, steam flows on the top and condensed liquid flows at the bottom of the zone. Gravity override causes sweep inhomogeneities that can be mitigated through foam flooding.

The tendency of fluids to stratify into different layers because of gravity forces. In gravity segregation, the heaviest fluid settles near the bottom and the lightest fluid rises to the top. Gravity segregation occurs inside reservoirs as well as in separator facilities.

The measurement of gravitational acceleration over an area, usually presented as a map or profile of Bouguer or free-air anomalies.

One-tenth of a milliGal: (10 g.u. = 1.0 mGal). Gravity units are sometimes used in old gravity maps.

A frontal advance in which gravity and viscous forces are in equilibrium, resulting in a stable, highly efficient frontal advance.

An assembly of components used to contain wellhead fluids and pressure during braided-line or wireline operations. The wireline passes through a close-tolerance tube assembly as it leaves the wellbore. High-pressure grease is pumped into the surrounding annulus to effect a pressure-tight dynamic seal that is maintained during the operation by injecting more grease as required. A slight leakage of grease is normal, and the addition of fresh grease enables the consistency of the seal to be maintained at an effective level.

The action in which mud particles (barite) become coated with an oily substance that causes the particles to agglomerate. This can cause barite settlement or removal by shaker screens. The problem can occur when mud additives, such as fatty-acid soaps or mud lubricants, are incompatible with the mud system or react adversely to contaminating ions.

A mathematical procedure used to improve computer usage in simulated annealing runs. The procedure uses a fast swapping technique to match model semivariograms to actual semivariogram data.

Untreated gas that leaves the well. This gas could contain impurities such as water, carbon dioxide [CO2], nitrogen [N2] and hydrogen sulfide [H2S], which will be removed in treating facilities.

To convert irregularly spaced points to a regular spacing by interpolation.

The act of determining values for grid elements on a map. The grid element values are chosen from nearby data points. Methods are deterministic and use linear and nonlinear interpolation methods, or may be statistical and use geostatistical approaches such as kriging. Gridding is usually applied to one characteristic per map, such as structure, thickness, porosity, permeability or saturation.

A computational procedure incorporated in methods for determining the values to be assigned to grid elements on a map. The deterministic gridding algorithms are mostly simpler interpolation; the stochastic approaches include the geostatistical methods, among them kriging.

The fineness to which cement is ground. Grind also may refer to a specific production of cement, such as the lot number.

The profiled blocks attached to the drive chains of a coiled tubing injector head. The gripper blocks are arranged in opposing pairs to secure the coiled tubing string in the injector-head chains. As the hydraulic drive system rotates the chains, the gripper blocks feed the tubing string into, or out of the well.

The total production of oil, gas or water from a well or field over a specific period of time.

A type of coherentnoise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters.

Water in the subsurface below the water table. Groundwater is held in the pores of rocks, and can be connate, from meteoric sources, or associated with igneous intrusions.

A set of seismometers whose output is sent to a common data channel to record a seismic trace. A large group is known as a patch.

The distance between geophones or groups of geophones.

A type of normal fault that develops and continues to move during sedimentation and typically has thicker strata on the downthrown, hanging wall side of the fault than in the footwall. Growth faults are common in the Gulf of Mexico and in other areas where the crust is subsiding rapidly or being pulled apart.

A hydrophilic polysaccharide from the seed of the guar plant. It is a galactomannan type of saccharide that is highly dispersible into water and brines of various types and salinity. Its water solutions are non-Newtonian and also can be cross-linked by borax to give very high gel strength for suspension. Such a structure is easily broken by breakers in fracturing fluids, so it serves as a carrier for placing sand far back into fractures. It is also used as a top-hole drilling fluid. Disadvantages of using guar gum include its lack of thermal stability and sensitivity to high pH and bacterial fermentation.

An electrode on a laterolog sonde that focuses the current sent by the central current-emitting electrode (A0). The guard electrode is held at the same potential as A0, thereby forcing the current from A0 to run approximately perpendicular to the sonde into the formation, and preventing it from running up the borehole to a great extent.

A measurement made by a type of electrode device in which the current flow and hence the measurement is focused in a disk that is concentric with and perpendicular to the sonde. The term usually refers to a Laterolog-3 device. Guard logs may be recorded by either wireline or measurements-while-drilling tools.

A tapered, often bullet-nosed piece of equipment often found on the bottom of a casing string. The device guides the casing toward the center of the hole and minimizes problems associated with hitting rock ledges or washouts in the wellbore as the casing is lowered into the well. The outer portions of the guide shoe are made from steel, generally matching the casing in size and threads, if not steel grade. The inside (including the taper) is generally made of cement or thermoplastic, since this material must be drilled out if the well is to be deepened beyond the casing point. It differs from a float shoe in that it lacks a check valve.

A nonspecific type of shale that becomes sticky when wet and adheres aggressively to surfaces. It forms mud rings and balls that can plug the annulus, the flowline and shale-shaker screens. Gumbo is likely to contain appreciable amounts of Ca+2smectite clays. It is dispersed in a water mud, causing rapid accumulations of colloidal solids.

(noun) A perforating device, typically a shaped-charge carrier assembly, that is lowered into a cased wellbore on wireline, tubing, or coiled tubing and detonated to create penetrating holes through the casing, cement sheath, and into the producing formation, establishing communication between the reservoir and the wellbore.

A settling tank used for treating oil. Oil and brine are separated only by gravity segregation forces. The clean oil floats to the top and brine is removed from the bottom of the tank. Gun barrels are found predominantly in older or marginal fields. A gun barrel is also called a wash tank.

The distance between the external surface of the gun assembly and the internal surface of the casing or liner. The gun clearance depends on the position of the gun within the tubular and will vary between phases of shots on any gun unless the gun assembly is centralized. This variation in clearance contributes to variable performance of the perforations. The greater the gun clearance, the smaller the entrance hole-with the effect increasing with depth (pressure).

The point on a gun assembly or perforating string that is used when correlating depth. With use of a surface depth reference or datum point, any convenient point may be used, providing it is applied accurately and consistently. For downhole correlation on wireline applications, the gun zero point will relate to the correlating device, such as the casing collar locator (CCL) or gamma ray log tool. For nonwireline applications, the gun zero point will relate to the locating device, such as the tubing-end locator. The correlation process is crucial to a successful perforating operation; and therefore the importance of ensuring correct placement of the gun assembly prior to firing cannot be overstated.

Debris, dirt, paraffin, oil, millscale and rust removed from a pipeline when a scraper or a pig is used for cleaning purposes.

A slurry that consists of bentonite, cement or polymers mixed into an oil; bentonite in diesel oil is commonly used as a gunk plug. A small batch of the slurry is pumped down a well that has lost circulation to seal the leaky zone. The gunk plug may or may not be squeezed by pressure into the zone. Water downhole interacts with the bentonite, cement or polymers to make a sticky gunk.

The operation of squeezing a gunk plug into a zone of lost circulation. When spotted next to the zone, the annular blowout preventers are closed and pressure is applied by further pumping to force the gunk into the loss zone.

The act of strongly agitating the mud pits, particularly on bottom and in corners, with the mud guns. Gunning the pits lifts settled barite, which can result in sudden, perhaps undesirable, increase in mud density in the pits.

A calcium-based water mud system containing gypsum. Gyp mud can be used for drilling shales, but it is also well-suited for drilling gypsum, anhydrite and salt stringers. An advantage of gyp over lime muds is that the pH of gyp mud need not be so high because it contains more soluble Ca+2 to inhibitshale swelling. Gypsum, CaSO4·2H2O, content is measured by an API test, and more can be added as needed. A calcium-tolerant claydeflocculant may be needed to control viscosity. Carboxymethylcellulose (CMC) and starch are used for fluid loss control along with a small amount of prehydrated bentonite.

[CaSO472H2O]A highly insoluble sulfate mineral that is the first to precipitate from evaporating seawater. Dehydration of gypsum can produce anhydrite. Fine-grained gypsum is called alabaster.