Glossary

gal	A unit of acceleration used in gravity measurements. One gal = 1 cm/sec² = 10^–2 m/sec². The earth's normal gravity is 980 gal. See milligal and gravity unit.

galvanometer	A sensitive ammeter which has a miniature mirror fastened to the moving coil. Through a system of precision-placed mirrors, light from a source is directed onto the galvanometer mirror from which it is reflected onto a moving photoraphic film where it traces a curve. Galvanometers often are part of the recorder or camera.

galvanometer drift	A change in the mechanical zero of a recording galvanometer during the course of a survey. The drift sometimes is a result of a change in temperature of the galvanometer or the occurrence of a bubble in the galvanometer fluid.

gamma-gamma log	See density log.

gamma ray	A photon having neither mass nor charge. It is a high-energy electromagnetic wave which is emitted by atomic nuclei as a form of radiation. Gamma rays are emitted by nuclei in their transition from an excited state to a lower energy state, in transmutations, and in radioactive disintegrations. Gamma rays have characteristic energy levels which can be used to identify the parent substance.

gamma-ray absorption	See mass absorption.

gamma-ray detectors	See ionization chamber, Geiger-Mueller counter and scintillation counter.

gamma-ray index	GRI. A clayiness index determined from the difference between the radioactivity level of the zone of interest and that of clean rock compared to the difference between the radioactivity level in clay shale and that in the clean rock.

gamma-ray interactions with matter	Gamma rays interact with matter in three different ways: see photoelectric absorption, Compton scattering, and pair production.

gamma-ray log	A well log of the natural formation radioactivity level. (1) In sediments the log mainly reflects clay content because clay contains the radioisotopes of potassium, uranium, and thorium. Potassium feldspars, volcanic ash, granite wash, and some salt deposits containing potassium (potash for example) may also give significant gamma-ray readings. The log often functions as a substitute for the SP for correlation purposes in nonconductive borehole fluids in open holes, for thick carbonate intervals, and to correlate cased-hole logs with open-hole logs. (2) Used in exploration for radioactive minerals.

gamma-ray source	An encapsulated radioactive material used in density logging. Usually Cesium-137. Compare test pill.

gamma spectometry log	The gamma spectometry tool (GST) measures both inelastic and capture gamma ray spectra, providing a detailed measurement of formation response to neutron bombardments. Eight essential elements are identified and their concentrations are determined. Measurements of carbon, oxygen, silicon, calcium, iron, chlorine, hydrogen, and sulfur are used to compute the hydrocarbon saturation, salinity, lithology, porosity, and shaliness of the formation. See also carbon-oxygen log. GST is a mark of Schlumberger.

gas	A fluid, compressible substance that completely fills any container in which it is confined. Its volume is dependent of the size of the container.

gas cap	A free-gas phase overlying an oil zone and occurring within the same reservoir as the oil. See reservoir.

gas-cap drive	Drive energy supplied naturally (as a reservoir is produced) by the expansion of gas in a cap overlying the oil in the reservoir. See reservoir-drive mechanism.

gas chromatograph	An analytical instrument used for separating the various components of a gas mixture, and measuring the relative concentrations of each. These instruments are used routinely by mud logging contractors to identify the types of hydrocarbons encountered in the drilling process. See partition gas chromatograph.

gas condensate	Liquid hydrocarbons present in casinghead gas that condense upon being brought to the surface; formerly distillate, now condensate. Also casinghead gasoline; white oil.

gas cone	See cone and coning.

gas-cut mud	Drilling mud aerated or charged with gas from formations downhole. Because a large amount of gas in mud lowers its density, gas-cut mud often must be treated to lessen the chance of blowout.

gas detector	An electrochemical device used by mud logging contractors to determine the presence and relative concentration of the total combustible fraction of the gas and air mixture drawn off the return stream of drilling mud.

gas drilling	See air drilling.

gas drive	The use of the energy that arises from gas compressed in a reservoir to move crude oil to a well bore. Gas drive is also used in a form of secondary recovery, in which gas is injected into input wells to sweep remaining oil to a producing well.

gas hydrate	A combination of natural gas and water under pressure which forms solid crystals at temperatures above the freezing point of water. These solids are an expanded form of ice (gas molecules trapped inside) and look like dirty ice.

gas in solution	(1) Gas dissolved in water or crude oil. (2) Gas dissolved in crude oil decreases the specific gravity and viscosity of the oil in a reservoir. Dissolved-gas energy is freed by the expansion of the gas released from solution as the pressure drops along the gradient from the reservoir, through the well. and to the tank As gas expands, it moves along the pressure gradient carrying or driving oil along with it.

gas lift	The process of raising or lifting fluid from a well by injecting gas down the well through tubing or through the tubing-casing annulus. Injected gas aerates the fluid to make it exert less pressure than formation pressure; consequently, the higher formation pressure forces the fluid out of the wellbore. Gas may be injected continuously or intermittently, depending on the producing characteristics of the well and the arrangement of the gas-lift equipment.

gas-lift valve	A device installed on a gas-lift mandrel, which in turn is put on the tubing string of a gas-lift well. Tubing and casing pressures cause the valve to open and close thus allowing gas to be injected into the fluid in the tubing to cause the fluid to rise to the surface.

gas-oil contact	The lowest depth (deepest depth in a well) opposite a formation at which virtually 100% gas can be produced. This depth is at the top of the gas-oil transition zone.

gas-oil interface	See gas-oil contact.

gas-oil ratio	GOR. A measure of the volume of gas produced with oil, expressed in cubic feet per barrel or cubic meters per metric ton.

gas tracer	A radioactive isotope (e.g., I¹³¹ in a methyl iodide carrier) used to follow gas flow in a well bore and determine a velocity flow profile.

gate	A window or opening, usually in time, during which certain measurements are made. The gate has specific beginning and ending time boundaries.

gauge hole	A borehole with a diameter the same size as the drill bit.

gauge ring	Gage ring. A nonrecording device run into cased boreholes in order to determine minimum inside diameter of the casing before running tools that might be easily damaged or become stuck because of the close tolerances involved.

Geiger-Mueller counter	A form of gamma-ray detector. Similar to the ionization chamber in that a center rod electrode is maintained at a positive potential relative to the cylindrical chamber wall. The difference is that the Geiger-Mueller tube chamber contains gas at a low pressure and maintains the center electrode at a high positive voltage (e.g., 900-l000 volts). Incident gamma rays cause the ejection of electrons from detector walls into the gas. As the ejected electron is drawn toward the highly charged center electrode, other collisions occur between each electron and gas atoms, thus producing additional electrons which in turn cause additional ionization by collision. That results in a multiplication of the ionization events, and an avalanche of electrons arrives at the central electrode. The surge of electrical current must be quenched before another surge can be determined. That results in a number of easily detectable pulses related to the number of gamma-ray interactions. Increased length and additional plates and baffles are used to increase the number of interactions by incident gamma rays. The increased length tends to decrease vertical resolution. Compare scintillation counter.

gel	A semisolid, jellylike state assumed by some colloidal dispersions at rest. When agitated, the gel converts to a fluid state. See thixotropy.

geofluid	Any natural fluid found in earth formations or rocks. Formation fluid.

Geolograph	A trade name for a patented device used on a drilling rig which records automatically the depth and rate of penetration during each consecutive 24-hour period.

geology	The science that relates to the study of the structure, origin, history, and development of the earth as revealed in the study of rocks, formations, and fossils

geometrical factor	The geometry-dependent weighting factor for determining how the conductivities of each medium in the vicinity of an induction logging tool affect the apparent conductivity measurement. The only well-logging devices for which this concept is sound are the induction tools, because when R_xo > R_t, only with these tools is the measuring geometry independent of variations in R_xo/R_t. When conductivities are not high, skin effect may be neglected, and the response of induction logs can be described in terms of conductivities and "geometrical factors" of the volumes surrounding the tool. The geometrical factor, (G), of a volume having a specific geometrical orientation with the sonde is simply the fraction of the total signal that would originate with that volume in an infinite homogeneous medium. For computation of geometrical factor to be practical. it is necessary to assume that the volumes conform to symmetry of revolution about the sonde. The magnitude of the signal in conductivity units is the product of the geometrical factor and the conductivity of the material, and the total signal sensed by the tool is the sum of these products for all volumes within range (which extends to infinity, but can be circumscribed to practical limits). Since the G's add up to unity by definition, this can be stated: C_IL = C₁G₁ + C₂G₂ + C₃G₃ . . . + C_NG_N where the C's and G's refer to the zones of differing conductivity, and N is the total number of such zones. The chief significance of this concept is the fact that a volume of space defined only by its geometry relative to the sonde has a fixed and computable geometric factor. That permits the construction of mathematically sound correction charts to account for the effects of borehole mud, the invaded zone, and adjacent beds on the R measurement, providing symmetry of revolution exists. Compare pseudo-geometrical factor.

geophone	An instrument that detects vibrations passing through the earth's crust, used in conjunction with seismography. The geophone, also referred to as a seismometer, converts seismic oscillations of the ground into electrical signals. The most common type consists of a coil and magnet: one fixed rigidly with respect to the ground and the other suspended from a fixed support by a spring. Relative motion between the coil and magnet produces a current in the coil. Vertical and horizontal ground motion can be measured. Exploration geophones operate in the 5-200 Hz range. They can be used on the surface or in special downhole packages. Downhole geophones must be "locked" to the borehole wall when recording. Downhole geophones are used in velocity check surveys and vertical seismic profiling.

geopressure	(1) Pressure pertaining to the earth, ground, or crust of the earth. A specific pressure regime, as geothermal pertains to a specific temperature regime. (2) Undisturbed formation pressure. The pore pressure existing within a body of interest prior to disturbance. (3) Historically misused to indicate high abnormal pore pressure.

geopressure gradient	The change in pore pressure observed per unit change in depth. If the pore pressures are normal, then the geopressure gradient pertains to the normal pressure gradient (on the Gulf Coast about 0.465 psi/foot depth).

geostatic	Of or pertaining to overburden or the aggregate of overlying rock column and fluid.

geostatic load	Overburden. The combined load of the overlying rock column and the fluids contained within the pores of the rock from the depth of the stratum of interest to the surface.

geostatic pressure	That pressure exerted by the weight of the entire geostatic load.

geothermal	Pertaining to temperatures in the earth, ground, or crust of the earth

geothermal energy	The internal energy of the earth. Available to human beings as heat from heated rocks or water.

geothermal gradient	The rate of increase of temperature in the earth with depth. The gradient near the surface of the earth varies from place to place depending on the heat flow in the region and on the thermal conductivity of the rock. Caused by the continuous flow of heat outward through the crust of the earth.

gilsonite	A mineral form of asphalt. Softens on heating and soluble in carbon disulfide.

GL	Ground level.

glands	Used in a stuffing box and hydraulic packing head. Rubber seals that can be squeezed down over logging cable to form a pressure seal in the event pressure develops at the surface. Cable can be moved in closed down glands if the cable is lubricated by fluid or injected grease. If glands should cut out, the blowout preventer can be closed and glands replaced. Compare rams.

globular saturation	Insular saturation. See description and illustration under saturation.

gooseneck	A curved device that fits over the fishing bell. Used during the process of lifting heavy downhole tools from a horizontal position to a vertical position. The logging cable or bridle fits into the curvature of the device thus minimizing localized stress and flexure on survey line and conductors.

graben	A block of the earth's crust that has slid downward between two faults; the opposite of a horst. See illustration in fault.

grade ore	Weight percent of a mineral of interest in a volume of ore containing the mineral.

grade thickness	The macroscopic ore grade determined by a logging instrument which investigates a volume of formation of greater than unit thickness. The average grade ore determined by the instrument (e.g., gross-count gamma-ray tool) would be grade thickness divided by thickness.

gradient	The change in any parameter per unit change of another parameter.

gradiomanometer	A device used to measure the average density of the fluids contained in a fixed length of the well bore located between sensitive membrane-type pressure sensors, irrespective of the fluid distribution. The measurement recorded as a function of depth is called specific gravity. The recorded curve would represent a specific gravity profile of fluids in the borehole for the conditions under which the survey was run. In some cases it may be affected by hole deviation, a friction component. and a kinetic component.

granite	In a broad sense, a coarse-grained igneous rock consisting essentially of quartz, feldspar, and mafic minerals, by far the most abundant of all plutonic rocks. See also pluton.

grain density	The density of a unit volume of a mineral or other rock matter at zero porosity. The density of the rock framework. Sometimes called matrix density. Usual units are g/cm³.

grand slam	A combination of logs or a computation procedure for calculating the depth of invasion and the resistivity of both invaded and uncontaminated zones, based on a dual induction-laterolog and a proximity log or microlaterolog.

graphic log	Strip log.

gravel pack	A mass of very fine gravel placed around a slotted liner in a well.

gravel packing	A method of well completion in which a slotted or perforated liner is placed in the well and surrounded by small-sized gravel. The well is enlarged by underreaming at the point where the gravel is packed. The mass of gravel prevents sand from flowing into the well but allows continuous rapid production.

gravel pack log	The gravel pack logging tool is a neutron-type device that evaluates the condition of the gravel pack. Count rates from two detectors are used to compute porosity. The count rates from both detectors are presented with the cased-hole porosity and compared to the porosity measured in the open hole if open-hole measurements are available. The log provides a quantitative analysis of the areas in the pack that need to be replaced before the well is placed on production.

gravimeter	An instrument for measuring variations in gravltational attraction; a gravity meter. Most present gravimeters are of the unstable or astatic type. The gravitational force on a mass in the meter is balanced by a spring arrangement, and a third force is provided which acts when the system is not in equilibrium. This third force intensifies the effect of changes of gravity and increases the sensitivity of the system. In the LaCoste-Romberg gravimeter, the main spring that balances out the gravitational pull on the weight is a "zero-length spring" inclined at an angle. A zero length spring has a stress-strain curve that passes through zero length when projected back to zero strain. (For example, a spring that requires an initial stress before the coils begin to separate.) Zero-length springs have very long periods and high sensitivity.

gravimetry	The measurement, correction, mapping, and interpretation of the earth's gravitational field by surface or borehole measurements.

gravity	(1) The force of attraction between bodies of finite mass. Usually expressed in milligals. (2) API gravity. A method for specifying the density of crude petroleum. The density in °API is equal to where P is the specific gravity of the oil measured at 60°F.

gravity anomaly	Difference between theoretical calculated and observed terrestrial gravity; excess observed gravity is positive and deficiency is negatively anomalous.

gravity drainage	The movement of oil in a reservoir toward a wellbore resulting from the force of gravity. In the absence of water drive or effective gas drive, gravity dramage is an important source of energy to produce oil. It is also called segregation drive.

gravity meter	See gravimeter.

gravity unit	A unit of gravitational acceleration, equal to 0.1 mgal or l0^–6m/sec². Sometimes called G unit.

graywacke	A variety of sandstone generally characterized by its hardness, dark color, and angular grains of quartz, feldspar, and small rock fragments set in a matrix of clay sized particles

green pattern	A convention used in dipmeter interpretation. The "green pattern" represents structural dip as denoted by a succession of dips of relatively constant direction and magnitude. Becomes more evident after removal of blue pattern and red pattern. Compare blue pattern and red pattern. See dip for illustration.

grid	See API log grid.

gross-count gamma-ray tool	A tool that can be applied to a much wider range of radioactivity than natural gamma-ray logging tools used in the petroleum industry. Responds to changes in gamma-ray activity at very high radiation levels. Used in the exploration for radioactive minerals.

ground electrode	A ground stake. With SP and resistivity logging, it is a portable surface grounding electrode placed at some remote location from the rig and logging unit and connected with a conductor line to provide ground connection for the instruments.

ground level	GL. Elevation of the undisturbed ground surface. Sometimes used as a depth datum.

ground line	(1) With SP and resistivity logging. It is the conductor line, to which is attached a portable surface electrode, which is extended to some remote location from the rig and logging unit to provide electrical ground connection. (2) With perforation safety. Ground strap. It is a heavy-gauge, low-resistance line provided with appropriate connectors or clamps to electrically connect together the masses of the hoist unit, rig, and surface casing (wellhead). This negates voltage differences which may occur between the masses.

ground safety line	A ground strap used for perforation safety. See ground line Part (2).

ground stake	Portable surface electrode used in SP and resistivity logging. Makes a remote electrical connection with earthen mass. A metal stake, connected to the end of an electrical ground line, which is driven into the ground. Compare fish. See also ground electrode.

guarded electrode	The short center electrode of a guard tool. The electrode on which most measurements are based.

guard electrode	(1) One of the long electrodes above and below the short center electrode, or guarded electrode, of a guard tool. (2) Sometimes used, incorrectly, to refer to the guarded electrode or the entire guard tool.

guard log	A well log of formation resistivity which involves the use of a guard tool.

guard logging tool	See guard tool.

guard logging sonde	See guard tooll.

guard tool	The guard tool behaves similarly to one elongated current electrode from which current flows radially in all directions to a distant current-return electrode. In practice, the current-emitting electrode is separated into three parts by insulation so that the center part, which is made short, can be treated as a discrete electrode without alteration of the current-flow pattern. The current from the center electrode, which serves as both a current and measure electrode, flows in a thin horizontal layer at an angle of 90° to the tool because of its central location in the configuration. Through the method of focusing described above, this arrangement provides good resolution of thin beds and permits the use of the tool in boreholes filled with saline muds. The log usually is presented with one resistivity curve and a gamma-ray curve and/or SP curve (recorded from an electrode not physically a part of the guard electrode). Compare laterolog.

gun	An assemblage including a carrier, electrical circuitry, explosive charges, and either percussion-type bullets or core barrels for taking formation samples, or projectiles or shaped charges for gun perforating or formation testing.

gun perforating	The wireline procedure involving the use of a gun in introducing holes through steel casing into a formation so that fluids can flow from the formation into the casing. The gun can be selectively positioned opposite the formation and has the capability of firing bullets or shaped charges.

gusher	An oil well which has come in with such great pressure that the oil jets out of the well like a geyser. In reality, a gusher is a blowout and is extremely wasteful of reservoir fluids and drive energy. In the early days of the oil industry, gushers were common, and many times were the only indications that a large reservoir of oil and gas had been found.

gypsum	A naturally occurring crystalline form of hydrated calcium sulfate. Gypsum is a calcium dihydrate (CaSO₄·2H₂O) in the monoclinic crystalline form. At elevated temperatures the dihydrate in a dry environment may lose some of its water of crystallization to become the unstable hemihydrate (which will rehydrate when exposed to water). A monoclinic crystal of gypsum cannot become an orthorhombic crystal of anhydrite by simple dehydration. Compare anhydrite.