G
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gamma ray |
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. |
gas cap |
A free-gas phase overlying an oil zone and occurring within the same reservoir as the oil. |
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. |
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 |
Hydrocarbon liquid dissolved in saturated natural gas that comes out of solution when the pressure drops below the dewpoint. |
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 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 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. |
gas in solution |
Gas dissolved in water or crude oil. |
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-oil contact |
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. |
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. |
gauge hole |
A borehole with a diameter the same size as the drill bit |
geophone |
A device used in surface seismic acquisition, 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. |
geopressure |
Undisturbed formation pressure. The pore pressure existing within a body of interest prior to disturbance. |
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). |
geothermal |
Pertaining to temperatures in the earth, ground, or crust of the earth |
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. |
graben |
A downthrown block between 2 parallel faults |
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. |
gravel pack |
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. |
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 drainage is an important source of energy to produce oil. It is also called segregation drive. |
gypsum |
An evaporite mineral,CaSO4ˇ2H2O, found in clays and limestones; sometimes associated with sulphur. |
