Oil and Gas Well Drilling and Servicing » Glossary of Terms - G

This is an abridged version of the Dictionary of Petroleum Terms provided by Petex and the University of Texas Austin. © Petex 2001

gamma ray log
n: a type of radioactivity well log that records natural radioactivity around the wellbore. Shales generally produce higher levels of gamma radiation and can be detected and studied with the gamma ray tool. See radioactivity well logging.

gas anchor
n: a tubular, perforated device attached to the bottom of a suckerrod pump that helps to prevent gas lock. The device works on the principle that gas, being lighter than oil, rises. As well fluids enter the anchor, gas breaks out of the fluid and exits from the anchor through perforations near the top. Remaining fluids enter the pump through a mosquito bill (a tube within the anchor), which has an opening near the bottom. In this way, all or most of the gas escapes before the fluids enter the pump.

gas cap
n: a free-gas phase overlying an oil zone and occurring within the same producing formation as the oil. See reservoir.

gas-cap drive
n: drive energy supplied naturally (as a reservoir is produced) by the expansion of the gas cap. In such a drive, the gas cap expands to force oil into the well and to the surface. See reservoir drive mechanism.

gas-cut mud
n: a drilling mud that contains entrained formation gas, giving the mud a characteristically fluffy texture. Gas cut mud may cause a lowering of mud weight.

gas drive
n: the use of the energy that arises from the expansion of compressed gas in a reservoir to move crude oil to a wellbore. Also called depletion drive. See dissolved-gas drive, gas-cap drive, reservoir drive mechanism.

gas injection
n: the injection of gas into a reservoir to maintain formation pressure by gas drive and to reduce the rate of decline of the original reservoir drive. One type of gas injection uses gas that does not mix (is not miscible) with the oil. Examples of these gases include natural gas, nitrogen, and flue gas. Another type uses gas that does mix (is miscible) with the oil. The gas may be naturally miscible or become miscible under high pressure. Examples of miscible gases include propane, methane enriched with other light hydrocarbons, methane under high pressure, and carbon dioxide under pressure. Frequently, water is also injected in alternating steps with the gas.

gas injection well
n: a well into which gas is injected for the purpose of maintaining or supplementing pressure in an oil reservoir.

n: any material (such as paper, cork, asbestos, stainless steel or other types of metal, or rubber) used to seal two essentially stationary surfaces.

gas lift
n: 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 the formation does; the resulting 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 mandrel
n: a device installed in the tubing string of a gas-lift well onto which or into which a gas-lift valve is fitted. There are two common types of mandrel. In the conventional gas-lift mandrel, the gas-lift valve is installed as the tubing is placed in the well. Thus, to replace or repair the valve, the tubing string must be pulled. In the sidepocket mandrel, however, the valve is installed and removed by wireline while the mandrel is still in the well, eliminating the need to pull the tubing to repair or replace the valve.

gas-lift valve
n: 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. See gas-lift mandrel.

gas-lift well
n: a well in which reservoir fluids are artificially lifted by the injection of gas.

gas lock
n: 1. 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. If the gas pressure is sufficient, the standing valve is locked shut, and no fluid enters the tubing. 2. a device fitted to the gauging hatch on a pressure tank that enables manual dipping and sampling without loss of vapor. 3. a condition that can occur when gas-cut mud is circulated by the mud pump. The gas breaks out of the mud, expands, and works against the operation of the piston and valves.

gas well
n: a well that primarily produces gas. Legal definitions vary among the states.

gear reducer
n: see speed reducer.

n: a semisolid, jellylike state assumed by some colloidal dispersions at rest.

n: a scientist who gathers and interprets data pertaining to the formations of the earth’s crust.

n: see safety slide.

go in the hole
v: to lower the drill stem, the tubing, the casing, or the sucker rods into the wellbore.

gone to water
adj: pertaining to a well in which production of oil has decreased and production of water has increased (for example, “the well has gone to water”).

n: the curved connection between the rotary hose and the swivel. See swivel.

n: sand or glass beads of uniform size and roundness used in gravel packing.

gravel packing
n: a method of well completion in which a slotted or perforated liner, often wire-wrapped, is placed in the well and surrounded by gravel. If open hole, the well is sometimes enlarged by underreaming at the point where the gravel is packed. The mass of gravel excludes sand from the wellbore but allows continued production.

guide shoe
n: 1. a short, heavy, cylindrical section of steel filled with concrete and rounded at the bottom, which is placed at the end of the casing string. It prevents the casing from snagging on irregularities in the borehole as it is lowered.

guy line anchor
n: a buried weight or anchor to which a guy line is attached.

guy wire
n: a rope or cable used to steady a mast or pole.