aquifer (water) drive: any process by which energy for the production of oil is derived principally from the pressure of water in the formation. A water-drive reservoir is one from which the oil or gas is wholly or partly expelled by pressure due to encroaching water. The water may have been in the reservoir initially, in which case the drive is natural, or it may have been introduced artificially.

aquifer encroachment: the movement of bottom or edge aquifer water into a petroleum reservoir as oil and gas are produced.

bubble-point pressure: the pressure at which gas, held in solution in crude oil, breaks out of solution as free gas. Syn: saturation pressure.

coning: the localized encroachment of water into the oil or gas column near and possibly into the well bore because of excessive drawdown.

dimensionless radii: the radii used in the unsteady-state water-influx theory and are the result of division of the aquifer radius by the outer oil-reservoir radius.

drawdown: (a) The lowering of the water level in a well as a result of withdrawal. (b) The difference between the height of the static (undisturbed) water table and that of the water level in a pumped well. (c) Reduction of the pressure head as a result of the withdrawal of water from a well. (d) The amount that the level of a reservoir, water level in a well, or head in an aquifer is lowered by the withdrawal of water; cf: cone of depression. (e) The pressure drop between the well bore and the reservoir.

edge water: the water around the margins of an oil or gas reservoir.

four-dimensional seismic: a time-lapse seismic survey of three-dimensional data, the fourth dimension being lapsed time; abbr.: 4-D.

gas-cap expansion (drive): the force exerted by energy released from the expanding gas of a gas cap; used to produce oil from the reservoir.

gas-material-balance plots: X-Y graphs consisting of the X axis being cumulative gas production and the Y axis being reservoir pressure divided by the gas compressibility factor. The graphs are used to estimate original gas in place under pressure-depletion conditions and can be used to interpret reservoir and production characteristics.

genetic units: sedimentary bodies representing complex genetically related facies formed in the same environment and mostly as the effect of a single leading process; e.g., alluvial, deltaic, lagoonal, or marine deposits.

gas:oil ratio (GOR): the quantity of gas produced with oil from an oil well, usually expressed as the number of cubic feet of gas per barrel of oil.

OGIP: abbreviation for original gas in place.

production time-series analysis: one that consists of graphing and mapping of oil, gas, water cut, fluid levels, and pressure-depletion variation over the producing life of a reservoir. Afterward, fluid-flow trends within the reservoir are established from a set of production-performance maps that illustrate initial potential, cumulative and current production, gas:oil ratios (GOR), fluid levels, water cut, and pressure depletion on a per-well basis. Anomalies in these maps can indicate barriers to fluid flow, which may also indicate reservoir compartmentalization.

solution-gas drive: in a solution-gas drive reservoir (known also as dissolved-gas drive), the lighter hydrocarbon components that are dissolved in the oil before it is produced come out in the form of gas as the reservoir pressure is depleted. The solution gas coming out of the oil expands to force the oil into the well bore.

spinner test: a production-logging method that uses a small propeller turned by fluid movement. According to a recording arrangement, the number of turns of the propeller can be related to fluid quantity flowing past the instrument to obtain a production log.

sweep efficiency: the efficiency at which water moves in and replaces oil as it is drawn from an aquifer or injection well.

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