Petroleum Reservoir Definltlons

Accumulations of oil and gas occur in underground traps that are formed by structural and/or stratigraphic features. A reservoir is the portion of the trap that contains the oil and/or gas in a hydraulically connected system. Many reservoirs are hydraulically connected to water-bearing rocks or aquifers that provide a source of natural energy to aid in hydrocarbon recovery. Oil and gas may be recovered by: fluid expansion, fluid displacement, gravitational drainage, and/or capillary expulsion.

In the case of a reservoir with no aquifer (which is referred to as a volumetric reservoir), hydrocarbon recovery occurs primarily by fluid expansion, which, in the case of oil, may be aided by gravity drainage. If there is water influx or encroachment from the aquifer, recovery occurs mainly by the fluid displacement mechanism which may be aided by gravity drainage or capillary expulsion. In many instances, recovery of hydrocarbon occurs by more than one mechanism. At initial conditions, hydrocarbon fluids in a reservoir may exist as a single phase or as two phases.

The single phase may be a gas phase or a liquid phase in which all of the gas present is dissolved in the oil. When there are hydrocarbons vaporized in the gas phase which are recoverable as liquids at the surface, the reservoir is called gas-condensate, and the produced liquids are referred to as condensates or distillates. For two-phase accumulations, the vapor phase is termed the gas cap and the underlying liquid phase is called the oil zone. In the two-phase ease, recovery of hydrocarbons includes the free gas in the gas cap, gas evolving from the oil (dissolved gas), recoverable liquid from the gas cap, and crude oil from the oil zone. If an aquifer or region of high water saturation is present, a transition zone can exist in which the water saturation can vary as a function of vertical depth and formation permeability.

Water that exists in the oil- or gas-bearing portion of the reservoir above the transition zone is called connate or interstitial water. All of these factors are important in the evaluation of the hydrocarbon reserves and recovery efficiency.

For reservoirs where the fluid at all pressures in the reservoir or on the surface is a single gaseous phase, estimates of reserves and recoveries are relatively simple. However, many gas reservoirs produce some hydrocarbon liquid or condensate. In the latter case, recovery calculations for the single-phase case can be modified to include the condensate if the reservoir fluid remains in a single phase at all pressures encountered. However, if the hydrocarbon liquid phase develops in the reservoir, additional methods are necessary to handle these retrograde, gas-condensate reservoirs.