Page 256 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Mechanisms & Recovery of Hydrocarbons by Natural Means 445

MECHANISMS & RECOVERY OF HYDROCARBONS BY NATURAL MEANS

Petroleum Reservoir Definltlons [17]
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 hydro-
carbons 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.

Natural Gas Reservoirs [17]
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.
Primary Recovery of Crude Oil

Initial crude oil production often takes place by the expansion of fluids which
were trapped under pressure in the rock. The expanding fluids may be gas
evolving from the oil, an expanding gas cap, a bottom- or edge-water drive, or
a combination of these mechanisms. After the initial pressure in the reservoir
falls to a low value, the oil no longer flows to the wellbore, and pumps are
installed to lift the crude oil to the surface. This mode of oil production is
referred to as primary production. Recovery of oil associated with natural