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184 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

(4) expansion of the compressed oil. The external energy is the pressure and elasticity
of the reservoir edge water and the elastic forces of the rocks. Figure 10.1 demon-
strates that the role of gravity drives declines from left to right. Outside the limits
shown in Fig. 10.1, in the ‘‘critical-state’’ accumulations the role of gravity force is
negligible, and the main active force is elasticity. A prolonged existence of the
‘‘critical-state’’ accumulations is possible only in the case of a strong preliminary
compression of fluids and rocks with the subsequent natural relaxation of stress in all
directions. Most favorable for the occurrence of the ‘‘critical-state’’ accumulations
are bedded (block-faulted) reservoirs. Gravity differentiation is more noticeable in
high column accumulations and at a relatively mild temperature and pressure. At
higher temperature and pressure, the effect of gravity is considerably lower on ap-
proaching the ‘‘critical-state’’ accumulations.

10.3.1. Solution gas drive

In solution gas drive, also called depletion drive, dissolved gas drive, or internal
gas drive, the major source to produce the oil and gas from a pool comes from the
evolution of dissolved gas from the oil with decline in pressure. No initial free gas
cap exists, and the free gas phase formed remains within the oil-producing section.
The reservoir is sealed off to a large extent from communication with contiguous
water zones by faults or permeability pinch-out. As a result, with declining pressure,
the water influx into the reservoir is minor.
Typical solution gas-drive performance is shown in Figs. 10.2 and 10.3. Initially,
there is no free gas phase and the instantaneous producing gas/oil ratio is equal to
the original solution gas/oil ratio. Except in case of undersaturated reservoirs, the

Fig. 10.2. Generalized performance of a solution gas-drive pool (after Torrey, 1961; courtesy of Prentice-
Hall, Inc.)