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CLASSIFICATION OF OIL AND GAS RESERVOIRS BASED ON DRIVE MECHANISM 183
Self-regulation of the temperature and pressure in an accumulation is one of its
most remarkable features. Similar phenomena occur in the upper portion of the
accumulation due to the surplus pressure (overpressure). It is especially true for the
gas accumulations. As a rule, some increase in the temperature and the heat flow is
recorded in the accumulations. This can be explained by the heat anisotropy of rocks
over the anticlines.

10.3. CLASSIFICATION OF OIL AND GAS RESERVOIRS BASED ON DRIVE MECHANISM

The classification of oil and gas reservoirs by producing mechanism 17 is imper-
ative for a careful study of the technology of oil and gas recovery.
The potential energy sources available to move the oil and gas to the wellbore
include the following:
(1) gravitational energy of the oil, acting over the vertical distance of the productive
column,
(2) energy of compression of the free gas in the gas cap or within the oil-producing
section,
(3) energy of compression of the solution gas dissolved in the oil or the water,
(4) energy of compression of the oil and water in the producing section of the
reservoir,
(5) energy of compression of the waters peripheral to the production zone,
(6) energy of capillary pressure effects,
(7) energy of the compression of the rock itself.
All these forces are active during the productive life of a reservoir.
The predominant producing mechanism operating to produce the oil and gas
reflects the relative influence each energy sources has on reservoir behavior. The
major drive mechanisms are (1) solution-gas drive, (2) gas-cap expansion drive, (3)
water drive, and (4) gravity drainage. Each drive mechanism when effective in a pool
will give rise to a certain characteristic form of reservoir behavior. Although in
practice most pools behave in a manner that represents a combination (‘‘mix’’) of
two or more drive mechanisms, each mechanism will be described in the context of a
single-drive pool. The common characteristics of each drive mechanism can be dis-
cussed for clastic reservoirs having intergranular porosity and then placed in the
context of the observed behavior of carbonate reservoirs with other types of po-
rosity. The interest is centered on the record of performance: variation of oil, gas,
and water production rates, gas/oil and water/oil ratios, and reservoir pressure with
time. Movement of the water–oil contact and creation or expansion of a free gas cap
is also of great importance.
Natural drive mechanisms may be subdivided into the ones caused by the energy
intrinsic in the reservoir and by the energy external to the reservoir. The internal
energy is supplied by (1) gravity force, (2) pressure of free gas, (3) solution gas, and

17
Originally prepared by H. H. Rieke, R. W. Mannon, G. V. Chilingar, and G. L. Langnes, in: Chilingar,
Mannon and Rieke, 1972. Also see Chilingarian et al. (1996, pp. 243–254).