Page 219 - Geology and Geochemistry of Oil and Gas

188 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

Fig. 10.5. Typical gas-cap-drive performance, Goldsmith San Andres Dolomite Pool in West Texas (after
Craze, 1950; courtesy of AIME).

problems. A comparison of the performance of these pools indicates that in the case
of thicker oil bands and correspondingly thinner gas caps the oil recoveries are
higher.

10.3.3. Water drive

A reservoir of high permeability, such as the fissured or cavernous limestone, in
contact with an aquifer of broad areal extent will normally have an active water
drive. The degree to which the reservoir withdrawals are replaced by water deter-
mines the efficiency of the water-drive mechanism. In complete water-drive systems,
which are not common, substantially all the fluid withdrawals are replaced by in-
truding water. Some excellent examples of complete water-drive pools in carbonate
rocks are the Arbuckle Limestone fields in Arkansas and Kansas.
If the reservoir is initially undersaturated, the natural maintenance of the pressure
by water influx may result in oil production above the bubble-point pressure for an
extended period. During this time a small portion of the reservoir voidage resulting
from production (withdrawals) is replaced by expanding oil. Later in the life of the
reservoir a free gas phase may form, which provide part of the energy for the oil
expulsion. The existence of the free gas will depend largely on the rate of withdrawal
of fluids.
In all types of water-drive reservoirs, including complete water-drive systems, an
initial pressure decline provides the necessary pressure differential at the reservoir
boundary to include water movement into the reservoir. This initial rapid decline
preceding water influx is illustrated in general by Fig. 10.6. The Schuler (Reynolds),
Magnolia, Buckner, and Midway fields are Smackover Limestone pools, and the