Page 218 - Geology and Geochemistry of Oil and Gas

CLASSIFICATION OF OIL AND GAS RESERVOIRS BASED ON DRIVE MECHANISM 187
10.3.2. Gas-cap drive

Oil pools with free gas caps are subject to a gas drive, which is external to the oil
zone and separate from the solution gas-drive mechanism. The oil expulsion mech-
anism is typically a combination of solution gas drive within the oil column plus the
added benefit of gas permeating and diffusing into the oil zone from the gas cap. The
idealized performance of a gas-cap-drive reservoir is presented in Fig. 10.4.
The decline in production rate and reservoir pressure is not as rapid as in solution
gas-drive reservoir. The gas/oil ratio performance is more favorable. Gas-cap-
drive reservoirs, however, are more sensitive to production rate than are solution
gas-drive pools.
Wells producing from intervals close to the gas cap must be produced at low rates
to prevent gas coning or recompleted to exclude these upper intervals. The overall
gas/oil ratio performance largely reflects such procedures. The performance of the
Goldsmith San Andres Dolomite Pool in West Texas (Fig. 10.5) early in its history
typifies gas-drive performance with a gradual increase in gas/oil ratio. The oil pro-
duction is curtailed, and no decline is evident.
Some gravity segregation of oil and gas takes place in virtually every gas-cap-drive
reservoir. More pronounced fluid segregation will promote the expansion of the gas
cap and downdip movement of the oil, with resultant higher oil recoveries. The size
of the gas cap will also affect the oil recovery. Normally, the thicker the gas cap, the
greater is the ultimate recovery. Notable exceptions are carbonate pools in the
Acheson–Homeglen–Rimbey reef trend, Alberta, Canada, which have large gas caps
underlain by thin oil bands. The estimated ultimate oil recoveries under primary
production are often very low (5–10%) owing to excessive gas and water coning

Fig. 10.4. Idealized performance of a gas-cap-drive reservoir (after Torrey, 1961; courtesy of Prentice-
Hall, Inc.)