Page 27 - Fundamentals of Enhanced Oil and Gas Recovery

An Introduction to Enhanced Oil Recovery 15

According to the low compressibility of oil and rock reservoir, the pressure drop is
comparatively rapid, and this leads to the minimum efficiency in this mechanism in
comparison to the rest of drive mechanisms.

1.10.2 Solution Gas Drive
This mechanism is active in a saturated oil reservoir, where the reservoir pressure is
below bubble point pressure. As the pressure decreases, the gas bubbles are liberated
from the oil and provide the needed force for oil production. The liberated gas sup-
ports reservoir pressure by gas bubble expansion and may help oil movement by oil
viscosity reduction. Lack of external fluid drive such as gas cap or water influx leads
to a relatively high reservoir pressure drop in this drive mechanism. As the reservoir
pressure drops, the liberated gas may move vertically and create a secondary gas cap.
This phenomenon definitely depends on the vertical permeability. Forming the sec-
ondary gas cap may significantly reduce reservoir pressure. The ultimate oil recovery
for solution gas drive varies from 5% to 30%.

1.10.3 Gas Cap Drive
In the saturated oil reservoir with a primary gas cap, the dominant drive mechanism is
gas cap drive. As the oil pressure decreases, gas expands and fills the extracted pore
volume. By gas expansion, stored energy in gas is evolved and the gas oil contact
comes down; therefore to avoid gas production from the cap, most wells are drilled in
oil zones. High compressibility of gas causes a slow pressure drop in the reservoir. The
level of pressure maintenance is relatively higher than the two abovementioned
mechanisms. Ultimate oil recovery of the gas cap drive varies from 20% to 40% of
original OIP.

1.10.4 Water Drive
The required energy for the water drive mechanism is provided by a bounded aqui-
fer. By oil production the water oil contact comes up, and water replaces the oil
withdrawals. Size of the aquifers varies from so small “which have negligible effects
on the reservoir performance” to so large “which act as an infinite source in com-
parison to the reservoir size.” According to the shape and structure of water
sources, the water influxes into the reservoir are divided into edge water and bot-
tom water. For a typical water drive reservoir the pressure decline is generally grad-
ual. Many reservoirs exist all over the world that have one psi pressure drop per
million barrel of oil. The ultimate oil recovery for water drive systems ranges from
30% to 70% of original OIP.