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In the case of a water drive in its pure form, oil withdrawn
from the reservoir is displaced, volume for volume, by
encroaching water. The perimeter of the oil drainage boundary
then continuously shifts and contracts. During the exploitation of a
water drive reservoir, there is first observed a certain decline in
reservoir pressure establishing a pressure differential which causes
the encroachment of water on the productive zone. The
stabilization of reservoir pressure with time in the case of a steady
rate of oil production proves that water drive operates with
complete replacement of the withdrawn oil by water. However, if
the rate of oil withdrawals from the reservoir continually increases,
a time may come when at the given pressure, through the capacity
of the water drive system becomes inadequate and the volume of
water entering the reservoir is less than the volume of oil
withdrawals. In this case, reservoir pressure begins to decline and
the water drive mechanism may be replaced by the solution gas
drive. As a result of the slow decline in reservoir pressure in water
drive reservoirs production from wells remains steady for a long
time. The gas factor too usually remains constant until the pressure
at bottom holes drops below saturation pressure.
In gas cap drive reservoirs the process of displacement of
oil by the expanding gas is usually accompanied by gravity effects:
the oil flows by gravity to the lowest parts of the reservoir and the
gas released from solution rises to the upper zone and replenishes
the gas cap. These effects are the more pronounced the greater are
the angle of dip of the strata, the permeability of reservoir rock,
and the lower the rate of liquid withdrawals, i.e., the rate of
filtration. The replenishment of the expanding gas cap by the gas
evolving from solution slows down the rate of decline of reservoir
pressure. Gravitation segregation of oil and gas in gas cap
reservoirs is also largely responsible for the fact that the gas-oil
ratio of wells remote from the gas zone of the reservoir can remain
low for a long time. In wells which are close to the gas-oil contact,
the gas-oil ratio usually increases rapidly, so that ultimately the

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