78 PETROLEUM TECHNOLOGY, ECONOMICS, AND POLITICS
and the total eventual recovery of petroleum from the reservoir
may be less than 20%.
If gas overlies the oil beneath the top of the trap, it is compressed
and can be utilized to drive the oil into wells situated at the bottom
of the oil-bearing zone. By producing oil only from below the gas
cap, it is possible to maintain a high gas-oil ratio in the reservoir
until almost the very end of the life of the pool. However, if the oil
deposit is not systematically developed so that bypassing of the gas
occurs, an undue proportion of oil is left behind. The usually recovery of petroleum from a reservoir in a gas cap field is 40 to 50%.
Usually the gas in a gas cap contains methane and other hydrocarbons that may be separated out by compressing the gas. A wellknown example is natural gasoline that was formerly referred to as
casinghead gasoline or natural gas gasoline. However, at high pressures, such as those existing in the deeper fields, the density of the
gas increases and the density of the oil decreases until they form a
single phase in the reservoir. These are the so-called retrograde condensate pools because a decrease (instead of an increase) in pressure brings about condensation of the liquid hydrocarbons. When
this reservoir fluid is brought to the surface and the condensate is
removed, a large volume of residual gas remains. The modern practice is to cycle this gas by compressing it and inject it back into the
reservoir, thus maintaining adequate pressure within the gas cap,
and condensation in the reservoir is prevented. Such condensation
prevents recovery of the oil, for the low percentage of liquid saturation in the reservoir precludes effective flow.
The most efficient propulsive force in driving oil into a well is
natural water drive, in which the pressure of the water forces the
lighter recoverable oil out of the reservoir into the producing wells.
In anticlinal accumulations, the structurally lowest wells around
the flanks of the dome are the first to come into water. Then the oilwater contact plane moves upward until only the wells at the top
of the anticline are still producing oil; eventually these also must be
abandoned as the water displaces the oil.
In a water drive field, it is essential that the removal rate be
adjusted so that the water moves up evenly as space is made available for it by the removal of the hydrocarbons. An appreciable
decline in bottom-hole pressure is necessary to provide the pressure gradient required to cause water influx. The pressure differential needed depends on the reservoir permeability; the greater
the permeability, the less the difference in pressure necessary.
and the total eventual recovery of petroleum from the reservoir
may be less than 20%.
If gas overlies the oil beneath the top of the trap, it is compressed
and can be utilized to drive the oil into wells situated at the bottom
of the oil-bearing zone. By producing oil only from below the gas
cap, it is possible to maintain a high gas-oil ratio in the reservoir
until almost the very end of the life of the pool. However, if the oil
deposit is not systematically developed so that bypassing of the gas
occurs, an undue proportion of oil is left behind. The usually recovery of petroleum from a reservoir in a gas cap field is 40 to 50%.
Usually the gas in a gas cap contains methane and other hydrocarbons that may be separated out by compressing the gas. A wellknown example is natural gasoline that was formerly referred to as
casinghead gasoline or natural gas gasoline. However, at high pressures, such as those existing in the deeper fields, the density of the
gas increases and the density of the oil decreases until they form a
single phase in the reservoir. These are the so-called retrograde condensate pools because a decrease (instead of an increase) in pressure brings about condensation of the liquid hydrocarbons. When
this reservoir fluid is brought to the surface and the condensate is
removed, a large volume of residual gas remains. The modern practice is to cycle this gas by compressing it and inject it back into the
reservoir, thus maintaining adequate pressure within the gas cap,
and condensation in the reservoir is prevented. Such condensation
prevents recovery of the oil, for the low percentage of liquid saturation in the reservoir precludes effective flow.
The most efficient propulsive force in driving oil into a well is
natural water drive, in which the pressure of the water forces the
lighter recoverable oil out of the reservoir into the producing wells.
In anticlinal accumulations, the structurally lowest wells around
the flanks of the dome are the first to come into water. Then the oilwater contact plane moves upward until only the wells at the top
of the anticline are still producing oil; eventually these also must be
abandoned as the water displaces the oil.
In a water drive field, it is essential that the removal rate be
adjusted so that the water moves up evenly as space is made available for it by the removal of the hydrocarbons. An appreciable
decline in bottom-hole pressure is necessary to provide the pressure gradient required to cause water influx. The pressure differential needed depends on the reservoir permeability; the greater
the permeability, the less the difference in pressure necessary.