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Exploration, Recovery, and Transportation
to producing wells. If the pressure on the fluid in the reservoir (reservoir energy) is great enough, the
oil flows into the well and up to the surface. Such driving energy may be derived from liquid expansion and evolution of dissolved gases from the oil as reservoir pressure is lowered during production,
expansion of free gas, or a gas cap, influx of natural water, gravity, or combinations of these effects.
Crude oil moves out of the reservoir into the well by one or more of three processes. These processes are: dissolved gas drive, gas cap drive, and water drive. Early recognition of the type of drive
involved is essential to the efficient development of an oil field.
In dissolved gas drive, the propulsive force is the gas in solution in the oil, which tends to come
out of solution because of the pressure release at the point of penetration of a well. Dissolved gas
drive is the least efficient type of natural drive as it is difficult to control the gas–oil ratio; the
bottom-hole pressure drops rapidly, 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 (gas cap
drive) 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. If, however, 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%–50%.
Usually the gas in a gas cap (associated natural gas) contains methane and other hydrocarbons
that may be separated out by compressing the gas. A well-known 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 oil-water 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. The recovery of petroleum from the
reservoir in properly operated water drive pools may run as high as 80%. The force behind the water
drive may be hydrostatic pressure, the expansion of the reservoir water, or a combination of both.
Water drive is also used in certain submarine fields.
Gravity drive is an important factor when oil columns of several thousands of feet exist, as they
do in some North American fields. Furthermore, the last bit of recoverable oil is produced in many
pools by gravity drainage of the reservoir. Another source of energy during the early stages of withdrawal from a reservoir containing under-saturated oil is the expansion of that oil as the pressure
reduction brings the oil to the bubble point (the pressure and temperature at which the gas starts to
come out of solution).
For primary recovery operations, no pumping equipment is required. If the reservoir energy is
not sufficient to force the oil to the surface, then the well must be pumped. In either case, nothing is
added to the reservoir to increase or maintain the reservoir energy or to sweep the oil toward the well.
Exploration, Recovery, and Transportation
to producing wells. If the pressure on the fluid in the reservoir (reservoir energy) is great enough, the
oil flows into the well and up to the surface. Such driving energy may be derived from liquid expansion and evolution of dissolved gases from the oil as reservoir pressure is lowered during production,
expansion of free gas, or a gas cap, influx of natural water, gravity, or combinations of these effects.
Crude oil moves out of the reservoir into the well by one or more of three processes. These processes are: dissolved gas drive, gas cap drive, and water drive. Early recognition of the type of drive
involved is essential to the efficient development of an oil field.
In dissolved gas drive, the propulsive force is the gas in solution in the oil, which tends to come
out of solution because of the pressure release at the point of penetration of a well. Dissolved gas
drive is the least efficient type of natural drive as it is difficult to control the gas–oil ratio; the
bottom-hole pressure drops rapidly, 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 (gas cap
drive) 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. If, however, 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%–50%.
Usually the gas in a gas cap (associated natural gas) contains methane and other hydrocarbons
that may be separated out by compressing the gas. A well-known 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 oil-water 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. The recovery of petroleum from the
reservoir in properly operated water drive pools may run as high as 80%. The force behind the water
drive may be hydrostatic pressure, the expansion of the reservoir water, or a combination of both.
Water drive is also used in certain submarine fields.
Gravity drive is an important factor when oil columns of several thousands of feet exist, as they
do in some North American fields. Furthermore, the last bit of recoverable oil is produced in many
pools by gravity drainage of the reservoir. Another source of energy during the early stages of withdrawal from a reservoir containing under-saturated oil is the expansion of that oil as the pressure
reduction brings the oil to the bubble point (the pressure and temperature at which the gas starts to
come out of solution).
For primary recovery operations, no pumping equipment is required. If the reservoir energy is
not sufficient to force the oil to the surface, then the well must be pumped. In either case, nothing is
added to the reservoir to increase or maintain the reservoir energy or to sweep the oil toward the well.