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Recovery of Heavy Oil and Tar Sand Bitumen
Some cracking of the bitumen also occurs and an upgraded product rather than bitumen itself is
the fluid recovered from the production wells.
The relatively small portion of the oil that remains after the displacement mechanisms have acted
becomes the fuel for the in situ combustion process. Production is obtained from wells offsetting the
injection locations. In some applications, the efficiency of the total in situ combustion operation can
be improved by alternating water and air injection. The injected water tends to improve the utilization of heat by transferring heat from the rock behind the combustion zone to the rock immediately
ahead of the combustion zone.
The performance of in situ combustion is predominantly determined by four factors: (1) the
quantity of oil that initially resides in the rock to be burned, (2) the quantity of air required to burn
the portion of the oil that fuels the process, (3) the distance to which vigorous combustion can be
sustained against heat losses, and (4) the mobility of the air or combustion product gases.
In many field projects, the high gas mobility has limited recovery through its adverse effect on
the sweep efficiency of the burning front. Because of the density contrast between air and reservoir
liquids, the burning front tends to override the reservoir liquids. To date, combustion has been most
effective for the recovery of viscous oils in moderately thick reservoirs in which reservoir dip and
continuity provide effective gravity drainage or operational factors permit close well spacing.
The use of combustion to stimulate oil production is regarded as attractive for deep reservoirs. In
contrast to steam injection, it usually involves no loss of heat. The duration of the combustion may
be less than 30 days or much as 90 days depending on requirements. In addition, backflow of the oil
through the hot zone must be prevented or coking will occur.
Forward combustion involves movement of the hot front in the same direction as the injected
air while reverse combustion involves movement of the hot front opposite to the direction of the
injected air. In forward combustion, the hydrocarbon products released from the zone of combustion move into a relatively cold portion of the formation. Thus, there is a definite upper limit of the
viscosity of the liquids that can be recovered by a forward-combustion process. On the other hand,
since the air passes through the hot formation before reaching the combustion zone, burning is
complete; the formation is left completely cleaned of hydrocarbons.
Forward combustion is particularly applicable to reservoirs containing mobile heavy oil and/or
with a high effective permeability. Even though lower effective reservoir permeability is required
for air injection compared with steam injection, the reservoir ahead of the combustion front is subject to plugging as the vaporized fluids cool and condense. Consequently, a relatively high permeability (400–1000 mD) and relatively low bitumen saturation (45%–65% of pore volume) are most
favorable for this process. The combustion process yields a partially upgraded product because the
temperature gradient ahead of the combustion front mobilizes the lighter hydrocarbon components
that move toward the cooler portion of the reservoir and mix with unheated bitumen. This mixture
is eventually produced through a production well. The heavier components (e.g., coke) are left on the
sand grains and are consumed as fuel for the combustion. Under certain operating conditions, a significant cost saving is attained by injecting oxygen or oxygen-enriched air rather than atmospheric
air because of reduced compression costs and a lower produced gas/oil ratio.
Reverse combustion is particularly applicable to reservoirs with lower effective permeability
(in contrast with forward combustion). It is more effective because the lower permeability would
cause the reservoir to be plugged by the mobilized fluids ahead of a forward combustion front. In the
reverse combustion process, the vaporized and mobilized fluids move through the heated portion
of the reservoir behind the combustion front. The reverse combustion partially cracks the bitumen,
consumes a portion of the bitumen as fuel, and deposits residual coke on the sand grains. In the process, part of the bitumen will be consumed as fuel and part will be deposited on the sand grains as
coke leaving 40%–60% recoverable. This coke deposition serves as a cementing material, reducing
movement and production of sand.
A modified combustion approach has been applied to the Athabasca deposit. The technique
involved a heat-up phase and production (or blowdown phase), followed by a displacement phase
Recovery of Heavy Oil and Tar Sand Bitumen
Some cracking of the bitumen also occurs and an upgraded product rather than bitumen itself is
the fluid recovered from the production wells.
The relatively small portion of the oil that remains after the displacement mechanisms have acted
becomes the fuel for the in situ combustion process. Production is obtained from wells offsetting the
injection locations. In some applications, the efficiency of the total in situ combustion operation can
be improved by alternating water and air injection. The injected water tends to improve the utilization of heat by transferring heat from the rock behind the combustion zone to the rock immediately
ahead of the combustion zone.
The performance of in situ combustion is predominantly determined by four factors: (1) the
quantity of oil that initially resides in the rock to be burned, (2) the quantity of air required to burn
the portion of the oil that fuels the process, (3) the distance to which vigorous combustion can be
sustained against heat losses, and (4) the mobility of the air or combustion product gases.
In many field projects, the high gas mobility has limited recovery through its adverse effect on
the sweep efficiency of the burning front. Because of the density contrast between air and reservoir
liquids, the burning front tends to override the reservoir liquids. To date, combustion has been most
effective for the recovery of viscous oils in moderately thick reservoirs in which reservoir dip and
continuity provide effective gravity drainage or operational factors permit close well spacing.
The use of combustion to stimulate oil production is regarded as attractive for deep reservoirs. In
contrast to steam injection, it usually involves no loss of heat. The duration of the combustion may
be less than 30 days or much as 90 days depending on requirements. In addition, backflow of the oil
through the hot zone must be prevented or coking will occur.
Forward combustion involves movement of the hot front in the same direction as the injected
air while reverse combustion involves movement of the hot front opposite to the direction of the
injected air. In forward combustion, the hydrocarbon products released from the zone of combustion move into a relatively cold portion of the formation. Thus, there is a definite upper limit of the
viscosity of the liquids that can be recovered by a forward-combustion process. On the other hand,
since the air passes through the hot formation before reaching the combustion zone, burning is
complete; the formation is left completely cleaned of hydrocarbons.
Forward combustion is particularly applicable to reservoirs containing mobile heavy oil and/or
with a high effective permeability. Even though lower effective reservoir permeability is required
for air injection compared with steam injection, the reservoir ahead of the combustion front is subject to plugging as the vaporized fluids cool and condense. Consequently, a relatively high permeability (400–1000 mD) and relatively low bitumen saturation (45%–65% of pore volume) are most
favorable for this process. The combustion process yields a partially upgraded product because the
temperature gradient ahead of the combustion front mobilizes the lighter hydrocarbon components
that move toward the cooler portion of the reservoir and mix with unheated bitumen. This mixture
is eventually produced through a production well. The heavier components (e.g., coke) are left on the
sand grains and are consumed as fuel for the combustion. Under certain operating conditions, a significant cost saving is attained by injecting oxygen or oxygen-enriched air rather than atmospheric
air because of reduced compression costs and a lower produced gas/oil ratio.
Reverse combustion is particularly applicable to reservoirs with lower effective permeability
(in contrast with forward combustion). It is more effective because the lower permeability would
cause the reservoir to be plugged by the mobilized fluids ahead of a forward combustion front. In the
reverse combustion process, the vaporized and mobilized fluids move through the heated portion
of the reservoir behind the combustion front. The reverse combustion partially cracks the bitumen,
consumes a portion of the bitumen as fuel, and deposits residual coke on the sand grains. In the process, part of the bitumen will be consumed as fuel and part will be deposited on the sand grains as
coke leaving 40%–60% recoverable. This coke deposition serves as a cementing material, reducing
movement and production of sand.
A modified combustion approach has been applied to the Athabasca deposit. The technique
involved a heat-up phase and production (or blowdown phase), followed by a displacement phase