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The Chemistry and Technology of Petroleum
using a combined fire flood and waterflood (COFCAW) process. In this manner, over a total
18-month period (heat-up: 8 months; blowdown: 4 months; displacement: 6 months), 29,000 bbl
of upgraded oil was produced from an estimated 90,000 bbl of oil in place.
The addition of water or steam to an in situ combustion process can result in a significant increase
in the overall efficiency of that process. Two major benefits may be derived. Heat transfer in the
reservoir is improved because the steam and condensate have greater heat-carrying capacity than
combustion gases and gaseous hydrocarbons. Sweep efficiency may also be improved because of
the more favorable mobility ratio of steam-bitumen compared with gas-bitumen.
Modes of application include injection of alternate slugs of air (oxygen) and water or co-injection
of air (oxygen) and steam. Again, the combination of air (oxygen) injection and steam or water injection increases injectivity costs that may be justified by increased bitumen recovery.
Process efficiency is affected by reservoir heterogeneity that will reduce horizontal sweep. The
underburden and overburden must provide effective seals to avoid loss of injected air and produced
bitumen. Process efficiency is enhanced by the presence of some interstitial water saturation. The
water is vaporized by the combustion and enhances the heat transfer by convection. The combustion
processes are subject to override because of differences in the densities of injected and reservoir
fluids. Production wells should be monitored for, and equipped to cool, excessively high temperatures (>1095°C, >2000°F) that may damage down-hole production tools and tubulars.
Applying a preheating phase before the bitumen recovery phase may significantly enhance the
steam or combustion extraction processes. Preheating can be particularly beneficial if the saturation
of highly viscous bitumen is sufficiently great as to lower the effective permeability to the point of
production being precluded by reservoir plugging. Preheating partially mobilizes the bitumen by
raising its temperature and lowering its viscosity. The result is a lower required pressure to inject
steam or air and move the bitumen.
Preheating may be accomplished by several methods. Conducting a reverse combustion phase
in a zone of relatively high effective permeability and low bitumen saturation is one method. Steam
or hot gases may be rapidly injected into a high-permeability zone in the lower portion of the reservoir. In the fracture-assisted steam technology (FAST) process, steam is injected rapidly into an
induced horizontal fracture near the bottom of the reservoir to preheat the reservoir. This process
has been applied successfully in three pilot projects in southwest Texas. Shell has accomplished the
same preheating goal by injecting steam into a high-permeability bottom-water zone in the Peace
River (Alberta) field. Electrical heating of the reservoir by radio-frequency waves may also be an
effective method.
Using combustion to stimulate oil production is regarded as attractive for deep reservoirs and, in
contrast to steam injection, usually involves no loss of heat. The duration of the combustion may be
less than 30 days, or approximately 90 days, depending upon requirements. In addition, backflow of
the oil through the hot zone must be prevented or coking occurs.
Using combustion to stimulate bitumen production is regarded as being attractive for deep reservoirs and, in contrast to steam injection usually involves no loss of heat. The duration of the combustion may be short (days) depending upon requirements. In addition, backflow of the oil through the
hot zone must be prevented or coking will occur. A variation of the combustion process involves use
of a heat-up phase, then a blowdown (production) phase, followed by a displacement phase using a
fire–water flood (a combination of forward combustion and waterflood, COFCAW).
Finally, by all definitions, the quality of the bitumen from tar sand deposits is poor as a refinery feedstock. As in any field in which primary recovery operations are followed by secondary or
enhanced recovery operations and there is a change in product quality, such is also the case for tar
sand recovery operations. Thus, product oils recovered by the thermal stimulation of tar sand deposits show some improvement in properties over those of the bitumen in place.
Although this improvement in properties may not appear to be too drastic, nevertheless, it usually is sufficient to have major advantages for refinery operators. Any incremental increase in the
units of hydrogen/carbon ratio can save amounts of costly hydrogen during upgrading. The same
The Chemistry and Technology of Petroleum
using a combined fire flood and waterflood (COFCAW) process. In this manner, over a total
18-month period (heat-up: 8 months; blowdown: 4 months; displacement: 6 months), 29,000 bbl
of upgraded oil was produced from an estimated 90,000 bbl of oil in place.
The addition of water or steam to an in situ combustion process can result in a significant increase
in the overall efficiency of that process. Two major benefits may be derived. Heat transfer in the
reservoir is improved because the steam and condensate have greater heat-carrying capacity than
combustion gases and gaseous hydrocarbons. Sweep efficiency may also be improved because of
the more favorable mobility ratio of steam-bitumen compared with gas-bitumen.
Modes of application include injection of alternate slugs of air (oxygen) and water or co-injection
of air (oxygen) and steam. Again, the combination of air (oxygen) injection and steam or water injection increases injectivity costs that may be justified by increased bitumen recovery.
Process efficiency is affected by reservoir heterogeneity that will reduce horizontal sweep. The
underburden and overburden must provide effective seals to avoid loss of injected air and produced
bitumen. Process efficiency is enhanced by the presence of some interstitial water saturation. The
water is vaporized by the combustion and enhances the heat transfer by convection. The combustion
processes are subject to override because of differences in the densities of injected and reservoir
fluids. Production wells should be monitored for, and equipped to cool, excessively high temperatures (>1095°C, >2000°F) that may damage down-hole production tools and tubulars.
Applying a preheating phase before the bitumen recovery phase may significantly enhance the
steam or combustion extraction processes. Preheating can be particularly beneficial if the saturation
of highly viscous bitumen is sufficiently great as to lower the effective permeability to the point of
production being precluded by reservoir plugging. Preheating partially mobilizes the bitumen by
raising its temperature and lowering its viscosity. The result is a lower required pressure to inject
steam or air and move the bitumen.
Preheating may be accomplished by several methods. Conducting a reverse combustion phase
in a zone of relatively high effective permeability and low bitumen saturation is one method. Steam
or hot gases may be rapidly injected into a high-permeability zone in the lower portion of the reservoir. In the fracture-assisted steam technology (FAST) process, steam is injected rapidly into an
induced horizontal fracture near the bottom of the reservoir to preheat the reservoir. This process
has been applied successfully in three pilot projects in southwest Texas. Shell has accomplished the
same preheating goal by injecting steam into a high-permeability bottom-water zone in the Peace
River (Alberta) field. Electrical heating of the reservoir by radio-frequency waves may also be an
effective method.
Using combustion to stimulate oil production is regarded as attractive for deep reservoirs and, in
contrast to steam injection, usually involves no loss of heat. The duration of the combustion may be
less than 30 days, or approximately 90 days, depending upon requirements. In addition, backflow of
the oil through the hot zone must be prevented or coking occurs.
Using combustion to stimulate bitumen production is regarded as being attractive for deep reservoirs and, in contrast to steam injection usually involves no loss of heat. The duration of the combustion may be short (days) depending upon requirements. In addition, backflow of the oil through the
hot zone must be prevented or coking will occur. A variation of the combustion process involves use
of a heat-up phase, then a blowdown (production) phase, followed by a displacement phase using a
fire–water flood (a combination of forward combustion and waterflood, COFCAW).
Finally, by all definitions, the quality of the bitumen from tar sand deposits is poor as a refinery feedstock. As in any field in which primary recovery operations are followed by secondary or
enhanced recovery operations and there is a change in product quality, such is also the case for tar
sand recovery operations. Thus, product oils recovered by the thermal stimulation of tar sand deposits show some improvement in properties over those of the bitumen in place.
Although this improvement in properties may not appear to be too drastic, nevertheless, it usually is sufficient to have major advantages for refinery operators. Any incremental increase in the
units of hydrogen/carbon ratio can save amounts of costly hydrogen during upgrading. The same