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The Chemistry and Technology of Petroleum
are broken by heat; essentially the vibrational energy exceeds the chemical energy in the carbon–
carbon bonds. Pyrolysis occurs in the absence of oxygen or a catalyst, but steam may be present.
For example, steam cracking and thermal cracking are done in refineries at temperatures at or above
800°C (1470°F). Such high temperatures are difficult to achieve in the reservoir. Pyrolysis can still
occur at lower temperatures, but at much, much slower rates.
For example, heavy oil produced under primary and fire flood conditions showed a gradual
increase in density, viscosity, and other properties over time. This indicates fractionation during
flow through the formation. There was also a decrease in viscosity, density, and other properties as
a result of the products of thermal cracking of the oil and the formation of smaller straight-chain
alkanes and small aromatics molecules (Reichert et al., 1989).
Thus, in situ upgrading is also an additional benefit to the combustion recovery process. As the
combustion front moves through the reservoir, the lower boiling constituents of the heavy oil are
vaporized, thermal cracking occurs, and the coke product is consumed as fuel. The resulting oil has
a much lower viscosity than the original bitumen.
It is also conceivable that under the appropriate conditions (as yet undefined), electric heating
(resistance, induction, or radio frequency) should also be able to achieve the high temperatures
required for in situ upgrading (Cunha, 2005).
7.4.2.3 Partial Combustion
The mobilization of heavy oil in the reservoir by partial combustion is not a new idea, and still has
many hurdles to overcome before it can be considered close to commercial. However, the product
oil is likely less viscous.
In situ combustion has long been used as an EOR method. For heavy oils, numerous field observations have shown upgrading of 2°–6°API for heavy oils undergoing combustion (Ramey et al.,
1992). During in situ combustion of heavy oils, temperatures of up to 700°C can be observed at the
combustion front.
In situ combustion is injection of an oxidizing gas (air or oxygen enriched air) to generate heat
by burning a portion of the oil. Most of the oil is driven toward the producers by a combination of
gas drive (from the combustion gases) and steam and water drive. This process is also called fire
flooding to describe the movement of the burning front inside the reservoir. Based on the respective
directions of front propagation and airflow, the process can be forward, when the combustion front
advances in the same direction as the airflow, or reverse, when the front moves against the airflow.
Forward combustion can be further characterized as dry when only air or enriched air is injected
or wet when air and water are co-injected. In the process, air is injected in the target formation for
a short time, usually a few days to a few weeks and the oil in the formation is ignited. Ignition can
be induced using downhole gas burners, electrical heaters, and/or injection of pyrophoric agents
(not recommended) or steam. In some cases, auto-ignition occurs when the reservoir temperature is
fairly high and the oil reasonably reactive. This often happens for California oils.
After ignition, the combustion front is propagated by a continuous flow of air. As the front
progresses into the reservoir, several zones can be found between the injector and the producer as
a result of heat, mass transport, and the chemical reactions occurring in the process. The burned
zone is the volume already burned. This zone is filled with air and may contain small amounts of
residual unburned organic solids. As it has been subjected to high temperatures, mineral alterations
are possible. Because of the continuous airflow from the injector to the burned zone, temperature
increases from injected air temperature at the injector to near combustion front temperature near the
combustion front. There is no oil left in this zone.
The combustion front is the highest temperature zone. It is very thin, often no more than several
inches thick. It is in this region that oxygen combines with the fuel and high temperature oxidation
occurs. The products of the burning reactions are water and carbon oxides. The fuel is often misnamed coke. In fact, it is not pure carbon but a hydrocarbon with H/C atomic ratios ranging from
about 1 to 2.0. This fuel is formed in the thermal cracking zone just ahead of the front and is the
The Chemistry and Technology of Petroleum
are broken by heat; essentially the vibrational energy exceeds the chemical energy in the carbon–
carbon bonds. Pyrolysis occurs in the absence of oxygen or a catalyst, but steam may be present.
For example, steam cracking and thermal cracking are done in refineries at temperatures at or above
800°C (1470°F). Such high temperatures are difficult to achieve in the reservoir. Pyrolysis can still
occur at lower temperatures, but at much, much slower rates.
For example, heavy oil produced under primary and fire flood conditions showed a gradual
increase in density, viscosity, and other properties over time. This indicates fractionation during
flow through the formation. There was also a decrease in viscosity, density, and other properties as
a result of the products of thermal cracking of the oil and the formation of smaller straight-chain
alkanes and small aromatics molecules (Reichert et al., 1989).
Thus, in situ upgrading is also an additional benefit to the combustion recovery process. As the
combustion front moves through the reservoir, the lower boiling constituents of the heavy oil are
vaporized, thermal cracking occurs, and the coke product is consumed as fuel. The resulting oil has
a much lower viscosity than the original bitumen.
It is also conceivable that under the appropriate conditions (as yet undefined), electric heating
(resistance, induction, or radio frequency) should also be able to achieve the high temperatures
required for in situ upgrading (Cunha, 2005).
7.4.2.3 Partial Combustion
The mobilization of heavy oil in the reservoir by partial combustion is not a new idea, and still has
many hurdles to overcome before it can be considered close to commercial. However, the product
oil is likely less viscous.
In situ combustion has long been used as an EOR method. For heavy oils, numerous field observations have shown upgrading of 2°–6°API for heavy oils undergoing combustion (Ramey et al.,
1992). During in situ combustion of heavy oils, temperatures of up to 700°C can be observed at the
combustion front.
In situ combustion is injection of an oxidizing gas (air or oxygen enriched air) to generate heat
by burning a portion of the oil. Most of the oil is driven toward the producers by a combination of
gas drive (from the combustion gases) and steam and water drive. This process is also called fire
flooding to describe the movement of the burning front inside the reservoir. Based on the respective
directions of front propagation and airflow, the process can be forward, when the combustion front
advances in the same direction as the airflow, or reverse, when the front moves against the airflow.
Forward combustion can be further characterized as dry when only air or enriched air is injected
or wet when air and water are co-injected. In the process, air is injected in the target formation for
a short time, usually a few days to a few weeks and the oil in the formation is ignited. Ignition can
be induced using downhole gas burners, electrical heaters, and/or injection of pyrophoric agents
(not recommended) or steam. In some cases, auto-ignition occurs when the reservoir temperature is
fairly high and the oil reasonably reactive. This often happens for California oils.
After ignition, the combustion front is propagated by a continuous flow of air. As the front
progresses into the reservoir, several zones can be found between the injector and the producer as
a result of heat, mass transport, and the chemical reactions occurring in the process. The burned
zone is the volume already burned. This zone is filled with air and may contain small amounts of
residual unburned organic solids. As it has been subjected to high temperatures, mineral alterations
are possible. Because of the continuous airflow from the injector to the burned zone, temperature
increases from injected air temperature at the injector to near combustion front temperature near the
combustion front. There is no oil left in this zone.
The combustion front is the highest temperature zone. It is very thin, often no more than several
inches thick. It is in this region that oxygen combines with the fuel and high temperature oxidation
occurs. The products of the burning reactions are water and carbon oxides. The fuel is often misnamed coke. In fact, it is not pure carbon but a hydrocarbon with H/C atomic ratios ranging from
about 1 to 2.0. This fuel is formed in the thermal cracking zone just ahead of the front and is the