151
Recovery of Heavy Oil and Tar Sand Bitumen
its mobility or to alter the mobility of the displacing fluids. In some cases, the effects of gravity
forces, which ordinarily cause vertical segregation of fluids of different densities, can be minimized
or even used to advantage. The various processes differ considerably in complexity, the physical
mechanisms responsible for oil recovery, and the amount of experience that has been derived from
field application. The degree to which the EOR methods are applicable in the future will depend
on development of improved process technology. It will also depend on improved understanding of
fluid chemistry, phase behavior, and physical properties, and on the accuracy of geology and reservoir engineering in characterizing the physical nature of individual reservoirs.
Variations of the EOR theme include the use of steam and solvents as the means of reducing interfacial tension. The solvent approach has had some success when applied to bitumen recovery from
mined tar sand, but when applied to nonmined material phenomenal losses of solvent and bitumen
are always a major obstacle. This approach should not be rejected out solvent hand since a novel concept may arise that guarantees minimal (acceptable) losses of bitumen and solvent. In fact, miscible
fluid displacement (miscible displacement) is a process in which an alcohol, a refined hydrocarbon,
a condensed petroleum gas, carbon dioxide, liquefied natural gas, or even exhaust gas is injected into
an oil reservoir, at pressure levels such that the injected gas or alcohol and reservoir oil are miscible;
the process may include the concurrent, alternating, or subsequent injection of water.
The procedures for miscible displacement are the same in each case and involve the injection of
a slug of solvent that is miscible with the reservoir oil followed by injection of either a liquid or a
gas to sweep up any remaining solvent. As the miscible slug of solvent becomes enriched with oil as
it passes through the reservoir, the composition changes, thereby reducing the effective scavenging
action. However, changes in the composition of the fluid can also lead to wax deposition as well as
deposition of asphaltene constituents (Speight, 2004).
Precipitation and deposition of high-boiling constituents, such as asphaltene constituents, inside
reservoirs, processing, and transportation facilities are a major concern in the petroleum industry.
The probability of asphaltene precipitation and deposition occurring during any EOR operation and
the subsequent effects on reservoir performance should be anticipated at earlier stages of any development project. Once the asphaltene deposition occurs, it causes severe permeability and porosity reduction and wettability alteration, changing relative permeability in the reservoir and, in the
severe cases plugging the wellbore and surface facilities. Therefore, caution is advised.
Microscopic observations of the leading edge of the miscible phase have shown that the displacement takes place at the boundary between the oil and the displacing phase. The small amount of
oil that is bypassed is entrained and dissolved in the rest of the slug of miscible fluids; mixing and
diffusion occur to permit complete recovery of the remaining oil. If a second miscible fluid is used
to displace the first, another zone of displacement and mixing follows. The distance between the
leading edge of the miscible slug and the bulk of pure solvent increases with the distance traveled,
as mixing and reservoir heterogeneity cause the solvent to be dispersed.
Other parameters affecting the miscible displacement process are reservoir length, injection rate,
porosity, and permeability of reservoir matrix, size and mobility ratio of miscible phases, gravitational effects, and chemical reactions.
Thermal recovery methods (Chapter 6) have found most use when heavy oil or bitumen has
an extremely high viscosity under reservoir conditions. For example, most heavy oils are highly
viscous, with a viscosity ranging from a thousand centipoises to a million centipoises or more at
the reservoir conditions. In addition, oil viscosity is also a function of temperature and API gravity
(Speight, 2000, 2009).
Thermal EOR processes (i.e., cyclic steam injection, steam flooding, and in situ combustion) add
heat to the reservoir to reduce oil viscosity and/or to vaporize the oil. In both instances, the oil is
made more mobile so that it can be more effectively driven to producing wells. In addition to adding
heat, these processes provide a driving force (pressure) to move oil to producing wells.
In situ combustion may make a comeback with a new concept. THAI (toe-to-heel air injection)
(Figure 7.3) is based on the geometry of horizontal wells that may solve the problems that have
Recovery of Heavy Oil and Tar Sand Bitumen
its mobility or to alter the mobility of the displacing fluids. In some cases, the effects of gravity
forces, which ordinarily cause vertical segregation of fluids of different densities, can be minimized
or even used to advantage. The various processes differ considerably in complexity, the physical
mechanisms responsible for oil recovery, and the amount of experience that has been derived from
field application. The degree to which the EOR methods are applicable in the future will depend
on development of improved process technology. It will also depend on improved understanding of
fluid chemistry, phase behavior, and physical properties, and on the accuracy of geology and reservoir engineering in characterizing the physical nature of individual reservoirs.
Variations of the EOR theme include the use of steam and solvents as the means of reducing interfacial tension. The solvent approach has had some success when applied to bitumen recovery from
mined tar sand, but when applied to nonmined material phenomenal losses of solvent and bitumen
are always a major obstacle. This approach should not be rejected out solvent hand since a novel concept may arise that guarantees minimal (acceptable) losses of bitumen and solvent. In fact, miscible
fluid displacement (miscible displacement) is a process in which an alcohol, a refined hydrocarbon,
a condensed petroleum gas, carbon dioxide, liquefied natural gas, or even exhaust gas is injected into
an oil reservoir, at pressure levels such that the injected gas or alcohol and reservoir oil are miscible;
the process may include the concurrent, alternating, or subsequent injection of water.
The procedures for miscible displacement are the same in each case and involve the injection of
a slug of solvent that is miscible with the reservoir oil followed by injection of either a liquid or a
gas to sweep up any remaining solvent. As the miscible slug of solvent becomes enriched with oil as
it passes through the reservoir, the composition changes, thereby reducing the effective scavenging
action. However, changes in the composition of the fluid can also lead to wax deposition as well as
deposition of asphaltene constituents (Speight, 2004).
Precipitation and deposition of high-boiling constituents, such as asphaltene constituents, inside
reservoirs, processing, and transportation facilities are a major concern in the petroleum industry.
The probability of asphaltene precipitation and deposition occurring during any EOR operation and
the subsequent effects on reservoir performance should be anticipated at earlier stages of any development project. Once the asphaltene deposition occurs, it causes severe permeability and porosity reduction and wettability alteration, changing relative permeability in the reservoir and, in the
severe cases plugging the wellbore and surface facilities. Therefore, caution is advised.
Microscopic observations of the leading edge of the miscible phase have shown that the displacement takes place at the boundary between the oil and the displacing phase. The small amount of
oil that is bypassed is entrained and dissolved in the rest of the slug of miscible fluids; mixing and
diffusion occur to permit complete recovery of the remaining oil. If a second miscible fluid is used
to displace the first, another zone of displacement and mixing follows. The distance between the
leading edge of the miscible slug and the bulk of pure solvent increases with the distance traveled,
as mixing and reservoir heterogeneity cause the solvent to be dispersed.
Other parameters affecting the miscible displacement process are reservoir length, injection rate,
porosity, and permeability of reservoir matrix, size and mobility ratio of miscible phases, gravitational effects, and chemical reactions.
Thermal recovery methods (Chapter 6) have found most use when heavy oil or bitumen has
an extremely high viscosity under reservoir conditions. For example, most heavy oils are highly
viscous, with a viscosity ranging from a thousand centipoises to a million centipoises or more at
the reservoir conditions. In addition, oil viscosity is also a function of temperature and API gravity
(Speight, 2000, 2009).
Thermal EOR processes (i.e., cyclic steam injection, steam flooding, and in situ combustion) add
heat to the reservoir to reduce oil viscosity and/or to vaporize the oil. In both instances, the oil is
made more mobile so that it can be more effectively driven to producing wells. In addition to adding
heat, these processes provide a driving force (pressure) to move oil to producing wells.
In situ combustion may make a comeback with a new concept. THAI (toe-to-heel air injection)
(Figure 7.3) is based on the geometry of horizontal wells that may solve the problems that have