137
Exploration, Recovery, and Transportation
the slug sizes depend upon the specific properties of the fluids and the rocks involved and upon
economic considerations.
Alkaline flooding adds inorganic alkaline chemicals, such as sodium hydroxide, sodium carbonate, or sodium orthosilicates, to the water to enhance oil recovery by one or more of the following mechanisms: interfacial tension reduction, spontaneous emulsification, or wettability alteration
(Morrow, 1996). These mechanisms rely on the in situ formation of surfactants during the neutralization of petroleum acids in the crude oil by the alkaline chemicals in the displacing fluids.
Although emulsification in alkaline flooding processes decreases injection fluid mobility to a
certain degree, emulsification alone may not provide adequate sweep efficiency. Sometimes polymer is included as an ancillary mobility control chemical in an alkaline water flood to augment any
mobility ratio improvements due to alkaline-generated emulsions.
Other variations on this theme include the use of steam and the means of reducing interfacial
tension by the use of various solvents. The solvent approach has had some success when applied
to bitumen recovery from mined tar sand but when applied to non-mined material losses of solvent and dissolved bitumen are always an issue. However, this approach should not be rejected out
of hand since a novel concept may arise that guarantees minimal (acceptable) losses of bitumen
and solvent.
Miscible fluid displacement (miscible displacement) is an oil displacement process in which an
alcohol, a refined hydrocarbon, a condensed petroleum gas, carbon dioxide, liquefied natural gas
(LNG), 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. It must be recognized that the miscible slug of solvent
becomes enriched with oil as it passes through the reservoir and its composition changes, thereby
reducing the effective scavenging action. However, changes in the composition of the fluid can
also lead to wax deposition (Weingarten and Euchner, 1986; Majeed et al., 1990; Pedersen et al.,
1991; Erickson et al., 1993; Pan and Firoozabadi, 1996; Calange et al., 1997) as well as deposition
of asphaltene constituents (Leontaritis et al., 1987 and references cited therein; Leontaritis, 1989;
Chung, 1992; Nghiem et al., 1993; Nor-Aziam and Adewumi, 1993; Kamath et al., 1994; Deo et al.,
1995; Rassamdana et al., 1999). 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. Miscible floods using carbon dioxide, nitrogen, or hydrocarbons as miscible solvents have their greatest potential for enhanced recovery of low-viscosity oils.
Commercial hydrocarbon-miscible floods have been operated since the 1950s, but carbon dioxidemiscible flooding on a large scale is relatively recent and is expected to make the most significant
contribution to miscible enhanced recovery in the future.
Carbon dioxide is capable of displacing many crude oils, thus permitting recovery of most of the
oil from the reservoir rock that is contacted (carbon dioxide-miscible flooding). The carbon dioxide
is not initially miscible with the oil. However, as the carbon dioxide contacts the in situ crude oil,
it extracts some of the hydrocarbon constituents of the crude oil into the carbon dioxide and carbon dioxide is also dissolved in the oil. Miscibility is achieved at the displacement front when no
Exploration, Recovery, and Transportation
the slug sizes depend upon the specific properties of the fluids and the rocks involved and upon
economic considerations.
Alkaline flooding adds inorganic alkaline chemicals, such as sodium hydroxide, sodium carbonate, or sodium orthosilicates, to the water to enhance oil recovery by one or more of the following mechanisms: interfacial tension reduction, spontaneous emulsification, or wettability alteration
(Morrow, 1996). These mechanisms rely on the in situ formation of surfactants during the neutralization of petroleum acids in the crude oil by the alkaline chemicals in the displacing fluids.
Although emulsification in alkaline flooding processes decreases injection fluid mobility to a
certain degree, emulsification alone may not provide adequate sweep efficiency. Sometimes polymer is included as an ancillary mobility control chemical in an alkaline water flood to augment any
mobility ratio improvements due to alkaline-generated emulsions.
Other variations on this theme include the use of steam and the means of reducing interfacial
tension by the use of various solvents. The solvent approach has had some success when applied
to bitumen recovery from mined tar sand but when applied to non-mined material losses of solvent and dissolved bitumen are always an issue. However, this approach should not be rejected out
of hand since a novel concept may arise that guarantees minimal (acceptable) losses of bitumen
and solvent.
Miscible fluid displacement (miscible displacement) is an oil displacement process in which an
alcohol, a refined hydrocarbon, a condensed petroleum gas, carbon dioxide, liquefied natural gas
(LNG), 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. It must be recognized that the miscible slug of solvent
becomes enriched with oil as it passes through the reservoir and its composition changes, thereby
reducing the effective scavenging action. However, changes in the composition of the fluid can
also lead to wax deposition (Weingarten and Euchner, 1986; Majeed et al., 1990; Pedersen et al.,
1991; Erickson et al., 1993; Pan and Firoozabadi, 1996; Calange et al., 1997) as well as deposition
of asphaltene constituents (Leontaritis et al., 1987 and references cited therein; Leontaritis, 1989;
Chung, 1992; Nghiem et al., 1993; Nor-Aziam and Adewumi, 1993; Kamath et al., 1994; Deo et al.,
1995; Rassamdana et al., 1999). 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. Miscible floods using carbon dioxide, nitrogen, or hydrocarbons as miscible solvents have their greatest potential for enhanced recovery of low-viscosity oils.
Commercial hydrocarbon-miscible floods have been operated since the 1950s, but carbon dioxidemiscible flooding on a large scale is relatively recent and is expected to make the most significant
contribution to miscible enhanced recovery in the future.
Carbon dioxide is capable of displacing many crude oils, thus permitting recovery of most of the
oil from the reservoir rock that is contacted (carbon dioxide-miscible flooding). The carbon dioxide
is not initially miscible with the oil. However, as the carbon dioxide contacts the in situ crude oil,
it extracts some of the hydrocarbon constituents of the crude oil into the carbon dioxide and carbon dioxide is also dissolved in the oil. Miscibility is achieved at the displacement front when no