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The Chemistry and Technology of Petroleum
the use of bacteria that can tolerate these conditions (Shennan and Levi, 1987). Bacteria producing
biosurfactants and polymers can grow at sodium concentrations up to 8% and selectively plug sandstone to create a biowall to recover additional oil (Raiders et al., 1989).
Organisms that participate in oil recovery produce a variety of fermentation products, for example, carbon dioxide, methane, hydrogen, biosurfactants, and polysaccharides from crude oil, pure
hydrocarbons, and a variety of nonhydrocarbon substrates. Organic acids produced through fermentation readily dissolve carbonates and can greatly enhance permeability in limestone reservoirs, and
attempts have been made to promote anaerobic production.
The MEOR process may modify the immediate reservoir environment in a number of ways
that could also damage the production hardware or the formation itself (Van Hamme et al., 2003).
Certain sulfate reducers can produce H 2 S, which can corrode pipeline and other components of the
recovery equipment. Thus, despite numerous MEOR tests, considerable uncertainty remains regarding process performance. Ensuring success requires an ability to manipulate environmental conditions to promote growth and/or product formation by the participating microorganisms. Exerting
such control over the microbial system in the subsurface is itself a serious challenge. In addition,
conditions vary from reservoir to reservoir, which calls for reservoir-specific customization of the
MEOR process, and this alone has the potential to undermine microbial process economic viability.
MEOR differs from chemical EOR in the method by which the enhancing products are introduced
into the reservoir. Thus, in oil recovery by the cyclic microbial method, a solution of nutrients and
microorganisms is introduced into the reservoir during injection. The injection well is then shut for
an incubation period allowing the microorganisms to produce carbon dioxide gas and surfactants
that assist in mobilization of the oil. The well is then opened and oil and oil products resulting from
the treatment are produced. The process is repeated as often as oil can be produced from the well.
Oil recovery by microbial flooding also involves the use of microorganisms but in this case, the reservoir is usually conditioned by a water flush after which a solution of microorganisms and nutrients
is injected into the formation. As this solution is pushed through the reservoir by water drive, gases
and surfactants are formed; the oil is mobilized and pumped through the well. However, even though
microbes produce the necessary chemical reactions in situ (Table 7.2), surface injected chemicals may
TABLE 7.2
Microbial Products and Their Contribution to Enhanced Oil Recovery
Microbial
Effect
Acids
Modification of reservoir rock
Improvement of porosity and permeability
Reaction with calcareous carbon dioxide production
Biomass
Selective or nonselective plugging
Emulsification through adherence to hydrocarbons
Modification of solid surfaces (e.g., wetting)
Degradation and alteration of oil
Reduction of oil viscosity and oil pour point
Desulfurization of oil
Gases (CO 2 , CH 4 , H 2 )
Reservoir repressurization
Oil swelling
Viscosity reduction
Increase of permeability due to solubilization of carbonate rocks by CO 2
Solvents
Oil dissolution
Surface active agent
Lowering interfacial tension
Polymers
Mobility control
Selective or nonselective plugging
The Chemistry and Technology of Petroleum
the use of bacteria that can tolerate these conditions (Shennan and Levi, 1987). Bacteria producing
biosurfactants and polymers can grow at sodium concentrations up to 8% and selectively plug sandstone to create a biowall to recover additional oil (Raiders et al., 1989).
Organisms that participate in oil recovery produce a variety of fermentation products, for example, carbon dioxide, methane, hydrogen, biosurfactants, and polysaccharides from crude oil, pure
hydrocarbons, and a variety of nonhydrocarbon substrates. Organic acids produced through fermentation readily dissolve carbonates and can greatly enhance permeability in limestone reservoirs, and
attempts have been made to promote anaerobic production.
The MEOR process may modify the immediate reservoir environment in a number of ways
that could also damage the production hardware or the formation itself (Van Hamme et al., 2003).
Certain sulfate reducers can produce H 2 S, which can corrode pipeline and other components of the
recovery equipment. Thus, despite numerous MEOR tests, considerable uncertainty remains regarding process performance. Ensuring success requires an ability to manipulate environmental conditions to promote growth and/or product formation by the participating microorganisms. Exerting
such control over the microbial system in the subsurface is itself a serious challenge. In addition,
conditions vary from reservoir to reservoir, which calls for reservoir-specific customization of the
MEOR process, and this alone has the potential to undermine microbial process economic viability.
MEOR differs from chemical EOR in the method by which the enhancing products are introduced
into the reservoir. Thus, in oil recovery by the cyclic microbial method, a solution of nutrients and
microorganisms is introduced into the reservoir during injection. The injection well is then shut for
an incubation period allowing the microorganisms to produce carbon dioxide gas and surfactants
that assist in mobilization of the oil. The well is then opened and oil and oil products resulting from
the treatment are produced. The process is repeated as often as oil can be produced from the well.
Oil recovery by microbial flooding also involves the use of microorganisms but in this case, the reservoir is usually conditioned by a water flush after which a solution of microorganisms and nutrients
is injected into the formation. As this solution is pushed through the reservoir by water drive, gases
and surfactants are formed; the oil is mobilized and pumped through the well. However, even though
microbes produce the necessary chemical reactions in situ (Table 7.2), surface injected chemicals may
TABLE 7.2
Microbial Products and Their Contribution to Enhanced Oil Recovery
Microbial
Effect
Acids
Modification of reservoir rock
Improvement of porosity and permeability
Reaction with calcareous carbon dioxide production
Biomass
Selective or nonselective plugging
Emulsification through adherence to hydrocarbons
Modification of solid surfaces (e.g., wetting)
Degradation and alteration of oil
Reduction of oil viscosity and oil pour point
Desulfurization of oil
Gases (CO 2 , CH 4 , H 2 )
Reservoir repressurization
Oil swelling
Viscosity reduction
Increase of permeability due to solubilization of carbonate rocks by CO 2
Solvents
Oil dissolution
Surface active agent
Lowering interfacial tension
Polymers
Mobility control
Selective or nonselective plugging