173
Recovery of Heavy Oil and Tar Sand Bitumen
tend to follow areas of higher permeability, resulting in decreased sweep efficiency. There is need for
caution and astute observation of the effects of the microorganisms on the reservoir chemistry.
The mechanism by which MEOR processes work can be quite complex and may involve multiple biochemical processes. In selective plugging approaches, microbial cell mass or biopolymers
plug high permeability zones and lead to a redirection of the waterflood. In other processes, biosurfactants are produced in situ, which leads to increased mobilization of residual oil. In still other
processes, microbial production of carbon dioxide and organic solvents reduces the oil viscosity as
the primary mechanism for EOR.
In an MEOR process, conditions for microbial metabolism are supported via injection of nutrients. In some processes, this involves injecting a fermentable carbohydrate into the reservoir. Some
reservoirs also require inorganic nutrients as substrates for cellular growth or for serving as alternative electron acceptors in place of oxygen or carbohydrates.
The stimulation of oil production by in situ bacterial fermentation is thought to proceed by one
or a combination of the following mechanisms:
1. Improvement of the relative mobility of oil to water by biosurfactants and biopolymers.
2. Partial repressurization of the reservoir by methane and carbon dioxide.
3. Reduction of oil viscosity through the dissolution of organic solvents in the oil phase.
4. Increase of reservoir permeability and widening of the fissures and channels through the
etching of carbonaceous rocks in limestone reservoirs by organic acids produced by anaerobic bacteria.
5. Cleaning the wellbore region through the acids and gas from in situ fermentation in which
the gas pushes oil from dead space and dislodge debris that plugs the pores. The average
pore size is increased and, as a result, the capillary pressure near the wellbore is made
more favorable for the flow of oil.
6. Selective plugging of highly permeable zones by injecting slime-forming bacteria followed
by sucrose solution that initiates the production of extracellular slimes and aerial sweep
efficiency is improved.
The target for EOR processes is the quantity of unrecoverable oil in known reservoirs and bitumen and known deposits. One of the major attributes of MEOR technologies is its low cost but there
must be the recognition that MEOR is a single process. Furthermore, reports on the deleterious
activities of microorganisms in the oil field contribute to the skepticism of employing technologies
using microorganisms. It is also clear that scientific knowledge of the fundamentals of microbiology
must be coupled with an understanding of the geological and engineering aspects of oil production
in order to develop MEOR technology.
However, MEOR has the advantages that microbes do not consume large amounts of energy and
they are independent of the price of crude oil, compared to other processes. Also, with increasing
subsurface depth, temperature appears to be the principal factor limiting microbial life, besides availability of suitable nutrients. They are also susceptible to salinity, which limits the use of microbes.
Finally, recent developments in upgrading of heavy oil and bitumen (Chapters 18 through 20)
indicate that the near future could see a reduction of the differential cost of upgrading heavy oil.
These processes are based on a better understanding of the issues of asphaltene solubility effects at
high temperatures, incorporation of a catalyst that is chemically precipitated internally during the
upgrading, and improving hydrogen addition or carbon rejection.
7.4 UPGRADING DURING RECOVERY
Crude oil upgrading is of major economic importance. Heavy crude oils exist in large quantities in
the Western Hemisphere, but are difficult to produce and transport because of their high viscosity.
Some crude oils contain compounds such as sulfur and/or heavy metals causing additional refining
Recovery of Heavy Oil and Tar Sand Bitumen
tend to follow areas of higher permeability, resulting in decreased sweep efficiency. There is need for
caution and astute observation of the effects of the microorganisms on the reservoir chemistry.
The mechanism by which MEOR processes work can be quite complex and may involve multiple biochemical processes. In selective plugging approaches, microbial cell mass or biopolymers
plug high permeability zones and lead to a redirection of the waterflood. In other processes, biosurfactants are produced in situ, which leads to increased mobilization of residual oil. In still other
processes, microbial production of carbon dioxide and organic solvents reduces the oil viscosity as
the primary mechanism for EOR.
In an MEOR process, conditions for microbial metabolism are supported via injection of nutrients. In some processes, this involves injecting a fermentable carbohydrate into the reservoir. Some
reservoirs also require inorganic nutrients as substrates for cellular growth or for serving as alternative electron acceptors in place of oxygen or carbohydrates.
The stimulation of oil production by in situ bacterial fermentation is thought to proceed by one
or a combination of the following mechanisms:
1. Improvement of the relative mobility of oil to water by biosurfactants and biopolymers.
2. Partial repressurization of the reservoir by methane and carbon dioxide.
3. Reduction of oil viscosity through the dissolution of organic solvents in the oil phase.
4. Increase of reservoir permeability and widening of the fissures and channels through the
etching of carbonaceous rocks in limestone reservoirs by organic acids produced by anaerobic bacteria.
5. Cleaning the wellbore region through the acids and gas from in situ fermentation in which
the gas pushes oil from dead space and dislodge debris that plugs the pores. The average
pore size is increased and, as a result, the capillary pressure near the wellbore is made
more favorable for the flow of oil.
6. Selective plugging of highly permeable zones by injecting slime-forming bacteria followed
by sucrose solution that initiates the production of extracellular slimes and aerial sweep
efficiency is improved.
The target for EOR processes is the quantity of unrecoverable oil in known reservoirs and bitumen and known deposits. One of the major attributes of MEOR technologies is its low cost but there
must be the recognition that MEOR is a single process. Furthermore, reports on the deleterious
activities of microorganisms in the oil field contribute to the skepticism of employing technologies
using microorganisms. It is also clear that scientific knowledge of the fundamentals of microbiology
must be coupled with an understanding of the geological and engineering aspects of oil production
in order to develop MEOR technology.
However, MEOR has the advantages that microbes do not consume large amounts of energy and
they are independent of the price of crude oil, compared to other processes. Also, with increasing
subsurface depth, temperature appears to be the principal factor limiting microbial life, besides availability of suitable nutrients. They are also susceptible to salinity, which limits the use of microbes.
Finally, recent developments in upgrading of heavy oil and bitumen (Chapters 18 through 20)
indicate that the near future could see a reduction of the differential cost of upgrading heavy oil.
These processes are based on a better understanding of the issues of asphaltene solubility effects at
high temperatures, incorporation of a catalyst that is chemically precipitated internally during the
upgrading, and improving hydrogen addition or carbon rejection.
7.4 UPGRADING DURING RECOVERY
Crude oil upgrading is of major economic importance. Heavy crude oils exist in large quantities in
the Western Hemisphere, but are difficult to produce and transport because of their high viscosity.
Some crude oils contain compounds such as sulfur and/or heavy metals causing additional refining