6.5
Microbial Enhanced Oil Recovery (mEOR)
Microbial enhanced oil recovery is the biotechnological approach for the tertiary oil
recovery process. This is applied by oil industries after the secondary oil recovery
process or some times during secondary oil recovery. mEOR process involves use of
microbes for enhancing residual oil production from the reservoir. In mEOR,
microbes enhance oil recovery by producing gas (CO 2 , CH 4 , H 2, and N 2 ), low
molecular weight acids, solvent, biosurfactant, polymers, and biomass production
(Kaster et al. 2012). These microbial products contribute to the enhanced oil
recovery process by different mechanisms. During the mEOR process, either microbial product (mEOR chemicals) may be injected in well or nutrients are added in the
injection water for in situ microbial growth. Adding microbes or microbial products
in an oil well is also known as bioaugmentation, while adding growth supplement for
increasing in situ microbial growth is termed as biostimulation.
During the mEOR process, biosurfactants mobilize the oil by reducing the
interfacial tension (IFT) between oil and water phases. Another mechanism is
microbial growth on reservoir rocks and biofilm formation changes its rocks wettability. Further microbial growth in reservoir rocks leads to bioclogging of the water
flow path. This bioclogging alters the water flow path and ultimately increases a
reservoir sweep efficiency. Other than these mechanisms, microbes generate gases
during metabolism (for example, methane and carbon dioxide) and these gases
increase pore pressure. These gases also dissolve in the oil phase, therefore reduces
the viscosity and swelling of the oil. Another mechanism is the biodegradation of the
hydrocarbon. Microorganisms utilize the crude oil components as a carbon source
and alter oil viscosity (Armstrong and Wildenschild 2012).
6.5.1 Conventional Methods of EOR
Conventional EOR methods are: thermal (steam injection, in situ combustion),
chemical (surfactant, polymer, alkali and solvents injection), and miscible gas
(CO 2 ) injection.
Thermal EOR (TEOR): This is a commercially available technique of EOR. In
this process, steam is generated from water using natural gas. This steam is injected
in the reservoir to heat heavy oil, for reducing its viscosity and also impact on
thermal expansion and crude oil vaporization. This process allows the oil to flow
easily. Other than using natural gas, alternatively solar energy is also used for steam
generation. This TEOR process is unsuitable where thin pay zone and increased
depth are present (Gbadamosi et al. 2019).
Chemical EOR (CEOR): CEOR enhances oil production by altering oil–water/
oil–rock interactions in the reservoir. Chemical is injected in water slug that lowers
the IFT between fluids (water–oil), alter the wettability of rock to increase oil
permeability, and increment in the viscosity of injection fluid improves the mobility.
Traditional CEOR methods are polymer, surfactant, and alkaline flooding. In
polymer flooding, polymer increases the viscosity of injected fluids and further
124
G. Prajapat et al.
Microbial Enhanced Oil Recovery (mEOR)
Microbial enhanced oil recovery is the biotechnological approach for the tertiary oil
recovery process. This is applied by oil industries after the secondary oil recovery
process or some times during secondary oil recovery. mEOR process involves use of
microbes for enhancing residual oil production from the reservoir. In mEOR,
microbes enhance oil recovery by producing gas (CO 2 , CH 4 , H 2, and N 2 ), low
molecular weight acids, solvent, biosurfactant, polymers, and biomass production
(Kaster et al. 2012). These microbial products contribute to the enhanced oil
recovery process by different mechanisms. During the mEOR process, either microbial product (mEOR chemicals) may be injected in well or nutrients are added in the
injection water for in situ microbial growth. Adding microbes or microbial products
in an oil well is also known as bioaugmentation, while adding growth supplement for
increasing in situ microbial growth is termed as biostimulation.
During the mEOR process, biosurfactants mobilize the oil by reducing the
interfacial tension (IFT) between oil and water phases. Another mechanism is
microbial growth on reservoir rocks and biofilm formation changes its rocks wettability. Further microbial growth in reservoir rocks leads to bioclogging of the water
flow path. This bioclogging alters the water flow path and ultimately increases a
reservoir sweep efficiency. Other than these mechanisms, microbes generate gases
during metabolism (for example, methane and carbon dioxide) and these gases
increase pore pressure. These gases also dissolve in the oil phase, therefore reduces
the viscosity and swelling of the oil. Another mechanism is the biodegradation of the
hydrocarbon. Microorganisms utilize the crude oil components as a carbon source
and alter oil viscosity (Armstrong and Wildenschild 2012).
6.5.1 Conventional Methods of EOR
Conventional EOR methods are: thermal (steam injection, in situ combustion),
chemical (surfactant, polymer, alkali and solvents injection), and miscible gas
(CO 2 ) injection.
Thermal EOR (TEOR): This is a commercially available technique of EOR. In
this process, steam is generated from water using natural gas. This steam is injected
in the reservoir to heat heavy oil, for reducing its viscosity and also impact on
thermal expansion and crude oil vaporization. This process allows the oil to flow
easily. Other than using natural gas, alternatively solar energy is also used for steam
generation. This TEOR process is unsuitable where thin pay zone and increased
depth are present (Gbadamosi et al. 2019).
Chemical EOR (CEOR): CEOR enhances oil production by altering oil–water/
oil–rock interactions in the reservoir. Chemical is injected in water slug that lowers
the IFT between fluids (water–oil), alter the wettability of rock to increase oil
permeability, and increment in the viscosity of injection fluid improves the mobility.
Traditional CEOR methods are polymer, surfactant, and alkaline flooding. In
polymer flooding, polymer increases the viscosity of injected fluids and further
124
G. Prajapat et al.