Rebecca S. Bryant
IIT Research Institute, National Institute for Petroleum and Energy Research
P.O. Box 2128, Bartlesville, Oklahoma 74005
Microorganisms were considered to be detrimental to the petroleum industry in the past. It is now known that they can also be beneficial in terms of oil recovery. There are three ways in which microorganisms may contribute to enhanced oil recovery (EOR): (a) microorganisms can produce biosurfactants and biopolymers on the surface; (b) microorganisms grow in reservoir rock pore throats to produce gases, surfactants, and other chemicals to recover trapped oil in reservoirs; and (c) microorganisms can selectively plug high-permeability channels in reservoir rock so that the sweep efficiency of the recovery process can be increased. In this paper progress in enhanced oil recovery through the use of microorganisms, whether in situ or on the surface, is reviewed, and field tests of the in situ process are reported.
Enhanced oil recovery (EOR) processes rely upon the use of chemical or thermal energy to recover crude oil that is trapped in pores of reservoir rock after primary and secondary (waterflood) crude oil production has ceased. The residual crude oil in reservoirs makes up about 67% of the total petroleum reserves, indicating the relative inefficiency of primary and secondary production.
Chemicals used for EOR include surfactants to reduce the interfacial tension between oil and water, and oil and rock interfaces. Many microorganisms produce biosurfactants and perform this activity by fermentation of inexpensive raw materials such as molasses. Several biosurfactants are being evaluated for use in EOR.
Polymers are used in EOR as water-thickening agents, allowing injected water to sweep the reservoir rock more efficiently. A biopolymer, xanthan gum, is already in use in oilfield EOR processes, and other biopolymers are being screened for improved properties such as increased stability at higher temperatures.
The chemicals used for EOR must be compatible with the physical and chemical environments of oil reservoirs. In the use of microorganisms in situ for EOR, it is necessary to use microbial cultures that can survive and grow at the temperatures, pressures, and salinities present. Microorganisms produce several compounds that have potential for enhanced oil recovery, including carbon dioxide, acids, and alcohols. Carbon dioxide may increase reservoir pressure and decrease the viscosity and gravity of the crude oil, allowing it to move more freely to the producing wells.
A major problem in EOR processes is the variation of permeability in petroleum reservoirs. When water is injected to displace oil the water will preferentially flow through areas of highest permeability, and bypass much of the oil. When bacteria are injected they also will initially flow preferentially into high-permeability zones with the water, but then will grow and block those zones. They will not be so likely to go into lower-permeability zones because of their size. When high-permeability zones are blocked, sweep efficiency is improved, and thus oil recovery. Another factor affecting bacterial penetration is the negative charge on the cell surface. This charge may affect both mobility and penetration of the bacteria.
Surfactants produced by microorganisms are usually lipids, and often they are glycolipids. Microorganisms, often coryneform bacteria, producing trehalose are described in the literature. Trehalose glycolipids are also produced by several strains of Arthrobacter, Mycobacterium, and Nocardia (1). The most commonly studied surfactants come from the cell wall of mycobacteria.
These are called cord factors, and are 6, 6 ' -dimycolates of trehalose.
Cooper (2) screened several organisms for their ability to release bitumen from tar sands. When the isolates that released bitumen were grown using kerosene as the carbon source, he showed that they produced large amounts of biosurfactant that release bitumen at ambient temperatures. The biosurfactant, after extraction from the medium, was added to the tar san