hemical 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 sands along with kerosene. The amount of released bitumen was compared with controls run with only kerosene added to tar sands. The biosurfactant enhanced the release of bitumen.
Generally the organisms that produce surfactants are aerobes. However, Cooper et al. (3) isolated a neutral lipid biosurfactant from the anaerobe Clostridium pasteurianum, and an anaerobic biosurfactant from a Bacillus sp. was recently reported (4).
Several combinations of surfactants and bacteria have been suggested for use in EOR drive fluids. Harvey (5) patented a process using dead bacterial and/or yeast cells to carry nonionic surfactants [poly-(ethylene oxide) ethers or thioethers] as EOR slugs through the reservoir. Using such slugs in laboratory sand column reservoir models, he obtained 20 to 30% recovery of residual oil. Hitzman (6) and Wagner and Stratton (7) claim that commercial surfactant-treated dead bacterial cells can be used effectively in EOR drive fluids, without the need for injecting live bacterial cells into a reservoir for effective channeling of the injection water.
Recently a Swiss biotechnology company announced plans to field test three biosurfactants in the United States. They utilize continuous culture methods (fermentation apparatus) for the production of these compounds (8).
Hussain et al. (9) suggested the use of bacterially produced polysaccharides (dextrans) as floodwater thickening agents. Lindblom et al. (10) documented the use of Xanthomonas sp. to ferment glucose and produce a thermally stable heteropolysaccharide that can be used in waterflooding systems. Using a model reservoir in the laboratory, they injected an aqueous mixture of the polysaccharide (xanthan gum) and 50 x 103 ppm sodium chloride. By using data from these experiments they showed that the properties of the xanthan gum were not altered by the sodium chloride. Precipitation and plugging of the model reservoir also did not occur. Processes based on the uses of xanthan gum in EOR are: (a) use of biopolysaccharides to thicken injection wate