rs; (b) use with other chemicals, which are sacrificial adsorption agents to prevent adsorption of the polysaccharide on to rock surfaces; (c) use with multivalent cations to bind the polysaccharide molecules together (crosslinking) for preferential partial plugging of the reservoir to prevent water channeling; and (d) use with gel particles, hydrolyzed polyacrylamides, and ferric chloride solutions to assist in the water-thickening process.
Since Lindblom's patent, several laboratories have investigated the production of xanthan gum. It is now used commercially as a mobility control agent for EOR. Its properties of viscosity, shear resistance, and temperature and salt tolerance make it almost an ideal polymer for use in EOR. It is considered superior to polyacrylamides for the above reasons, but it is more expensive (11) and more susceptible to bacterial degradation (12).
Another biopolymer, scleroglucan, is produced by species of the fungus Sclerotium. These polymers were examined for possible use in EOR (13). The scleroglucans are neutral polysaccharides with a cellulose-type backbone (14). Scleroglucan and xanthan gum possess similar rheological properties. Sclerotium sp. can grow on a variety of carbon and energy sources. It is postulated that the fungus can grow on waste plant material such as hydrolyzed lignin and other industrial wastes, and produce the polymer. These nutrients would be less expensive than hydrocarbons or most other commercially available ones (13).
Under favorable conditions and in a liquid culture, bacteria can ferment carbohydrates to produce gases such as CO2 and H2. Such gases produced in situ can contribute to re-pressurization of a pres-
sure-depleted petroleum reservoir. These gases also may dissolve in the crude oil and reduce its viscosity. Additional benefits of this bacteria-induced fermentation process are the production of acids, such as acetic and propionic acids, which can dissolve carbonate rock and increase its permeability and porosity, and the production of solvents, such as acetone, ethanol, 1-butanol, and butanone.
Grula et al. (15) used a microbial screening program to isolate an anaerobic Clostridium species that produced gases, acids, alcohols, and surfactants. They isolated several such strains, but tolerance of the bacteria to high salt concentrations (>7%) remains a problem. Success of in situ microbial EOR (MEOR) operations depends upon finding microorganisms that can survive and produce the desired metabolic products in reservoirs containing hydrocarbons and saline water. Other researchers continue to investigate Clostridium species, Bacillus species, and Enterobacter for better adaptation to reservoir conditions.
C.E. Zobell, in 1946, patented a process for the secondary recovery of petroleum, using anaerobic, hydrocarbon-utilizing, sulfate-reducing bacteria such as Desulfovibrio species in situ (16). The mechanism of oil recovery with this organism was thought to be similar to those with other microorganisms such as Clostridium sp., i.e., through the production of organic acids, gases, and surfactants. He reported that an aqueous mixture of nutrients, Clostridium, and Desulfovibrio resulted in hydrogenase production by the bacterial cells. These microorganisms were considered to use this enzyme, hydrogenase, as a catalyst for producing acetic acid, butyric acid, and ammonium hydroxide from CO2, water, and nitrates in the reservoir. Some of these products are known to enhance the release of oil from reservoir rock. Other patents were granted to Updegraff and Wren (17), who proposed injecting Desulfovibrio and molasses downhole for the production of organic acids and gases. In later work, other fermentation-type bacteria were used to produce substantial amounts of organic acids and carbon dioxide to enhance oil recovery in laboratory experiments (18). A drawback to this early work was the fact that Desulfovibrio species produce hydrogen sulfide and thus should not be considered for MEOR processes.
Hitzman (19) patented a process for the injection of bacterial spores 