uch 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 along with nutrients into a reservoir. He asserted that the spores would germinate in the reservoir and enhance removal of oil from reservoir rock. The hypothesis was tested in the laboratory by using an oil-saturated sandpacked column. An aqueous solution containing spores of Clostridium roseum and molasses was passed through the column and an improved release of oil (about 30%) was obtained.
Later patents by Hitzman concern the use of microorganisms that feed on injected polymers and the byproducts of CO2 floods, to produce chemicals for EOR, i.e. gases, acids, solvents, and surfactants (20). In polymer floods the injected organisms feed on polymer that was adsorbed on the reservoir rock. In CO2 floods, the microbes feed on soluble compounds of carbon, nitrogen, and sulfur left behind the CO2-crude oil slug. The process has mobilized residual oil in sandpack tests but no core or field tests are reported.
Knapp et al. (21) reported the isolation of 22 microorganisms that produce biopolymers and emulsifiers. One strain was isolated that grew in 10% salt concentrations, over a pH range of 4.6 to 9.0, at temperatures up to 50 °C, and in the presence of crude oil. They demonstrated in the laboratory that glucose, ammonium sulfate, and potassium phosphate were easily transported through sandstone cores. Viable bacterial cells in aqueous solutions of 2% NaCl and 0.01% CaCl2 injected into these cores were not recovered in the effluent. The addition of glucose to cores previously inoculated with bacteria resulted in a significant decrease in permeability, indicating that bacteria were multiplying and plugging the pores. Bacteria indigenous to all of the cores treated were Pseudomonas sp., Bacillus sp., and Actinomycetes. A major problem in these experiments was in determining how much of the plugging was caused by
injected bacteria and how much by indigenous bacteria. The problem persisted even when cores were steam-cleaned and autoclaved. "Sterilization" of cores with chlorine dioxide diminished the problem, but the ba