No abstract

World requirements for fossil energy are expected to grow by more than 50% within the next 25 years, despite advances in alternative technologies. Since conventional production methods retrieve only about one-third of the oil in place, either large new fields or innovative strategies for recovering energy resources from existing fields are needed to meet the burgeoning demand. The anaerobic biodegradation of n-alkanes to methane gas has now been documented in a few studies, and it was speculated that this process might be useful for recovering energy from existing petroleum reservoirs. We found that residual oil entrained in a marginal sandstone reservoir core could be converted to methane, a key component of natural gas, by an oil-degrading methanogenic consortium. Methane production required inoculation, and rates ranged from 0.15 to 0.40 mol/day/g core (or 11 to 31 mol/day/g oil), with yields of up to 3 mmol CH 4 /g residual oil. Concomitant alterations in the hydrocarbon profile of the oil-bearing core revealed that alkanes were preferentially metabolized. The consortium was found to produce comparable amounts of methane in the absence or presence of sulfate as an alternate electron acceptor. Cloning and sequencing exercises revealed that the inoculum comprised sulfate-reducing, syntrophic, and fermentative bacteria acting in concert with aceticlastic and hydrogenotrophic methanogens. Collectively, the cells generated methane from a variety of petroliferous rocks. Such microbe-based methane production holds promise for producing a clean-burning and efficient form of energy from underutilized hydrocarbon-bearing resources.The recognition that the earth's petroleum supplies are finite and dwindling has sparked the development of many nonfossil-fuel-based energy alternatives. However, even optimistic projections suggest that such energy sources will comprise less than 10% of world requirements through 2030 (10). In fact, the demand for oil is expected to increase given world population growth, the accompanying human dependence on fossil fuels for power, the existing energy infrastructure, and the use of petroleum components for manufacturing feedstocks. Currently, oil recovery techniques can extract only up to 40% of existing resources, leaving the remainder stranded in mature fields (38). Developing new technologies to recover even a fraction of such a large energy pool is of great interest to help nations reduce reliance on foreign imports and increase the value of domestic reserves (www.fossil.energy.gov/programs/oilgas /marginalwells/index.html). Here, we investigated whether it is possible to convert at least a fraction of trapped oil in marginal fields into methane gas as an alternate energy source by using a hydrocarbon-degrading methanogenic consortium.

Corrosion of metallic oilfield pipelines by microorganisms is a costly but poorly understood phenomenon, with standard treatment methods targeting mesophilic sulfatereducing bacteria. In assessing biocorrosion potential at an Alaskan North Slope oil field, we identified thermophilic hydrogen-using methanogens, syntrophic bacteria, peptideand amino acid-fermenting bacteria, iron reducers, sulfur/thiosulfate-reducing bacteria and sulfate-reducing archaea. These microbes can stimulate metal corrosion through production of organic acids, CO 2 , sulfur species, and via hydrogen oxidation and iron reduction, implicating many more types of organisms than are currently targeted.Micromolar quantities of putative anaerobic metabolites of C 1 -C 4 n-alkanes in pipeline fluids were detected, implying that these low molecular weight hydrocarbons, routinely injected into reservoirs for oil recovery purposes, are biodegraded and provide biocorrosive microbial communities with an important source of nutrients.

Biosurfactant-mediated oil recovery may be an economic approach for recovery of significant amounts of oil entrapped in reservoirs, but evidence that biosurfactants can be produced in situ at concentrations needed to mobilize oil is lacking. We tested whether two Bacillus strains that produce lipopeptide biosurfactants can metabolize and produce their biosurfactants in an oil reservoir. Five wells that produce from the same Viola limestone formation were used. Two wells received an inoculum (a mixture of Bacillus strain RS-1 and Bacillus subtilis subsp. spizizenii NRRL B-23049) and nutrients (glucose, sodium nitrate, and trace metals), two wells received just nutrients, and one well received only formation water. Results showed in situ metabolism and biosurfactant production. The average concentration of lipopeptide biosurfactant in the produced fluids of the inoculated wells was about 90 mg/liter. This concentration is approximately nine times the minimum concentration required to mobilize entrapped oil from sandstone cores. Carbon dioxide, acetate, lactate, ethanol, and 2,3-butanediol were detected in the produced fluids of the inoculated wells. Only CO 2 and ethanol were detected in the produced fluids of the nutrient-only-treated wells. Microbiological and molecular data showed that the microorganisms injected into the formation were retrieved in the produced fluids of the inoculated wells. We provide essential data for modeling microbial oil recovery processes in situ, including growth rates (0.06 ؎ 0.01 h ؊1 ), carbon balances (107% ؎ 34%), biosurfactant production rates (0.02 ؎ 0.001 h ؊1 ), and biosurfactant yields (0.015 ؎ 0.001 mol biosurfactant/mol glucose). The data demonstrate the technical feasibility of microbial processes for oil recovery.