Basic Study on the Microbial Conversion of CO₂ into CH₄ in Depleted Oil Reservoir by using Hydrogen-Producing Bacteria and Hydrogenotrophic Methanogens

Abstract

CO₂ sequestration into a depleted oil reservoir has been expected to be a method of reducing the CO₂ emission. We focus on the in-situ microbial conversion of CO₂ into CH₄ by hydrogenotrophic methanogens that inhabit oil reservoirs universally. It is important for this conversion process to accelerate the supply of H₂ for the CH₄ production by methanogens in reservoirs. This study aims to search for oil-degrading and H₂-producing thermophilic bacteria (ODHPTB) that can produce H₂ from oil in reservoir brine.
Reservoir brine was extracted from 10 producing wells in Yabase Oilfield in Japan. Indigenous bacteria in brine were incubated with sterilized oil under anaerobic conditions (10% CO₂ and 90% N₂) at 50°C and/or 75°C. Both the production of H₂, CH₄, and the consumption of CO₂ were observed in almost all culture systems after 2 months incubation. The maximum production of H₂ was 1267 Nml/l-med. for 4 months incubation. Petrotoga sp. and Thermotoga sp. which were reported as ODHPTB were detected as dominant bacteria from each enrichment culture solution by gene analysis.
These culture solutions and raw brine were inoculated into nutrient agar medium and incubated under anaerobic conditions at 50°C and/or 75°C. Microbial single colonies which were formed in the nutrient agar medium after 2 weeks incubation were picked and inoculated into sterilized brine including sterilized oil as sole hydrogen source. More than 38 strains were isolated and incubated in the brine medium, and then producing hydrogen from oil were observed from 38 strains after 1 month incubation. The maximum production of H₂ was 26 Nml/l-med. for 3 months incubation.
These results show the in-situ microbial conversion of CO₂ and residual oil into CH₄ using ODHPTB and hydrogenotrophic methanogens is promising. Moreover, the valuable characteristics of ODHPTB isolated in this study, made it suitable to be injected into reservoirs to stimulate the conversion of CO₂ into CH₄.