Experimental studies on indigenous hydrocarbon-degrading and hydrogen-producing bacteria in an oilfield for microbial restoration of natural gas deposits with CO2 sequestration

Sequestration of carbon dioxide (CO2) into depleted oil reservoirs may be a method of reducing CO2 emissions. We focus on microbial restoration of natural gas deposits with CO2 sequestration via in situ microbial conversion of CO2 into methane (CH4) by hydrogenotrophic methanogens (HM) that universally inhabit oil reservoirs. The means of supplying HM with H2 for their CH4 production is central to this process. This study considers the potential of this process by evaluating the H2 productivity of hydrocarbon-degrading and hydrogen-producing bacteria (HD–HPB) that inhabit oil reservoirs.Reservoir brine was extracted from 10 producing wellheads in the Yabase Oilfield of Japan. Indigenous bacteria in the brine samples were incubated with sterile oil under anaerobic conditions with 10%-CO2 (N2 balanced) at 50 °C and 75 °C. Production of H2 and CH4 and consumption of CO2 were observed in almost all the brine at both temperature, especially, larger amount of gases were produced at 50 °C. Those gases production was significantly stimulated with the additional yeast extract, on the other hand, it became lower under high pressure condition.Nutrient agar inoculated with raw brine was incubated under anaerobic conditions at 50 °C and 75 °C. Microbial single colonies formed in the nutrient agar media after 2 weeks were selected and inoculated into sterile brine including sterile oil. More than 20 isolates were isolated and incubated in the brine media and 14 strains were observed to produce H2 after 3 months incubation. The maximum rate of H2 production by HD–HPB was 0.38 NmL/L-medium/day.These results suggest that in situ microbial conversion of sequestrated CO2 and H2 generated by HD–HPB into CH4 using HM can be expected in many oilfields universally. Moreover, the most capable HD–HPB isolated in this study can be injected into reservoirs to stimulate the restoration of natural gas deposits with CO2 sequestration.

► Microbial conversion of CO2 and oil into CH4 is demonstrated without H2 supplement.
► Yeast extract stimulated H2 productivity of the indigenous H2-producing bacteria.
► H2 productivity of the bacteria was dependent on the components of reservoir brine.
► Oil-degrading and H2-producing bacteria were successfully isolated from brine.