Abstract

CO2 sequestration into depleted oil reservoir has been expected as a method of reducing CO2 emission. Moreover, the authors focus on in-situ microbial conversion process of carbon dioxide into methane by hydrogenotrophic methanogens that inhabit oil reservoir universally. It is important for this process how to supply hydrogenotrophic methanogens with hydrogen for their methane production in reservoir. This study is aimed at searching for the oil-degrading and hydrogen-producing thermophilic bacteria (ODHPTB) which can produce hydrogen from oil in reservoir brine.

Reservoir brine was extracted from 10 producing wells in Yabase oilfield in Japan. Indigenous bacteria in brine samples were incubated with sterilized oil under anaerobic conditions (10% CO2 balance N2) at 50°C and/or 75°C. Both the production of hydrogen and methane and the consumption of carbon dioxide were observed in almost all culture solutions after 2months incubation. The maximum rate of hydrogen production was 20.9 Nml/L-medium/day.

These culture solution and raw brine were inoculated into nutrient agar medium and incubated under anaerobic conditions at 50°C and 75°C. Microbial single colonies formed in the nutrient agar medium after 2weeks incubation were picked and inoculated into sterilized brine including sterilized oil as a hydrogen source. More than 40 strains were isolated and incubated in the brine medium and 24 strains were observed to produce hydrogen from oil after 1 month incubation. The maximum rate of hydrogen production was 1.0 Nml/L-medium/day.

These results show that the in-situ microbial conversion process of carbon dioxide and residual oil into methane using ODHPTB and hydrogenotrophic methanogens is promising. Moreover, the most talented ODHPTB that was isolated in this study can be injected into reservoir in order to stimulate the conversion of carbon dioxide into methane.

Introduction

The techniques for CO2 sequestration in subsurface has been studied and developed actively. Depleted oil reservoir, coal seams and aquifers are expected as subsurface sites for CO2 sequestration. In particular, depleted oil reservoir are expected to be utilized effectively because they have huge capacities with tight cap rocks and there are already injecting system such as injecting pumps and wells which can be utilized for CO2 sequestration. On the other hand, the consumption of natural gas is expected to increase significantly in the future because it is an energy source with low environmental load1). It will be increasingly significant to supply natural gas stably for long term2). Therefore, it is important to develop new valid techniques which can solve both issues; reduction of CO2 emission and long-term stable supply of natural gas.

The authors focus on in-situ microbial conversion process of carbon dioxide into methane by hydrogenotrophic methanogens which inhabit oil reservoir universally. This process has not only a potential of reducing CO2 emission but also a potential of producing methane in reservoir.

Fujiwara et al. investigated indigenous hydrogen-producing thermophilic bacteria (HPTB) and methane-producing thermophilic archaea (MPTA) which inhabit oil reservoir3). It was shown that there were many kinds of HPTB and MPTA in reservoir universally and three kinds of HPTB and MPTA were isolated from reservoir brine samples. They demonstrated the potential of the microbial conversion process of carbon dioxide into methane by enrichment culture experiments using indigenous bacteria which inhabit oil reservoir. In addition, they also observed oil-degrading and hydrogen-producing thermophilic bacteria (ODHPTB) which could produce hydrogen from oil in reservoir in their another study4). It should be preferable for the residual oil in reservoir to become usable as a suitable and economical hydrogen source of HPTB.

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