Sequestration of CO2 in oil reservoirs is considered to be one of the feasible options for mitigating atmospheric CO2 building up and also for the in situ potential bioconversion of stored CO2 to methane. However, the information on these functional microbial communities and the impact of CO2 storage on them is hardly available. In this paper a comprehensive molecular survey was performed on microbial communities in production water samples from oil reservoirs experienced CO2-flooding by analysis of functional genes involved in the process, including cbbM, cbbL, fthfs, [FeFe]-hydrogenase, and mcrA. As a comparison, these functional genes in the production water samples from oil reservoir only experienced water-flooding in areas of the same oil bearing bed were also analyzed. It showed that these functional genes were all of rich diversity in these samples, and the functional microbial communities and their diversity were strongly affected by a long-term exposure to injected CO2. More interestingly, microorganisms affiliated with members of the genera Methanothemobacter, Acetobacterium, and Halothiobacillus as well as hydrogen producers in CO2 injected area either increased or remained unchanged in relative abundance compared to that in water-flooded area, which implied that these microorganisms could adapt to CO2 injection and, if so, demonstrated the potential for microbial fixation and conversion of CO2 into methane in subsurface oil reservoirs.

CO2 sequestration in deep-subsurface formations including oil reservoirs is a potential measure to reduce the CO2 concentration in the atmosphere. However, the fate of the CO2 and the ecological influences in carbon dioxide capture and storage (CDCS) facilities is not understood clearly. In the current study, the fate of CO2 (in bicarbonate form; 0∼90 mM) with 10 mM of formate as electron donor and carbon source was investigated with high-temperature production water from oilfield in China. The isotope data showed that bicarbonate could be reduced to methane by methanogens and major pathway of methanogenesis could be syntrophic formate oxidation coupled with CO2 reduction and formate methanogenesis under the anaerobic conditions. The bicarbonate addition induced the shift of microbial community. Addition of bicarbonate and formate was associated with a decrease of Methanosarcinales, but promotion of Methanobacteriales in all treatments. Thermodesulfovibrio was the major group in all the samples and Thermacetogenium dominated in the high bicarbonate treatments. The results indicated that CO2 from CDCS could be transformed to methane and the possibility of microbial CO2 conversion for enhanced microbial energy recovery in oil reservoirs.

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Transformation of CO2 in both carbon capture and storage (CCS) to biogenic methane in petroleum reservoirs is an attractive and promising strategy for not only mitigating the greenhouse impact but also facilitating energy recovery in order to meet societal needs for energy. Available sources of petroleum in the reservoirs reduction play an essential role in the biotransformation of CO2 stored in petroleum reservoirs into clean energy methane. Here, the feasibility and potential on the reduction of CO2 injected into methane as bioenergy by indigenous microorganisms residing in oilfields in the presence of the fermentative metabolite ethanol were assessed in high-temperature petroleum reservoir production water. The bio-methane production from CO2 was achieved in enrichment with ethanol as the hydrogen source by syntrophic cooperation between the fermentative bacterium Synergistetes and CO2-reducing Methanothermobacter via interspecies hydrogen transfer based upon analyses of molecular microbiology and stable carbon isotope labeling. The thermodynamic analysis shows that CO2-reducing methanogenesis and the methanogenic metabolism of ethanol are mutually beneficial at a low concentration of injected CO2 but inhibited by the high partial pressure of CO2. Our results offer a potentially valuable opportunity for clean bioenergy recovery from CCS in oilfields.

<p><strong>Abstract.</strong> Sequestration of CO₂ in oil reservoir is one of the feasible options for mitigating atmospheric CO₂ building up. The in situ bioconversion of sequestrated CO₂ to methane by microorganisms inhabiting oil reservoirs is feasible. To evaluate the potential of in situ microbial fixation and conversion of CO₂ into CH₄ in oil reservoirs, a comprehensive molecular survey was performed to reveal microbial communities inhabiting four oil reservoirs with different temperatures by analysis of functional genes involved in the biochemical pathways of CO₂ fixation and CH₄ synthesis (<i>cbbM</i>, <i>cbbL</i>, <i>fthfs, [FeFe]-hydrogenase</i> encoding gene, and <i>mcrA</i>). A rich diversity of these functional genes was found in all the samples with both high and low temperatures and they were affiliated to members of the <i>Proteobacteria</i> (<i>cbbL</i> and <i>cbbM</i>, <i>fthfs</i>), <i>Firmicutes</i> and <i>Actinobacteria</i> (<i>fthfs</i>), uncultured bacteria (<i>[FeFe]</i>-hydrogenase), and <i>Methanomirobiales</i>, <i>Methanobacteriales</i> and <i>Methanosarcinales</i> (<i>mcrA</i>). The predominant methanogens were all identified to be hydrogenotrophic CO₂-reducing physiological types. These results showed that functional microbial communities capable of microbial fixation and bioconversion of CO₂ into methane inhabit widely in oil reservoirs, which is helpful to microbial recycling of sequestrated CO₂ to further new energy in oil reservoirs.</p>