Development of Microbial Conversion Process of Residual Oil to Methane in Depleted Oil Fields

Abstract

Abstract Methane is a clean-burning fuel that is the main component of natural gas. We have attempted to develop a methane-producing system using indigenous microbes in depleted oil fields. In particular, we aim to combine a microbial conversion of the residual oil into methane with the geological sequestration of carbon dioxide: First, hydrogen-producing bacteria are harnessed to produce hydrogen from residual petroleum components in the depleted oil reservoir. Next, methane-producing microbes (methanogens) utilize the hydrogen and carbon dioxide, which is injected for geological sequestration, to generate methane. Resulting methane is therefore a carbon-neutral fuel. As the first step, we successfully isolated multiple hydrogen- and methane-producing microbes (10 hydrogen-producing bacteria and 4 methanogens) from depleted oil fields in Japan. Strikingly, our analysis of inoculums incubated under high temperature and high pressure, the condition similar to native conditions inside of petroleum reservoirs, revealed that indigenous microbes in the reservoir brine are capable of generating methane by utilizing crude oil and carbon dioxide. Consumption/production rate of gases (methane, carbon dioxide) and acetic acid indicated that the methane production under reservoir conditions is likely mediated through two pathways; the aceticlastic (acetic-acid utilizing) and the hydrogenotrophic (hydrogen and carbondioxide utilizing) pathways. Furthermore, by analyzing methane-producing ability of isolated microbes, we found that the syntrophic cooperation between hydrogen-producing bacteria and methanogens is critical for the methane production under the resavoir condition. We are currently determining optimal combination(s) of these microbes to achieve an efficient methane production in the depleted oil field. Our next step will be to establish a method to enhance methanogenic activity of the microbes as well as an effective methanogenesis in porous media, the condition inside of actual reservoirs (porous rock formations). Ultimately, our endeavor will enable a sustainable carbon-cycling system that converts residual oil and stored carbon dioxide in depleted oil fields into environmentally friendly bio-methane.