MICROBIAL METHANOGENESIS IN OIL RESERVOIRS SUBJECTED TO THE INJECTION OF CO2

Geochemical evidences showing that 1) CH₄ is a major end product of oil biodegradation in oil reservoirs, and 2) over 20% of Earth’s biogenic CH₄ have been formed by methanogenic microbes via the reduction pathway of CO₂ with H₂, suggest that the combined injection of CO₂ and stimulating nutrients should result in the biogenic conversion of oil and CO₂ to CH₄ by indigenous microbes. To prove this hypothesis, batch experiments have been carried out using a mixture culture of H₂-using methanogenic microbes and H₂-forming fermentative microbes. It has been found that moderate acidic pH levels and the addition of protein-rich matter stimulates the biogenic formation of CH₄ via the reduction pathway of CO₂ with H₂. To assess the feasibility of microbial methanogenesis in geological CO₂ storage sites subjected to the injection of CO₂ and stimulating nutrients, multiphase reactive transport simulations of microbial methanogenesis have been performed using a reactive transport model that accounts for 1) water-rock geochemical reactions, 2) microbial competition between methanogenic microbes and sulfate reducing bacteria (SRB) for H₂ and acetate, and 3) the effect of pH on the activity of the microbial community. Simulation results indicate that oil reservoirs containing calcite in the mineral phase and low concentrations of sulfate in the formation water are most suitable for microbial methanogenesis from CO₂ in geological CO₂ storage sites. On the basis of these preliminary experimental and numerical simulation studies, in this presentation the feasibility of stimulating the microbial methanogenesis from CO₂ and oil in depleted oil reservoirs will be discussed. The concretization of this possibility can result in the attenuation of the depletion of fossil energy resources and reduction of CO₂ emissions into the atmosphere.