Efficiency assessment of underground biomethanation with hydrogen and carbon dioxide in depleted gas reservoirs: A biogeochemical simulation

Underground biomethanation, which converts hydrogen and carbon dioxide to methane with the catalysis of methanogens in geological formations, has great potential for carbon dioxide utilization and sequestration, renewable natural gas production, and large-scale energy storage. However, the efficient conversion of hydrogen and carbon dioxide in a complex reservoir environment has not been explored. To address this issue, a novel biogeochemical model is developed for underground biomethanation that considers reservoir environment factors (e.g. pH, temperature, and salinity) and integrated into PHREEQC software. The biogeochemical model is validated with a laboratory experiment and utilized to investigate the effects of reservoir parameters and injection parameters on biomethanation efficiency in depleted gas reservoirs. Results show that the biomethanation efficiency is 94.2% after 360 days in both sandstone and carbonate reservoirs. Underground biomethanation can be completed in 30 days if initial biomass and optimum specific growth rate increase and decay rate decreases. Additionally, the optimal ratio of injected hydrogen and carbon dioxide for biomethanation is greater than 4:1 and increases with total pressure if it is above 70 atm. To improve the biomethanation efficiency, this study suggests utilizing geothermal energy and pre-injecting highly active methanogens cultured on the ground before mixed gas injection.