Microbial-mediated CO2 methanation and renewable natural gas storage in depleted petroleum reservoirs: A review of biogeochemical mechanism and perspective

Renewable natural gas (RNG) production by microbial-mediated CO2 methanation has been identified as a key for future systems relying on renewable and zero-carbon power, which provides a twin dividend of energy-environmental significance and is critical to facilitate China's pledge to be carbon neutral by 2060. Here we provide an overview of the hydrogenotrophic methanation coupled with subsurface CO2 utilization and renewable natural gas storage, giving special focuses on the biogeochemical mechanism and potential applications for renewable energy conversion and storage. Taking injected CO2 and H2 as substrates and subsurface petroleum reservoir as a bioreactor, indigenous methanogenic archaea can induce bio-methane generation by offering essential functional groups and electron carriers. Detailed reviews are provided on the subsurface biogeochemical mechanism including microorganisms, metabolic pathways, environmental conditions, and substrates utilization. Bio-methane-related energy conversion and storage can be achieved by coupling “microbial restoration of methane deposit” (MRMD) with CO2 emission-free power plants, which provides flexibility to balance the unstable power demands and bring additional commercial value for CO2 geological storage. Biological methanation can also be integrated into the “power-gas-power” (PGP) system, yielding advantages in flexibility, large-scale capacity, long-term storage cycle and CO2 emission reduction. The techno-economic status and perspectives are discussed for the purpose of enhancing substrate utilization, energy efficiency and economic feasibility that hold great promise for further practical implementation. Its early potential in northwest China is particularly emphasized. Based on the current technical barriers and energy policy, challenges and future research emphasis of biological methanation in depleted petroleum reservoirs are also presented.