Physical simulation of in-situ microbial methanation in coal reservoirs with the participation of supercritical CO₂

Supercritical CO₂ can destroy the molecular structure of coal and increase the production of biomethane. To investigate the gas production potential under the in-situ conditions of coal reservoirs with the participation of supercritical CO₂, the original reservoir pressure, reservoir temperature, and gas composition of the target coal seam in a coalbed methane block in Xinjiang were used as the in-situ reservoir conditions. The self-designed anaerobic fermentation device for coal reservoirs was used to simulate the in-situ anaerobic fermentation process, and the biogas production, coal surface functional groups and microbial community structure under the in-situ conditions were analyzed. The results show that under the in-situ conditions of coal reservoirs with supercritical CO₂, the biomethane production reaches 32.9 mL/g, and the CO₂ bioconversion rate is 17.44%. FTIR spectroscopy shows that the ability of microorganisms to degrade oxygen-containing groups in phenol, alcohol, ether and ester under in-situ conditions is stronger than that under the conventional anaerobic fermentation conditions. In the in-situ anaerobic fermentation system with the participation of supercritical CO₂, methanogens with multiple methanogenic metabolic pathways (hydrogenotrophic, acetic acidotrophic and methylotrophic) gradually evolved to single hydrogenotrophic methanogens. Under high pressure, the genus of Solibacillus silvestris became the dominant bacteria in the fermentation stage of hydrolysis and acid production. This article provides an experimental basis for the on-site implementation of coalbed gas bioengineering and carbon emission reduction.