Coal reservoir as a natural bioreactor for methane formation from coal and carbon dioxide

Coalbed gas bioengineering is an emerging approach for enhancing coalbed methane production while enabling biological carbon utilization. Here, a static-flow two-phase anaerobic digestion system is established to comprehensively investigate the synergistic methanation of coal and carbon dioxide through activation of indigenous microbial communities under in situ reservoir conditions. The coupled static and dynamic stages simulate the shut-in period following hydraulic fracturing and the subsequent production phase, respectively. The experimental results demonstrate that methane production is dominated by indigenous microbial communities whose core composition remains stable, indicating low ecological disturbance. Injection of nutrient and CO₂ into coal seams via hydraulic fracturing effectively activates microbial activity, yielding a methane production of 13.28 mL/g coal. Notably, during the late gas-production stage, when nutrients are supplied only by replenished coal-seam water, the daily CH₄ yield is maintained at 0.013 mL/g coal, indicating persistent methanogenic activity. Upscaled to the control volume of a single well, the CO₂-enriched environment significantly enhances the activity and functional expression of hydrogenotrophic methanogens, with cumulative CO₂-derived methane accounting for approximately 65% of the total gas production. These results demonstrate the strong potential of coal reservoirs as subsurface platforms for coupling microbial methanation with carbon management, providing a sustainable pathway toward integrated energy production and carbon-neutral development.