Coal Reservoir Modification by Supercritical CO ₂ Extraction and Anaerobic Digestion of Coal: Implications for Carbon Emission Reduction

Coal reservoir is considered as the ideal sites of coalbed methane (CBM) occurrence, biomethanation and CO2 sequestration. Four coal samples with different rank from China were selected to conduct supercritical CO2(SC-CO2) extraction and anaerobic digestion (AD). X-ray photoelectron spectroscopy, mercury intrusion and isothermal adsorption experiments were performed on raw coal samples and the remaining coal samples after SC-CO2-AD to analyze the mechanism of enhanced CBM by reservoir modification. Results show that the content of oxygen-containing functional groups and the specific surface area of micropores in coal decrease continually after SC-CO2-AD, which contributes a significant increase in methane desorption capacity of coal. The diffusion capacity of CBM increases because of the continuous decrease of pore tortuosity in coal after SC-CO2-AD. The increase of coal permeability is determined by CO2 phase transformation and the increase of large pores and total pore volume. The reservoir modification by increasing desorption, diffusion and permeability after SC-CO2-AD is beneficial to CBM production. Additionally, the generation of biomethane increases the gas phase pressure, the dynamic force of fluid migration and the critical desorption pressure, which promotes CBM production. This novel approach can provide a new thinking for CO2 emission reduction.