Influence of CO2 Exposure on High-Pressure Methane and CO2 Adsorption on Various Rank Coals: Implications for CO2 Sequestration in Coal Seams

There exist complex interactions between coal and CO2 during the process of CO2 sequestration in coal seams with enhanced coalbed methane recovery (CO2-ECBM). This work concentrated on the influence of CO2 exposure on high-pressure methane and CO2 (up to 10 MPa) adsorption behavior of three types of bituminous coal and one type of anthracite. The possible mechanism of the dependence of CO2 exposure on adsorption performance of coal was also provided. The results indicate that the maximum methane adsorption capacities of various rank coals after CO2 exposure increase by 3.45%-10.37%. However, the maximum CO2 adsorption capacities of various rank coals decrease by 9.99%-23.93%. TG and pore structure analyses do not observe the obvious changes on the inorganic component and pore morphology of the coals after CO2 exposure. In contrast, CO2 exposure makes changes in surface chemistry of the coals, according to the results from FTIR analysis, which is the main reason for increases in the maximum adsorption capacity of methane and decreases in the maximum adsorption capacity of CO2 for the coals after CO2 exposure. The different role of CO2 exposure on methane and CO2 adsorption is detrimental to CO2-ECBM. Thus, the implementation of CO2-ECBM must take into account the influence of CO2 exposure on the adsorption performance of the target coal seams.