Understanding the adsorption behaviors of CO2 and CH4 is of significance to exploit shale gas by CO2 injection. This paper simulates the adsorption of pure CO2, CH4 and their mixtures on kerogen for different pressures and compositions. The stability of adsorption, the affinity between gas and kerogen, and the efficiency of injecting CO2 to displace adsorbed CH4 are discussed. Results show that kerogen adsorbs CO2 more preferentially than CH4. The presence of CO2 causes a minimum and maximum in the adsorption isotherms of CH4 with increasing pressure. CO2 is adsorbed more stably than CH4 overall but adsorbed CO2 molecules are more heterogeneous in stability than CH4. The increasing pressure and CO2 mole fraction in free gas result in the decrease in adsorption selectivity of CO2 over CH4 under high pressure, while the trends are reversed under low pressure. Within the exploitable pressure range, the effects of injecting CO2 for displacing adsorbed CH4 are similar between different reservoir pressures. The adsorbed CH4 will be displaced more intensively in the early stage of CO2 injection than other stages. This research provides some recognitions for recovering shale gas by injecting CO2.
Keywords carbon dioxide, shale gas, adsorption, carbon utilization, carbon storage, molecular simulation