Effects of Sub- and Supercritical CO2 on Coal Diffusivity and Surface Thermodynamics

The diffusion mechanism of CH4 and CO2 on the coal matrix is fundamentally important for enhanced coal seam gas recovery and long-term CO2 sequestration in coal reservoirs. To investigate the influence of different-phase CO2 on the gas mass transfer and surface thermodynamics in the coal matrix, high volatile bituminous coal samples were prepared and treated with subcritical CO2 (SbCO2; 5 MPa and 318 K) and supercritical CO2 (ScCO2; 10 and 15 MPa, 318 K). The bidisperse model is used for sorption kinetics analysis. The results show that both effective macropore and micropore diffusivities of coal decreased after SbCO2 treatment but increased after ScCO2 treatment. The CO2 exposure pressure has a significant influence on sorption kinetics. A positive correlation existed between the effective micropore diffusivity and micropore surface area, demonstrating that the slow diffusion stage is dominated by surface diffusion in the micropores. In addition, micropore diffusion selectivity of coal multiplied after 10 MPa ScCO2 treatment. The standard enthalpy of adsorption increased with the CO2 pressure, enhancing the adsorption energy. The enhanced adsorption energy further hinders the gas mass transfer on the adsorbent surface.