Influence of Fluid Exposure on Surface Chemistry and Pore-Fracture Morphology of Various Rank Coals: Implications for Methane Recovery and CO2 Storage

The surface chemistry and pore-fracture morphology of coals are critical to the process of CO2 sequestration in coal seams with enhanced coalbed methane (CH4) recovery (CO2-ECBM). To assess the influence of deionized water–CO2 mixture (DH2O–CO2) exposure on these properties, the interaction of DH2O–CO2 with three rank coals, i.e., sub-bituminous coal (SBC), high volatile bituminous coal (HVBC), and anthracite, was conducted on a dynamic supercritical fluid extraction system with a temperature of 45 °C and an equilibrium pressure of 12 MPa. Characterization methods including proximate analysis (PA), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), probe molecule (N2/CO2) adsorption, and low-field nuclear magnetic resonance (NMR) were adopted to fully address the changes in surface functional groups and pore-fracture characteristics. The results indicate that the geochemical interaction occurs between the mineral matters and DH2O–CO2 as demonstrated by the change in th...