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 CO 2 sequestration in coal seams with enhanced coalbed methane (CH 4 ) recovery (CO 2 -ECBM). To assess the influence of deionized water–CO 2 mixture (DH 2 O–CO 2 ) exposure on these properties, the interaction of DH 2 O–CO 2 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 (N 2 /CO 2 ) 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 DH 2 O–CO 2 as demonstrated by the change in the content of clays, carbonates, and sulfates in the coal matrix. DH 2 O–CO 2 exposure also causes a decrease in the content of organic oxygen and carbon–oxygen functional groups, especially for COOH groups, but an increase in C–C/C-H species, and the impact is strengthened with the decreasing coal rank. The aforementioned aspects illustrate the reconfiguration of surface geometry and the chemical interaction between DH 2 O–CO 2 and the oxygen-containing functional groups. In combination with the reduced volatile matter and organic sulfur groups, the results imply that the extraction effect and chemical interaction may contribute to the change in coal surface chemistry. DH 2 O–CO 2 interaction degrades the accessibility of micropores of all the coals, whereas the reverse trend is found for macropores and fractures. DH 2 O–CO 2 interaction facilitates the development of macropores and fractures and thus improves the permeability of coal seams, which can be attributed to the dissolution and mobilization of mineral matters by the acid water and the shrinkage of coal induced by the water loss. The variation of mesopores due to DH 2 O–CO 2 exposure is strongly related to coal rank and surface chemistry. Specifically, the decreasing mesoporosity is found for SBC, whereas this trend is opposite for HVBC and anthracite. The relationship between adsorption pores (<100 nm) and macropores and fractures of SBC decreases, while the opposite trend is recorded in HVBC and anthracite after fluid exposure. The results obtained from this work indicate that the net effect of the surface chemistry and the accessibility of porosity in various rank coals after fluid exposure is crucial for the potential of CH 4 recovery and CO 2 storage during the implementation of the CO 2 -ECBM process.