| Summary: | Carbon dioxide (CO<sub>2</sub>) has been used to replace coal seam gas for recovery enhancement and carbon sequestration. To better understand the alternations of coal seam in response to CO<sub>2</sub> sequestration, the properties of four different coals before and after supercritical CO<sub>2</sub> (ScCO<sub>2</sub>) exposure at 40 °C and 16 MPa were analyzed with Fourier Transform infrared spectroscopy (FTIR), low-pressure nitrogen, and CO<sub>2</sub> adsorption methods. Further, high-pressure CO<sub>2</sub> adsorption isotherms were performed at 40 °C using a gravimetric method. The results indicate that the density of functional groups and mineral matters on coal surface decreased after ScCO<sub>2</sub> exposure, especially for low-rank coal. With ScCO<sub>2</sub> exposure, only minimal changes in pore shape were observed for various rank coals. However, the micropore specific surface area (SSA) and pore volume increased while the values for mesopore decreased as determined by low-pressure N<sub>2</sub> and CO<sub>2</sub> adsorption. The combined effects of surface property and pore structure alterations lead to a higher CO<sub>2</sub> adsorption capacity at lower pressures but lower CO<sub>2</sub> adsorption capacity at higher pressures. Langmuir model fitting shows a decreasing trend in monolayer capacity after ScCO<sub>2</sub> exposure, indicating an elimination of the adsorption sites. The results provide new insights for the long-term safety for the evaluation of CO<sub>2</sub>-enhanced coal seam gas recovery.
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