Impacts of Long-Term Exposure to Supercritical Carbon Dioxide on Physicochemical Properties and Adsorption and Desorption Capabilities of Moisture-Equilibrated Coals

Regarding CO2 geologic sequestration in the coal seam, injected CO2 in the coal reservoir at an optimum depth often presents as supercritical fluid (scCO2), which is capable of changing the physicochemical properties of coal and further affecting its CO2 storage capacity. Aiming to better explore the aforementioned influences, we conducted a long-term scCO2–H2O–coal static interaction in the laboratory. The influences of scCO2 exposure on the pore structure characteristics, the main oxygenic functional groups, and the high-pressure CO2 adsorption and desorption capabilities of four rank coals were explored. The results show that long-term scCO2–H2O exposure has minor effects on the coal micropore surface area and volume. On the contrary, it remarkably decreases the mesopore of all of the coals. The alterations in the coal pore structure parameters are mainly relevant to coal matrix swelling and mineral dissolution due to long-term scCO2–H2O exposure. In general, long-term scCO2–H2O exposure increases the Neimark fractal dimension of coals, further implying an elevated pore surface roughness and a more complex pore structure. Moreover, the main oxygenic functional groups, including C–O, C═O, and −COOH, of coals after long-term scCO2 exposure are reduced. The aforementioned changes in the physicochemical properties of coals further affect their CO2 adsorption and desorption capabilities. In particular, the maximum CO2 adsorption capacity of high-rank coals decreases after exposure, whereas that of low-rank coals increases. Furthermore, the CO2 adsorption and desorption hystereses of coals become pronounced after long-term scCO2–H2O exposure, implying more stable CO2 sequestration in the coal seam. In conclusion, the implementation of CO2 sequestration in the coal seam should focus on the long-term scCO2–H2O–coal interaction.