High-Pressure Adsorption–Desorption Mechanism of CH ₄ /CO ₂ in Supercritical CO ₂ Sequestration and Impact on Residual Sequestration Capacity

This study investigates the adsorption–desorption behavior of supercritical CH4 and CO2 in coals from the Liuzhuang and Qidong mines (Huainan-Huaibei coalfield, Anhui, China) through high-pressure isothermal adsorption experiments under varying temperature and pressure conditions. Excess adsorption data were corrected using the intercept method to obtain characteristic adsorption/desorption parameters, and an improved Dubinin-Astakhov (D–A) model was applied to construct adsorption phase density evolution curves, enabling precise calculation of absolute adsorption and quantitative characterization of hysteresis indices. Multiscale fractal analysis was employed to elucidate the relationships between pore structural features, adsorption capacity, and hysteresis. Results show that the excess adsorption peaks of CH4 and CO2 occur at approximately 7.5 and 6 MPa, respectively, with CH4 peaks shifting to lower pressures as temperature increases. A strong linear correlation between free-phase density and excess adsorption in the postpeak high-pressure region supports the use of the intercept method for parameter correction, with the modified D–A model accurately describing supercritical adsorption. Both CH4 and CO2 exhibit pronounced hysteresis, with hysteresis indices decreasing nonlinearly with increasing pressure and showing nonmonotonic temperature dependence. Pore fractal dimension is negatively correlated with adsorption capacity but positively correlated with hysteresis, indicating that reduced structural complexity enhances adsorption capacity and lowers hysteresis. The irreversible adsorption induced by hysteresis provides a key indicator for evaluating the residual sequestration potential of coalbed methane.