Segmented Desorption Hysteresis of CO2-Rich Coalbed Methane in High-Volatile Bituminous Coal: Possible Associations with Pore Structure

To address engineering challenges from large fluctuations in gas concentration and low CH4 recovery during the development of CO2-rich coalbed methane in the southern Junggar Basin, high-volatile bituminous coal samples were selected. Adsorption–desorption experiments were conducted separately with pure CH4, pure CO2, and their binary mixture. Multiscale pore characterization was used to examine possible associations between pore structure and desorption hysteresis. The results indicate that ① Both the gas adsorption affinity and the degree of desorption irreversibility follow the order of pure CO2 > the binary-mixture > pure CH4. The fitted Langmuir volume of pure CO2 was 1.60–2.64 times that of pure CH4. This difference suggests that stronger CO2 adsorption may inhibit CH4 uptake. The binary-mixture results represent only apparent total adsorption. ② CH4 showed early release at high pressures, and its desorption hysteresis decreased at higher temperatures. CO2 showed limited desorption over 9–3 MPa and a marked increase in the amount desorbed per unit pressure decrease over the measured 3–1 MPa interval. ③ Comparisons among the four coal samples suggest gas-dependent associations between pore characteristics and desorption hysteresis. The pore-structure comparisons suggest possible associations between CH4 hysteresis and mesopore structural complexity, and between CO2 hysteresis and micropore development. ④ Hysteresis in CO2-containing systems may reflect the combined effects of pore confinement and transport restrictions. CO2-induced matrix swelling may change slit-pore geometry and pore accessibility during desorption. This study provides evidence on gas-dependent desorption hysteresis in high-volatile bituminous coal, offering theoretical support for gas component prediction, recovery enhancement, and CO2 sequestration.