Combined Effects of In Situ Stress and Moisture on CO2 Adsorption Characteristics in Deep Coal Seams

Abstract Deep coalbed methane (CBM) has emerged as an indispensable resource for expanding worldwide natural gas production and reserves. Nevertheless, the combined impact of in situ stress and moisture on gas adsorption properties under deep geological conditions remains poorly documented, severely restricting the efficient exploitation of deep CBM. This work comprehensively explored the combined effects of stress and moisture on the gas adsorption capacity. By introducing a moisture attenuation coefficient and a stress correction factor, we developed a modified Langmuir model to account for these synergistic effects. CO2 isothermal adsorption experiments were carried out on both dry and moist coal samples at varying hydrostatic pressures (4, 8, and 12 MPa). The findings demonstrate that CO2 adsorption in coal conformed to typical Langmuir behavior, with the proposed modified model exhibiting excellent agreement with the experimental data. Notably, hydrostatic pressure severely inhibited the CO2 adsorption capacity of coal, an effect that was markedly amplified in the moist samples. At an equilibrium pressure of 1 MPa, increasing the hydrostatic pressure from 4 to 12 MPa reduced the CO2 adsorption capacity of the dry coal samples by 29.81%, whereas the moist samples experienced a drastic decrease of 59.95%. Furthermore, moisture consistently suppressed the level of CO2 adsorption across the entire equilibrium pressure range. Investigations into the evolution of the specific surface area (SSA) revealed that both the hydrostatic pressure and moisture independently restricted the SSA of coal to varying degrees. Finally, parametric sensitivity analysis was conducted to quantify how the Langmuir maximum adsorption capacity (a) and adsorption equilibrium constant (b) affected CO2 adsorption predictions. These findings provided a theoretical reference for analyzing gas adsorption rules in deep coal seams and optimizing CBM extraction schemes.