High-Pressure adsorption of supercritical methane and carbon dioxide on Coal: Analysis of adsorbed phase density

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The occurrence of supercritical CH4 and CO2 in coal is significant for both coalbed methane recovery and CO2 storage. Adsorption isotherms of CH4 and CO2 on two coals have been measured at three temperatures up to pressures of 25 MPa. Three modified supercritical adsorption models (SL-V, SBET-V, and SDR-V) were used to describe the excess adsorption behavior, with the pressure-independent adsorbed phase volume being considered. The adsorbed phase density (APD) and absolute adsorption amount were calculated based on the modified models and compared with those estimated by previous models. The results indicate that SDR-V is the most suitable for CH4 adsorption due to the dominant role of filling of micropores by CH4, while SBET-V is the most appropriate model for CO2 adsorption because of multilayer adsorption of CO2 in both micropores and meso-macropores. The APD increases with pressure until reaching a maximum value but decreases as temperature increases following a Langmuir-style trend. The previous models, which use a fixed APD, seriously underestimate the absolute adsorption, the extent of which increases with increasing pressure and temperature, leading to significant errors in deep formations with high temperature and pressure conditions. Furthermore, as burial depth increases, the adsorption process can be divided into three stages based on changes in APD and free phase density (FPD). These findings contribute to a comprehensive understanding of the behavior and occurrence of CH4 and CO2 within deep coal seams, enhancing the applicability of CO2-ECBM (CO2 Geological Storage Enhanced Coalbed Methane Recovery).