Re-cognition of adsorption phase density and adsorption phase volume: Insights from methane adsorption behavior

The determination of adsorption phase density significantly influences the assessment of gas occurrence state and gas content within in-situ reservoirs, particularly for the evaluation of gas content in deep coalbed reservoirs. Despite the extensive research on adsorption phase density, various calculation methods encounter limitations due to the inability to directly measure it. Simultaneously, the considerable disparity in results obtained from various adsorption phase density methods hampers the accurate determination of adsorbed gas and free gas content in deep coal seams. To address these challenges, this study systematically reviews calculation methods and highlights issues related to adsorption phase density. Considering practical experimental conditions, two new calculation methods are proposed: the adsorption potential adsorption phase density (APD) method and limit adsorption phase density (LAD) method. These methods enable the directional calculation of adsorption phase density based on the unknown or known absolute adsorption amount. The obtained results reveal that the adsorption phase density obtained through the APD method exhibits variations in temperature and pressure. For different coal rank samples, the range of actual adsorption phase density at different temperatures under high pressures is expected to fall between 0.30 g/cm3 and 0.40 g/cm3. The range can serve as a theoretical upper limit value for refining adsorption phased density models. The adsorption phase density acquired through the LAD method exhibits variations in temperature and pressure, whereas the adsorption phase volume remains unaffected by changes in temperature and pressure. The computed outcomes from the LAD method align with those derived from nuclear magnetic resonance (NMR), adsorption phase volume (APV), and molecular simulation methods, affirming the accuracy and applicability of the adsorption phase density determined via the LAD method. It is envisaged that these research findings will enhance the precise determination of adsorption phase density and assist in discerning the occurrence state of deep coalbed methane and, potentially, carbon dioxide sequestration.