Thermodynamic adsorption mechanism of supercritical methane and application to Ordos coal

Coal, as a heterogeneous porous medium, exhibits substantial adsorption capacity for coalbed methane, highlighting the necessity to comprehend methane storage mechanisms for reserve evaluation and production optimization. To investigate the adsorption characteristics of methane, a novel approach for calculating thermodynamic parameters is developed based on phase equilibrium and fugacity principles. Isothermal adsorption experimental results are utilized to assess the influence of moisture on the adsorption mechanism, disclosing variations in enthalpy, entropy, and Gibbs free energy for supercritical methane. The findings reveal a pronounced reduction in adsorption capacity under hydrated conditions. Methane adsorption onto coal is identified as an exothermic, entropy-decreasing, non-spontaneous process dominated by microporous filling and monolayer adsorption. The supercritical Langmuir and Dubinin–Radushkevich model confirms that microporous filling accounts for over 10% of the total adsorption capacity under high-temperature and high-pressure conditions. Additionally, temperature changes within coal seams during distinct production phases, ranging from 1 to 18 K, are quantified. A predictive method for isothermal adsorption curves using thermodynamic parameters is introduced, with an accuracy within a 6% error margin. These insights offer theoretical support for modeling gas–water flow dynamics in coalbed methane reservoirs.