A novel approach for predicting adsorbed and free gas content: Insights from adsorbed phase density and volume in supercritical conditions

The quantitative characterization of the storage characteristics of methane in different phases within coal is crucial for assessing the gas content structure and production dynamics of deep coal reservoirs. This study conducts methane isothermal adsorption experiments under high temperature (313.15 K, 323.15 K, and 333.15 K) and high pressure (P > 20 MPa) conditions. Based on a modified adsorbed phase density (ρ_a), adsorbed phase volume (V_ad) and adsorption potential models (ε-modified), the gas content was reevaluated. The results indicate that the supercritical Dubinin-Radushkevich (SDR, ρ_a-calculated) model more effectively captures the adsorption behavior of methane and the differences in intermolecular interactions. The study reveals that micropore filling adsorption exhibits higher isosteric heat of adsorption and lower entropy change compared to monolayer adsorption. The ε-modified (ρ_a-calculated) model not only better matches with the characteristics of dispersion forces, maintaining the adsorption potential independently of temperature, sheds light on the interactions between coal molecules and methane molecules. Based on the differences in densities and content of different phase methane, the adsorbed gas is characterized by a critical content depth and a critical density depth. Before reaching the critical density depth, the adsorption gas content initially increases with depth and then decreases. After surpassing the critical density depth, the adsorption gas content increases again with depth. This study introduces a novel gas content prediction model based on the modified ρ_a, ρ_g and ε parameters, align well with theoretical values. The model provides a more precise method for evaluating the adsorption behavior and occurrence state of methane in in-situ reservoirs.