Mechanisms of Methane Occurrence in Strong and Weak Adsorption Zones of Coal Micropores: A Multifactor Decoupling Analysis

Methane adsorption is governed by multifactor synergies, leading to a complex occurrence mechanism. Based on micropore filling and adsorption potential theories, this study develops a coupled adsorption model differentiating strong and weak adsorption zones. This study utilizes molecular simulations to analyze the independent effects of the temperature, pressure, and pore size on methane occurrence in strong and weak adsorption zones. By integration of isothermal adsorption experiments and pore size distribution measurements, the influence of coal chemical properties was decoupled. Furthermore, an XGBoost-based predictive model was developed to estimate methane occurrence parameters under the combined influence of multiple factors. The results indicate that increasing pore size and temperature amplify differences in methane density and content between strong and weak adsorption zones, whereas elevated pressure attenuates these effects. Temperature and pore size reduce the adsorption constant <i>b</i> (1/<i>P</i>_L) in both zones, with a more pronounced effect in the weak adsorption zone. Temperature has a minimal influence on the <i>b</i>₁/<i>b</i>₂ ratio, while pore size exerts a significant impact. Additionally, the proportion of strongly adsorbed methane increases slightly with temperature but decreases markedly with larger pore sizes and elevated pressures. Adsorption parameters, including gas density, generally rise rapidly before stabilizing as the coal rank (<i>R</i>_o,max) increases, with a turning point near 1.5%. Methane densities computed from 12 coal ranks using actual molecular structures align with model predictions with over 85% accuracy, confirming the model's reliability.