A Modified Simplified Local Density Model for Characterizing and Predicting High-Pressure Methane Adsorption in Deep Coal Seams

Abstract Methane, the primary component of coalbed methane (CBM), occurs predominantly in the adsorbed state in coal. Accurate characterization and prediction of methane adsorption under high-temperature and high-pressure conditions are therefore essential for evaluating deep CBM resources. However, conventional adsorption models generally have limited extrapolation capacity and often oversimplify the temperature dependence of the adsorbed-phase density. In this study, a modified simplified local density (mSLD) model was developed by incorporating a thermal-expansion correction for the adsorbed phase into the original SLD framework. The model was evaluated using more than 1000 adsorption data points for 29 coal samples reported in the literature. It accurately represented multiple methane adsorption isotherms over broad temperature and pressure ranges, yielding an average absolute deviation of 2.53%, a root-mean-square error of 0.021 mmol/g, and a coefficient of determination of 0.995. Compared with the original SLD model, the mSLD model substantially improved high-pressure prediction accuracy. It also maintained robust performance under pressure extrapolation, temperature extrapolation, and temperature interpolation, while requiring only approximately two-thirds of the experimental data to achieve nearly the same accuracy as fitting the complete dataset. The model results further showed that specific surface area was the principal factor controlling methane adsorption capacity and that methane adsorption occurred preferentially in micropores (<2 nm). When coupled geothermal and pressure gradients were considered, the absolute adsorption capacity initially increased with burial depth, reached a maximum at an adsorption critical depth of approximately 1250 m, and subsequently decreased because of the increasing inhibitory effect of temperature. Accordingly, adsorbed gas dominated in shallow coal seams, whereas the proportion of free gas gradually increased with depth. The total gas content reached a maximum at approximately 2700 m and decreased only slightly at greater depths. These results demonstrate that the mSLD model provides a robust tool for characterizing methane adsorption over broad temperature and pressure ranges and for predicting gas content in deep coal seams.