Comprehending the mechanism of methane adsorption in shales is a crucial step towards optimizing the development of deep-buried shale gas. This is because the methane adsorbed in shale represents a significant proportion of the subsurface shale gas resource. To properly characterize the methane adsorption on shale, which exhibits diverse mineral compositions and multi-scale pore sizes, it is crucial to capture the energy heterogeneity of the adsorption sites. In this paper, a dual-site Langmuir model is proposed, which accounts for the temperature and pressure dependence of the density of the adsorbed phase. The model is applied to the isothermals of methane adsorption on shale, at pressures of up to 30 MPa and temperatures ranging from 40 to 100 °C. The results show that the proposed model can describe the adsorption behavior of methane on shale more accurately than conventional models, which assume a constant value for the density of adsorbed phase. Furthermore, the proposed model can be extrapolated to higher temperatures and pressures. Thermodynamic parameters were analyzed using correctly derived equations. The results indicate that the widely used, but incorrect, equation would underestimate the isosteric heat of adsorption. Neglecting the real gas behavior, volume of the adsorbed phase, and energy heterogeneity of the adsorption sites can lead to overestimation of the isosteric heat of adsorption. Furthermore, the isosteric heat evaluated from excess adsorption data can only be used to make a rough estimate of the real isosteric heat at very low pressure.

Shale gas has attracted increasing attention as a potential alternative gas in recent years. Because a large fraction of gas in shale formation is in an adsorbed state, knowledge of the supercritical methane adsorption behavior on shales is fundamental for gas-in-place predictions and optimum gas recovery. A practical model with rigorous physical significance is necessary to describe the methane adsorption behavior at high pressures and high temperatures on shales. In this study, methane adsorption experiments were carried out on three Lower Silurian Longmaxi shale samples from the Sichuan Basin, South China, at pressures of up to 30 MPa and temperatures of 40, 60, 80, and 100 °C. The simplified local density/Elliott–Suresh–Donohue model was adopted to fit the experimental data in this study and the published methane adsorption data. The results demonstrate that this model is suitable to represent the adsorption data from the experiments and literature for a wide range of temperatures and pressures, and the average absolute deviation is within 10%. The methane adsorption capacity of the Longmaxi shale exhibited a strong linear positive correlation with the total organic carbon content and a linear negative correlation with increasing temperature. The rate of decrease in the methane adsorption capacity with swing temperature increased with the total organic carbon content, indicating that the organic matter is sensitive to temperature.