Thermodynamic analysis of high pressure methane adsorption in Longmaxi shale

Abstract

Thermodynamic analyses of high pressure methane adsorption in shale are rarely reported because of the lack of a reliable approach for obtaining the true adsorption uptake from observed adsorption isotherms and the routinely used, oversimplified Clausius–Clapeyron (C-C) approximation. This work extends our previously proposed dual-site Langmuir adsorption model to calculate the isosteric heat of adsorption analytically from the observed adsorption isotherms for high pressure methane adsorption isotherms on Longmaxi shale from Sichuan, China (up to 27MPa and 355.15K). The calculated isosteric heat of adsorption considers both the real gas behavior of bulk methane and the adsorbed phase volume, which are neglected in the C–C approximation. By this method, the temperature dependence as well as the uptake dependence of the isosteric heat can be readily investigated, where the former cannot be revealed using the C–C approximation. The influence of the adsorbed phase and the gas behavior (real gas or ideal gas) on the isosteric heat of adsorption are also investigated, which shows that neglecting either the real gas behavior or the adsorbed phase volume always results in an overestimation of the isosteric heat of adsorption. In the Henry’s law regime of low pressure and low adsorption uptake (and up to a surface occupancy of <0.5), the isosteric heat of adsorption of methane on Longmaxi shale is approximately constant at 15–17kJ/mol, but then decreases significantly at higher pressures. This work therefore justifies the method to obtain the true isosteric heat of adsorption for high pressure methane in shale, which lays the foundation for future investigations of the thermodynamics and heat transfer characteristics of the interaction between high pressure methane and shale.