Study on the Adsorption and Thermodynamic Characteristics of Methane under High Temperature and Pressure

The understanding of the adsorption and thermodynamic characteristics of methane adsorbed by shale under high pressure and high temperature (HPHT) is of great significance for evaluating deep shale gas reserves and for optimizing shale gas development. In this study, a strict framework for analyzing the adsorption-thermodynamic characteristics of methane adsorption in shale under HPHT conditions was proposed. Further, data processing and explanation of the framework were carried out through reference to published experimental data. 'When real gas behavior and phase density changes were considered, the temperature-dependent parameters were corrected on the dual-site Langmiur model (DSL) model to reflect the absolute adsorption isotherm at each temperature. These were successfully calculated by combining those with the global fitting method. Compared with other adsorption models that reflected surface heterogeneity, the modified DSL model was found to well characterize the adsorption behavior of shale under HPHT conditions. The thermodynamic potentials of the methane adsorption process were further calculated from the results obtained from the modified DSL model. It was found that in the two groups of shale samples with different total organic carbon (TOC) contents, the change law of isosteric enthalpy with adsorption capacity was different. The abnormal entropy change behavior of methane adsorption in shale with higher TOC content was explained, and a new position concerning the long-standing controversy relating to entropy change was put forward. In addition, both the total entropy and internal energy during the methane adsorbed phase under HPHT increased with an increase in adsorption capacity, and the total entropy in the adsorbed phase revealed a positive relationship with temperature. By introducing a thermodynamic graphic method to draw a clearer and more intuitive U-a-n(a)-S-a three-dimensional surface, more thermodynamic potential information could be obtained. The framework proposed in this study could also be used to help future research on the influence of basic physical parameters on adsorption and thermodynamic properties and to provide a foundation for improving the evaluation of deep shale gas reserves.