Methane adsorption on shales and application of temperature-related composite models based on dual adsorption modes

There are some controversies in shale gas about adsorption mechanisms and thermodynamics. This paper studied the adsorption of methane on two shale samples at pressures up to 20 MPa and temperatures in the range of 40–80 °C. Two composite models based on monolayer adsorption and filling of micropores were proposed and used for the fitting of the experimental isotherms under multiple temperatures. The fitting results show that the composite models lead to better fitting effects for isotherms and under low pressure compared to the Langmuir models thus being appropriate models for fitting. Using the composite models, the competition of two adsorption modes was investigated that the filling of micropores hardly appear until pressure rises to approximately 2 MPa. The isosteric heat and entropy change were determined by the Clapeyron equation and weighted average method, showing their dependence on the competition of two adsorption modes at different adsorbed amounts. These findings not only determine two novel models for describing shale gas adsorption but also reveal the adsorption process and associated thermodynamics.