Thermodynamic characteristics of high-pressure CH4 adsorption on longmaxi shale subjected to supercritical CO2-water saturation

Abstract

The shale-CH4 interaction after supercritical CO2 (ScCO2) injection can be well interpreted by adsorption thermodynamics study. Using a gravimetric method, the methane adsorption experiments over wide range of pressure (up to 35 MPa) and temperature (30–90 °C) were conducted on Longmaxi shale before and after ScCO2-water saturation (60days/20Mpa/70°C). In addition, low-temperature CO2 and N2 adsorption experiments were performed to analyze the variations of pore structure of shale. Results indicates that the high-pressure CH4 adsorption curves of shale exhibit obvious excess adsorption characteristics, and the excess adsorption capacity of shale to CH4 decreased after ScCO2-water saturation, which is mainly caused by the decrease of the specific surface area of shale. Comparing to Brunauer-Emmett-Teller, Dubinin-Radushkevich and Ono-Kondo models, the excess adsorption data can be well-fitted by Langmuir model through combining Henry equation. Additionally, the thermodynamic parameters of shale-CH4 adsorption pair, including the isosteric heat of adsorption, enthalpy change, Gibbs free energy change and entropy change, displayed strong dependence on adsorption capacity and temperature, and altered in varying degrees after ScCO2-water saturation, demonstrating that the shale gas recovery can be enhanced by ScCO2-water-shale interaction from the perspective of thermodynamics. This study provides a reference for CO2 enhanced shale gas recovery.