Measuring and modeling of CO₂ adsorption on shale of Sichuan Basin, China

In this study, carbon dioxide (CO₂) sorption measurements on shales obtained from the Sichuan Basin of China were carried out at 50°C up to 15 MPa by a volumetric method. The compositions of shales were characterized by X-ray diffraction analysis and total organic carbon (C) content tests. The pore structures of samples were characterized by liquid nitrogen (N₂) adsorption experiments at 77 K based on the standard static volumetric method. The compositions of the samples are mainly clay, detrital and authigenic minerals. The pore sizes of the samples show wide distribution from micropores to mesopores to macropores. The carbon dioxide adsorption isotherms measured in this study can be divided into three sections: rapid rise (<7 MPa), sharp decline (7–11 MPa) and finally slow rise (>11 MPa). A new model, which combines the Dubinin–Astakhov equation, Langmuir equation and ‘k’ term (DLK), is proposed to fit the three-segment curve. The fitting results show that the DLK model can fit carbon dioxide isotherms well. According to the physical meaning of the parameters of the model, the sorption of carbon dioxide on shales can be expressed as pore filling at lower pressures and monolayer sorption at higher pressures, the sharp decrease of the sorption in the middle of the curve may be due to the steep increase of the gas density near the critical pressure.