Re-evaluating the methane adsorption behavior in shale kerogen: Unifying experiment and molecular simulation

The methane adsorption capacity is crucial for evaluating gas-in-place resources and the gas production potential in shale gas reservoirs. There are many reports concerning the interfacial interaction between methane fluid and rock using various thermodynamic models; however, little research has been performed to reveal how methane is adsorbed into nanopores with different scales. In this study, we did methane adsorption experiments on nine Longmaxi Formation shale kerogen. Then, molecular simulation and an improved Ono–Kondo model were used to analyze the methane adsorption behaviors. Results show that methane is preferentially adsorbed in sulfur-containing sites by surface adsorption and pore-filling adsorption, and methane adsorbed in the form of pore-filling contributes dominantly to the total methane adsorption amount. Surface adsorption capacity nS increases with increasing mesopore volume, while pore-filling adsorption capacity nF is affected by both micropore (<2 nm) development and the micropore accessibility of methane. On the one hand, nF increases logarithmically with increasing micropore volume. On the other hand, the mean interplanar distance of the aromatic layers d002 is the key parameter in determining nF because the micropores within the aromatic layer with d002 less than 0.38 nm will be inaccessible for methane. This study is essential for understanding the methane adsorption mechanism and lay the foundation for future investigation of fluids–rock interactions.