Adsorption and diffusion of shale gas in kerogen matrix: Insights from molecular simulations

The low recovery rate of shale gas highlights the necessity of understanding how it interacts with and diffuses in kerogen matrices. In this study, by using molecular simulation techniques, the adsorption and diffusion of shale gas in kerogen matrices were systematically investigated. Five kerogen models (i.e., I-A, II-A, II-B, II-C, and II-D) were constructed. I-A and II-D type kerogens were found to exhibit the lowest and highest densities, respectively, and the porosity of kerogens increased monotonically with the thermal maturity. Furthermore, a linear correlation was found between methane adsorption capacity and porosity. The diffusivity of methane in I-A type kerogen was significantly higher than that in type II kerogen, which was attributed to the flexibility of I-A kerogen. Additionally, isotope fractionation for methane diffusion was characterized and it was found that the negative isotope fractionation can be explained by the slightly higher diffusion activation energy of 13CH4. The findings presented in this study provide a molecular-level insight into methane adsorption and diffusion in kerogen and can help understand the methane-kerogen interactions in the production field.