Carbon Isotope Fractionation during Shale Gas Transport through Organic and Inorganic Nanopores from Molecular Simulations

Carbon isotope fractionation is a promising method to predict gas-in-place content and evaluate the shale gas production stage. In this study, molecular simulations are conducted to investigate fractionation characteristics of (CH4)-C-12 and (CH4)-C-13 in high-Kn (Knudsen number) flows (Kn > 0.1) within organic and inorganic pores under shale reservoir conditions (353 K, 5-25 MPa). The results show that isotope fractionation is more obvious (i.e., the difference in transport capacities between (CH4)-C-12 and (CH4)-C-13 is larger) in organic pores than in inorganic pores. Methane adsorption capacity and surface roughness of pore walls are two major reasons. High-coverage adsorption in organic pores reduces the effective pore size, and the Knudsen diffusion becomes significant. Moreover, the specular reflection of molecules occurs frequently on the smooth surfaces of organic pores, which enlarges the difference in isotope diffusion capacity. Indeed, the difference in energy (specific enthalpy) transport of methane isotopes in organic and inorganic pores is the intrinsic reason for fractionation. Furthermore, the fractionation level is positively correlated with Kn due to the enhanced contribution of Knudsen diffusion and surface diffusion in high-Kn flows. In addition, the isotope fractionation level decreases as pore size increases because Kn and the contribution of the adsorbed phase to the total molar flux reduce in a large pore. Our findings and related analyses may help us to understand isotope fractionation in different pore types and sizes from the atomic level and assist future applications in engineering.