Carbon isotope fractionation during shale gas release: Experimental results and molecular modeling of mechanisms

Carbon isotope fractionation during the release of gas from shales is a potential tool to evaluate the remaining gas content. However, the mechanism of carbon isotope fractionation during this process is still unclear. In this study, the δ13CH4 ratios of gases desorbed from two shale samples over a period of 800 min were analyzed. The results show that during gas release, the δ13CH4 values change in four stages: firstly decreasing from −27‰ to −31‰; then increasing significantly from −31‰ to −23‰. Subsequently, they decrease, and then finally increase slightly. Molecular simulations were also employed to calculate the adsorption and transport of 12CH4 and 13CH4. Based on simulation results, the adsorption amount, adsorption energy and density of 13CH4 are very similar to those of 12CH4. However, the molecular simulation showed significant differences in the transport of 13CH4 and 12CH4. By integrating the experimental and simulation results, a molecular mechanism for carbon isotope fraction is proposed. In Stage 1, gas expelled from the shale is mainly stored in fractures and macropores, and without the interaction of the gas with pore walls, 13CH4 is transported more slowly than 12CH4. In Stage 2, the high-pressure gas initially stored in micropores is expelled from the shale. The 13CH4 is transported more quickly in the small nanopores than 12CH4 when pressure is > 1 MPa, and the δ13C values increase correspondingly. In Stage 3, the gas pressure in the whole pore system is relatively low, and 13CH4 is transported more slowly than 12CH4 when migrating through small micropores. As a result, δ13C values show a slight decrease. In the last stage, since relatively more 13CH4 molecules remain in shale, the δ13CH4 values increase slightly again.