Impacts of gas pressure on carbon isotope fractionation during methane degassing—An experimental study on shales from Wufeng and Longmaxi Formations in southeast Sichuan, China

Abstract Methane carbon isotope fractionation (CIF) during shale gas desorption provides a new method to evaluating in-situ gas content and the ratios of adsorbed and free gas as well as predicting the production status of shale gas wells. Considering anisotropic pressures in shale gas in-situ reservoirs, five gas pressure values were selected for this study to perform methane isotherm adsorption-desorption experiments for four shale samples from the Wufeng and Longmaxi Formations in the southeastern Sichuan Basin, and the characteristics of carbon isotope fractionation during methane desorption processes were investigated. Results show that: (1) under different gas pressures, CIF of the in-methane transport process shows four stages: stable stage (I), lighter stage (II), heavier stage (III), and another lighter stage (IV); (2) CIF during methane desorption is impacted by multiple factors including the total organic carbon (TOC) content, pore structure, gas pressure, and adsorbed gas ratio, while material composition and pore structure largely determine the characteristics of isotopic fractionation in methane desorption, which are extremely important control factors; and (3) gas pressure has an obvious effect on CIF characteristics by changing the proportion of adsorbed gas, especially in stages II and IV, and the sample with developed organic matter (OM) had a stronger response. The lower the gas pressure, the earlier the desorption process corresponding to the start of significant isotope fractionation. The higher the ratio of adsorbed gas, the larger the range of isotope fractionation. This demonstrates the significance of adsorption-desorption for CIF in shale gas degassing. Furthermore, we used a technique integrating the Amoco curve fit (ACF) method and CIF to calculate in-situ gas content and the dynamic change in adsorbed and free gas ratios during gas degassing.