Carbon isotope fractionation during shale gas transport: Mechanism, characterization and significance

The gas in-place (GIP) content and the ratio of adsorbed/free gas are two key parameters for the assessment of shale gas resources and have thus received extensive attention. A variety of methods have been proposed to solve these issues, however none have gained widespread acceptance. Carbon isotope fractionation during the methane transport process provides abundant information, serving as an effective method for differentiating the gas transport processes of adsorbed gas and free gas and ultimately evaluating the two key parameters. In this study, four stages of methane carbon isotope fractionation were documented during a laboratory experiment that simulated gas transport through shale. The four stages reflect different transport processes: the free gas seepage stage (I), transition stage (II), adsorbed gas desorption stage (III) and concentration diffusion stage (IV). Combined with the results of decoupling experiments, the isotope fractionation characteristics donated by the single effect (seepage, adsorption-desorption and diffusion) were clearly revealed. We further propose a technique integrating the Amoco curve fit (ACF) method and carbon isotope fractionation (CIF) to determine the dynamic change in adsorbed and free gas ratios during gas production. We find that the gases produced in stage I are primarily composed of free gas and that carbon isotope ratios of methane (δ13C1) are stable and equal to the ratios of source gas (δ13C01). In stage II, the contribution of free gas decreases, while the proportion of adsorbed gas increases, and the δ13C1 gradually becomes lighter. With the depletion of free gas, the adsorbed gas contribution in stage III reaches 100%, and the δ13C1becomes heavier. Finally, in stage IV, the desorbed gas remaining in the pore spaces diffuses out under the concentration difference, and the δ13C1becomes lighter again and finally stabilizes. In addition, a kinetic model for the quantitative description of isotope fractionation during desorption and diffusion was established.