Physical selectivity on isotopologues of gaseous alkanes by shale pore network: Evidence from dynamic adsorption process of natural gas

In this study, frontal chromatography experiments have been conducted with hydrocarbon gases (methane and ethane) on selected stationary phases (quartz, 3 angstrom, 5 angstrom and 13X molecular sieves, activated carbon, coal and shale) to study compositional and carbon isotope fractionation during adsorption process. The results show that the diffusion process cannot leads to obvious isotopic fractionation larger than similar to 1.5 per mil in a relatively short distance. However, carbon isotope fractionation varies in different adsorption matrix. Gas molecules transporting in a relatively short distance can cause the isotopically light molecules weakly enriched in the front part of the alkane gases flows. The pore size distribution acts as the leading factor influencing the isotopic fractionation during hydrocarbon gases adsorption. In adsorption matrix with relatively low pore radius (i.e., activated carbon, 5 A molecular sieve and coal), the isotopically light molecules can occupy the adsorption site preferentially and almost cannot be replaced due to the strong adsorption force. Hence, isotopically heavy molecules flow out of the system preferentially. To contrast, in adsorption matrix with relatively high pore radius (i.e., 13X molecular sieve and shale), the isotopically light molecules can occupy the adsorption site and desorb preferentially. Hence, isotopically light molecules flow out of the system preferentially after repeated adsorption and desorption. In addition, physical selectivity on isotopologues of gaseous alkanes by pore network also can occur in natural shale gas system. The Niutitang shale mainly is contributed by micropore. Whereas, Wufeng-Longmaxi shale mainly is contributed by mesopore. In general, initial adsorbed gases (residual gases) in natural shale samples from Niutitang shale were of more negative isotopic compositon in CH4 and C2H6 compared to Wufeng-Longmaxi shale.