Molecular dynamics simulations about isotope fractionation of methane in shale nanopores

Methane isotope gas fractionation is an interesting topic during pressure depletion process. In this study, molecular dynamics (MD) simulations were conducted to investigate transport characteristics of isotopologues (12CH4 and 13CH4) in 2 nm and 6.8 nm diameter carbon nanotubes (CNT) at temperature of 353 K. Pressure differential (Δp) are set to be 6, 7, 8, 9, and 10 MPa for 2 nm pore, and 1, 3, 4, 5, and 6 MPa for 6.8 nm pore, respectively. In this regard, isotopologues flow derived by pressure differential were simulated in both pores. The simulation results showed that transport diffusion coefficient ratio of 13CH4 to 12CH4 (D*/D) exhibited different variation trends as pressure dropped. Gas became “light” (D*/D decreased) first at the beginning and then it became “heavy” (D*/D increased) toward the end, which was coincident with the experimental results. Our analysis manifested that 13CH4 with a stronger adsorption affinity had a lower desorption rate, indicating that more 13CH4 molecules were accumulated in adsorption layers and free state gas were enriched in 12CH4, which resulted in the evident fractionation at the early gas desorption stage. As pressure dropped further, more 13CH4 molecules were triggered to desorb from pore surfaces and became free state gas, which made production gas be enriched in 13CH4 and fractionation correspondingly became less evident. Moreover, fractionation was obvious in smaller pores as gas transport shifted to high Knudsen (Kn) flow. Our simulation result bridges the nano-scaled isotope gas transport in porous medium with the reservoir engineering.