Molecular dynamics simulations about isotope fractionation of methane in shale nanopores

摘要

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 ((CH4)-C-12 and (CH4)-C-13) in 2 nm and 6.8 nm diameter carbon nanotubes (CNT) at temperature of 353 K. Pressure differential (4p) 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 (CH4)-C-13 to (CH4)-C-12 (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 (CH4)-C-13 with a stronger adsorption affinity had a lower desorption rate, indicating that more (CH4)-C-13 molecules were accumulated in adsorption layers and free state gas were enriched in (CH4)-C-12, which resulted in the evident fractionation at the early gas desorption stage. As pressure dropped further, more (CH4)-C-13 molecules were triggered to desorb from pore surfaces and became free state gas, which made production gas be enriched in (CH4)-C-13 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.