Nanoconfined Transport Characteristic of Methane in Organic Shale Nanopores: The Applicability of the Continuous Model

In recent years, since the fast mass transport emerges from some low-dimensional nanostructures (e.g., graphene and CNTs), the applicability of the continuous model (e.g., hydrodynamics) for describing the nanoscale flow has been intensely challenged, even for shale gas. As the typical tight rock with numerous nanopores, most scholars considered that gas transport capacity (permeability) within a shale organic matrix (i.e., kerogen) would be significantly enhanced as well, due to the nanoscale slip effect. Herein, we perform comprehensive molecular dynamics (MD) simulations to reveal the realistic gas transport behavior through shale kerogen nanopores. The results show that, interestingly, all the velocity profiles for different kerogen (type I, II, and III) nanopores display no-slip parabolic shape, which are quantitatively satisfied with the continuous model under various conditions, including pressure drop (0.25-1 MPa), pore size (2-8 nm), and ambient pressure (5-50 MPa) and temperature (300-390 K). In particular, using potential energy surface (PES) and particle trajectory capture (PTC) technologies, we find that, confined by the rough kerogen walls and ultra-high reservoir pressure, the methane molecules collide with the walls frequently but just go round and round without moving along the walls (confined reflection), and thus, the tangential momentum (slip velocity) is negligible at the walls. Importantly, this work demonstrates that traditional consideration of the slip effect is redundant for methane transport in organic shale (kerogen) nanopores and will overestimate the gas permeability immensely (as much as 100 times). These new insights would be helpful for the precise understanding and accurate modeling of methane transport within nanoporous shale rocks.