Breakdown of Fast Mass Transport of Methane through Calcite Nanopores

Fast mass transport was identified for gas through nanoscale pores, especially those fabricated from carbon nanotubes and graphene sheets. Even in porous media such as sedimentary rock, it is commonly believed that there exists the well-known Klinkenberg effect due to gas slip. Here, we use molecular simulation to show that the flow enhancement of methane breaks down in calcite nanopores of shale reservoirs. The Klinkenberg effect fails to characterize methane transport through interparticle pores of calcites, and the molecules travel even slower than the prediction of Hagen–Poiseuille equation. The comparison of methane transport in graphene, quartz, and calcite nanopores suggests that this behavior arises from the strong attractive potential and the lack of atomically ultrasmooth surface in calcite, thus leading to the presence of particles sticking at the interface. The Navier–Stokes equation, coupled with a negative slip length and bulk viscosity, can provide a reasonable description of methane flow through calcite nanoslits having apertures greater than 2 nm. Moreover, it is evident that as the pore size increases, the confined methane transforms from a single-file chain to two symmetrical adsorbed layers (extending to ∼0.8 nm), above which a bulk fluid region is present in the central slit. Beyond the theoretical value, the insights gained from this study will advance the exploitation of shale resources and shed light on, more generally, mass transport in nanoporous media