Slip length of methane flow under shale reservoir conditions: Effect of pore size and pressure

The conventional hydrodynamic equations (Navier-Stokes, Hagen-Poiseuille equations and Darcy's law) are no longer applicable in shale gas recovery due to strong surface adsorption and slip flow effect on gas transport in nanopores. Understanding the shale gas transport behavior is important for reservoir evaluation and production optimization. Herein, we report a molecular simulation study of methane flow in organic nanopores under shale reservoir conditions (temperature: 300-450 K, pressure: 10-60 MPa), with pore sizes ranging from 2 to 20 nm. We use the grand-canonical Monte Carlo simulations to determine the methane content in nanopores. We analyze the density distribution using equilibrium molecular dynamics. The surface adsorption effect is almost negligible under high pressure (60 MPa) conditions on average density, while it plays an important role under low-pressure conditions. Finally, we use nonequilibrium molecular dynamics simulation to study the transport behavior of nano-confined methane molecules. We find that the pore size has a significant effect on the slip length under low pressure (10 MPa) conditions. In contrast, the slip length is almost constant under high-pressure conditions. Under most conditions, the slip length decreases with the increase of pressures. Such a trend is more obvious in small pores under high-temperature conditions. Our work should provide important insights into the quantification of slip length in shale reservoir conditions.