Abstract
Slip phenomenon is one of major characteristic of gas flow through porous media, of particular in unconventional gas reservoirs with small pore throat, such as tight sands, coal seams, and shale formations. Consequently, a permeability correction needs to be considered for evaluating the flow ability of gas in these reservoirs. There are various analytically derived correction models for engineering applications. However, it is not well understood which one should be implemented in the real problems of shale reservoirs. In this paper, slip velocity and permeability correction for gas flow in nanopores are studied by molecular dynamics simulations. For simplicity, the considered system is methane gas flow in parallel plate channel of quartz. The fluid flow is characterized by the Knudsen number defined as ratio between the mean free path and the representative length of the pores. The studies with various Knudsen number were conducted either by (1) changing the methane density (the mean free path) or (2) plate-spacing (pore size). The simulation results show that the relations between slip velocity (normalized by the maximum velocity) versus Knudsen number and between permeability correction factors versus Knudsen number agree well with Beskok and Karniadakis' analytical solution (BK model) for large nanopores (10-34 nm). This model has considered both rarefaction and compressibility effects to gas microflows and been tested by the experimental results with characteristic dimension of the order of one micrometer. Our simulation results indicate that this model can be further extended to nanoflows as they are in unconventional reservoirs. Some deviations from BK model were noted for small nanopores (2-10 nm), where variance and the apparent gap from BK model became significant. We suggest that the limit for application of BK model may be about 8-10 nm, and it is ascribed to the fact that the overall fluid is no more homogenous (the fluid is structured in the proximity of the walls due to the interaction from the wall molecules).
