Slip Velocity of Methane Flow in Nanopores With Kerogen and Quartz Surfaces

Abstract

Summary Slip phenomenon is one of the major characteristics of gas flow through porous media—in particular, in unconventional gas reservoirs with small pore throats, such as tight sands, coal seams, and shale formations. Consequently, a permeability correction needs to be considered to evaluate the gas-flow ability in these reservoirs. Various analytically derived and empirical correction models exist for engineering applications. However, it is not well-understood which one should be implemented in real-shale-reservoir problems. In this paper, slip velocity and permeability for gas flow in nanopores are studied by molecular-dynamics (MD) simulations. For simplicity, the system considered is methane gas flow in a parallel-plate channel of quartz and kerogen. The fluid flow is characterized by the Knudsen number (Kn), which is defined as the ratio between the mean free path and the representative length of the pores. Studies with various Knudsen numbers were conducted by changing (1) the methane density (the mean free path) or (2) the plate-spacing (pore size). Simulation results show that the relationships between slip velocity and Knudsen number and between the permeability-correction factors and Knudsen number agree well with the Beskok and Karniadakis (1994) analytical solution (BK model) for large nanopores (12–34 nm) in both quartz and kerogen cases. This model considered rarefaction and compressibility effects on gas microflows, and was tested experimentally with characteristic dimensions of one-micrometer order. Our simulation results indicate that this model can be extended to nanoflows existing in unconventional reservoirs. Under temperature and pressure conditions that we studied, deviation from the BK model is noted for small nanopores (<12 nm), but for a pore size smaller than 6 nm, it converges to a constant value in the quartz slit pore. In contrast, a radical increase of slip velocity is observed in the kerogen pore. The deviation from the BK model for a pore size smaller than 12 nm is ascribed to the fact that the overall fluid is no longer homogeneous (i.e., the fluid at the interface region plays a crucial role in the overall flow behavior). An adsorption structure is observed in the proximity of the solid walls because of the interaction from the wall molecules. Moreover, it is found that the effect of roughness becomes significant in an extremely small kerogen nanopore.