Pore-scale study of non-ideal gas dynamics under tight confinement considering rarefaction, denseness and molecular interactions

Abstract

Non-ideal gas flow behaviors are investigated by an Enskog-Vlasov type kinetic model considering the simultaneous effects of gas molecule size (volume exclusion) and long-range intermolecular attractions at the molecular level, which corresponds to the real gas equation of state at the macroscopic level. The Knudsen minimum is captured and a local Knudsen maximum may appear if gas molecule sizes or intermolecular attractive forces are considered. Although the Boltzmann equation is applicable to all the flow regimes in rarefied gas dynamics, it is invalid for a dense gas system, such as a tight or a shale gas reservoir. The Boltzmann-BGK model and the Enskog-BGK model overestimate and underestimate the mass flow rate of real gases, respectively, while Guo's model is more accurate to investigate real gas dynamics under tight confinements from a physical perspective. As the channel width increases or the solid fraction decreases, the impact of intermolecular interactions reduces. An anomalous slip regime occurs if both the volume exclusion and long-range intermolecular attraction are considered. Although the rarefaction effect is more prominent at larger Knudsen numbers, the flow at a smaller Knudsen number (a larger solid fraction or channel width) contributes to more practical gas production. The effect of long-range intermolecular attractions on gas dynamics decreases with temperature for real gases. This paper provides a novel insight into the real gas flow characteristics in a dense gas system, such as tight and shale gas reservoirs, from molecular natures of fluids.