Establishment of a new slip permeability model of gas flow in shale nanopores based on experimental and molecular dynamics simulations studies

Abstract

It is difficult to describe the gas flow in micro-nanometer shale pores by conventional seepage theory. In this study, based on a large number of experimental data, the pore characteristics of shale gas and the mechanism of solid-gas interaction are analyzed. The gas flow capability was tested by a steady-state flow experimental system, and a new apparent permeability model considering the effect of boundary layer was proposed. The result shows Micropores(<2 nm) in shale formation contribute 20% of the pore volume, while the proportion of macropore (>50 nm) determines the flow ability of shale gas. The molecular dynamics simulations confirm that the thickness of the adsorption layer is about 0.7–1 nm, and the pore size and average pressure are the main factors affecting the thickness of the adsorption layer and slippage effect. As Knudsen diffusion and slippage effect are both caused by the interaction of pore surface and gas molecules in the adsorption layer, they shouldn't be both considered in flow calculation. The flow ability for different pressures and pore sizes can be calculated by slippage flow model based on the steady-state flow experimental results. With the decrease of the average experimental pressure, the ratio of slip permeability to Darcy permeability can be a few tens to several hundred. The study provides a new unified slip permeability model considering gas transport behaviors in nanopores and applies the flow model to macroscale numerical simulation.