Gas Slippage in Microscale Fractures of Partially Saturated Shale of Different Matric Potentials

Abstract

Summary The gas slippage phenomenon in microscale fractures is extremely important to better understand subsurface gas flow in many engineering activities in shales, but the effects of water on gas slippage and the gas flow process have rarely been investigated. In this study, two shale samples, each with a single fracture, were obtained to investigate gas slippage in partially water-saturated microscale fractures. A new experimental approach for monitoring gas slippage in partially saturated fractures of different matric potentials is presented. The fractures were moistened to different matric potentials in an environment under a certain confining pressure and in the presence of both water and gas, and the gas permeability at different gas pressures was measured with the steady-state flow method under five different matric potentials. The experimental results suggest that water in microscale fractures greatly reduces the gas permeability by one to three orders of magnitude and that capillary condensation is the primary mechanism underlying the observed dramatic decrease. The gas permeability decreases with increasing matric potential, and this relationship is quantitatively described. The pressure dependence of the gas permeability indicates that gas flow in the partially saturated fracture manifests strong slippage. Without considering the effect of gas slippage, the gas permeability obtained by continuum hydrodynamics prediction is overestimated by up to 15–20 times. Gas slippage is enhanced with increasing matric potential, and the corresponding relationship can be expressed with an exponential function.