Gas sorption-induced pore radius change and their impact on gas apparent permeability

Abstract

Shale gas reservoirs contain small pore network that typically range from macro to nano meters and abundant gas storage as sorbed gas. In addition, the storage medium in such pore network varies and may be affected by physical parameters of shale matrix. This work presents the influence of gas sorption on pore radius and thickness at low and high pressure and analyzes the gas apparent permeability. Langmuir model is generally used to quantify the adsorbed layer thickness; however, such model undertakes mono-layer gas sorption on pore surfaces and provides poor results especially at high pressure. In this work, supercritical dubinin radushkevich (SDR) model, micropore-filling mechanism based, has been used to quantify the adsorbed layer thickness and their results have been compared with Langmuir model. The impact of adsorbed gas density on gas sorption and the impact of gas sorption crucial factors on pore radius were examined. At the end, gas slippage, geo-mechanical and adsorbed layers impacts were analyzed for evolution of gas apparent permeability. The proposed gas apparent permeability model was also validated with experimental data. The results show that assumed adsorbed gas density in Langmuir model is misleading the accurate measurement of gas sorption. The SDR model does not only provide an accurate adsorbed gas density but also its values are very close to the experimental at both low and high pressure. Gas sorption-induced pore radius based on SDR model was altered when gas sorption behavior alter at same pressure than Langmuir model-based. Gas apparent permeability changes due to gas slippage, geo-mechanical and gas adsorbed layers impacts. High gas adsorbed layer impact was observed when using proposed model as compared with Langmuir model especially at high pressure.