Experimental and analytical investigation of adsorption effects on shale gas transport in organic nanopores

Gas production from shale gas reservoirs is determined by original gas-in-place and gas transport mechanism in shale. Gas storage in shale consists of three different states: first, free gas in pores and natural fractures; second, adsorbed gas on the organic and inorganic pore walls; and third, absorbed or dissolved gas into formation water and organic matter. On the other hand, gas transport in shale may be dominated by noncontinuum flow in nanopores, which depends on gas slippage, bulk diffusion, Knudsen diffusion and adsorbed surface diffusion. In this study, a gas velocity model involving all of these parameters is proposed to describe the gas adsorption and Non-Darcy flow effects on gas transport behavior in shale gas reservoirs. Adsorption capacity of six shale core samples is measured by the gravimetric method using magnetic suspension sorption system. Regression analysis of the measured data has been performed using the Simplified Local-Density model coupled with modified Peng-Robinson Equation of State (SLD-PR). The presence of the moisture content is taken into consideration to distinguish between gas adsorption on the surface of organic matter and clay minerals. The hydrophilic feature of clay minerals and hydrophobic feature of organic matter result in water molecules preferring adsorbing on the surface of clays, while gas molecules preferring adsorbing on the surface of organic matter. In the developed velocity model, adsorbed gas is treated as mobile gas molecules adsorbing on the surface of organic nanopores, which moves by means of adsorbed surface diffusion. The surface diffusion velocity is characterized by adsorbed surface diffusion coefficient. Simultaneously, bulk gas is governed by second-order gas slippage, which is calculated using Navier-Stokes (N-S) equation associated with second-order slip boundary condition. Finally, the adsorbed surface diffusion is incorporated into bulk gas flow using Langmuir slip model to account for the total gas flow in the organic nanopores. The results of this study indicate that adsorbed surface diffusion contributes to the enhancement or impediment of total gas flow velocity in organic nanopores. Therefore, properly handling the adsorption effect on gas transport in organic nanopores is critical to correctly understand gas production from shale gas reservoirs.