Shale gas transport in nanopores coupled with real gas effect and surface diffusion

The physical properties and transport behaviors of shale gas have a significant influence on shale reservoirs' effective development. In this study, based on the real gas equation of state and dense gas theory, a new method with concise form and good generality was proposed to acquire the dynamic viscosity of the shale gas. On account of this method, a real gas model was developed to theoretically investigate the shale gas transport characteristics in nanopores, considering real gas effect, gas slippage and surface diffusion. In this model, the second-order slippage boundary condition coupled with surface diffusion was presented for the free gas, and the Langmuir adsorption theory was adopted for the adsorbed gas. The results show that the dynamic viscosity of the shale gas is a function of the pore pressure and temperature, and it increases with the pressure, while decreases with the increase of temperature. For free gas, the slippage effect is promoted dramatically through the surface diffusion effect in small pores, whereas this effect becomes weak as the pressure increases. And the absorbed gas shouldn't be neglected when the pore size is less than ten nanometers. Furthermore, the ideal gas model has some deviations in calculating the shale gas dynamic viscosity and Knudsen number, hence overestimating the gas production.