Transport Model for Gas and Water in Nanopores of Shale Gas Reservoirs

Abstract

Because of the many nanoscale pores in shale gas reservoirs (SGRs), the fluid transport mechanisms in shale are complex. Also, previous research has shown that there exists water in the shale plays. Hence, two-phase gas-water transport model construction becomes very important so as to increase accuracy in numerical simulation work. However, most of the current study is still focused on single-phase gas transport. In this paper, based on the second slip model coupled with the fluid single-pipe flow equation, the Knudsen and surface diffusions, and combined with the fractal theory, the relative permeability model for gas-water in shale was constructed. The model reliability was proven by using the available two-phase gas-water relative permeability data. A sensitivity analysis has been carried out based on the proposed model. The results show that with the pressure decreasing, the relative permeability of gas increases. The increase of the pore size distribution fractal dimensions (Df) and fractal dimension (DT) caused the gas relative permeability (Krg) to increase. The Krg increases with the increase of Df and DT. The influence of the viscous slip flow, Knudsen diffusion, and surface diffusion are trade-offs, which are mainly controlled by water saturation (SW) and pressure (P). The Krg is extremely sensitive when P<1 MPa. Under low pressure and low water saturation, the effect of viscous slip flow is secondary. And its contribution increases gradually and becomes the main role with the increase of water saturation or pressure. The effect of the Knudsen diffusion is negligible when P>1 MPa and the water saturation SW>40%. However, it cannot be ignored under other conditions. The influence of surface diffusion reached 21.64%–72.78% when P<1 MPa and SW<10%. A surface diffusion contribution of less than 4.25% was obtained when P>1 MPa and SW>70%.