Impact of water film on methane surface diffusion in gas shale organic nanopores

Abstract

Abstract When a shale pore radius is less than 10 nm, the contribution of surface diffusion to methane transport in shale multiscale pore structure will significantly increase. It is critical to understand whether the adsorbed water film in shale nanoscale organic pores is beneficial to multiscale methane transport. Thus, a model considering the effect of a water film on methane surface diffusion is established. Furthermore, the influences of a water film on different diffusion mechanisms are discussed. The results show that, first, when water molecule enters shale organic pores, an adsorption interface changes from gas-solid to gas-water, i.e., the fractional surface coverage of gas in the surface increases so the surface diffusion ability increases; second, when a pore radius is less than 6 nm, the overall diffusion capacity of methane in pores increases, even though a water film can decrease the slip flow and Knudsen diffusion. However, when a pore radius is 8- 10 nm, the apparent permeability (Kapp) increases with pressure. Since a pore surface contains a water film, Kapp is not always greater than that for a pore surface without water film. Our investigation shows that the impact of fracturing fluid retention in shale multiscale pore structure is not always negative for methane multiscale transport if the volume of a fracturing fluid and shut-in time are reasonable.