Behaviours of methane and water in heterogeneous shale nanopores: effect of water saturation and pore size

Understanding the structure and flow behaviours of methane and water co-existing in shale nanopores is crucial for the optimal exploitation of shale reservoirs after hydro-fracking stimulation. In this study, a number of MD simulations are used to analyse the structure and flow pattern of such a two-phase fluid in quartz and kerogen nanoslits. Our findings indicate that water molecules preferentially adsorb near walls, generating water films and finally constructing a water bridge in quartz nanopores, but in kerogen nanopores they form a plug-like water cluster. As pore size decreases, both organic and inorganic nanoslits enclose increasingly complex water/methane complexes. For two-phase transport behaviour, methane flow has a parabolic profile with fading peaks as S_w increases, but water films near the wall of quartz nanopores appear to be static. In contrast, the methane and water phases flow jointly in kerogen nanopores for all S_w values. As methane primarily exists as an adsorption gas in both quartz and kerogen nanopores, its flow rate is further slowed by approximately-one order of magnitude in narrower nanoslits. Notably, the water restriction in organic pores is slightly weaker than that in quartz pores, around 10 % less in average flow velocity. Our findings will contribute to the advancement of numerical models of shale gas movement in wet shale reservoirs.