Nanoscale Two-Phase Flow of Methane and Water in Shale Inorganic Matrix

Both connate water and the injected water through hydraulic fracturing can coexist with methane inside shale nanopores where two-phase flow possibly occurs. Few studies have been pertaining to two-phase flow of water and methane in shale reservoirs at nanoscale. In this work, molecular dynamics simulations are employed to investigate two-phase flow of water and methane in slit-shaped silica nanopores with hydrophilic surfaces. A sandwich structure of water film-methane-water film or a structure of the methane gas bubble wrapped in water bridge exists in the nanopore because water wets the surfaces. Darcy's law breaks down for methane single-phase flow in the nanopore because of the slippage near the surfaces. For two-phase flow of water and methane within the nanopore, water flow pattern varies in the form of water film, water bridge and water pillar at different applied accelerations (different pressure gradients), showing the breakdown of Darcy's law for water flow. This is attributed to the inhomogeneous number density distribution near the surfaces, which arises from the electrostatic interactions and the hydrogen bonds between water molecules and the surfaces. However, the varied water flow patterns have no effect on methane flow rate, suggesting that Darcy's law holds for methane flow in two-phase flow of water and methane inside the nanopore. This can be explained by the increased friction between methane and fluctuating water films. The results will advance understanding the mechanism of water and gas transport in nanoporous media and the exploitation of shale resources.