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

Abstract

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.

Oil-Water Transport in Clay-Hosted Nanopores: Effects of Long-Range Electrostatic Forces

“…Water preferentially adsorbs on the surfaces because of the significant electrostatic force and hydrogen bond between water molecules and the surfaces 75 . This observation is analogous to water adsorption in silica-based pore structures 69,76 . Increasing the water concentration to 58.82% as shown in Fig.4b leads to an increase in the adsorbed film thickness.…”

“…Fig. 13b shows an increased hydrocarbon velocity for an initial increase in the water concentration which has been attributed to the creation of smoother surfaces for hydrocarbon flow 69 . However, when the water concentration is increased, the width of the water bridge progressively increases (shown in Fig S10), thereby hampering hydrocarbon flow.…”

“…Several methods exist for inducing flow in molecular dynamics, such as forced [60][61][62] , surface-induced, pressure difference 63 , osmotic pressure 64 and gravitational approaches [65][66][67] . In our simulations, we adopt the gravitational technique because other methods have been known to cause a distortion of the velocity-field and density profile along the flow direction [67][68][69] . A uniform external force (along the x-direction as shown in Fig.…”

“…However, in NEMD simulations, the molecule velocities in the direction of the driving force consists of both thermal velocities and the imposed center-of-mass velocity. To ensure the temperature is maintained, our fluid temperature calculations do not include the center-of-mass velocity 69 . The temperature and pressure during the NEMD simulations are shown in Fig.…”

“…7b indicates that increasing water concentration promotes hydrocarbon flow up to a point. The initial increase has been attributed to the creation of smooth surfaces following adsorption of water 69 . Subsequent increases decrease the effective flow radius for hydrocarbon flow as shown in Fig.…”