Investigation on gas/water two-phase flow in quartz nanopores from molecular perspectives

Abstract

Understanding the gas/water two-phase occurrence, flow patterns, and underlying mechanisms in quartz nanopores is crucial for gas transport and production in shale reservoirs. In this study, we investigated the flow characteristics of methane and water in hydroxylated quartz nanopores employing the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations and focused on elucidating the effects of water saturation, pressure gradient and ion concentration in various aperture nanopores. Results show that water molecules preferentially adsorb on the hydrophilic quartz surface to form water film, increasing methane flow friction and reducing its slippage velocity significantly. Water bridges are formed to wrap gas bubbles and hinder the gas flow with increased water saturation. The water phase occupies a larger pore space with increasing water saturation resulting in an increase in methane effective viscosity and a decrease in apparent permeability. The water bridge is broken under the joint action of external force and gas driving when the pressure gradient is much larger, leading to a dramatic growth of gas flow, and the high-speed gas will produce a shear driving effect on the thick water layers. Ions will weaken the formation of hydrogen bonds between water molecules, but the flux of the water phase decreases with the increasing ion concentration due to the combined impact of ion spatial structure and electroviscous effects and counteracts the gas phase, thus impeding its flow. This study shed light on the flow phenomena of gas/water phases in hydrophilic nanopores. The results are expected to provide some insight into energy and environmental issues, such as fracturing fluid optimization, enhanced gas recovery (EGR) by water flooding, and the fabrication of microfluidic chips.