Effects of water on gas flow in quartz and kerogen nano-slits in shale gas formations

Abstract

Understanding the gas-water two-phase flow behavior in a shale gas formation is important for reservoir simulation and production optimization. A molecular simulation study of gas-water flow in quartz and kerogen nano-slits (2–6 nm) at shale reservoir conditions (temperature: 313.15–393.15 K, pressure: 20–60 MPa) is reported in this work. The simulation results show that the existence of water in the hydrophilic quartz slits will form water films on the slit walls; while the presence of water in the hydrophobic kerogen slits will form water clusters in the central of the gas phase at low water saturation and a water layer at high water saturation. In both wetting conditions, water will take flow space and reduce gas flow path. However, water affects gas flow velocities in the two wetting types nano-slits in different ways due to the two opposite occupancies in the slits. The momentum transfer between water and methane molecules in the gas-water interface region plays an important role in the gas-water two-phase flow. The gas flow is more readily affected by water content in the quartz slit with an aperture greater than 2 nm. When the slit aperture is reduced to 2 nm, it is difficult to form a continuous gas or water phase, and the existence of water in both types of slits will reduce the velocity of methane. Increasing the temperature will accelerate the flow of methane and water because hydrogen bonds between water molecules as well as hydrogen bonds between water molecules and the walls are reduced. High pressure promotes the mixing of the methane and water molecules, resulting in the gas velocity decreasing in both quartz and kerogen slits. The flow mechanism of methane and water in nano-slits provide insights into theoretical models for shale gas production.