Simulation of Multiphase Fluid Motion in Pore-Scale Fractures

Abstract

Small scale environmental and geophysical flows are very important, but are usually difficult to simulate because of the associated multiple fluid phases and multiple physics, as well as the existence of complex geometries and arbitrarily moving interfaces. For example, fluid motion in the vadose zone is very critical for groundwater recharge, fluid motion and contaminant transport. Flow through fractures and fractured porous media can lead to exceptionally rapid movement of liquids and associated contaminants. [1,2] The physics of fluid flows in unsaturated fractures and porous media is still poorly understood due to the complexity of multiple phase flow dynamics. Experimental studies of fluid flow in fractures and fractured porous media are limited, and in computer simulations it is usually difficult to take into account the fracture surface properties and microscopic roughness. A broadly applicable model must be able to simulate a variety of phenomena including film flow with free surfaces, stable rivulets, snapping rivulets, fluid fragmentation and coalescence (including coalescence/fragmentation cascades), droplet migration and the formation of isolated single-phase islands trapped due to aperture variability. Realistic models for multiphase fluid flows in fracture and fractured porous media must be able to handle moving interfaces, large density ratios (e.g., ≈1000:1 for water and air), and large viscosity ratios (e.g., ≈100:1 for water and air). These requirements combined with the complex geometries of natural fractures present severe challenges to mechanistic models. Grid based numerical methods such as finite difference methods, finite volume methods and Eulerian finite element methods require special algorithms to treat and track the interface between different phases. However, continuum grid based numerical models usually do not take account of the detailed void and obstacle geometries, fluid-fluid interface dynamics within pores and complex fluid-fluid-solid contact line dynamics. They rely on constitutive equations that describe the coarse-grained behaviour and can, at least in principle, be derived from the results of pore scale simulations or experiments. Therefore, small-scale simulations with mechanistic models are needed to develop a better understanding of the temporal and spatial dynamics of multiphase flow through pore-scale structures such as fractures and fractured porous media. Pore-scale flows have been studied extensively using grid based methods including finite difference method, [3] finite volume method, [4] and finite element method, [5] However, due to the difficulties associated with geometrically complex boundaries, fluidfluid-solid contact line dynamics, and fluid-fluid interface dynamics, it is difficult to apply conventional grid based multiphase simulation methods coupled with interface tracking algorithms to pore-scale multiphase flow modelling. Dissipative particle dynamics is a meso-scale particle method. Though it may be less computationally efficient than the grid-based methods, it is advantageous in simulating pore-scale multiphase flow modelling in fractures. DPD is a Lagrangian method, and conserves mass exactly. In DPD method, there is no explicit interface tracking ‐ the motion of the fluid is