Numerical simulation of multiphase flow and phase partitioning in discretely fractured geologic media

Abstract

The three-dimensional compositional model CompFlow has been extended to allow the simulation of the multiphase advective, dispersive and diffusive flux of non-aqueous phase liquid (NAPL) contaminants in a discrete-fracture network, allowing for phase partitioning and dynamic interactions between the fracture network and the surrounding low-permeability rock matrix. The approach used to couple fluxes between the fractures and matrix allows representation of capillary pressure differences within the fractures and matrix and makes no assumption of equilibrium hydraulic conditions between the two. The model is verified for the case of aqueous-phase solute transport by comparison with an analytical solution. An example problem is presented involving the migration of a dense non-aqueous phase liquid (DNAPL) consisting of trichlorethylene (TCE) in a single vertical fracture within a low-permeability material with significant matrix porosity. The simulation results demonstrate that matrix diffusion acts to transfer significant amounts of contaminant to the matrix in the aqueous phase. After the DNAPL source is removed, the NAPL ultimately disappears from the fracture due to partitioning of contaminant into the aqueous phase with concomitant matrix diffusion. It is shown that as the porosity of the matrix increases, the rate of migration of the TCE DNAPL front within fractures is retarded, due to dissolution and matrix diffusion. The sensitivity of DNAPL migration within the fracture to the form of the relative permeability relationship is also discussed. The model is then used to highlight the potential for deep DNAPL penetration through a vertical cross-section consisting of a shallow unconfined sand aquifer and a deeper sand aquifer separated by a layer of fractured clay. Vertical fractures through the clay that hydraulically connects the shallow and deep aquifers are shown to be capable of transmitting both dissolved contaminant and DNAPL to the underlying aquifer, with DNAPL travel times through the 5-m thick clay unit being on the order of days. For the scenarios examined, the DNAPL entering the lower aquifer via the fractured clay unit may completely dissolve at the aquifer–aquitard interface due to lateral groundwater flow in the lower aquifer, provided the aperture of the vertical fractures in the clay layer are less than about 30 μm; however, some DNAPLs may penetrate into the lower aquifer and exist as a non-aqueous phase if the fracture apertures in the clay layer are 50 μm in size or larger. In this latter case, the presence of the DNAPL in the lower aquifer acts as a persistent source of groundwater contamination and can produce an extensive plume in the direction of groundwater flow.