Modelling transport in fractured media using the fracture continuum approach

Abstract

Two of the commonly used approaches in modelling flow and transport in fractured geological media are the discrete fracture network (DFN) approach and the stochastic continuum (SC) approach. The former approach is computationally demanding and requires large input parameters, whereas the latter approach is computationally efficient and requires fewer parameters but at the expense of not preserving fracture network details and properties. The fracture continuum (FC) approach combines the advantages of both the DFN and SC approaches. The main objective of this research is to develop a mapping technique utilizing the FC approach to preserve transport characteristics between DFN and the mapped continuum. A two-dimensional particle tracking model is developed to simulate conservative contaminant transport through DFN. The fracture network is randomly generated in a stochastic Monte Carlo framework, and the flow problem is solved using mass conservation at the fracture junctions. The transport problem is then solved via the developed particle tracking model on the DFN. The obtained transport solution is used as a reference solution. The fracture network is mapped onto a finite difference grid at four different grid cell sizes: 1 m × 1 m, 2 m × 2 m, 5 m × 5 m, and 10 m × 10 m, and the flow problem is solved via MODFLOW. The transport problem is solved on the grid using a particle tracking method. Comparisons between the transport characteristics for both approaches (DFN and FC) are performed, and the percentage error in each case is quantified. It is found that a new correction factor is needed to preserve conservative transport characteristics on the grid. The developed correction factor improves the ability of the FC technique to preserve transport on the grid for the case of conservative contaminant transport.