Characteristics of flow and transport in low-permeability fractured rock based on a channel network model

Abstract

Discrete Fracture Network (DFN) models for evaluating flow and transport in low-permeability fractured rocks are important tools in safety assessments of nuclear waste repositories, and also important for other geoengineering and environmental applications. The well-known phenomena of flow channeling, arising from both intra-fracture and inter-fracture heterogeneities, is in general difficult to implement in these models. The present study uses the Channel Network Model (CNM) concept as a complementary approach to DFN models, with focus on channelized flow within fracture planes and in the fracture network. A method used to generate CNMs based on channels connecting centroids of fracture planes was implemented within a pychan3d library and applied to a 3D DFN model based on field data from Forsmark, Sweden. Three sets of realizations of the channel network are used to characterize the flow and transport system between deformation zones in the granitic host rock. The results indicate the significance of very low-conductivity fractures in providing critical flow connections in these rocks. It is shown that only a few (4 to 6 in our cases) key flow bridges within a network of 9000 or more fractures control its flow and transport. The use of CNMs together with DFN models enhances confidence in safety assessments for nuclear waste repositories and other applications, while providing valuable insights into complex flow and transport behavior in low-fracture-permeability rocks.