Linking structural and transport properties in three-dimensional fracture networks

We investigate large-scale particle motion and solute breakthrough in sparse three-dimensional discrete fracture networks characterized by power law distributed fracture lengths. The three networks we consider have the same fracture intensity values but exhibit different percolation densities, geometric properties, and topological structures. We considered two different average transport models to predict solute breakthrough, a streamtube model and a Bernoulli continuous time random walk model, both of which provide insights into the flow fields within the networks. The streamtube model provides acceptable predictions at short distances in two of the networks but fails in all cases to predict breakthrough times at the outlet plane, which indicates that particle motion in such fracture networks cannot be characterized by a constant velocity between the inlet and control plane at which the breakthrough curve is detected. Rather, the structure of the network requires that frequent velocity transitions be made as particles move through the system. Despite the relatively broad distribution of fracture radii and relatively small number of independent velocity transitions, the continuous time random walk approach conditioned on the initial velocity distribution provides reasonable predictions for the breakthrough curves at different distances from the inlet. The application of these averaged transport models provides a richer understanding of the link from the fracture network structure to flow and transport properties. ©2019. American Geophysical Union. All Rights Reserved.J. D. H. and A. H. are thankful for support from Department of Energy at Los Alamos National Laboratory under contract DE-AC52-06NA25 396 through the Laboratory-Directed Research and Development Program. J. D. H. acknowledges support from the LANL LDRD program office Grant 20180621ECR. M. D. gratefully acknowledges the support of the European Research Council (ERC) through the project MHetScale (617511). P. K. K. acknowledges a grant from Korea Environment Industry & Technology Institute(KEITI) through Subsurface Environmental Management (SEM) Project, funded by Korea Ministry of Environment(MOE; 2018002440003). dfnworks can be obtained at https://github.com/lanl/dfnWorks and run files for simulations can obtained at https://gitlab.com/hymanjd/ hyman-jgr-2018.git.Peer reviewe