Rigid, Smooth, and Impermeable? Complex Fracture Hydraulics Revealed by Oscillatory Flow Interference Testing

Fractured bedrock aquifers, especially sedimentary bedrock aquifers, represent idealized targets for wastewater storage and alternative energy development because of their large storage volumes. Characterizing the physical properties that govern fluid flow and storage in these aquifers remains a fundamental challenge, which is complicated by the combination of fracture-dominated fast-flow pathways and porous-media dominated slow-flow pathways contributing to the overall flow field. Recent field and modeling experiments highlight oscillatory flow interference testing as a novel pressure-based approach to characterize flow properties in an isolated bedrock fracture. These studies use simple modeling approaches that conceptualize the tested fracture as a non-deforming, parallel-plate fracture bounded by impermeable host rock assuming Darcian flow and find that the returned effective flow parameters are dependent on the period of the pumping signal. In this presentation, we highlight field experiments in an isolated sedimentary bedrock fracture near Madison, WI, and show that simple analytical modeling approaches, such as non-Darcian flow effects and fracture leakage cannot account for the apparent period dependence in estimated fracture flow parameters. We then present more complex numerical modeling approaches to explore the effects of aperture heterogeneity, fracture - host rock interactions, and fracture hydromechanical behavior on effective flow parameter estimates under oscillatory flow conditions. Through these synthetic modeling experiments, we show that each of flow processes produces a diagnostic period dependence on estimated flow parameters, and fracture hydromechanical behavior reproduces the period dependence reported in the literature.