Tracer-test-based dimensionality reduction model for characterizing fracture network and predicting flow and transport in fracture aquifer

Abstract

Dimensionality reduction models (DRM) are widely utilized for well-posed inversion of highly heterogeneous aquifer properties. However, these models predominantly rely on predefined aquifer properties, which are often unknown in realistic aquifers; moreover, DRM for fracture network characterization is still lacking. In this study, we develop a novel DRM based on the tracer breakthrough curve (TBC) obtained from cross-hole tracer test. This model enables integrated characterization of fracture networks and prediction of flow and transport in fractured aquifers. Firstly, the relationship between TBC shape and flow and transport in fracture networks is elucidated through 3D-printing physical modeling and numerical simulations. It is observed that in complex fracture networks, flow and transport dominantly occur through a limited number of preferential flow paths under pumping scenarios involving simultaneous injection and extraction. The preferential flow paths remain stable under different pumping rates, with their number aligning with the number of inflection points, including concentration peaks in TBC and evident slowness point during the decline of tracer concentration. Building upon this finding, DRM composed of parallel fractures with the number of fractures equal to the number of inflection points in TBC is constructed. Parallel fractures in DRM represent preferential flow paths for flow and transport within the original fracture network. Parameters including sizes, apertures, and separation of parallel fractures are then inferred by TBC inversion. It is tested by the 3D-printing physical model that under pumping rates differential to that implemented in the tracer test for DRM parameters estimation, the established DRM is capable of predicting the tracer concentration and outflow temperature at the extraction well with a relative error lower than 10%. Moreover, the implementation of proposed DRM concept with synthetic large fracture network demonstrates its genericity for the characterization of fracture networks at field scale. This new DRM concept provides a feasible and effective way for fracture network characterization and flow and transport prediction.