Effects of internal fracture variability on flow channelling in discrete fracture network models

There is a need for improved understanding of flow channelling and solute transport in fractured crystalline rock in the context of safety analysis of geological repositories for spent nuclear fuel. Numerical discrete fracture network (DFN) models of sparsely fracture rock often employ an assumption of effectively homogeneous properties at the scale of individual fractures. However, real-world fractures have rough surfaces which translates to internal variability in aperture and permeability which can impact transport properties. Although it is known that internal variability controls flow channelling at the scale of single fractures, there is a lack of understanding of effects at the bedrock scale of multiple connected fractures forming networks. Therefore, it is relevant to study internal fracture variability in DFN models to better understand potential impacts on flow and transport.

In this contribution, flow channelling and solute transport in three-dimensional fracture networks with internal variability in permeability is investigated using a numerical DFN flow model with a stochastic Lagrangian transport framework. The fracture network properties are obtained from field measurements and data of fractured rock from the Forsmark site in Sweden, which is a planned location for the construction of the spent nuclear fuel repository. Different assumptions for describing the correlation length of the textures used for internal variability of fracture permeability are considered. Multiple realisations are generated and it is shown that cases with strong correlation length can lead to reduced travel times and reduced solute retention when compared to cases assuming homogenous fractures. The changes observed occur only for a small fraction of early arrivals, whereas bulk mass is essentially unaltered, and late mass breakthrough with strong retention is generally enhanced. Cases with weak correlation lengths generally have minor impacts. Thereby key thresholds for cases where flow channelling is controlled by internal fracture variability versus network scale connectivity are discussed, and a need to understand and evaluate correlation structures for real-world fractures is highlighted.