Comparing flows to a tunnel for single porosity, double porosity and discrete fracture representations of the EDZ

Abstract

Andra is studying the feasibility of constructing a repository for radioactive waste in the Callovo-Oxfordian mudstones. The excavation of a repository leads to the formation of an Excavation Damaged Zone (EDZ) adjacent to the repository. The EDZ contains fractures and micro-fissures modifying the hydraulic properties of this region. Conventionally, porous medium representations are used to model two-phase flow around repositories. However, it is not possible to represent the detailed heterogeneity inherent to a fractured medium in such models. Conversely, discrete fracture network (DFN) models can be used to provide a direct representation of the fractures, but currently computational limitations mean that is at the expense of the representation of two-phase flow behaviour. In this study the different types of models that may be used to model two-phase flows through an EDZ to a tunnel are compared to help to determine a suitable approach for future modelling of the EDZ. In particular, calculations are performed using an approach in which the properties for a porous medium model are determined by calculation of the effective properties of a fracture network set up in a DFN model. This combined approach is examined as a possible way to improve on modelling performed using either approach alone.