Modeling the Hydraulic and Two-phase Flow Behaviour of the Heterogeneous, Fractured EDZ in the Opalinus Clay During the HG-A Experiment at the Mont Terri URL

The characterization of gas migration through low-permeability clay formations has been a focus of R&D programs for radioactive waste disposal. Nagra has developed a comprehensive program to characterize gas flow in the Opalinus Clay including the excavation disturbed zone (EDZ) around the underground openings of the repository. The experiment (“Gas path through host rock and along seal sections / HG-A”) as part of the Mont Terri research programme was designed as a long-term water & gas injection experiment in a backfilled microtunnel, to investigate both water/gas leak-off rates and gas release paths from a sealed tunnel section in a low permeability host rock. The experiment combines field investigations with extensive hydro-mechanical instrumentation in and around the HG-A microtunnel with geotechnical laboratory studies on core samples and extensive modelling activities for the validation of coupled hydro-mechanical models. For the numerical modeling of the HG-A experiment, a 3D model was developed, taking into account the heterogeneity associated with the fractured EDZ. In addition, the relevant coupled hydro-mechanical phenomena were implemented which include (a) the gradual sealing of the fractures in the EDZ during resaturation, and (b) a pressure-dependent permeability associated the water- and gas injection tests.