Engineered damage zone sealing during a water injection test at the Tournemire URL

Low-permeability clay formations provide good candidate host rocks for geological disposal of radioactive waste, because there is expected to be limited movement of gas or water through the formation. However, when constructing tunnels, the stress state in the formation around the tunnel will change, which can lead to damage to the formation, changing the bulk hydraulic properties of the formation close to the tunnel. There is the potential for this damaged zone to act as a preferential pathway for fluid flow and radionuclide transport. A water injection experiment is ongoing at the Tournemire underground rock laboratory to investigate the hydraulic properties of the Toarcian argillite in which the laboratory is constructed. Water is injected into the formation at the end of a sealed borehole and moves preferentially in the damaged zone along the borehole walls. The rate of water injection into the rock changed over the first year of the experiment and the causes of this change are investigated in this paper by numerical modelling. The study demonstrates that the change in water injection rate into the argillite can be explained by the evolving hydraulic properties of the damaged zone around the borehole. The findings of the modelling study are discussed in the context of long-term radioactive waste disposal.