Evaluation of damage-induced diffusion parameters in Tournemire argillite using an in-situ experiment

Full text of publication follows: Preliminary studies carried out on 4 m-length boreholes drilled around drifts in the Tournemire argilites showed that the values of hydraulic conductivity in the excavation-damaged zone can be up to 4 orders of magnitude higher than those measured in the undisturbed zone [1]. The present study, by means of an in-situ experiment, aims to verify whether the diffusion parameters display a similar behaviour. The experiment has consisted in circulating, at a constant flow rate, a pore water-like synthetic fluid containing tracers in a 30 cm packed-off test interval located at 3.7 meters from the drift wall. Tracers were deuterium- and oxygen-18- depleted water, iodide, bromide and caesium. The tracer concentrations and main ion compositions in the borehole were regularly monitored. Interpretation of hydraulic tests performed in the interval prior to tracer injection led to values of hydraulic conductivity that are 2 orders of magnitude higher than those estimated for the undisturbed zone, confirming the previous results [1]. Tracer concentration evolution in the test interval suggests the existence of a dual porosity system. For most of the tracers, this is evidenced by two diffusion rates, diffusion soliciting first macro-porosity and then micro-porosity. Caesium evolution, which did not show this behaviour, is probably more controlled by sorption than by pure diffusion. The macro-porosity could correspond to micro-fissures, induced either by hydration/dehydration cycles occurring in the galleries or by the damaged zone around the borehole. The experiment has demonstrated that both diffusion and advection parameters are sensitive to the presence of a damaged zone. [1] Extension of the de-saturated zone around structures excavated in the Tournemire argillite by means of petrophysical measurements coupled to gas tests. J.-M. Matray, S. Savoye, S. Lemius, M. Keich. In Proceedings of the International Conference 'Clays in natural and engineered barriers for radioactive waste confinement', Tours, 2005. (authors)