Experiments to estimate gas intrusion in Callovo-oxfordian argillites

Abstract

Gases, especially hydrogen, will be generated by corrosion and radiolysis of high radioactive waste containers in deep geological repositories. Pressure build up resulting from those long-term processes may create a risk of geological barrier fracturation and of potential creation of preferential pathways for radionuclide migration. Good knowledge of gas transfer mechanisms in the potential host rock formations is therefore necessary. The present work focuses on Callovo-oxfordian argillites from the Bure (Meuse, France) underground laboratory operated by Andra, the French national agency for radioactive waste management. Its goal is to understand the pore structure of that rock formation as a first step towards characterizing and modelling gas intrusion and transfer. Our approach was to use various techniques in combination. Firstly, mercury intrusion experiments were made. Besides supplying the classical pore size distribution curve, they bear some resemblance with the real-life problem since mercury (a non-wetting phase) invades the porous structure like a gas would. Secondly, the water vapor sorption curve was measured by submitting initially dry samples to atmospheres with increasing vapor content. Comparison of the mercury intrusion/water vapour sorption data sets yielded information on the connectivity of the pore structure. Thirdly, the gas permeability of dry argillites was measured in a specifically designed set-up, using helium as an acceptable (non-detonating) substitute for hydrogen. The main observation was the existence of a large Klinkenberg effect. Finally, all the available information was used to build and constrain a multi-scale models for the pore network using the XDQ technique. The resulting models were used to predict the properties of the material at various water saturation levels.