Characterization of hydromechanical damage of claystones using X-ray tomography

During the excavation of underground galleries in a rock mass, reorganisation of the stress field around the site results in damage to the surrounding material. In the framework of research undertaken at the ANDRA underground laboratory in France, galleries have been excavated in a Callovo-Oxfordian argillite formation. The damage has manifest as a number of fracture systems in the adjacent rock. Observations have shown that fractures generated during excavation and storage exploitation are able to self-seal during re-saturation of the site in the post-closure stage. For the present study, we performed a range of self-sealing experiments using a newly-developed triaxial compression cell placed within a nano-CT scanner with a 10 mu m resolution. The samples contained an artificial fracture that had been generated in the laboratory, into which a fluid was injected. Two facies of claystone were tested. In the first facies (CaCO3 content of 29.7%), an important but not total reduction in volume after 34 days of injection was observed, as well as a high decrease in permeability, even if safe claystone values were not recovered. In the second facies (CaCO3 content of 59.4%), a low volume reduction and decrease in permeability were observed after 15 days of injection.