Experimental study of the impact of cracks propagation and self-sealing on water and gas permeability of the Callovo-Oxfordian claystone

The excavation of underground galleries generally results in a zone damaged by fractures. In the context of the storage of radioactive waste in deep clay layers, this network of fractures desaturates the host rock and causes it to lose its confinement properties (very low permeability, good mechanical strength). This thesis focuses on the Callovo-Oxfordian claystone (COx), a geological formation chosen in France (Bure, Meuse/Haute-Marne) for its potential as a containment barrier for radioactive waste. Using an experimental approach, the impact of crack propagation and self-sealing on the water and gas permeability of the COx claystone was studied. Self-sealing tests were carried out on cylindrical specimens artificially fractured under X-ray tomography, under different orientations (parallel and perpendicular to the bedding plane) and temperatures, with different calcite contents, different opening widths of the initial fracture, and with the injection of different fluids such as water and gas. The objective being to better understand the impact of these different factors on the self-sealing process of the COx claystone (changes in permeability and fracture closure). Furthermore, fracturing tests with measurement of gas permeability were also carried out for the two orientations in order to study the impact of microcracking damage on the hydraulic properties of the COx claystone. It has been observed that the mineralogical composition of the claystone plays a determining role in the effectiveness of self-sealing. High calcite content, combined with low clay content, has been identified as a limiting factor, thereby compromising the rock's ability to self-seal effectively. To guarantee effective sealing, it is necessary that the calcite content remains below 40%. In general, the self-sealing process is rapid at the start of the test and then stabilizes after a month. The water permeability of the samples is partially restored (∼10^-18-10^-19 m²) compared to the initial permeability of healthy claystone (i.e. without fracture) (∼10^-20-10^-21 m²) and the closure of the fracture is almost complete. Furthermore, the self-sealing process appears to be equally effective for both parallel and perpendicular orientations. The impact of temperature on the self-sealing process was not significant. Only a slight delay effect was observed at the start of tests at high temperatures (80 deg. C). Regarding the simultaneous injection of inert gas and water, our study demonstrated a retarding effect on the reduction of water permeability, mainly due to gas-induced desaturation. Despite this delay, the self-sealing process proved effective, inducing a significant reduction in permeability. Pilot tests have shown us that it is important to pay attention to the procedure for re-saturating samples before using them. As a result, it is preferable not to re-saturate the specimens. The fracturing tests (triaxial compression tests) showed that the gas permeability gradually decreases for both orientations with increasing deviatoric stress, illustrating the closure of the initial cracks due to the axial compression of the samples. For higher deviatoric stresses, a significant increase in gas permeability is observed near the dilatancy threshold, particularly in samples oriented parallel to the bedding plane. This suggests a close link between the opening of microcracks and the increase in permeability. (author)