In Situ Thermal Hydrofracturing Behavior of the Callovo-Oxfordian Claystone within the Context of the Deep Geological Disposal of Radioactive Waste in France

A deep geological facility (Cigéo project) is planned, if authorized, in the Callovo-Oxfordian claystone (COx) for the disposal of all long-lived radioactive waste produced in France. The heat emitted from the most exothermic wastes, emplaced in horizontal parallel micro-tunnels, will lead to a temperature increment within the COx. The resulting thermal pressurization may lead to tensile effective stresses and induce thermal hydrofracturing. One of the design criteria for this area is to avoid any damage induced by thermal loading, especially in the far field. The French National Radioactive Waste Management Agency (Andra) has conducted an in situ heater experiment at the Meuse/Haute-Marne Underground Research Laboratory to better understand the thermal hydrofracturing process of the COx. The experiment’s design was conceived in a way that the thermal pressurization will be sufficiently high to generate effective tensile stresses larger than the effective tensile strength of the COx. To induce thermal hydrofracturing, the COx was submitted to temperatures and pore pressures higher than those expected in the Cigéo project. A breakdown pore pressure, a sign of a fracture event, was registered by the measurements during the second heating cycle. These observations were also confirmed by the post-test core drilling. Furthermore, the numerical modelling of the experiment shows that the thermo-poro-elastic approach reproduces the temperature and pore pressure measurements well in a wide range of locations up to just before the occurrence of the fracture event. This satisfactory reproduction provides confidence for the design of the Cigéo project.