The understanding of the coupled thermo-hydro-mechanical behaviour of fault zones in naturally fractured reservoirs is of fundamental importance for a variety of societal and economic reasons, such as the sustainable energy transition for the safe use of natural resources (energy storage, nuclear waste disposal or geothermal energy). The overall objective of this project is to better understand the physical processes resulting from a thermal and hydric load on an existing, identified and characterized fault zone. The idea here is to carry out a thermally controlled in situ fluid injection experiment in one of the faults accessible from the Tournemire underground research laboratory (URL). A heating system will be installed around the injection area and enable a precise and controlled incremental increase of the thermal load. In addition, an important monitoring system will be installed to measure the seismic and aseismic events induced either by thermal or by hydraulic loading. The monitoring system will be composed of acoustic and broadband sensors that will measure low to medium magnitude events. Furthermore, it is also planned to install a fibre optic network to measure temperature and displacement in a distributed manner in the investigation area. The active seismic methods will be deployed before and after the experiment to determine the structural network but also to detect the appearance of new structures triggered from the hydro-thermal pressurisation of the fault. A series of laboratory experiments will be conducted to understand the chemical and structural evolution occurring within the fault zones during the thermal and hydraulic loading. Experiments in climatic chambers exposing the samples to the same heat treatment as that of the in situ experiment will be carried out in order to compare the mineralogical composition evolution of the samples with those taken from the field investigated zone. Finally, a rock mechanical study, from the microscopic to the centimeteric scale with monitoring of the acoustic properties will be carried out. This study will include experiments from Scanning Electron Microscope with Energy Dispersive Spectroscopy (SEM-EDS) allowing the identification of the micro-scale mechanisms of deformation localization to which it is planned to add an acoustic measurement system. In order to study the evolution of mechanical behaviour as a function of scale, experiments in triaxial press, again with acoustic monitoring, are planned.

Section 4.8: DTS monitoring of heating experiment, thermal charaterization of the URL