Long-term geophysical monitoring of fault-heating and fluid-injection processes in shale rocks at the underground research facility Tournemire, Southern France

The international research program CHENILLE (Coupled beHavior undErstaNdIng of faulLts: from the Laboratory to the fiEld) addresses key questions regarding the impact of high temperatures on shear zones and fault reactivation processes in shale formations. We report on results of CHENILLE from a thermally controlled in situ fluid injection experiment conducted on a strike-slip fault zone cropping out at IRSN’s Tournemire Underground Research Laboratory (URL). The research focuses on understanding the mechanical, hydraulic, structural and thermal evolution of fault zone under thermal and hydraulic loading. Key findings were derived using various geophysical monitoring methods including Acoustic Emission (AE), active seismic surveys, thermal diffusion, and Distributed Temperature Sensing (DTS) to capture in situ temperature evolution and structural changes within the fault core and damage zone.<br>The Tournemire URL is located on the western margin of the Mesozoic Causses Basin in southwestern France. It transects an about 250 m thick Toarcian shale formation, bounded above and below by aquiferous limestone units The URL is intersected by several strike-slip fault zones that played a significant role in the tectonic evolution of the Causses Basin. Among them is the F2 fault, where the CHENILLE experiment is being conducted, a particularly well-exposed, subvertical strike-slip fault that cuts through the Toarcian shale. It has been extensively studied due to its structural and hydrogeological properties. The fault zone comprises a fault core and a fault damage zone, with a total thickness of approximately 5 to 10 m, and includes a clay-rich fault core up to 1.2 m thick. The CHENILLE experiment is realized by means of borehole-supported observation methods applying DTS installed in four boreholes, 12 AE sensors installed in four wells, 18 boreholes for the installation of 3C-geophone receivers for active seismic recording, two wells with 3 m long heater units and a fluid injection probe with double packer system. The latter enables separate stimulation of two intervals with 1.85 m length within the fault core and 3.1 m length within the damage zone.<br>Heating started in November 2023 and ended in June 2024. The rock could not be heated to more than 40°C at a distance of about 1.5 m. Three series of injection tests with gas (Nitrogen and Argon) and water have been carried out in June/July October/November 2024 and June/July 2025 in the core and damage zone of the fault. For more information on injection procedures and pressure tests, please refer to the article by Zimmermann et al. in this volume. Injection tests periods were characterized by a significant increase in AE activity, with up to 1000 events per day in the fault damage zone. Persistent AE event activity is seen throughout the monitoring period in the southwest of the AE network, where a low P wave velocity zone is imaged. Seismic tomography was carried out before and after heating and gas injection of 2024. In an area extending from the injection interval to the gallery, a decrease in P wave velocity of 0.1 to 0.3 km s-1 was observed after stimulation of the fault zone. Further analyses will include data from geophysical monitoring during thermal decay behavior and injection tests in 2025.