The CHENILLE experiment: monitoring of fault-heating and fluid-injection in shale rocks at the underground research facility Tournemire, Southern France

The international research programme 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. This paper presents 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 methods including acoustic emission (AE), active seismic surveys, thermal diffusion and distributed temperature sensing to capture in situ temperature evolution within the fault core and damage zone. Heating started in November 2023 and ended in July 2024, at which the fault core temperature could not exceed 40°C. Two series of injection tests using gas (nitrogen and argon) and water were conducted in June–July and October–November 2024 within both the fault core and damage zone. Monotonic, cyclic and stepwise progressive flow rate injection protocols were adapted for the Tournemire shale environment. The average gas permeabilities were determined to be 10 −17 m 2 in the damage zone and 10 −16 m 2 in the fault core, while the corresponding water permeabilities were 10 −16 m 2 in the damage zone and 10 −17 m 2 in the fault core. Injection tests periods were characterized by a significant increase in AE activity, with up to 1000 events per day in the fault damage zone, compared to a baseline daily mean of 0–30 events without work activities in the URL. Persistent AE event activity is seen throughout the monitoring period in the SW 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. In an area extending from the injection interval to the gallery, a decrease in P-wave velocity of 0.1–0.3 km s −1 was observed after stimulation of the fault zone.