CHENILLE: Coupled beHavior undErstaNdIng of fauLts: from the Laboratory to the fiEld

The understanding of the coupled thermo-hydro-mechanical behaviour of fault zones in naturally fractured reservoirs is essential both for fundamental and applied sciences and in particular for the safety assessment of radioactive waste disposal facilities. In this framework, an international research program callled CHENILLE was built to address key questions related to the impact of high temperatures (up to 150°C) on shear zones as well as fault reactivation processes in shale formations. The project includes a thermally controlled in situ fluid injection experiment on a strike-slip fault zone outcropping atIRSN’s Tournemire Underground Research Laboratory (URL) and a series of laboratory experiments to understand the chemical and structural evolution occurring within the fault zones during the thermal and hydraulic loading. The in situ experiment includes a heating system installed around an injection borehole will enable a precise and controlled incremental increase of the thermal load. The injection borehole will be equiped with a Step-Rate Injection Method for Fracture In-Situ Properties (SIMFIP) probe, in order to perform step pressure tests. The probe will not only measure the flow and pressure rate inside the injection borehole but also allow to monitor the borehole’s 3D deformation during the hydraulic and thermal loading steps. In addition, an array of seismicifferent sensors will be implemented around the injection area to measure the seismic and aseismic deformation induced either by thermal or by hydraulic loading. The seismic monitoring system is composed of Acoustic Emission (sensitive between 1kHz and 60kHz) enabling monitoring fracturing processes of sub-decimeter size. Furthermore, a fibre optic network will be installed in the heating boreholes to measure spatially temperature variationsvia Distributed Temperature Sensing technology in the investigation area. Active seismic surveys, using different source types, are scheduled before and after the experiment to determine the structural network but also to detect the appearance of new structures triggered from the hydro-thermal pressurization of the fault by tomography and reflection seismic methods. The overall goal of our work is to present the interaction between the different geophysical methods that we are using as well as some preliminary results. A first part is dedicated to the description of the fault zone through field and core samples observations as well as borehole to borehole correlation, whereas the second is dedicated to preliminary results on the thermal diffusion expected in the fault.