International audienceIn the Upper Rhine Graben, several innovative projects based on enhanced geothermal system (EGS) technology exploit local deep-fractured geothermal reservoirs. The principle underlying this technology consists of increasing the hydraulic performances of the natural fractures using different stimulation methods in order to circulate the natural brine at commercial flow rates. For this purpose, knowledge of the in situ stress state is of central importance to predict the response of the rock mass to different stimulation programs. Here, we propose a characterization of the in situ stress state from the analysis of ultrasonic borehole imager (UBI) data acquired at different key moments of the reservoir development using a specific image correlation technique. This unique dataset has been obtained from the open-hole sections of the two deep wells (GRT-1 and GRT-2, ∼ 2500 m) at the geothermal site of Rittershoffen, France. We based our analysis on the geometry of breakouts and drilling-induced tension fractures (DITFs). A transitional stress regime between strike-slip and normal faulting consistent with the neighboring site of Soultz-sous-Forêts is evident. The time-lapse dataset enables us to analyze both in time and space the evolution of the structures over 2 years after drilling. The image correlation approach developed for time-lapse UBI images shows that breakouts extend along the borehole with time and widen (i.e., angular opening between the edges of the breakouts) but do not deepen (i.e., increase in the maximal radius of the breakouts). The breakout widening is explained by wellbore thermal equilibration. A significant stress rotation at depth is evident. It is shown to be controlled by a major fault zone and not by the sediment-basement interface. Our analysis does not reveal any significant change in the stress magnitude in the reservoir
