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. In the Upper Rhine Graben, several innovative projects based onenhanced geothermal system (EGS) technology exploit local deep-fracturedgeothermal reservoirs. The principle underlying this technology consists ofincreasing the hydraulic performances of the natural fractures usingdifferent stimulation methods in order to circulate the natural brine atcommercial flow rates. For this purpose, knowledge of the in situ stressstate is of central importance to predict the response of the rock mass todifferent stimulation programs. Here, we propose a characterization ofthe in situ stress state from the analysis of ultrasonic borehole imager (UBI) dataacquired at different key moments of the reservoir development using aspecific image correlation technique. This unique dataset has been obtainedfrom the open-hole sections of the two deep wells (GRT-1 and GRT-2, ∼2500 m ) at the geothermal site of Rittershoffen, France. Webased our analysis on the geometry of breakouts and drilling-inducedtension fractures (DITFs). A transitional stress regime between strike-slipand normal faulting consistent with the neighboring site ofSoultz-sous-Forets is evident. The time-lapse dataset enables us toanalyze both in time and space the evolution of the structures over2 years after drilling. The image correlation approach developed for time-lapse UBI images shows that breakouts extend along the borehole with time andwiden (i.e., angular opening between the edges of the breakouts) but do notdeepen (i.e., increase in the maximal radius of the breakouts). The breakoutwidening is explained by wellbore thermal equilibration. A significantstress rotation at depth is evident. It is shown to be controlled by amajor fault zone and not by the sediment–basement interface. Our analysisdoes not reveal any significant change in the stress magnitude in thereservoir.

Stress characterization and temporal evolution of borehole failure at the Rittershoffen geothermal project