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Discussion papers
https://doi.org/10.5194/se-2019-72
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-72
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Apr 2019

Research article | 09 Apr 2019

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.

Stress Characterization and Temporal Evolution of Borehole Failure at the Rittershoffen Geothermal Project

Jérôme Azzola1, Benoît Valley2, Jean Schmittbuhl1, and Albert Genter3 Jérôme Azzola et al.
  • 1Institut de Physique du Globe de Strasbourg/EOST, University of Strasbourg/CNRS, Strasbourg, France
  • 2Center for Hydrogeology and Geothermics, University of Neuchâtel, Neuchâtel, Switzerland
  • 3ÉS géothermie, Schiltigheim, France

Abstract. In the Upper Rhine Graben, several innovative projects based on the 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 with commercially flow rates. For this purpose, the knowledge of the in-situ stress state is of central importance to predict the response of the rock mass to the 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 GRT2, ~ 2500 m) at the geothermal site of Rittershoffen, France. We based our analysis on the geometry of breakouts and of drilling induced tension fractures (DITF). A transitional stress regime between strike-slip and normal faulting consistently with the neighbour site of Soultz-sous-Forêts is evidenced. The time lapse dataset enables to analyse both in time and space the evolution of the structures over two years after drilling. The image correlation approach developed for time lapse UBI images shows that breakouts extend along the borehole with time, widen (i.e. angular opening between the edges of the breakouts) but do not deepen (i.e. increase of the maximal radius of the breakouts). The breakout widening is explained by wellbore thermal equilibration. A significant stress rotation at depth is evidenced. 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.

Jérôme Azzola et al.
Jérôme Azzola et al.
Jérôme Azzola et al.
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Short summary
In projects based on the Enhanced Geothermal System (EGS) technology the knowledge of the in-situ stress state is of central importance to predict the response of the rock mass to the different stimulation programs. 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. We discuss a significant stress rotation at depth and the absence of significant change in the stress magnitude.
In projects based on the Enhanced Geothermal System (EGS) technology the knowledge of the...
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