Impact of induced thermal stresses during circulation tests in an engineered fractured geothermal reservoir - Example of the Soultz-sous-Forets European Hot Fractured Rock Geothermal Project, Rhine Graben, France

The wider concept of hot fractured rock geothermal energy is put forward to overcome the natural limitations of classic geothermal fields. A suite of numerical modules based on a discrete fracture network approach has been developed, specifically devoted to the understanding of flow through faulted and fractured deep rock masses. The presented application concerns the geothermal project developed at the Soultz-sous-Forets site (Alsace, France) in a graben geological setting. This area seems well suited for such a project, since most of the faults and fractures are nearly optimally-oriented for frictional slip in the present day stress field. A 3D structural model is generated from a variety of measurements collected during the 1993-1997 experimental program at depths ranging in 2850 down to 3600 m. These include induced microsismicity, geophysical logs, flow logs, hydraulic interference tests between wells 500 in apart, a four months long circulation test and two tracer tests. An interesting output of the modeling work is that rock shrinkage due to cooling at the reinjection well is likely to happen within some months of fluid circulation, and this would fit with many observations, such as a pressure decrease at the re-injection well, noticeable changes in the flow logs or a significant evolution in tracer breakthrough curves.