Overview of the Fracture Network at Different Scales Within the Granite Reservoir of the EGS Soultz Site (Alsace, France)

In EGS concepts like the one at Soultz, knowledge of the fracture network is essential to understand reservoir behavior and plan further stimulations and long term hydraulic circulation. At Soultz, the targeted reservoir is located at a 5km depth within the granite basement of the Upper Rhine Graben near the western border of France. This granite underwent a complex tectonic history including the Hercynian and Alpine orogeneses, leading to the currently observed fracturing. This fracturing is dominated by steeply dipping fracture sets which are distributed at various scales in the granite basement. This structural work presents the characterization of the fracture network at different scales in order to understand and model the hydro-thermo-mechanical behavior of the granite geothermal reservoir through hydraulic stimulation and subsequent circulation tests. The small-scale fracture network has been characterized based on several kilometers of high resolution borehole image logs, allowing the characterization of different sets. There was a focus on large-scale fracture zones, which are the major fluid pathways. A total of 39 fracture zones have been described in detail based on borehole data. They have been compared to large-scale geophysical investigations using Vertical Seismic Profile (VSP) and passive microseismicity in order to build an extended fracture network in 3D. Hydraulically active fractures have been investigated in more detail in the different open-hole sections based on image logs, flow logs and temperature logs. These near-well borehole fractures represent potential fluid pathway for connecting the borehole to the far-field geothermal reservoir. The fracture network in the granite Soultz reservoir is constituted by large-scale fracture zones, which are connected to a dense network of small-scale fractures. However, fluid pathways are more complex, with channelized structures, because only a limited number of fractures support fluid flow.