| Title | The geomechanical significance of clay in geothermal reservoirs |
|---|---|
| Authors | Carola Meller, Emmanuel Gaucher, Thomas Kohl |
| Year | 2014 |
| Conference | European Geothermal Workshop |
| Keywords | Hydrothermal alteration, clay, fracture zones, geomechanics, induced seismicity |
| Abstract | 25 years' experience during EGS development in the Upper Rhine Graben (URG) in Central Europe highlighted the importance of a sound reservoir characterization for the safety and success of a geothermal power plant, especially for enhanced geothermal systems (EGS). Especially seismic events with their immediate impact on the environment of a geothermal power plant have become a major task in EGS associated research. Being a direct effect of the geological and mechanical processes underground, controllability of induced seismicity can only be reached by a deep understanding of the reservoir geology and associated rock mechanical processes. Large mechanical contrasts in the crystalline geothermal reservoir of Soultz‐sous‐Forêts are expected between the primary granitic body and hydrothermally altered fracture zones. The present study aims on the investigation of relationships between the occurrence of hydrothermally altered, clay‐ bearing zones and observable geomechanical processes. The basis for this study is synthetic clay content logs (SCCL) created with a neural network. These logs display the clay content inside fractures along a geothermal well. Their high resolution in the order of few decimeters allows detailed interpretation of induced seismicity. The basis for this study is the microseismic catalogue of the 1993 stimulation of the well GPK1. With a probabilistic model of fracture orientations in the open hole section of this well combined with a modification of their mechanical parameters according to their clay content, a model of the distribution of the critical pressure inside the reservoir is created. With a comparison between the probabilistic model and recorded seismicity during the 1993 stimulation it is demonstrated that the presence of weak fractures inside the reservoir can explain the evolution of induced seismicity. |