| Title | A Conceptual Model for Post-Injection Seismicity at Soultz-sous-Forets |
|---|---|
| Authors | Baisch, Stefan; Weidler, Ralph; Voros, Robert; Jung, Reinhard |
| Year | 2006 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Induced Seismicity; microearthquakes, geothermics, shear dilation, reservoir stimulation, seismic risk |
| Abstract | Shear dilation stimulation, which implies injection of large fluid volumes at high pressure, has become a standard technology for the development of high enthalpy geothermal reservoirs. In the crystalline crust, these measures are accompanied by the occurrence of induced seismicity. Observations from different geothermal sites indicate that comparatively large magnitude events (i.e. up to Ml~3) occur in the shut-in period after the injection has been terminated. Based on observations from the geothermal site at Soultz-sous-Forêts (France) we present a conceptual model to explain post-injection seismicity and its tendency to produce relatively strong seismic events. In this model, fluid overpressures on pre-existing fractures control the triggering of induced seismicity. Fracture zones are assumed to consist of adjacent fracture patches which may slip individually on a millimetre scale and are coupled similar to a “block-spring system”. Depending on the spatial fluid pressure gradient, adjacent fracture patches may become overcritical simultaneously and slip in terms of a single seismic event of comparatively large magnitude. Our proposed model suggests that observed post-injection seismicity can be described by the simple process of ongoing pressure diffusion without invoking additional triggering mechanisms. |