| Title | Induced Seismic and Aseismic Slip in EGS Reservoir: Case Studies from Alsace, France |
|---|---|
| Authors | Olivier LENGLINE, Léna CAUCHIE, Jean SCHMITTBUHL |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | induced seismicity, repeaters, hydraulic stimulation, stress drop |
| Abstract | The injection of fluid in the upper crust, notably for the development or exploitation of deep geo-thermal reservoirs, is often associated with the rise of induced seismicity. The occurrence of large seismic events during such operations needs to be reduced in order to preserve infrastruc-tures and population nearby the injection site. However it is not clear how the injected fluid in-fluences the characteristics of the induced events. Here we investigate the micro-earthquakes that occurred during one of the hydraulic stimulation in Soultz-sous-Forêts, France in 1993 and study the link between the injected fluid and the source properties of the induced earthquakes when aseismic slip is known to occur. We take advantage of the deep borehole accelerometers that were running during this experiment in the vicinity of the injection zone. We estimated the moments and radii of all recorded events using a spectral analysis and classified them into 663 repeating sequences. We show that events follow the typical scaling law between radius and moment but fluctuations of moments are important while the radius of the events remain simi-lar. Repeating events on a given asperity with a well-defined radius, are shown to experience very different stress drops questioning the involved mechanisms. We also evidence an increase of the average event radius (and moments) over the course of the injection that follows the in-crease of the wellhead pressure but also an evolving access to preexisting fault zones of the reservoir (see Figure). Two different populations of events are characterized. These observations suggest that the fluid pressure has both a direct and an indirect control on the rupture size of the induced events. We also compare this behavior with those of other deep geothermal circula-tions in Northern Alsace, France (Soultz-sous-Forêts and Rittershoffen). |