Fault network development during repeated fluid injection at the Rittershoffen deep geothermal site, France

Fault network development during repeated fluid injection at the Rittershoffen deep geothermal site, France

Rike Köpke

Rike Köpke1,2, Olivier Lengliné2, Emmanuel Gaucher1, Jean Schmittbuhl2, Thomas Kohl1

1Karlsruhe Institut of Technology, Deutschland; 2University of Strasbourg, France

At the deep geothermal site of Rittershoffen (Alsace, France), the well GRT1 underwent a series of stimulations. In particular, a thermal and a hydraulic stimulation took place subsequently in less than three month. During both stimulations, seismicity was induced, which can be used to follow the development of the fault network and the mechanical state of the reservoir over the course of thermal and hydraulic stimulation.

To follow the temporal and spatial details of the fault network development, an extensive, robust seismic database is crucial. To obtain it, we applied a template matching detection algorithm to the continuous waveforms recorded by the seismic network that monitored the development of the Rittershoffen site, resulting in the detection of about 300 events for the thermal stimulation and about 3000 events for the hydraulic stimulation. All events were relocated with HypoDD, using the travel time differences between the events to gain precise relative locations.

We use the obtained detections and locations to highlight the development of the fault network in time and space over the course of the different stimulation operations. This also shows the impact the two stimulations had on the reservoir. We can show that during both stimulations the same fault was seismically active. Yet, the mechanical state of the fault changed, resulting in a seismic activation during hydraulic stimulation at much lower injection pressure. At the end of the hydraulic stimulation, aseismic stress transfer took place, leading to the triggering of seismicity on a second fault.

We also performed a clustering analysis to get insight on the mechanisms behind the induced seismicity. It shows that the event waveforms during thermal and hydraulic stimulation are very similar and waveform clusters remain stable over the whole course of both stimulations. This suggests the same fault activation mechanism took place during both stimulations. The clustering analysis also shows that seismicity triggered at the second fault has different waveforms compared to the seismicity directly linked to injection, confirming a different fault activation mechanism.

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