| Title | On-going seismic monitoring of the Rittershoffen project EGS project (Alsace, France) |
|---|---|
| Authors | Maurer, V; Grunberg, M; Richard, A; Baujard, C; Doubre, C; Lehujeur, M |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | seismic monitoring, induced seismicity, natural seismicity, enhanced geothermal system, Alsace |
| Abstract | The ECOGI joint venture (ES group, Roquette Frères and Caisse des Dépôts et de Consignation) is in charge of the development and the exploitation of the Rittershoffen enhanced geothermal system located 6 km east of Soultz-sous-Forêts, in Northern Alsace. This EGS reservoir is one of the very few currently under development in Europe and is designed to produce 24 MWth (170 °C, 70 l/s) out of 90 MWth, which will be delivered to a bio-refinery located 15 km away. Currently the geothermal plant is under construction as well as the heating network that will deliver the heat to a bio-refinery located 15 km away. A seismic network has been deployed in order to monitor the induced seismic activity during drilling operations or future geothermal exploitation. Two deep wells have been drilled between 2012 and 2014 to 2500 m TVD (True Vertical Depth) for targeting local normal-faults located at the interface between the clastic Triassic sediments and the top crystalline basement. Due to a poor initial injectivity index, the first well was developed by using various thermo-mechanical and chemical treatments. The results were positive, since the initial well injectivity index was multiplied by a factor of five. The second well was good enough hydraulically after drilling operation and thus, it was not necessary to enhance its natural permeability. Thus, before, during and after drilling operations, induced seismicity was continuously monitored by a series of surface seismological stations. In order to detect any rise of micro-seismicity induced by the geothermal operation, the drilling authorities required the deployment of a permanent seismic network, composed of 5 surface stations. Since 2012, the micro-seismicity activity of the Rittershoffen geothermal project has been monitored by a permanent seismic network composed of 12 surface stations: The Soultz seismic network is composed of short-period (1 Hz) seismometers, one or three components (L4C/L4C 3D), deployed at surface. Signals are digitized on site by 15-bit GEOSTAR data loggers and sampled at 150 Hz. The Rittershoffen seismic network is composed of short-period (1 Hz) three components seismometers (L4C 3D), deployed at the surface. Signals are digitized by Quanterra Q330S directly in miniSEED format at a sampling rate of 100 Hz, increased to 200 Hz by beginning of 2014. In order to locate the induced micro-seismic events, a 1D velocity model was built based on both sonic logs, VSP, and litho-stratigraphic logs performed in the two deep wells. Since the end of the drilling of the 2nd well on July 2014, no induced micro-seismic activity was detected. However early 2016, the down-hole pump will be installed at depth in the production well, and the geothermal circulation tests will start As low magnitude seismic events are occasionally associated to deep circulations of such projects, the micro-seismic activity will be carefully monitored in real-time during starting operational tests of production of the plant as well as later, during exploitation. |