| Title | Ground vibrations caused by geothermal drilling operations: a case study from the Rittershoffen EGS project (Alsace, France) |
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| Authors | Maurer, V; Lehujeur, M; Richard, A; Vergne, J |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | Seismology, geothermal energy, ground vibration, drilling, Alsace |
| Abstract | Designed to produce 24 MWth (170 °C, 70 l/s) with a doublet the Rittershoffen EGS project is located 6 km eastwards of well-known Soultz-sous-Forêts EGS project, in Northern Alsace, France. The first well, GRT1, reached its final depth of 2580 m end of 2012 for targeting local normal-faults located at the interface between the clastic Triassic sandstones and the top crystalline basement. Numerous logs and hydraulic tests were performed beginning of 2013 and it has been decided to develop the connexion between the well and the reservoir. A reservoir development strategy has been performed and the results were positive, since the injectivity of the well was multiplied by a factor of five. The second well, GRT2, was drilled from March to August 2014 using a HH300 drilling machine. This machine is able to hang 270 t, was used to drill a deviated well of 3200 m length down to 2600 m depth. After the drilling phase, production tests and circulations tests were performed. No reservoir development was required since production tests showed that the well was artesian, naturally producing about 40 l/s. Since some induced seismicity was expected during the reservoir development of GRT1 the permanent seismic monitoring network has been reinforced with a temporary short-period network. In total, 31 short-period velocimeters were installed in a range of 5 km around the drilling platform from the very beginning of the drilling of GRT2. These stations remained installed till November 2014, after the production tests, so five months after the end of the drilling phase. This densely spatially distributed seismic network allowed to measure the level of vibration in terms of velocity displacement at each site during and after the drilling phase. It revealed that the vibrations caused by the drilling rig is visible by the sensors at least up to more than 2 km away from the drilling platform, but without being perceptible by population. Moreover, during the drilling of GRT-2, some micro-seismicity events were detected and located due to some drilling issues. Since the drill bit was stuck in the well due to clay collapse, the drilling operator has to increase the mud pressure to be able to pull out the drill bit from the well. This minor seismic cluster was temporally concentrated on two hours with a maximum seismic rate of about 100 evt/hour. Since these micro-seismic events occurred in May 2014, this seismic activity has been recorded by the whole seismological network. Since the maximum magnitude recorded was 1.2 Mlv, the seismic network allowed to measure velocity displacement for this event over the whole network. It shows that the velocity displacement was below the human being threshold of detection. This work allowed quantifying the level of vibration caused by a drilling operation in terms of velocity displacement, which represent an acceptability issue for implantation of geothermal project, especially in urbanized areas. We also show that the induced seismic activity recorded during the drilling of GRT-2 on the Rittershoffen EGS project was negligible and could not be felt by the local population. |