Abstract
The development of the Rittershoffen deep geothermal field (Alsace, Upper Rhine Graben) between 2012 and 2014 induced unfelt seismicity with a local magnitude of less than 1.6. This seismicity occurred during two types of operations: (1) mud losses in the Muschelkalk formation during the drilling of both wells of the doublet and (2) thermal and hydraulic stimulations of the GRT-1 well. Seismicity was also observed 4 days after the main hydraulic stimulation, although no specific operation was performed. During chemical stimulation, however, no induced seismicity was detected. In the context of all field development operations and their injection parameters (flow rates, overpressures, volumes), we detail the occurrence or lack of seismicity, its magnitude distribution and its spatial distribution. The observations suggest the presence of the rock stress memory effect (Kaiser effect) of the geothermal reservoir as well as uncritically stressed zones connected to the GRT-1 well and/or rock cohesion. A reduction of the seismic rate concurrent with an increase of injectivity was noticed as well as the reactivation of a couple of faults, including the Rittershoffen fault, which was targeted by the wells. These results are derived from the homogeneous and consistent catalogue of more than 1300 local earthquakes that is provided. This reference catalogue is based on a standard detection method, whose output was manually verified and improved. The given absolute locations have been computed in a calibrated, geologically realistic 3D velocity model. Our work builds on previous analyses addressing the seismicity induced by the GRT-1 hydraulic stimulation and places the results into a historical context, thus considering the full dynamics of the observed phenomena. This paper also complements existing descriptions of the hydrothermal characteristics of the deep reservoir by providing insights separate from the wells.
Highlights
The Upper Rhine Graben (URG) currently hosts several enhanced geothermal system (EGS) plants that are in the exploitation phase, such as those in Rittershoffen and Soultzsous-Forêts in France (Baujard et al 2017; Genter et al 2018), Landau and Insheim in Germany (Ganz et al 2013; Küperkoch et al 2018) and Riehen in Switzerland
The processing procedure was applied to the continuous seismic data recorded during and around all major development operations: GRT-1 drilling, stimulations and testing; and GRT-2 drilling (Fig. 3)
We presented and discussed the seismicity associated with the Rittershoffen field development, its occurrence, its magnitude distribution and its spatial distribution in the light of all field operations and their injection parameters
Summary
The Upper Rhine Graben (URG) currently hosts several enhanced geothermal system (EGS) plants that are in the exploitation phase, such as those in Rittershoffen and Soultzsous-Forêts in France (Baujard et al 2017; Genter et al 2018), Landau and Insheim in Germany (Ganz et al 2013; Küperkoch et al 2018) and Riehen in Switzerland. Other EGS projects in France are in the drilling phase (Vendenheim and Illkirch-Graffenstaden) or Maurer et al Geotherm Energy (2020) 8:5 in an earlier stage (Eckbolsheim, Hurtigheim, Wissembourg), demonstrating the growing interest and viability in utilizing deep geothermal resources in the current energy mix. All of these EGS fields exploit hot local geothermal brine that is circulating in fracture and fault networks of Triassic sediments and the underlying granitic fractured basement. EGS technology consists of increasing the low natural hydraulic performance of deep geothermal reservoirs by thermal, chemical and/or hydraulic stimulations. Seismicity can be temporarily or continuously induced during the exploitation itself, i.e., during the interwell circulation of the geothermal fluid (Baujard et al 2018; Cuenot and Genter 2015; Evans et al 2012; Megies and Wassermann 2014)
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