Seismic response of the fractured and faulted granite of Soultz-sous-Forêts (France) to 5 km deep massive water injections

Abstract

S U M M A R YThe European Enhanced Geothermal System (EGS, formerly Hot Dry Rock, HDR) programme of Soultz-sous-Forêts is organized around three wells drilled to a depth of about 5000 m. Hydraulic stimulations were performed in the wells in 2000 (GPK2 well), 2003 (GPK3 well) and 2005 (GPK4 well). The stimulation of GPK2 induced more than 700 seismic events with a magnitude greater than 1.0. The seismicity depicts a dense, homogeneous cloud, without any apparent structure. Medium-size earthquakes represent more than 80 per cent of the cumulative seismic moment. The b-value of the Gutenberg and Richter law is larger than 1.2. The injectivity has been increased by a factor 20. These characteristics indicate that the stimulation reactivated a 3-D dense network of fractures. The stimulation of GPK3 induced only about 250 events with a magnitude greater than 1.0 but with a greater proportion of large events, up to 2.9. The hypocentres form clear structures identified as large faults, the b-value is about 0.9 and the large events (M > 2.0) account for the greater part of the cumulative seismic moment. The injectivity of the well, which was already high before the stimulation, remained unchanged. The stimulation of GPK4 was achieved in two stages. This stimulation produced even less induced events, making the interpretation difficult. The differences between the seismic response of GPK2 and GPK3 are due to the presence of large faults cut by GPK3 or in its close vicinity and reached by the injected water. Once a seismic event occurs on a fault, a sequence of earthquakes is triggered and the seismicity behaves, for a large part, independent of the injected flow rate. The stimulations also show some evidence that creeping could be a major source of deformation, if not the main one. The future EGS programme will have to drill wells in zones free of large faults to avoid poor hydraulic performance and inconvenience to the population.

Analysis of induced seismicity in geothermal reservoirs – An overview

“…A total of four events with magnitudes greater than 2 were induced by these activities: during the first phase, a magnitude 2.3 event occurred a few hours after injection was stopped; during the second phase, a magnitude 2.7 event occurred at the height of the injection; during the third phase, a magnitude 2.2 event occurred during stimulation and a magnitude 2.3 event occurred about eight days later, when both the well-head pressure and the seismic activity had declined to almost normal levels again. Dorbath et al (2009) showed that the relative size distribution of earthquakes, as expressed by the -value of the Gutenberg-Richter relation, was significantly different for the events induced during the stimulation of GPK2 and GPK3: whereas the average -value was around 1.23 at GPK2, it was only 0.94 at GPK3. The difference seems to be linked to the observation that the seismic cloud induced at GPK2 is diffuse and structureless, whereas at GPK3 the hypocentres align along major faults that can be traced back to the wells, where they were identified in the borehole logs.…”

“…GPK2 was stimulated in June 2000, GPK3 in May 2003and GPK4 in September 2004and February 2005. Events with magnitudes as high as 2.5 were recorded during the stimulation of both GPK2 and GPK3, and in each case, the strongest event occurred after shut in -a magnitude 2.6 event some 10 days after shut-in of GPK2 and two magnitude 2.9 and 2.7 events three-and-a-half and five days after shut-in of GPK3 (Dorbath et al, 2009). The stimulation of GPK4 occurred in three phases -the first two as conventional freshwater injections and the third with the addition of acid.…”

“…The difference seems to be linked to the observation that the seismic cloud induced at GPK2 is diffuse and structureless, whereas at GPK3 the hypocentres align along major faults that can be traced back to the wells, where they were identified in the borehole logs. Furthermore, Dorbath et al (2009) reported that the two 2003 events with magnitude 2.9 and 2.7 were located on two of these major faults.…”

“…Geothermal sites in the Rhine Graben near Basel , Landau (Grünthal, 2014) and Soultz-sousForêts (Dorbath et al, 2009), however, have experienced 2.5 events due to EGS activities (Table 1). Although this seismicity has been short lived it has attracted public concern due to its proximity to populated areas (Kraft et al, 2009).…”

Bridging aero-fracture evolution with the characteristics of the acoustic emissions in a porous medium

“…Monitoring, predicting and controlling fracture evolution during hydraulic-fracturing, steam-assisted gravity drainage, or CO 2 sequestration operations is a key goal [13][14][15][16]. One possibility for monitoring is to use generated acoustic emissions during those operations.…”