Microseismic Activity Induced Under Circulation Conditions at the EGS Project of Soultz-Sous-Forêts (France)

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1 Proceedings World Geothermal Congress 2010 Bali, Indonesia, April 2010 Microseismic Activity Induced Under Circulation Conditions at the EGS Project of Soultz-Sous-Forêts (France) Nicolas Cuenot 1, Louis Dorbath 2, Michel Frogneux 2 and Nadège Langet 2 1 GEIE Exploitation Minière de la Chaleur, Route de Soultz, BP 40038, Kutzenhausen, France; 2 EOST-IPGS, 5 rue René Descartes, Strasbourg Cedex, France cuenot@soultz.net; louis.dorbath@eost.u-strasbg.fr; michel.frogneux@eost.u-strasbg.fr Keywords: EGS, Soultz-sous-Forêts, induced seismicity, circulation ABSTRACT After 20 years dedicated to research and development, the European EGS pilot project of Soultz-sous-Forêts (France) has entered a new phase, aiming at the building, testing and commissioning of a geothermal power plant. module of 1.5 MWe was installed, as well as all surface facilities. The heat-power conversion system is based on ORC (Organic Rankine Cycle) technology. Since the beginning of 2008, several circulation tests were performed so as to test the different components of the power plant, as well as the medium- to long-term behaviour of the reservoir. One of the major concern of the project was, and still is, the induced seismicity. During the development of the project, several hydraulic and chemical stimulation tests generated thousands of seismic events. Some of them were felt at the surface, provoking unwanted nuisance for the neighbouring population. As the power plant is expected to run continuously for years, it is necessary to study the behaviour of the geothermal reservoir under circulation conditions, in order to observe and understand the occurrence of microseismic activity. Three circulation tests were carried out at 5 km depth using the 3 deep geothermal wells. During the first 6-months test, operated in 2005 under artesian conditions, around 600 earthquakes were induced. Among them, 4 were above magnitude 2 and thus likely to be felt. During the two other tests, involving downhole production pumps installed in the wells, the seismic activity was of the same order, but no event reached magnitude as large as in The location of seismic events shows that the same zones of the reservoir were activated in the three tests. Finally the microseismic activity is linked to the hydraulic parameters of the circulation tests, so as to better understand the conditions of generation of the seismicity and try to avoid any repetitive disturbance for the population. 1. INTRODUCTION The European EGS (Enhanced Geothermal System) project is located in Soultz-sous-Forêts, Alsace, France in the western part of the Upper Rhine Graben (Figure 1). It started in 1987 from the will of the European Commission to develop new sources for power production. The aim of the project is to produce electricity from the heat stored in deep, fractured crystalline rocks. The first phase of the project mainly consisted in the characterization of the underground geothermal reservoir and in a better understanding of the natural hydrothermal circulation. Thus, 3 deep geothermal wells were drilled down to 5 km depth, where the geothermal fluid reached a temperature of 200 C. These wells were stimulated both hydraulically and chemically, in order to improve their hydraulic performance. The current phase of the project is dedicated to the building and testing of a pilot power plant. A first demonstration 1 Figure 1: Location of the site of Soultz-sous-Forêts; the area in grey colour corresponds to the hottest geothermal anomaly In 2005, a first circulation test, involving the three deep wells GPK2, GPK4 (production wells) and GPK3 (injection well) was performed under artesian conditions. During this test, a continuous microseismic activity was observed. Two circulation tests were performed in 2008 involving either 2 or 3 wells. For both test the production wells were equipped with production pumps, which correspond to the future conditions of continuous exploitation of the power plant. Thus the monitoring of microseismicity during both tests would bring important observations about what we can expect, in terms of microseismic activity, when the power plant will be fully in operation. In this paper, we will first briefly present the site of Soultzsous-Forêts and its geothermal characteristics. Then a focus will be made on the three circulation tests (July-December 2005, July-August 2008 and November-December 2008) and the associated microseismic activity.

2 2. THE SITE OF SOULTZ-SOUS-FORÊTS The geothermal project is situated is the northwestern part of the Upper Rhine Graben, which is a Tertiary Graben. This site was selected because of the presence of a large geothermal anomaly and a good knowledge of the shallow geology, both characterized by exploration studies performed for former oil exploitation in the region. 2.1 Geology / Fracturation The shallow geology (0 to 1400 m depth) consists in sedimentary layers, overlaying the crystalline basement, made of altered and fractured granitic rocks, which are older than 330 My (Cocherie et al., 2004) and thus not related to the present-day geothermal anomaly. The shape of the geothermal gradient is related to the presence of convective cells and fluid circulation within the granitic basement (Clauser and Villinger, 1990; Le Carlier et al., 1994; Benderitter and Elsass, 1995). 2.3 The Deep Boreholes 5 deep boreholes were drilled in the frame of the project. Figure 3 shows their trajectory on a vertical cross-section. The geothermal reservoir is developed in the crystalline rock at a depth between 4 and 5 km and supplied by natural underground fluid circulations which take place within a dense network of permeable fractures. Extensive research was performed in order to characterize the main properties of the fracture network. Geophysical borehole measurements, coring and cuttings analysis allowed determining the following (Genter et al., 1997): - The fractures are mainly oriented in a North- South direction and exhibit large values of dip. - Most of the fractures are naturally permeable, but the permeability values are very low, because the fractures are often sealed with hydrothermal deposits, such as calcite, silica and clays. 2.2 Temperature Gradient Figure 2 presents the temperature gradient form the surface down to 5 km depth, observed by borehole measurements. Figure 3: North-South vertical cross-section showing the trajectory of the deep boreholes at Soultz. In brackets is the measured (or logging) depth (Dezayes et al., 2005) EPS1 (in green) was a former oil well, which was deepened to 2.2 km depth; it was cored form 930 m depth to bottom. GPK1 (in magenta) is 3.6 km depth and was the first drilled, geothermal well. It was hydraulically stimulated in GPK2 (in blue), GPK3 (in red) and GPK4 (in purple) reach 5 km depth and form the geothermal triplet. GPK2 was stimulated in 2000, GPK3 in 2003 and GPK4, twice in 2004 and The wellheads of GPK-2, -3 and -4 are 6 m apart, while, at the bottom holes, the distance between the wells is around 650 m: this distance is required to allow the re-injected water to be in contact with hot rocks on rather long pathways so that it could be re-heated before being pumped again. GPK-2, -3 and -4 are cased from the surface to about 4.5 km depth. The last 500 m correspond to the open-hole section of each borehole. Figure 2: Temperature profile from the surface to 5 km depth The gradient shows an irregular shape: from the surface to 1 km depth, the gradient is very high (10 C/100 m), then it decreases to 1.5 C/100 m in the depth interval 1 km 3.5 km and increases below 3.5 km depth to about 3 C/100 m (Schellschmidt and Schultz, 1991; Pribnow and Schellschmidt, 2000). The temperature at 5 km depth is around 200 C. GPK2 and GPK4 are production wells, into which downhole production pumps are installed. GPK3 is used to re-inject the produced geothermal fluid, once it has been cooled after using the heat in the power production process. GPK1 is also planned to be a re-injection well, if the total volume of produced fluid cannot be safely absorbed by GPK3. EPS1 is currently used as a seismic observation well or for testing borehole sondes. 3. THE CIRCULATION TESTS After a series of hydraulic and chemical stimulation, which brought the wells to acceptable levels of injectivity and productivity, several circulation tests were performed. The first 3-wells circulation test was operated in 2005 for 6 months, with artesian production. The second and third 2

3 ones were performed after the building of the power plant. The experimental conditions are thus slightly different from the 2005 test: it involves production pumps installed in the production wells (GPK2 and GPK4). 3.1 The 2005 Circulation Test The first circulation test involving the geothermal triplet GPK-2, -3 and -4 was performed for about 6 months form mid-july to December Production from GPK2 and GPK4 was driven only by buoyancy effect, and the produced geothermal fluid was re-injected into GPK3 after being cooled through a heat exchanger to simulate the use of thermal energy. More details about this test are given by Gérard et al. (2006), Cuenot et al. (2006) and Sanjuan et al. (2006). Figure 4 presents the main hydraulic parameters observed during the circulation test. maintenance or repairing, the longest, uninterrupted period of circulation started at the beginning of July 2008 and lasted to the 17 th of August At that time, a failure of the production pump led to stop the test. The production of geothermal fluid was performed at a flowrate of around 90 m 3.h -1, that is, around 25 L.s -1. Reinjection was operated at around 82 m 3.h -1. The wellhead pressure of GPK2 was set to about 1.8 MPa and GPK3 wellhead pressure increased continuously during the test to reach a maximum value of about 7.3 MPa. The temperature of the produced fluid reached 163 C at the end of the test. The re-injection temperature varied between 40 C and a maximum of 110 C, depending on the cooling capacities. Figure 5 summarizes the hydraulic data from GPK2 and GPK3. Figure 4: Hydraulic parameters recorded during the circulation test. Top: production flowrates at GPK2 and GPK4 and injection flowrate at GPK3; Bottom: GPK2, GPK3 and GPK4 wellhead pressure Around m 3 of geothermal fluid were produced from GPK2 at an average flowrate of 12 L.s -1. The temperature of the produced fluid was around 160 C. From GPK4, we were able to produce only m 3 at a flowrate of about 3 L.s -1 for a production temperature of 120 C. The sum of GPK2 and GPK4 contributions (i.e m 3 ) was reinjected into GPK3 after being cooled down to 60 C by passing through a heat exchanger. The injection was performed at a flowrate of 15 L.s -1, except at the end of the test, when an additional flow of 4 L.s -1 was added to get a total injection flowrate of 19 L.s -1. The wellhead pressure of both production boreholes was maintained between 1 and 2 MPa, so as to prevent scaling. The pressure at GPK3 wellhead was almost stable at around 4 MPa during the first part of the test, and then increased to a maximum of 7 MPa, when the flowrate rose to 19 L.s The Circulation Test of July-August 2008 This test started after the building of the power plant and the installation of a production pump (LSP Line Shaft Pump) into GPK2. The circulation was aimed at observing the LSP performances and testing the components of the ORC conversion unit. Thus it only involved GPK2 as a producer and GPK3 as re-injection borehole. As the beginning of the test was subjected to several stop and restart procedures due to parameters adjustments, 3 Figure 5: Hydraulic parameters of the July-August circulation test (Schindler, 2009). In red: GPK2 data; in blue: GPK3 data. The upper picture shows the wellhead temperature of the produced and injected fluid; the middle graph presents the production and injection flowrates; on the lower figure are plotted GPK2 and GPK3 wellhead pressures. The jump on GPK3 pressure curve is an artifact caused by the acquisition system. Note also several bumps on GPK3 pressure curve: they correlate with increases of the injection temperature 3.3 The Circulation Test of November-December 2008 The circulation test of November-December 2008 was performed after the installation of the second production pump (ESP Electro-Submersible Pump) into GPK4 and the re-installation of the LSP into GPK2. At first, the ESP was started on the 17 th of November, then the LSP one week later. Unfortunately, the LSP encountered problems and had to be stopped. It was restarted on the 1 st of December. Since this date, the circulation involved the three deep boreholes until the 17 th of December: at that time a problem of the automation system caused the LSP to stop. 3 days later, the ESP was also stopped due to a problem on the air cooling system. Fluid was extracted from GPK2 at a mean flowrate of 60 m 3.h -1 (around 17 L.s -1 ) and the produced fluid temperature was 163 C at the end of the test. From GPK4, the geothermal water was produced at an initial flowrate of 60 m 3.h -1, quickly deceasing to a stable value of 44 m 3.h -1 (around 12 L.s -1 ) for a final production temperature of about 155 C. For both production wells, the wellhead pressure was maintained at 2 MPa in order to prevent scaling. At the beginning of the test, that is, when only GPK4 was producing, the re-injection into GPK3 was performed at a flowrate of about 60 m 3.h -1, then about 42 m 3.h -1 and the wellhead pressure increased to a maximum of 2.8 MPa. As

4 soon as the second well was put in production, the reinjection flowrate rose to a maximum of 98 m 3.h -1 and the wellhead pressure increased up to 8.6 MPa. Figure 6 presents the hydraulic parameters from GPK2, GPK3 and GPK4. above a magnitude 1.8 and 4 show magnitude equal or higher than 2. Some of the strongest earthquakes were felt by the population in the neighbourhood Number of events per day /07/ /07/ /08/ /08/ /08/ /08/ /08/ /09/ /09/ /09/ /09/ /10/ /10/ /10/ /10/2005 Date 31/10/ /11/ /11/ /11/ /11/ /12/ /12/ /12/ /12/ /01/ /01/2006 Figure 6: Hydraulic parameters of the November- December circulation test (Schindler, 2009). In red: GPK2 data; in blue: GPK3 data; in green: GPK4 data. The first plot presents the temperature recorded at the wellheads, the second picture shows the production and injection flowrates and the last one the observed wellhead pressures 4. MICROSEISMIC ACTIVITY In 1997, a first circulation test, which lasted 4 months, involved GPK2 (production well, equipped with a downhole production pump) and GPK1 (injection well). At that time, GPK1 was at a depth of 3600 m and GPK m. The test was performed with a flowrate of 90 m 3.h -1 for a production temperature of about 140 C (Baumgaertner et al., 1997). The most important information regarding the microseismic activity during this test was that NO seismic event was observed. Thus, the three circulation tests described above represented a good opportunity to get observations on microseismic activity under hydraulic conditions approaching those of future exploitation, at greater depth and in three different configurations: circulation between three wells with artesian flow (2005), between 2 wells with a production pump (July-August 2008) and between 3 wells with 2 production pumps (November-December 2008). 4.1 Microseismic Activity During the 2005 Circulation Test Microseismic Activity Around 600 microseismic events were recorded during this test (Cuenot et al., 2006). The number of microearthquakes per day varied between 0 and a maximum of 45 (Figure 7). During the first 4 months, the activity was restrained to a maximum of 10 events per day. The most intense seismic activity was observed during the last 2 months of the test, when the injection flow rate had been increased from 15 l.s - 1 to 20 l.s -1 (see Figure 4) Magnitudes For all tests, magnitudes were calculated from the length (duration) of the seismic signal coda. They were compared to other estimates performed with different methods and the results are in agreement. The observed magnitudes range between -1 and seismic events reached a magnitude 1.3 or higher; 7 were 4 Figure 7: Number of microseismic events recorded per day during the 6-month circulation test of During the first 4 months of the test, only 12 earthquakes in the magnitude range could be observed. But, as soon as the injection flowrate was increased, a series of larger magnitude events occurred almost immediately. An example is given in Figure 8. Figure 8: Series of larger magnitude earthquakes occurred in the period 29/10/05 to 31/10/05 following the increase of injection flowrate. The wellhead pressure of GPK-2, -3 and -4 are plotted in cyan, green and red respectively. The blue curve represents the injection flowrate. The red stars marks the origin time of the earthquakes. In this example we can observe a series of 7 seismic events which took place within a few hours and show large magnitude, including the highest one (2.3). They occurred after the increase of injection rate, which led to an increase of GPK3 wellhead pressure from 4.5 to 7 MPa. A similar sequence occurred within 2 days (November, 20 th and 21 st ), with one event of magnitude 1.9, one of magnitude 2, and two of magnitude Locations Figure 9 (a and b) shows the location of the observed microseismic event in plan view and in North-South vertical cross-section. Three distinct active zones can be noticed. A few earthquakes (mainly in blue and cyan on Figure 8) are located in the vicinity of GPK4 and form the first active

5 area. They occurred at the beginning of the test, but no other seismic event took place within this zone afterwards. a) located in the area to the North of GPK2 and the two M=2.2 earthquakes (large red circles) occurred in the vicinity of GPK Microseismic activity during the July-August 2008 circulation test Microseismic activity A total of around 190 microseismic events were detected during the July-August 2008 circulation test (Cuenot et al., 2008). The first observed microearthquake occurred on the 30 th of July (Figure 10), when GPK3 wellhead pressure reached around 6 MPa (see Figure 5). Before, no seismic event could be detected Number Date b) Figure 10: Number of microearthquakes per day during the July-August 2008 circulation test On figure 9, we can observe that the seismic activity varies between 1 and 26 seismic events per day. 2 peaks of activity can be distinguished, although they cannot be linked with any significant hydraulic event, as both production and injection flowrates were stable and GPK3 injection pressure was continuously increasing without any sharp change. This behaviour is quite different from what was observed during the 2005 test: the variations of seismic activity were closely linked with changes of hydraulic parameters. Figure 9: Locations of the microseismic events recorded during the 2005 circulation test. a) Plane view; b) North-South vertical cross-section. The legend is common for both figures. Earthquakes of magnitude equal or higher than 1.4 are represented by circles which diameter is proportional to the magnitude The second zone is located on the North of GPK2 bottom hole between 5 and 5.5 km depth. The third active area is situated on the North-West of GPK3 bottom hole between 4.8 and 5.4 km depth. Both were active all along the experiment. Several earthquakes are located in the volume between GPK2 and GPK3, but the seismicity does not extend a lot between GPK3 and GPK4 and seems to keep concentrated around GPK3. Both main active areas contain events of small and large magnitude. For instance the M=2.3 earthquake (large yellow circle on Figure 9) is Magnitude Data The observed magnitudes are in the interval -0.3 to 1.4. For some of the smallest earthquakes, the magnitude was arbitrarily set to -0.3, because it was impossible to calculate reliable value of magnitude. 11 seismic events reached a magnitude equal or higher than 1. None of them was felt on surface. The two strongest earthquakes reached a magnitude of 1.4 and occurred on the 11 th and 12 th of August. They cannot be clearly linked with any significant hydraulic variations Locations Figure 11 presents the location of the seismic activity recorded during the July-August Three main areas were active during this test: the first is situated North of GPK2 bottom well, between 5 and 5.4 km depth, the second extends between GPK2 and GPK3 bottom hole between 4.7 and 5.4 km depth and the third one is located South-West of GPK3. However, no earthquake occurred in the vicinity of GPK4, which is not surprising, as this well was not active during the circulation test. The early seismicity appears in the zone located north of GPK2, and then propagates toward GPK3. The latest activated area is located South-West of GPK3. 5

6 a) events/day during most of the test under stable hydraulic conditions, only two days exhibit a larger rate: the 17 th and 18 th of December (12 and 19 events per day respectively). This is probably linked with the sudden stop of the LSP pump, which provoked a sort of mini shut in, as the reinjection pressure dropped from around 9 MPa down to 5 MPa. Moreover, between 20h00 (17/12/08), that is, a few hours after the stop, and 8h00 (18/12/08), 21 microseismic events occurred; that represents almost the half of the total number of earthquakes recorded during the test. This behaviour is similar to what was observed in 2005: the sudden change of hydraulic parameters leads to an increase of the seismic activity (see paragraph 4.1.1) Nombre de séismes b) 0 17/11/ /11/ /11/ /11/ /11/ /11/ /11/ /12/ /12/ /12/ /12/2008 Date 09/12/ /12/ /12/ /12/ /12/ /12/ /12/ /12/ /12/2008 Figure 11: Location of the microseismic activity recorded during the July-August 2008 circulation test. a) Plane view; b) North-South vertical cross-section. The legend is common for both figures. The colour code corresponds to the origin time of the earthquakes and the diameter of the circles is proportional to the magnitude The two largest seismic events (M=1.4) are located in the zone between GPK2 and GPK3 (large yellow and green circles). 4.3 Microseismic Activity During the November- December Circulation Test Microseismic Activity 53 microearthquakes were detected during this experiment (Cuenot, 2009). The first detected event occurred on the 9 th of December, that is, more than 3 weeks after the beginning of the test (Figure 12). Then the number of microearthquakes per day varies between 0 and a maximum of 19. While the seismic rate was not higher than 4 6 Figure 12: Number of microearthquakes per day during the November-December 2008 circulation test Magnitude Data The observed magnitudes are in the range -0.2 to events reached a magnitude 1 or higher: two of magnitude 1.2, one of magnitude 1.3 and one of magnitude 1.7. Among them, one of the M=1.2 events occurred on the 18 th of December, a few hours after the stop of the LSP pump. The M=1.7 earthquake took place on the 25 th of December, 5 days after the complete end of the test, that is, during the shut in period. The occurrence of stronger earthquakes during the shut in phase was already observed, mainly during stimulation tests and has become one of the major issues related to induced seismicity on geothermal plant. For instance, at Soultz-sous-Forêts, the strongest seismic events recorded during the hydraulic stimulations of GPK2 in 2000 (magnitude: 2.6) and GPK3 in 2003 (magnitude 2.9) took place during the post-injection period (e.g Weidler et al, 2002; Cuenot et al., 2008; Baria et al., 2004; Charléty et al., 2007) Locations Figure 13 shows the locations of the microseismic events recorded during the November-December 2008 circulation test. The seismicity is mainly concentrated around the well GPK2 in a zone situated to the North of GPK2 bottom hole. It extends between 5 and 5.5 km depth. Several earthquakes can be observed to the West of GPK3 well and a few in between GPK2 and GPK3. No seismicity is located around GPK4, although this well was in production and equipped with the ESP downhole pump. The sudden stop of the ESP did even not induce any earthquake in the vicinity of GPK4. Around GPK2, the microseismic activity began to develop in the deepest part of the activated area and then migrates to shallower levels. It can be noticed that the microearthquakes occurred during the mini seismic crisis

7 following the stop of the LSP pump constitute a small cluster in the volume to the North of GPK2 (yellow and green circles on figure 13). a) 5. COMPARISON BETWEEN THE THREE TESTS The behaviour of the reservoir during the three circulation tests presents both common features and significant differences, which can be related to the hydraulic parameters of the tests and to the hydraulic performance of each well. The interpretation of the observations is somehow not an easy task, as the experimental conditions were really different from one test to the other (artesian pumping or production pumps, 2 or 3 active wells,...). Table 1 summarizes the main hydraulic parameters and the main seismological observations for the three circulation tests. Table 1. Summary of the Main Hydraulic and Seismological Observations for the Three Circulation Tests. Jul.-Dec Jul.-Aug Nov.-Dec GPK2 prod. flowrate ~12 l.s -1 ~25 l.s -1 ~17 l.s -1 GPK3 inj. flowrate ~15 l.s -1 then ~20 l.s -1 ~23 l.s -1 ~12 l.s -1 then ~27 l.s -1 GPK4 prod. flowrate ~3 l.s -1 - ~12 l.s -1 b) GPK3 max. pressure 4 MPa then 7 MPa 7.3 MPa 2.8 MPa then 8.6 MPa Number of earthquakes ~600 ~ Highest magnitude The most striking similarity concerns the location of the active zones during each test: when comparing figures 9, 11 and 13, one can clearly observe that the microseismic activity develops in the same zones. It is especially remarkable for the volume to the North of GPK2 bottom well, for the area situated to the South-West of GPK3 and for GPK2-GPK3 inter-well domain. The lack of seismicity around GPK4 is also common for the three tests. Figure 13: Location of the microseismic activity induced during the November-December circulation test. a) Plane view; b) North-South vertical crosssection. The colour code correspond to the origin time of the earthquakes and the diameter of the circles is proportional to the magnitude Concerning the strongest seismic events, 3 are situated in the vicinity of GPK2 and the last one is located in betxeen GPK2 and GPK3. The M=1.3 and M=1.7 earthquakes are located near GPK2 bottom hole and are very close to each other (blue and pink circles). The location of the M=1.7 event could be quite surprising, as it occurred in the shut in phase following the stop of the ESP pump, which is installed in GPK4. The seismic activity occurred in the vicinity of GPK3 is clearly related to the re-injection process, which creates overpressures in the medium. This is confirmed by the fact that any variation of the injection flowrate, which generates pressure variations, has an impact on the microseismic activity. However it is difficult without any other information, such as downhole pressure measurements for example, to assess whether the seismicity on production side is linked with the production process or with the propagation of the overpressure induced by the injection. On one hand, if we assume that the production process is responsible for the induced seismicity, then the microearthquakes occurred in the vicinity of GPK2 can be explained by the pressure variations caused by the production. However, this mechanism should also play in the case of the other production borehole GPK4, but no seismic activity occurred in the vicinity of this well. As the production flowrates were lower than for GPK2, especially during the 2005 experiment, the pressure disturbances induced by the production process may be too small to trigger microseismic activity. 7

8 On another hand, if we consider that the induced microseismic activity is related to the propagation of pressure waves due to the re-injection into GPK3, then the observed dissymmetry of the reservoir response between GPK2 side and GPK4 side could be associated with different pressure connection between GPK3-GPK2 and GPK3-GPK4: a tracer test performed during the 2005 circulation test showed that a fast and strong connection exists between GPK2 and GPK3, while the connection between GPK3 and GPK4 is rather poor (Sanjuan et al., 2006). The few seismic events occurred at the beginning of the 2005 test in the vicinity of GPK4 (figure 9, dark blue circles) may not be directly linked to the circulation, but may result from stress redistribution, as two massive hydraulic stimulations were performed through GPK4 in September 2004 and February 2005, which induced several thousands of microearthquakes. Despite the fact that the experimental conditions of the 2005 test and the July-August 2008 test were different, the total number of microseismic events occurred during both tests is roughly of the same order, if we consider the different duration of the experiments. One can notice that the injection pressure was almost the same for both tests. On the contrary, the November-December 2008 test exhibits a lower seismic activity, despite a higher injection pressure: as both production wells were active, the overpressure caused by re-injection, which was though higher than both other tests, may be much better balanced by the higher production rate. Another striking difference between the tests concerns the origin time of the first detected earthquake: while the onset of seismicity happened almost instantaneously after the beginning of the 2005 test, the first seismic events of both 2008 experiments occurred several weeks after the start of circulation. This behaviour is likely to be related to the fact that a rock volume, which had already been seismically activated during hydraulic tests (stimulation and/or circulation), needs to be subjected to, at least, equivalent pressure levels so that seismic activity could be triggered. The main issue concerning the project and the future longterm exploitation of the geothermal plant is actually the larger magnitude earthquakes. The largest magnitudes and the global level of magnitude were the highest during the 2005 circulation test, compared to the 2008 tests. During the July-August 2008 test, a higher number of earthquakes of magnitude equal or larger than 1 was observed, compared to the test, but the strongest 2008 seismic events occurred during the November-December test. The only common observation that can be made corresponds to the occurrence of several large magnitude earthquakes following sudden variations of hydraulic parameters (2005 and November-December 2008 tests), which can be either an increase of injection flowrate or a decrease, down to 0 in the case of shut in period. However, during the November-December test, no large magnitude event occurred when the second production pump was started, implying an increase of the injection flowrate and pressure. Finally one question is still remaining: why did several earthquakes reach magnitude higher than 2 in 2005 and not in 2008, while the circulation flowrates were generally higher in 2008? In 2005, the series of M 2 seismic events took place after the increase of injection flowrate from 15 l.s -1 to about 20 l.s -1, implying an increase of injection pressure from 4 MPa to 7 MPa. In November-December , the same process was not followed by a series of large magnitude earthquakes. So the change of hydraulic parameters alone cannot explain the M 2 earthquakes. However, 3 other possible factors could be taken into account to explain the occurrence of these strong earthquakes: - In 2005, the series of M 2 earthquakes happened quite lately in the course of the test (around 4 months after the beginning). It means that a large volume of geothermal fluid had already circulated before, compared to the 2008 tests. So the total volume of circulating fluid may have an influence on seismicity and especially on the level of magnitude: some water-rock interactions may develop on the fault planes, as long as water is circulating, and may reduce the shearing resistance of the fractures. - It is possible that the major part of the seismic energy accumulated on major faults was released by the largest earthquakes of Until the tests of 2008, the seismic loading may have not retrieved its previous level so that the strongest magnitudes of 2008 are generally lower than those of The enhanced production due to downhole pumping may help limiting the overpressure effect, which is responsible in many cases for the occurrence of the larger seismic events. CONCLUSIONS The three circulation tests performed in 2005 with artesian pumping, in July-August 2008 and in November-December 2008 with downhole pumping, allowed us to get some insights about the reservoir seismic behaviour under conditions close to those of the future exploitation of the geothermal power plant. The main information is that, in the three cases, seismic activity was observed. The number of microseismic event is much lower than during stimulation test, but several earthquakes reached similar level of magnitude, especially during the 2005 test. It appears that the seismic activity stays at a low level when the hydraulic regime is stable, but, as soon as there are variations of hydraulic parameters (increase or decrease of flowrate, leading to pressure change), especially if these variations are sudden, the seismic activity tends to increase and several earthquakes of magnitude higher than 2 may occur. However, this latter observation was not confirmed during the 2008 test, as no seismic events reached a magnitude 2. So either the duration of both 2008 tests (around 2 months) was too short to observe medium- to long-term effects, such as in 2005, or the higher production rate obtained with the help of downhole production pumps may prevent the effects of overpressure in the reservoir. Nevertheless, the testing of the geothermal power plant (figure 14) is still ongoing at Soultz-sous-Forêts, as well as reservoir monitoring. New seismic data are still recorded and planned long-term circulation tests between the three wells with downhole pumping will allow us to get further information about the seismic response of the reservoir. ACKNOWLEDGMENTS This work was done in the framework of the European EGS Pilot Plant project which is supported by the European Commission, BMU and Forschungszentrum Jülich (Germany), ADEME (France), the Swiss Federal Office of Energy (OFEN) and by a consortium of French and German

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