Faulting mechanisms and stress regime at the European HDR site of Soultz-sous-Forêts, France

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1 Geothermics 35 (2006) Faulting mechanisms and stress regime at the European HDR site of Soultz-sous-Forêts, France Nicolas Cuenot a,1, Jean Charléty a,, Louis Dorbath a,b, Henri Haessler a a Institut de Physique du Globe de Strasbourg, Ecole et Observatoire des Sciences de la Terre (IPGS-EOST), 5 rue René Descartes, Strasbourg Cedex, France b Institut de Recherche pour le Développement, Laboratoire des Mécanismes et Transferts en Géologie (IRD, LMTG), 14 Avenue Edouard Belin, Toulouse, France Received 20 December 2005; accepted 6 November 2006 Available online 11 January 2007 Abstract The state of stress and its implications for shear on fault planes during fluid injection are crucial issues for the HDR (Hot Dry Rock) or EGS (Enhanced or Engineered Geothermal System) concept. This is especially true for hydraulic stimulation experiments, aimed at enhancing the connectivity of a borehole to the natural fracture network, since they tend to induce the shearing of fractures, which is controlled by the local stress regime. During the 2000 and 2003 stimulation tests at Soultz-sous-Forêts, France, about 10,000 microearthquakes were located with a surface seismological network. Hundreds of double-couple (DC) focal mechanisms were automatically determined from first-motion polarities using the FPFIT program [Reasenberg, P.A., Oppenheimer, D., FPFIT, FPPLOT and FPPAGE: Fortran computer programs for calculating and displaying earthquake fault-plane solutions. US Geological Survey Open-File Report , 25 pp.]. The majority of these mechanisms indicate normal-faulting movement with a more or less pronounced strike-slip component. Some quasi-pure strike-slip events also occurred, especially in the deeper part of the stimulated rock volume, at more than 5 km depth. Although we found a double-couple solution for all events, we tried to observe and quantify the proportion of the non-double-couple (NDC) component in the seismic moment tensor for several microseisms from the 2003 data. The study shows that the NDC is higher for the events in the vicinity of the injection well than for the events far from the well. Corresponding author. Tel.: ; fax: address: jean.charlety@eost.u-strasbg.fr (J. Charléty). 1 Present address: European Economic Interest Grouping (EEIG) Heat Mining, Route de Soultz, BP 38, Kutzenhausen, France /$ Published by Elsevier Ltd on behalf of CNR. doi: /j.geothermics

2 562 N. Cuenot et al. / Geothermics 35 (2006) We used the method of Rivera and Cisternas [Rivera, L., Cisternas, A., Stress tensor and fault-plane solutions for a population of earthquakes. Bull. Seismol. Soc. Am. 80, ] to perform the inversion of the deviatoric part of the stress tensor from P-wave polarities. This method was applied to different datasets from the 2000 test, taken from the shallower and deeper parts of the stimulated region. The results show a stable, horizontal, NE-SW-oriented trend of the minor horizontal stress, but a rotation of the major stress from a sub-vertical direction (top of the stimulated region) to a sub-horizontal one (bottom of the stimulated region). This implies a change from a normal-faulting to a strike-slip regime, in agreement with our fault-plane solutions. Finally, we applied the stress components to the nodal planes of several events and were able to determine their fault plane and obtain a 3D image of the fracture network, based on real data Published by Elsevier Ltd on behalf of CNR. Keywords: Microseismicity; Stress regime; Faulting mechanisms; Enhanced Geothermal Systems; Soultz-sous-Forêts; France 1. Introduction After the completion of boreholes GPK2 in 1999 and GPK3 in 2002 to about 5 km depth, two massive hydraulic stimulation tests were performed in 2000 and 2003 in order to develop the permeability and connectivity of the fractured (or jointed) rocks needed to create an underground geothermal heat exchange system. The tests were conducted at flow rates up to 50 L/s, with short-time periods of up to 90 L/s in The huge number of microseismic events induced by these experiments presented us with the opportunity of studying both the faulting mechanisms and the state of stress within the geothermal reservoir. The studies of Ohtsu (1991), Sasaki (1998), Jost et al. (1998), Dahm et al. (1999), Nolen-Hoeksema and Ruff (2001) as well as others, indicate that the dominant source mechanism for fluidinduced microseismicity is the shear or double-couple mechanism and that there are only small non-double-couple (tensile or volumetric) components to the radiated seismic energy. Nolen- Hoeksema and Ruff (2001) propose three different mechanisms for the microseismicity: shearing along favorably oriented weakness planes that fail because either the prevailing shear stress is high, or the prevailing normal stress has been reduced by an increase in fluid pressure, or both. The deployment and operation of more than 20 surface seismological stations by EOST, University of Strasbourg, in addition to the existing downhole seismic array operated by the EEIG Heat Mining, allowed us to determine automatically several hundreds of focal mechanisms from first-motion polarity data, which give information about the type of movements on fracture planes. Furthermore, in order to better characterize the faulting mechanisms within the Soultz geothermal reservoir, we investigated the seismic moment tensor of several events to determine whether the seismic rupture exhibits a non-doublecouple component, which could correspond to a tensional opening of fractures in addition to shear. Finally, as we wanted to check the validity of our fault-plane solutions and to compare our results to existing stress data, we performed the inversion of the deviatoric stresses. By applying the stress tensor solutions to the nodal planes of a number of microseismic events, we were able to determine the active fault planes associated with several of these events and to obtain a realistic 3D view of the fracture system.

3 N. Cuenot et al. / Geothermics 35 (2006) Fig. 1. Injection rate and pressure recorded at the wellhead of well GPK2 during the 2000 Soultz hydraulic stimulation test. 2. The 2000 stimulation 2.1. Hydraulic parameters The injection scheme was rather simple in 2000 (Fig. 1). The injection lasted about 6 days (30 June 6 July) with rates of 30 L/s over 24 h, 40 L/s for 27 h, and 50 L/s during the last 90 h into the open-hole section of GPK2 ( m). The wellhead pressure instantaneously reached a peak of 12 MPa and then declined during the first two steps. On the other hand, at a rate of 50 L/s, the pressure initially rose to 12 MPa, but continued to increase until it had reached 13 MPa at shut-in time (Weidler et al., 2002). When injection was stopped, an instantaneous pressure drop of about 5 MPa was observed, which was followed by a very slow pressure decrease Seismic monitoring network In addition to the downhole seismic network, which consisted of three 4-component accelerometers and two hydrophones, several surface stations were installed by EOST, University of Strasbourg. The surface network comprised eight vertical seismometers, six 3-component seismometers, one broadband station and three permanent stations belonging to ReNaSS (French National Seismic Network). A plane view of the 2000 seismic monitoring system is given in Fig Microseismic activity During the 2000 stimulation more than 10,000 microseismic events were recorded by the surface network, which was deployed from 30 June to 11 July. They range in magnitude between

4 564 N. Cuenot et al. / Geothermics 35 (2006) Fig. 2. Downhole and surface seismological networks deployed for the 2000 and 2003 stimulation tests. 0.9 and 2.6. About 7200 of these events could be located precisely using the method of Thurber (1983), which is a simultaneous tomographic inversion of the velocity structure and location parameters. The events form a NNW-SSE oriented cloud about 1.5 km long and 0.5 km wide, ranging in depth between 4 and 5.5 km (Cuenot et al., 2005) (Fig. 3). The orientation of the microseismic cloud seems to be consistent with the N-S to NNW-SSE trend of the majority of the natural fractures suggested by Genter and Traineau (1996). 3. The 2003 stimulation 3.1. Hydraulic parameters The 2003 stimulation strategy used in borehole GPK3 was rather different from the 2000 stimulation of borehole GPK2 (Fig. 4). Baria et al. (2004) described four phases during the 2003 stimulation. During phase I ( ), injection was at 30 L/s and was later increased to 50 L/s, with two short periods at 60 and 90 L/s. From to the concept of dual, focused injection was introduced by injecting water at a rate of 50 L/s into GPK3 and at 20 L/s into GPK2.

5 N. Cuenot et al. / Geothermics 35 (2006) Fig. 3. Three-dimensional view of the volumes defined by the microseismic clouds of the 2000 (black) and 2003 (grey) stimulation experiments. Trajectories of wells GPK2 and GPK3 are shown in yellow and black, respectively. From to 06.06, GPK2 was shut in, whereas in GPK3, after a short increase in injection rate (up to 90 L/s), injection was decreased in three steps. Finally, because of the microseismic activity remaining after GPK3 was shut-in, GPK2 was discharged at a rate of 10 L/s to reduce the pressure in the reservoir Seismic monitoring network The three downhole 4-axis accelerometers were used in the same configuration as in 2000; 3- component geophones were also added. On the surface, the main improvement was the permanent monitoring network installed by EOST (three 3-component sensors and six vertical sensors). A temporary surface network (six 3-component stations and eight vertical stations) was also deployed during the stimulation test (Fig. 2) Microseismic activity About 5000 microseismic events in the 0.9 to 2.9 magnitude range were recorded by the surface network; 2250 were located using the TomoDD code (Zhang and Thurber, 2003). As in 2000, the cloud of events in 2003 was oriented in a NNW-SSE direction, but located further to the south (Fig. 3). The cloud was about 2 km long and 1 km wide, and extended between 3 and

6 566 N. Cuenot et al. / Geothermics 35 (2006) Fig. 4. Soultz 2003 hydraulic stimulation test. Fluid injection rates and wellhead pressures in wells GPK3 (top) and GPK2 (bottom). 7 km depth. In Fig. 3, parts of the 2000 and 2003 clouds seem to overlap, which mean that at least a part of the seismic structures that were stimulated in 2000 did not slip in Focal mechanisms We automatically determined several thousands of focal mechanisms using the program FPFIT (Reasenberg and Oppenheimer, 1985): nodal planes were calculated from the first-motion polarities by a maximum likelihood procedure and manually checked afterwards. On average, more than 14 polarities are available for the 2000 events and more than 16 for those of The results indicate a majority of normal-faulting movements, pure or with a more or less pronounced strike-slip component. However, a strike-slip regime does appear to dominate in the deepest part of the reservoir, with some quasi-pure strike-slip events. Some representative focal mechanisms for the 2000 and 2003 stimulation tests are shown in Figs. 5 and Non-double-couple component The full determination of the seismic moment tensor (first order) was performed by considering a homogenous half space. The source-time function was considered identical for all stations and the amplitude of the P-wave was corrected from the plateau of each instrument. The seismic moment tensor was then inverted using a common least square method (Nolen-Hoeksema and Ruff, 2001).

7 N. Cuenot et al. / Geothermics 35 (2006) Fig. 5. Representative focal mechanisms for the 2000 stimulation test.

8 568 N. Cuenot et al. / Geothermics 35 (2006) Fig. 6. Representative focal mechanisms for the 2000 and 2003 stimulation tests. The color represents the depth of the events. These are events of magnitudes equal or larger than 1.4. The moment tensor describes the equivalent forces at the source, which can be correlated with the physical processes involved at the source. Moreover, it can be decomposed into a doublecouple component and a non-double-couple component. Giardini (1983) defined the deviation from the DC mechanism as the ratio of the minimum to the maximum eigenvalue of the moment tensor, in the sense of absolute value. This ratio, ε, ranges between 0.5 and 0.5. The deviation is taken as positive if the eigenvalue of the maximum absolute value is tensional and negative if it is compressional. If ε ranges between 0.25 and 0.25, the DC component dominates. The results indicate a value for the NDC component that may be the result of three mechanisms. The first of these is related to the simplest assumptions made for its determination, so that the NDC component is an artifact of errors; this must not be neglected. The other two are tied to the mechanics of faulting. Kuge and Lay (1994) and Houston (1993) show that the NDC is the result of the fact that a fault is the sum of several small segments that have their own associated shear, but since these segments do not have the same orientation, their summation produces this NDC component. On the other hand, it could also be the result of an opening and shearing mode of failure. The significance of the result can be assessed by considering the variation between each of the events considered in this study. As the ray paths of the seismic events are similar and the events are close to one another within the entire studied space, we consider that the error in the determination of the moment tensor is approximately identical for all events. Thus the observed NDCs are not the result of an artifact of errors. In Fig. 7, several of the 2003 events are represented as colored spheres whose radii are proportional to the magnitudes of the events; the colors correspond to the value of ε. For each seismic event this ratio is less than 0.25, which means that the DC mechanism prevails. This is verified by the fact that we were always able to fit the polarity with a DC. We also noted that the events occurring in the direct vicinity of the injection well GPK3 showed high values of ε, whereas events far from the well had smaller ratios. This may indicate that events in the vicinity of GPK3 have a non-negligible NDC component, and that the fractures that rupture may undergo tension in addition to shearing, or that the fracture is composed of a complex system

9 N. Cuenot et al. / Geothermics 35 (2006) Fig. 7. Proportion (ε) of non-double-couple component for several 2003 microseismic events as indicated by the color scale. The radius of each colored sphere is proportional to the magnitude of the event. Trajectories of wells GPK2 and GPK3 are shown in red and blue, respectively. of segments. Charléty and Dorbath (2005) showed that the fracture zone consists of a complicated system of fractures with heterogeneous orientations, in agreement with the geological studies of Dezayes et al. (2003). Cooling around the injection well may be another cause of the NDC component observed for the events near the well. Indeed this thermal effect can be invoked because the events very close to the well took place in the early stages of the hydraulic stimulation when the temperature difference between the rock mass (at about 200 C) and the injected fluid (50 70 C) was large. The higher NDC component for events near the well may also be the consequence of a large increase in overpressure near the well due to massive fluid injection, which can cause fractures to open slightly. On the contrary, away from the injection well the fracture tensional component seems to decrease. This would mean that the overpressure there is less effective, maybe because it drops quickly with distance from the injection well. The prominent pressure effect is confirmed by another observation. As shown in Fig. 7, one event near injection well GPK3 does not show a high NDC component. This particular event occurred after shut-in, at a time when the area near the well was no longer being subjected to high fluid pressures, so that no tension was involved in the seismic rupture. In conclusion, we determined a double-couple solution for each microseismic event, which may indicate that the dominant process of the faulting movements is shearing. This result also seems quite common at other HDR sites (e.g. Pearson, 1981; Pine and Batchelor, 1984; Sasaki, 1998). However, our analysis of the seismic moment tensor showed that the rupture process involves an NDC component, which may indicate a proportion of tensional opening of the fracture planes.

10 570 N. Cuenot et al. / Geothermics 35 (2006) This NDC component is, moreover, significantly higher for events close to the injection well, probably because of greater pressure effects. 6. Stress tensor inversion We performed a stress tensor inversion because of two observations. First, our results on focal mechanisms showed a higher proportion of strike-slip events in the deepest part of the geothermal reservoir. Second, the results of a stress profile by Klee and Rummel (1993) at Soultz-sous-Forêts based on hydrofracturing stress measurements indicated a possible crossover between the vertical stress S V and the maximum horizontal stress S H at around m depth. This would imply a change in the faulting regime with depth, from a normal-faulting to a strike-slip regime. It was in order to check this hypothesis that we decided to perform the stress tensor inversion Method We used the method of Rivera and Cisternas (1990), which involves the direct inversion of the deviatoric part of the stress tensor and of focal mechanisms from first-motion polarity data. The stress tensor is defined by three Euler angles and a shape factor, which indicates the faulting regime. From an initial trial solution (tensor and focal mechanisms), theoretical polarities are calculated and compared with the observed data at each iteration. The solution is then modified in order to maximize a likelihood function. The quality of the solution is expressed in terms of likelihood and score (the score describes the fit between observed and theoretical polarities) Data We performed two inversions with two different data sets of events from the 2000 stimulation experiment. The first set contains microseisms that occurred in the upper part of the reservoir (depth 4.5 km), and the second, events in its bottom part (depth 5 km). For each set, about 60 microseismic events were randomly selected from those exhibiting the largest number of available polarity data. Each selected event shows a number of polarities between 14 and 18. In order to check the reliability of the inversion, we performed several calculations using various sets containing different arrangements of events. The calculations gave similar results Results The results of the inversion are presented in Figs. 8 and 9. In both, the figure at the top corresponds to the 100 best tensor solutions and that at the bottom shows the best estimate of the stress tensor. Fig. 8 shows the results of the inversion for the upper part of the reservoir, and Fig. 9 the results for the bottom part. Stresses are expressed in terms of σ 1, σ 2 and σ 3, where σ 1 > σ 2 > σ 3. Our first observation deals with the stability of the orientation of the minimum horizontal stress S h, which trends in both cases NE-SW to NNE-SSW (orientation of S h at the regional scale in the upper Rhine Graben). In Figs. 8 and 9 the maximum horizontal stress S H is oriented NW-SE to NNW-SSE. This result is also consistent with regional estimates of S H. However, at the local scale, other studies show a more N-S orientation of the maximum horizontal stress (e.g. Bérard and Cornet, 2003). The method of Rivera and Cisternas assumes that the stress tensor is homogenous over the study region. In the case of Soultz-sous-Forêts, fluid injection may introduce strong local

11 N. Cuenot et al. / Geothermics 35 (2006) Fig. 8. Results of the stress tensor inversion for the top of the geothermal reservoir. Top: 100 best tensor solutions; bottom: best tensor solution. stress variations that we cannot see with our inversion method. Our results may correspond to an average stress field, which could be more representative of the regional stress field. The relative scatter of the solutions may reflect these stress heterogeneities. The most important result, however, concerns the rotation of the maximum stress σ 1 from a subvertical orientation at the top of the reservoir (Fig. 8) to a horizontal direction (Fig. 9). We effectively observed this feature, which had been predicted by other measurements. This means that the maximum horizontal stress S H becomes the maximum stress at the bottom of the

12 572 N. Cuenot et al. / Geothermics 35 (2006) Fig. 9. Results of the stress tensor inversion for the bottom of the geothermal reservoir. Top: 100 best tensor solutions; bottom: best tensor solution. reservoir, implying a change in the failure mode. At the top of the reservoir, the dominant regime is normal faulting whereas strike slip is likely to occur in the deepest part of the reservoir. This is in agreement with the focal mechanism results. Nevertheless, Figs. 8 and 9 both show a relative dispersion of the solutions. In Fig. 9 in particular, some solutions still indicate a subvertical trend for σ 1 and a subhorizontal direction for σ 2, which suggests that the faulting regime may not have completely changed at the bottom of the reservoir; that is, the stimulated volume is located within

13 N. Cuenot et al. / Geothermics 35 (2006) Fig. 10. Three-dimensional representation of the fracture network at Soultz. the region of stress rotation. It also confirms that the magnitudes of S V and S H are very close, as suggested by Klee and Rummel (1993). 7. Three-dimensional imaging of the fracture network We applied the stress tensor on the nodal planes we determined for the 2000 stimulation in order to define the shear plane. Fig. 10 shows the result in a 3D view. The majority of the fault planes are oriented NNW-SSE to NW-SE with a dip either to the west or east. We can also observe that most of the planes dipping to the west are subvertical, while those dipping to the east seem more subhorizontal. In addition, several fault planes exhibit an en echelon structure. Nevertheless, the fracture system appears to be somewhat heterogeneous. 8. Conclusions Our analysis of the faulting mechanisms associated with the hydraulic stimulation experiments performed at Soultz-sous-Forêts suggests that the main process involved is shear on fault planes, which has already been highlighted at several other HDR sites. At Soultz the faulting mechanisms are strongly related to the extensional regime, which is dominant in the Rhine Graben.

14 574 N. Cuenot et al. / Geothermics 35 (2006) From determination of the focal mechanisms, it appears that the failure mode is mainly normal faulting, with a more or less marked strike-slip component. However, our observations suggest a strike-slip regime in the deepest part of the Soultz geothermal reservoir. The inversion of the deviatoric part of the stress tensor confirms this feature. That is, the maximum stress rotates from a subvertical trend to a subhorizontal stress with depth, implying a change from a normal-faulting regime at the top of the reservoir to a strike-slip regime at the bottom. The change is in fact quite gradual and it is possible that the geothermal reservoir crosses the zone of stress rotation. In any case, the stress conditions, as well as the pre-existing seismogenic structures, seem to be suitable to ensure good hydraulic stimulation results in terms of enhancement of fracture permeability and connectivity. We were also able to determine a double-couple solution for each injection-induced seismic event in both 2000 and 2003, indicating that shear is the dominant process. By calculating the seismic moment tensor of several 2003 events, however, we show that at least some of these events have a non-negligible NDC component. It is striking that they tend to occur in the vicinity of the injection well, whereas those located at greater distance do not exhibit such high NDC components, or none at all. This means that events near the injector show part of a tensional opening as well as the main shearing process. Such behavior is closely related to the overpressure induced by fluid injection, which is highly effective near the injection well and able to slightly open the fractures, and has evident implications for the creation of permeability in the vicinity of the injector and for connection between the well and the fracture network. The extent of tensional opening is on the other hand close to zero for events occurring far from the injection well, indicating that the pressure effects on fracture opening are limited to the vicinity of the well. Finally, by applying the stress tensor to the nodal planes of events, we were able to construct a 3-D representation of the fracture system at Soultz based on real data. As a next step, it would be interesting to obtain a similar picture with fault lengths scaled to the magnitude of microseismic events. Acknowledgments This work was funded by a grant from Ademe (French National Energy Agency) and the Conseil Régional d Alsace. The authors would like to thank the staff of the EEIG Heat Mining for kindly providing the hydrological data and seismological downhole data. Thanks are also extended to Hervé Blumentritt, Michel Frogneux and Jacky Sahr for their active participation in the installation of the surface seismological networks. References Baria, R., Michelet, S., Baumgärtner, J., Dyer, B., Gérard, A., Nicholls, J., Hettkamp, T., Teza, D., Soma, N., Asanuma, H., Garnish, J., Megel, T., Microseismic monitoring of the world s largest potential HDR reservoir. In: Proceedings of the 29th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, USA, pp Bérard, T., Cornet, F.H., Evidence of thermally induced borehole elongation; a case study at Soultz, France. Int. J. Rock Mech. Mining Sci. 40, Charléty, J., Dorbath, L., Evolution of the seismicity on the HDR site of Soultz-sous-Forêts: stimulation experiment of J. Volcanol. Geotherm. Res, submitted for publication. Cuenot, N., Dorbath, C., Dorbath, L., Analysis of the microseismicity induced by fluid injections at the Hot Dry Rock site of Soultz-sous-Forêts (Alsace, France): implications for the characterization of the geothermal reservoir properties, submitted for publication. Dahm, T., Manthei, G., Eisenblätter, J., Automated moment tensor inversion to estimate source mechanisms of hydraulically induced micro-seismicity in salt rock. Tectonophysics 306, 1 17.

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