faulting mechanisms and stress regime at the european...
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Geothermics 35 (2006) 561–575
Faulting mechanisms and stress regime at the EuropeanHDR site of Soultz-sous-Forets, France
Nicolas Cuenot a,1, Jean Charlety 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 Rene Descartes, 67084 Strasbourg Cedex, France
b Institut de Recherche pour le Developpement, Laboratoire des Mecanismes et Transferts en Geologie (IRD, LMTG),14 Avenue Edouard Belin, 31400 Toulouse, France
Received 20 December 2005; accepted 6 November 2006Available online 11 January 2007
The state of stress and its implications for shear on fault planes during fluid injection are crucial issues forthe HDR (Hot Dry Rock) or EGS (Enhanced or Engineered Geothermal System) concept. This is especiallytrue for hydraulic stimulation experiments, aimed at enhancing the connectivity of a borehole to the naturalfracture network, since they tend to induce the shearing of fractures, which is controlled by the local stressregime.During the 2000 and 2003 stimulation tests at Soultz-sous-Forets, France, about 10,000 microearthquakes
were located with a surface seismological network. Hundreds of double-couple (DC) focal mechanismswere automatically determined from first-motion polarities using the FPFIT program [Reasenberg, P.A.,Oppenheimer, D., 1985. FPFIT, FPPLOT and FPPAGE: Fortran computer programs for calculating anddisplaying earthquake fault-plane solutions. US Geological Survey Open-File Report 85-739, 25 pp.]. Themajority of these mechanisms indicate normal-faulting movement with a more or less pronounced strike-slipcomponent. Some quasi-pure strike-slip events also occurred, especially in the deeper part of the stimulatedrock 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 the2003 data. The study shows that the NDC is higher for the events in the vicinity of the injection well thanfor the events far from the well.
∗ Corresponding author. Tel.: +33 3 90 24 00 62; fax: +33 3 90 24 01 25.E-mail address: [email protected] (J. Charlety).
1 Present address: European Economic Interest Grouping (EEIG) “Heat Mining”, Route de Soultz, BP 38, 67250Kutzenhausen, France.
0375-6505/$30.00 © 2006 Published by Elsevier Ltd on behalf of CNR.doi:10.1016/j.geothermics.2006.11.007
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We used the method of Rivera and Cisternas [Rivera, L., Cisternas, A., 1990. Stress tensor and fault-planesolutions for a population of earthquakes. Bull. Seismol. Soc. Am. 80, 600–614.] to perform the inversionof the deviatoric part of the stress tensor from P-wave polarities. This method was applied to differentdatasets from the 2000 test, taken from the shallower and deeper parts of the stimulated region. The resultsshow a stable, horizontal, NE-SW-oriented trend of the minor horizontal stress, but a rotation of the majorstress from a sub-vertical direction (top of the stimulated region) to a sub-horizontal one (bottom of thestimulated region). This implies a change from a normal-faulting to a strike-slip regime, in agreement withour fault-plane solutions. Finally, we applied the stress components to the nodal planes of several eventsand were able to determine their fault plane and obtain a 3D image of the fracture network, based on realdata.© 2006 Published by Elsevier Ltd on behalf of CNR.
Keywords: Microseismicity; Stress regime; Faulting mechanisms; Enhanced Geothermal Systems; Soultz-sous-Forets;France
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 developthe permeability and connectivity of the fractured (or jointed) rocks needed to create anunderground geothermal heat exchange system. The tests were conducted at flow rates upto 50 L/s, with short-time periods of up to 90 L/s in 2003. The huge number of microseis-mic events induced by these experiments presented us with the opportunity of studying boththe faulting mechanisms and the state of stress within the geothermal reservoir. The stud-ies of Ohtsu (1991), Sasaki (1998), Jost et al. (1998), Dahm et al. (1999), Nolen-Hoeksemaand Ruff (2001) as well as others, indicate that the dominant source mechanism for fluid-induced microseismicity is the shear or double-couple mechanism and that there are only smallnon-double-couple (tensile or volumetric) components to the radiated seismic energy. Nolen-Hoeksema and Ruff (2001) propose three different mechanisms for the microseismicity: shearingalong favorably oriented weakness planes that fail because either the prevailing shear stressis high, or the prevailing normal stress has been reduced by an increase in fluid pressure, orboth.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 bythe EEIG “Heat Mining”, allowed us to determine automatically several hundreds of focalmechanisms from first-motion polarity data, which give information about the type ofmovements on fracture planes. Furthermore, in order to better characterize the faultingmechanisms within the Soultz geothermal reservoir, we investigated the seismic momenttensor of several events to determine whether the seismic rupture exhibits a non-double-couple component, which could correspond to a tensional opening of fractures in addition toshear.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 applyingthe stress tensor solutions to the nodal planes of a number of microseismic events, we were ableto determine the active fault planes associated with several of these events and to obtain a realistic3D view of the fracture system.
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Fig. 1. Injection rate and pressure recorded at the wellhead of well GPK2 during the 2000 Soultz hydraulic stimulationtest.
2. The 2000 stimulation
2.1. Hydraulic parameters
The injection scheme was rather simple in 2000 (Fig. 1). The injection lasted about 6 days (30June–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 intothe open-hole section of GPK2 (4400–5000m). The wellhead pressure instantaneously reached apeak of 12MPa and then declined during the first two steps. On the other hand, at a rate of 50 L/s,the pressure initially rose to 12MPa, but continued to increase until it had reached 13MPa atshut-in time (Weidler et al., 2002). When injection was stopped, an instantaneous pressure dropof about 5MPa was observed, which was followed by a very slow pressure decrease.
2.2. Seismic monitoring network
In addition to the downhole seismic network,which consisted of three 4-component accelerom-eters and two hydrophones, several surface stations were installed by EOST, University ofStrasbourg. The surface network comprised eight vertical seismometers, six 3-component seis-mometers, one broadband station and three permanent stations belonging to ReNaSS (FrenchNational SeismicNetwork). A plane view of the 2000 seismicmonitoring system is given in Fig. 2.
2.3. Microseismic activity
During the 2000 stimulation more than 10,000 microseismic events were recorded by thesurface network, which was deployed from 30 June to 11 July. They range in magnitude between
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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 locationparameters. 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 themicroseismic cloud seems to be consistent with the N-S to NNW-SSE trend of the majority ofthe 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 2000stimulation of borehole GPK2 (Fig. 4). Baria et al. (2004) described four phases during the 2003stimulation. During phase I (25.05–02.06), injection was at 30 L/s and was later increased to50 L/s, with two short periods at 60 and 90 L/s. From 02.06 to 04.06 the concept of dual, focusedinjection was introduced by injecting water at a rate of 50 L/s into GPK3 and at 20 L/s into GPK2.
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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 04.06 to 06.06, GPK2 was shut in, whereas in GPK3, after a short increase in injectionrate (up to 90 L/s), injection was decreased in three steps. Finally, because of the microseismicactivity remaining after GPK3 was shut-in, GPK2 was discharged at a rate of 10 L/s to reduce thepressure in the reservoir.
3.2. 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 permanentmonitoring network installed by EOST (three 3-component sensors and six vertical sensors). Atemporary surface network (six 3-component stations and eight vertical stations)was alsodeployedduring the stimulation test (Fig. 2).
3.3. Microseismic activity
About 5000 microseismic events in the −0.9 to 2.9 magnitude range were recorded by thesurface network; 2250 were located using the TomoDD code (Zhang and Thurber, 2003). As in2000, the cloud of events in 2003 was oriented in a NNW-SSE direction, but located further tothe south (Fig. 3). The cloud was about 2 km long and 1 km wide, and extended between 3 and
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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 leasta part of the seismic structures that were stimulated in 2000 did not slip in 2003.
4. 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 polar-ities by a maximum likelihood procedure and manually checked afterwards. On average, morethan 14 polarities are available for the 2000 events and more than 16 for those of 2003. Theresults indicate a majority of normal-faulting movements, pure or with a more or less pronouncedstrike-slip component. However, a strike-slip regime does appear to dominate in the deepest partof the reservoir, with some quasi-pure strike-slip events. Some representative focal mechanismsfor the 2000 and 2003 stimulation tests are shown in Figs. 5 and 6.
5. Non-double-couple component
The full determination of the seismicmoment tensor (first order) was performed by consideringa homogenous half space. The source-time function was considered identical for all stations andthe amplitude of the P-wave was corrected from the plateau of each instrument. The seismicmoment tensor was then inverted using a common least square method (Nolen-Hoeksema andRuff, 2001).
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Fig. 5. Representative focal mechanisms for the 2000 stimulation test.
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Fig. 6. Representative focal mechanisms for the 2000 and 2003 stimulation tests. The color represents the depth of theevents. 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 withthe physical processes involved at the source. Moreover, it can be decomposed into a double-couple component and a non-double-couple component. Giardini (1983) defined the deviationfrom the DC mechanism as the ratio of the minimum to the maximum eigenvalue of the momenttensor, in the sense of absolute value. This ratio, ε, ranges between −0.5 and 0.5. The deviationis taken as positive if the eigenvalue of the maximum absolute value is tensional and negative ifit is compressional. If ε ranges between −0.25 and 0.25, the DC component dominates.The results indicate a value for theNDC component thatmay be the result of threemechanisms.
The first of these is related to the simplest assumptions made for its determination, so that theNDC component is an artifact of errors; this must not be neglected. The other two are tied to themechanics of faulting. Kuge and Lay (1994) and Houston (1993) show that the NDC is the resultof 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 NDCcomponent. On the other hand, it could also be the result of an opening and shearing mode offailure. The significance of the result can be assessed by considering the variation between eachof the events considered in this study. As the ray paths of the seismic events are similar and theevents are close to one another within the entire studied space, we consider that the error in thedetermination of the moment tensor is approximately identical for all events. Thus the observedNDCs 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 propor-
tional to the magnitudes of the events; the colors correspond to the value of ε. For each seismicevent this ratio is less than 0.25, which means that the DC mechanism prevails. This is verifiedby 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 eventsin the vicinity of GPK3 have a non-negligible NDC component, and that the fractures that rupturemay undergo tension in addition to shearing, or that the fracture is composed of a complex system
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Fig. 7. Proportion (ε) of non-double-couple component for several 2003 microseismic events as indicated by the colorscale. The radius of each colored sphere is proportional to the magnitude of the event. Trajectories of wells GPK2 andGPK3 are shown in red and blue, respectively.
of segments. Charlety and Dorbath (2005) showed that the fracture zone consists of a complicatedsystem of fractures with heterogeneous orientations, in agreement with the geological studies ofDezayes 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 veryclose to the well took place in the early stages of the hydraulic stimulation when the temperaturedifference 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 fracturesto open slightly. On the contrary, away from the injection well the fracture tensional componentseems to decrease. This would mean that the overpressure there is less effective, maybe becauseit drops quickly with distance from the injection well. The prominent pressure effect is confirmedby another observation. As shown in Fig. 7, one event near injection well GPK3 does not show ahigh NDC component. This particular event occurred after shut-in, at a time when the area nearthe well was no longer being subjected to high fluid pressures, so that no tension was involved inthe seismic rupture.In conclusion,we determined a double-couple solution for eachmicroseismic event,whichmay
indicate that the dominant process of the faulting movements is shearing. This result also seemsquite 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 anNDC component, which may indicate a proportion of tensional opening of the fracture planes.
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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 focalmechanisms showed a higher proportion of strike-slip events in the deepest part of the geothermalreservoir. Second, the results of a stress profile by Klee and Rummel (1993) at Soultz-sous-Foretsbased on hydrofracturing stress measurements indicated a possible crossover between the verticalstress SV and the maximum horizontal stress SH at around 3500–4000m depth. This would implya change in the faulting regime with depth, from a normal-faulting to a strike-slip regime. It wasin order to check this hypothesis that we decided to perform the stress tensor inversion.
We used the method of Rivera and Cisternas (1990), which involves the direct inversion ofthe 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 faultingregime. From an initial trial solution (tensor and focal mechanisms), theoretical polarities arecalculated and compared with the observed data at each iteration. The solution is then modifiedin order to maximize a likelihood function. The quality of the solution is expressed in terms oflikelihood and score (the score describes the fit between observed and theoretical polarities).
We performed two inversions with two different data sets of events from the 2000 stimulationexperiment. 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, about60 microseismic events were randomly selected from those exhibiting the largest number ofavailable polarity data. Each selected event shows a number of polarities between 14 and 18. Inorder to check the reliability of the inversion, we performed several calculations using varioussets containing different arrangements of events. The calculations gave similar results.
The results of the inversion are presented in Figs. 8 and 9. In both, the figure at the topcorresponds to the 100 best tensor solutions and that at the bottom shows the best estimate ofthe stress tensor. Fig. 8 shows the results of the inversion for the upper part of the reservoir, andFig. 9 the results for the bottom part. Stresses are expressed in terms of σ1, σ2 and σ3, whereσ1 > σ2 > σ3.Our first observation deals with the stability of the orientation of the minimum horizontal stress
Sh, which trends in both cases NE-SW to NNE-SSW (orientation of Sh at the regional scale inthe upper Rhine Graben). In Figs. 8 and 9 the maximum horizontal stress SH is oriented NW-SEto NNW-SSE. This result is also consistent with regional estimates of SH. However, at the localscale, other studies show amoreN-S orientation of themaximumhorizontal stress (e.g. Berard andCornet, 2003). The method of Rivera and Cisternas assumes that the stress tensor is homogenousover the study region. In the case of Soultz-sous-Forets, fluid injection may introduce strong local
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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 relativescatter of the solutions may reflect these stress heterogeneities.The most important result, however, concerns the rotation of the maximum stress σ1 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. Thismeans that the maximum horizontal stress SH becomes the maximum stress at the bottom of the
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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 regimeis normal faulting whereas strike slip is likely to occur in the deepest part of the reservoir. This isin agreement with the focal mechanism results. Nevertheless, Figs. 8 and 9 both show a relativedispersion of the solutions. In Fig. 9 in particular, some solutions still indicate a subvertical trendfor σ1 and a subhorizontal direction for σ2, which suggests that the faulting regime may not havecompletely changed at the bottom of the reservoir; that is, the stimulated volume is located within
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Fig. 10. Three-dimensional representation of the fracture network at Soultz.
the region of stress rotation. It also confirms that the magnitudes of SV and SH are very close, assuggested by Klee and Rummel (1993).
7. Three-dimensional imaging of the fracture network
Weapplied the stress tensor on the nodal planeswe determined for the 2000 stimulation in orderto define the shear plane. Fig. 10 shows the result in a 3D view. The majority of the fault planesare oriented NNW-SSE to NW-SE with a dip either to the west or east. We can also observe thatmost of the planes dipping to the west are subvertical, while those dipping to the east seem moresubhorizontal. In addition, several fault planes exhibit an “en echelon” structure. Nevertheless,the fracture system appears to be somewhat heterogeneous.
Our analysis of the faulting mechanisms associated with the hydraulic stimulation experimentsperformed at Soultz-sous-Forets 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 mechanismsare strongly related to the extensional regime, which is dominant in the Rhine Graben.
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From determination of the focal mechanisms, it appears that the failure mode is mainly normalfaulting, with a more or less marked strike-slip component. However, our observations suggesta strike-slip regime in the deepest part of the Soultz geothermal reservoir. The inversion of thedeviatoric part of the stress tensor confirms this feature. That is, the maximum stress rotates froma subvertical trend to a subhorizontal stress with depth, implying a change from a normal-faultingregime at the top of the reservoir to a strike-slip regime at the bottom. The change is in fact quitegradual and it is possible that the geothermal reservoir crosses the zone of stress rotation. In anycase, the stress conditions, as well as the pre-existing seismogenic structures, seem to be suitableto ensure good hydraulic stimulation results in terms of enhancement of fracture permeability andconnectivity.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 theseismicmoment tensor of several 2003 events, however, we show that at least some of these eventshave a non-negligible NDC component. It is striking that they tend to occur in the vicinity of theinjectionwell,whereas those located at greater distance donot exhibit such highNDCcomponents,or none at all. This means that events near the injector show part of a tensional opening as well asthe main shearing process. Such behavior is closely related to the overpressure induced by fluidinjection, 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 andfor connection between the well and the fracture network. The extent of tensional opening is onthe other hand close to zero for events occurring far from the injection well, indicating that thepressure 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 beinteresting to obtain a similar picture with fault lengths scaled to the magnitude of microseismicevents.
This work was funded by a grant from Ademe (French National Energy Agency) and theConseil Regional 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 alsoextended to Herve Blumentritt, Michel Frogneux and Jacky Sahr for their active participation inthe installation of the surface seismological networks.
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