Overview of the Fracture Network at Different Scales Within the Granite Reservoir of the ECS SOULTZ SITE

8/11/2019 Overview of the Fracture Network at Different Scales Within the Granite Reservoir of the ECS SOULTZ SITE

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Proceedings World Geothermal Congress 2010Bali, Indonesia, 25-29 April 2010

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Overview of the Fracture Network at Different Scales Within the Granite Reservoir of theEGS Soultz Site (Alsace, France)

Chrystel Dezayes 1, Albert Genter 2, Benot Valley 3 1,BRGM, Geothermal Department, 3, avenue Cl. Guillemin, BP36009, F-45060 Orlans Cedex 2

2GEIE Exploitation Minire de la Chaleur, Route de Soultz, BP40038, F-67250 Kutzenhausen3ETH Zrich, Engineering Geology, CH-8093 Zrich; now: MIRARCO/Laurentian University, 935 Ramsey Lake Road, Sudbury,

ON P3E 2C6, [email protected], [email protected], [email protected]

Keywords: Rhine graben, fractures, fracture zones, cores,borehole images, Enhanced Geothermal System

ABSTRACT

In EGS concepts like the one at Soultz, knowledge of thefracture network is essential to understand reservoirbehavior and plan further stimulations and long term

hydraulic circulation. At Soultz, the targeted reservoir islocated at a 5km depth within the granite basement of theUpper Rhine Graben near the western border of France.This granite underwent a complex tectonic historyincluding the Hercynian and Alpine orogeneses, leading tothe currently observed fracturing. This fracturing isdominated by steeply dipping fracture sets which aredistributed at various scales in the granite basement.

This structural work presents the characterization of thefracture network at different scales in order to understandand model the hydro-thermo-mechanical behavior of thegranite geothermal reservoir through hydraulic stimulationand subsequent circulation tests. The small-scale fracturenetwork has been characterized based on several kilometersof high resolution borehole image logs, allowing thecharacterization of different sets. There was a focus onlarge-scale fracture zones, which are the major fluidpathways. A total of 39 fracture zones have been describedin detail based on borehole data. They have been comparedto large-scale geophysical investigations using VerticalSeismic Profile (VSP) and passive microseismicity in orderto build an extended fracture network in 3D. Hydraulicallyactive fractures have been investigated in more detail in thedifferent open-hole sections based on image logs, flow logsand temperature logs. These near-well borehole fracturesrepresent potential fluid pathway for connecting theborehole to the far-field geothermal reservoir.

The fracture network in the granite Soultz reservoir isconstituted by large-scale fracture zones, which areconnected to a dense network of small-scale fractures.However, fluid pathways are more complex, withchannelized structures, because only a limited number offractures support fluid flow.

1. INTRODUCTION

Since 1980 (Grard et al. , 1984; Grard and Kappelmeyer,1987), the goals of the EGS project at Soultz (France) havebeen to experiment and develop a new geothermaltechnology. After the initial Hot Dry Rock (HDR) conceptof artificial fracture creation in a homogeneous rock byhydraulic fracturing was realized, the concept at Soultz hasprogressively evolved toward an Enhanced GeothermalSystem (EGS), where reservoir development involved thereactivation of pre-existing fractures in the granite (Evans etal. , 2005; Grard et al. , 2006). Thus, a good knowledge and

the geometrical characterization of the rock mass andparticularly of the fracture network are essential for manyreasons, from the optimization of the borehole design to theunderstanding of the flow distribution at depth. At Soultz,this fracture network is structured at different scales, frommajor fracture zones cross-cutting the granite batholith tointra-crystalline microfractures that induce weakness in the

rock mass (Dezayeset al.

, 2000).Several studies have shown that some fracture zones arewater-bearing prior to stimulation (Vuataz et al. , 1990;Genter et al. , 1995) and form the main flow channels afterstimulation and during circulation (Evans et al. , 2005;Sanjuan et al. , 2006). These fracture zones are probablyreactivated by shearing during hydraulic stimulation.Moreover, other fracture zones that were not permeableprior stimulation have been activated during stimulationand have been taken into account in this study.

Thus, fracture zones form the main potential fluid pathwaysconnected to the dense network of meso-scale fractures toform the geothermal reservoir, which is about 1,125 km 3 in

the upper reservoir and 8 km3 in the deeper reservoir, based

on induced micro-seismicity study (Cuenot et al. , 2006).

This paper presents a characterization of intermediate scaleto the large-scale fracture zones in order to make ageometrical update of the fracture data base likely tosupport conductive fluid flow. Small-scale fracturedistribution and characterization have been extensivelystudied (Dezayes et al., 2000; Genter at al., 2000). The goalof this study is to define the geometry of the fracturenetwork for a better understanding of the fluid circulation ina deep fractured granite reservoir dedicated to geothermalexploitation. These results are used to model the hydraulicstimulations and circulations in the Soultz geothermalreservoir (Sausse et al. , 2009; Rachez et al., 2006; Rachezand Gentier 2008; Baujard and Bruel, 2005; Kohl andMegel, 2005) and to make geophysical interpretations orinversions (Place et al. , 2009; Schill and Geiermann, 2009).

2. GEOLOGICAL AND STRUCTURAL CONTEXT

The Soultz site is located within the Upper Rhine Graben,which is a Cenozoic rift structure belonging to the WestEuropean Rift System, as shown in the geological map ofthe Graben in Figure 1 (Ziegler, 1992). The fillingsediments are marine and lacustrine limestones, marls andevaporites (including the petroleum layers of Pechelbronn)overlaying in unconformity the Jurassic limestones and theGermanic Trias (indicated by (b) in Figure 1-II). These

Cenozoic and Mesozoic sediments have been deposited onthe Paleozoic basement, which includes porphyritic monzogranite and two-mica granite (Genter et al. , 1999; Stussi etal. , 2002; Cocherie et al. , 2004; Hooijkaas et al. , 2006).


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This granite unit is the target of the geothermal project andhosts the geothermal reservoir. This massif underwent amultiphase tectonic history, including the Hercynian andAlpine orogeneses. The Cenozoic tectonic history isreflected by the orientations of the current structuresforming the Upper Rhine Graben (Figure 1). The regionalmajor border faults mapped on the surface or derived frompetroleum seismic reflection studies within the Upper Rhine

Graben show a N0-30E direction in relation to the threemain directions of the graben (Figure 1). In the southernpart of the graben, the main direction is about N10E. Thisdirection rotates clockwise at the center of the graben toN25E-N30E and becomes N0E in its northern part. TheRhenish fracture orientation was formed during theOligocene opening of the Rhine graben (Villemin andBergerat, 1985), probably reactivating some Hercynianstructures (Illies, 1972, Illies, 1975; Rotstein et al. , 2006).At the Soultz site, the Upper Rhine Graben rotates, and theregional border faults have a N45E orientation. Thisdirection is also present in the Triassic sediments in theVosges area west of the graben fault (Mnillet et al., 1989).

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Figure 1: Location of the EGS Soultz site and geologyof the Upper Rhine Graben. I) Location of theUpper Rhine Graben within the West EuropeanRift System. (1) Hercynian basement; (2)Tertiary basins; (3) Alpine molasse. II): WEcross-section through the Rhine Graben borderand the Soultz site. (a) Cenozoic; (b) Mesozoic;(c) Hercynian basement. III) Geological andstructural map of the Rhine graben. (1) Cenozoicsediments; (2) Cenozoic volcanics; (3) Jurassic;(4) Triassic; (5) Permo-Carbonifeous basins; (6)Hercynian basement; (7) boundary faults; (8)other faults; (9) overthrusts

However, the granitic basement has been affected by ante-Cenozoic tectonics, particularly the Hercynian orogen. The

strike of the major dislocations delimiting the differentHercynian tectonic blocks is roughly N60E (Figure 1).Geological mapping, borehole data and interpretation ofseismic profiles acquired on a local scale around the Soultzsite during the exploration of the Pechelbronn oil fieldprovided lots of data to characterize the major fault system(Foehn, 1985; Place et al. , 2009). These data were compiledto build a 3D geological model of the sedimentary cover

(Renard and Courrioux, 1994; Castera et al. , 2008). Thesestudies show that in the sedimentary cover, the faults have aN20E strike, i.e. a Rhenish direction (Figure 1). In mapview, the faults have a trace length of about 2 to 20 km andoccur with a spacing range of about 800 m to 3 km (Valleyet al. , 2007). At depth, below the Soultz site, a horststructure is present, and the top of the basement is at a 1.4km depth (Figure 1-II). Within this horst structure, somelocal faults were detected on the seismic profiles, whichmainly dip to the west (Figure 1-II).

On the Soultz site, five boreholes have been drilled since1987. Two relatively shallow, fully cored, exploratory wellscalled GPK1 and EPS1 were drilled to depths of 3600 mand 2200 m respectively. Three wells called GPK2, GPK3and GPK4 were drilled for reservoir development and heatexploitation to depths of approximately 5000 m, wheretemperatures reach 200C. (Baumgrtner et al. , 2004).These wells are illustrated in Figure 2. The exploitationtriplet consists of GPK2, GPK3 and GPK4, which weredrilled from the same well pad and are slightly inclinedwith depth. The three wells are aligned at 5000 m depth in aN165E direction, and the horizontal distance betweenadjacent wells is about 700 m at depth (Figure 2).

Figure 2: Map view and N-S cross-section of the EGSwell trajectories at Soultz

3. METHODOLOGYThe orientation and classification of several thousandfractures were observed from oriented borehole images.High-resolution acoustic image logs such as BoreHoleTeleViewer (BHTV) were acquired in GPK1 and EPS1,and Ultrasonic Borehole Imager (UBI) in GPK2, GPK3 andGPK4, as shown in Figure 3. Various electrical image logs(FMS, FMI, ARI) were run in the GPK1 and GPK2 wellsand analysed in previous studies (Genter et al. , 1995;

Dezayes et al. , 1995; Genter et al. , 1997).

In the cores, all the fractures are systematically sealed byhydrothermal filling but only 20% of them are visible on


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the borehole images (Genter et al. , 1997). However, onlythe EPS1 borehole is fully cored (Figure 3) and some spotcoring is available in GPK1 and GPK2. But the boreholeimages are suitable for detecting and measuring theorientation of the meso-scale fractures and fracture zones.

For the analysis of the small-scale fractures, two main typesof fractures have been defined along the wells based ontheir origin: the natural fractures, which are older than thedrilling and related to the tectonic history of the granitemassif, and the induced fractures formed during the drillingthat are related to the present-day stress field. In this study,only the natural fractures have been taken into account.Induced fractures have been studied in other papers likeTenzer et al. (1991), Genter et al. (1995), Dezayes et al. (1995), and Valley and Evans (2005).

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ARI: Azimuthal Resistivity Imager. Depth:measured depth along the well (MD)

The natural fracture typology was defined in order to obtainhomogenous fracture datasets in the GPK3 and GPK4 wells(Dezayes et al. , 2005). This typology is based on two maincriteria:

- The comparison of transit time and amplitude imagelogs can be used to determine if the fracture is open(dark traces) or not (no trace).

- The continuity of the sinusoidal trace on the amplitudeimage can be used to distinguish the scale of fractures(100% to 80% of the sinusoidal trace visible

representing large fracture, between 80% and 50% andlower than 50% of the sinusoidal trace representingmedium-sized fracture, and uncertain sinusoidal fittingrepresenting small fracture). The occurrence of an

alteration halo surrounding the fracture is also takeninto account.

In order to better characterize low permeability fracturezones, the geological database was used based onpetrographical descriptions of cuttings, borehole image logand geophysical log analyses, and caliper, spectral gammaray and drilling parameters (Figure 4). The spectral gammaray data, such as potassium, thorium and uranium contents,were used to detect radioactive element concentrations dueto the hydrothermal alteration related to fracture zones orsome petrographical variations.

However, evidence of natural permeability in a givenfracture cannot be determined by interpreting geologicaldata only. Thus, temperature and flow log analyses wereused as well to determine the zone of fluid lost in theboreholes (Evans, 2000; Dezayes et al. , 2004).

On borehole images, fracture orientations are sometimesdifficult to measure due to the fact that fractures are notperfectly planar as it is assumed for the dip calculation.Also, fracture zones are complex, and their orientation is

difficult to define. Often, several individual fracture tracesare visible on the log image within a given fracture zone,which includes brecciated to microbrecciated zones. Todetermine the overall orientation of a fracture zone, it wasconsidered that the orientation of the border of thebrecciated zone is representative of the overall orientation ifthis limit is clearly visible and defined and if it forms awell-marked planar structure. However, when severalplanar structures were present and roughly parallel, it wasassumed that the orientation of the fracture zone is wellapproximated by the 3D average orientation of theindividual parallel planes.

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In the open-hole sections of GPK3 and GPK4, severaltemperature and flow logs were acquired during thestimulation of the granite reservoir. The detailed studies ofthese logs coupled with the borehole image logs enabled thedefinition of the fluid exit zone, which connected the wellswith the reservoir.


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In the GPK3 well, the high density value (0.8 fract./m)observed in the upper part of the granite is in agreementwith the previous studies done at Soultz (Genter et al. ,1997). From 1800 m to about 2900 m, the fracture densityis quite constant, with a low density (0.4 fract./m). Thesetwo sections correspond in terms of petrography to thestandard pophyritic granite (Hooijkaas et al. , 2006; Figure6).

In the intermediate part of the well, two high fracturedensity zones (1.4 fract./m and 0.6 fract/m) surround a lowdensity zone (0.4 fract./m). These three zones correspond tothe upper part (between 2900 m and 3300 m MD) of thehighly fractured and altered standard granite facies (Figure6).

Below these zones, there are two low and moderate fracturedensity zones, which are embedded into the biotite andamphibole rich granite (Figure 6).

In the lower part of the well (from 4800 m MD), a veryhigh fracture density zone occurs. This zone corresponds tothe boundary between two granite facies: the standard

porphyritic granite and the two-mica granite. In this deepfine-grained facies, the fracture density is very low anddecreases significantly to 0.4 fract./m (Figure 6).

In the neighboring GPK4 well, a high density value (0.8fract./m) occurs in the upper part of the granite, as in theGPK3 well. Between about 1800 m and 1900 m MD, thefracture density increases to 1.25 fract./m. Below 1900 mand to about 2700 m, the fracture density is quite constantwith a low density (0.55 fract./m). These three sectionscorrespond in terms of petrography to the standardporphyritic granite and are illustrated in Figure 7.

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Figure 7: Cumulative number of fractures with depthincluding all fracture types derived fromborehole image analysis in the GPK4 well. Depth

in the left column corresponds to the measureddepth along the borehole from the drilling floor.The depth boundary of the granite facies in the

right column corresponds to the vertical depth,from the drilling floor

In the intermediate part of the well, two very high fracturedensity zones (2.86 fract./m and 1.76 fract/m) alternate withtwo moderate density zones (0.71 fract./m). These fourzones correspond to a highly fractured and altered standardporphyritic granite facies, between 2900 m and 3500 m MD(Figure 7).

Below these zones, between 3500 m and 4750 m MD, thefracture density is very low and quite constant with a valueof 0.23 fract/m (Figure 7).

In the lower part of the well (from 4750 m MD to thebottom of the well), a high fracture density zone (0.89fract./m) occurs in the lower part of the biotite andamphibole rich granite facies and the two mica-granite(Figure 7).

5. FRACTURE ZONE NETWORK

Characterization of Fracture Zones

In the six wells of the Soultz site, 39 fracture zones wereconsidered, which indicate some potential traces of fluidflow, as shown in Table 1 and Figure 8. This list of fracturezones is probably not exhaustive and could be completedlater by further data acquisition or processing.

Table 1. All fracture zones determined in this study.TVDss: True Vertical Depth Sub Sea. Depth:measured depth along the well. Fracture zones oflevel 1 are bold; fracture zones in level 2 arenormal text; fracture zones in level 3 areitalicized.

Well Name Depth Level TVDssEPS1 EPS1-FZ1010 1012 3 836.637512

EPS1 EPS1-FZ1200 1198 1 1021.84821EPS1 EPS1-FZ1640 1643 2 1466.48914EPS1 EPS1-FZ2180 2179 1 1988.09058

GPK1 GPK1-FZ1015 1015 3 862.00885 GPK1 GPK1-FZ1220 1220 1 1066.59717GPK1 GPK1-FZ1820 1820 1 1666.24219

GPK1 GPK1-FZ2815 2815 2 2657.19238GPK1 GPK1-FZ3220 3223 3 3064.05273

GPK1 GPK1-FZ3490 3492 2 3332.64502 GPK2 GPK2-FZ2120 2123 1 1953.37292

GPK2 GPK2-FZ3240 3242 3 3069.90625 GPK2 GPK2-FZ3350 3347 3 3174.62988 GPK2 GPK2-FZ3515 3514 3 3341.70801

GPK2 GPK2-FZ3900 3900 2 3726.61133GPK2 GPK2-FZ4760 4760 2 4544.82227GPK2 GPK2-FZ4890 4890 3 4668.3252

GPK2 GPK2-FZ5060 5060 2 4831.29248GPK3 GPK3-FZ1580 1579 3 1410.36487 GPK3 GPK3-FZ1640 1637 3 1468.78821GPK3 GPK3-FZ1820 1820 3 1651.01147 GPK3 GPK3-FZ2040 2042 3 1873.30823 GPK3 GPK3-FZ2045 2046 3 1876.76367 GPK3 GPK3-FZ2090 2092 3 1923.16223 GPK3 GPK3-FZ2970 2970 2 2798.43579

GPK3 GPK3-FZ3270 3271 2 3092.95361GPK3 GPK3-FZ4090 4089 3 3856.2417 GPK3 GPK3-FZ4770 4775 1 4538.90137

GPK4 GPK4-FZ1720 1723 3 1554.30212 GPK4 GPK4-FZ1800 1801 3 1632.22925 GPK4 GPK4-FZ2820 2817 3 2762.20264 GPK4 GPK4-FZ3940 3940 3 3603.97852 GPK4 GPK4-FZ4360 4361 2 3963.29565 GPK4 GPK4-FZ4620 4620 3 4195.97852

GPK4 GPK4-FZ4710 4712 2 4279.59717GPK4 GPK4-FZ4970 4973 3 4530.36914 GPK4 GPK4-FZ5050 5012 3 4568.51904 GPK4 GPK4-FZ5100 5100 3 4655.44922

4550 4550-FZ1265 1265 1 1107.95679

The fracture zones have been classified within three levelsin attempting to reflect their relative scale and importance

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lines and orange dots: cyclographic traces andpoles of fracture zones of level 2 (dotted orangeline: supposed orientation); green line and greendot: cyclographic traces and poles of fracturezones of level 3 (Schmidts projection, lowerhemisphere)

Organization of the Fracture Zones

Most of the fracture zones characterized in the frameworkof this study (Table 1) can be attributed to three mainclusters of fracture zones at about 1800-2000 m, 3000-3400m and 4500-5000 m TVD (True Vertical Depth), asillustrated in Figure 9. In that regard, the GPK4 borehole issomehow atypical, presenting less clearly clustered, steeplydipping fractures (Figure 9). In the other wells, fracturezones more consistently correspond to these three fracturezone clusters. This was reported previously in the upperpart of the granite body, before the deepening of GPK2 andthe drilling of GPK3 and GPK4 (Genter et al. , 1995; Genterand Castaing, 1997). These three clusters of fracture zonesare considered to reflect a major fault, equivalent to thefault detected in the sedimentary cover based on seismicreflection (Genter and Castaing, 1997). If it is assumed thatthese faults are steeply dipping structures with valueshigher than 60, the raw spacing between two consecutiveclusters of fracture zones is around 500 m, which isequivalent to the fault spacing in the sedimentary cover(Valley et al. , 2007).

Figure 9: Synthesis of the all fracture zones determinedin this study along a N-S cross-section throughthe Soultz wells. (1) Sedimentary cover,(2) standard porphyritic granite, (3) Standardgranite with intense vein alteration, (4) Biotiteand amphibole rich granite gradually giving way

to standard granite with depth, (5) Two micagranite and biotite rich granite

The upper cluster at 1800-2000 m depth MD (Cluster I inFigure 9) is located in the unaltered porphyritic granite.This cluster includes major fracture zones qualified as level1 with permeable zones. The cluster II does not includemajor level 1 faults. It is located within the fractured andaltered granite zone (Figure 9). This granite facies ischaracterized by high pervasive alteration related tonumerous meso-scale fractures present in this zone(Hooijkaas et al. , 2006). This facies contains a highproportion of clay and hydrothermal minerals. The highfracturing of this facies possibly leads to a generally weakerrock mass, where the strain is distributed in smallincrements over a dense network of smaller fractures,instead of being concentrated by a few major faults. Thislocation corresponds to the upper reservoir stimulated in thefirst phase of the project in 1997 (Baumgrtner et al. ,1996). In this reservoir, a four month circulation loop testwith tracer injection was performed with success (Bruel,2002). Based on this test, it appears that there is a goodfluid connection in this reservoir between GPK1 and GPK2.

The lower cluster (Cluster III in the Figure 9) is close to theinterface between the 2 granite units at 4700 m TVD. Thedeep facies is characterized by massive granite with a lowdegree of pervasive alteration. Cluster III contains majorfracture zone (GPK3-FZ4770 in Table 1). Thus, it appearsthat the deformation in this facies tends to be localizedalong major isolated fracture zones. This last cluster islocated in the lower reservoir, which was stimulatedpreviously and is currently a part of the deep geothermalloop under testing for the electrical production test.Conceptually, this reservoir appears to behave as afractured reservoir embedded in a low permeability matrix.

Based on this characterization of fracture zones at aborehole scale, these major planes have been compared toother fracture information derived from other detectionmethods in order to build a 3D model (Sausse et al. , 2009).These other methods are microseismicity data collectedwithin the reservoir during the stimulations of GPK2,GPK3 and GPK4 (Dorbath et al. , 2009) and a VSP(Vertical Seismic Profiling) investigation (Place et al. ,2009). Some of the fracture zones observed at the well scalematch the microseismicity or VSP results. They correspondto fracture zones qualified by level 1 or 2 such as GPK1-FZ3490, GPK2-FZ3900 and GPK3-FZ4770, which matchmicroseismicity and VSP analysis. Moreover, the diporientations of the fracture zones are very close and seem todefine a large-scale fault intersecting the Soultz basement(Figure 9). The orientation of these major fracture zones is

about N230E-70 (N140E-70W).

In GPK1, two fracture zones at 1820m MD and 2860m MDappear to match with the VSP analysis (Place et al. , 2009;Sausse et al. , 2009).

In the lower part of the reservoir, two smaller fracture zonesin GPK4 match geometrically with the microseismicityanalysis at 4620 m and 4970 m MD (Sausse et al. , 2009).These fracture zones strike N10E and dip steeply to thewest (Figure 9).

6. FLOW PATTERN IN THE OPEN HOLESECTIONS

Some of the fluid exit zones in the open holes correspond tofracture zones, such as the large ones in GPK3 at 4770 mMD describe above. In this well, six other zones have beendefined as fluid exit zones. These exit zones are described


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The zone at 4920 m MD also consists of several fractures,one of which stands out (Figure 10). This zone did not takeany fluid during the last hydraulic test, which includedinjection at a rate of 50 l/s after two peaks at 90 l/s. Theaverage orientation is N12E-68W (Figure 10). The zoneat 4972 m MD showed a single open fracture without anyalteration. The fracture is oriented N135E-65E and tookapproximately 4% of the fluid during all of the hydraulic

tests. A deeper zone at 5025 m MD also consists to a singlefracture oriented N22E-58W (Figure 10). The fracturetakes 10 to 15% of the total injection accepted by thewellbore during all the hydraulic tests.

Two zones, namely 4940 and 4990m MD, do notcorrespond to open fractures based on UBI image logsacquired shortly after the end of the drilling. However,these two zones appear to be hydraulically active undercertain conditions. The zone at 4940 m MD accepted about2-4% of the injected fluid at injection rates of 25 and 30 l/s,but it did not accept any fluid during injection at 50 l/s orafter the 90 l/s period. The zone at 4990 m MD seemed tobe closed at the beginning of the stimulation (at an injectionrate of 25 l/s), but later accepted 4% of the fluid flow.These zones may have initially been closed but were laterre-opened during the hydraulic experiments.

In GPK4, as the flow logs have a bad quality, we based thedetermination of the fluid exit zones on the temperatureanalysis. The temperature variation was located, andfractures were characterized matching depth on boreholeimages. Then, nine fluid exit zones were determined, asshown in Figure 11. As there were less data than in GPK3,the zones in GPK4 are less constrained, and three degreesof fluid exit zones were used to describe them: certain,probable, and supposed (Figure 11). Three of them (ZA3,ZA5, and ZA7) were in the global fracture zone network.

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Figure 11: Synthetic log of the open hole section of theGPK4 well showing characterization of the fluidexit fractures. Column 2: Rate of Penetration.Column 3: Facies variation log: (1) two micagranite, (2) biotite rich granite; Column 4:Standard granite: porphyritic MFK-rich granite;

Column 5: Occurrence of minerals indicatingfracture zones; Column 6: Standard Gammaray; Column 7: Potassium content; Column 8:Thorium content; Column 9: Uranium content;

Column 10: hole diameter; Column 11: holearea; Column 12-14: HAC; Column 15:temperature variation

Five zones were described as certain fluid exit zones at4923 m, 4973 m, 5073 m, 5100 m and 5238 m MD. At4923 m MD, two N-S high dipping fractures are associatedwith the borehole cave and that kind of breakout (Figure11). A 4973 m MD, one N-S high dipping fracture ispresent with a 1 m thick alteration halo on the both sides ofthe fracture. This alteration is also indicated by a highpotassium and uranium content (Figure 11). A series ofNW-SE parallel fractures forms the zone at 5073 m MDthat exhibited a Gamma Ray anomaly. The potassium andthorium contents were found to be very low here, whereasthe uranium content was high (Figure 11). At 5100 m MD,a brecciated zone is present with N-S and NNW-SSEborders. This zone is associated to a cave, observed on thecaliper log, with low thorium content and high uraniumcontent (Figure 11). The deeper zone at 5238 m MD is aseries of several NNE-SSW parallel fractures. This zonehas been detected with the cutting observations and presentsa high uranium content (Figure 11).

The probable fluid exit zones are located at 5010 m and5135 m MD. The first one is a large 15 m thick zone withmany fractures. The average direction of these fractures isNNW-SSE with a high dip. This zone is characterized byhigh uranium content (Figure 11). At 5135 m MD, there is aseries of fractures with a NNW-SSE average direction(Figure 11).

The two other zones were supposed exit zones. At4822 m MD, two N-S high dipping fractures were present,but did not exhibit Gamma Ray anomaly (Figure 11). Acave was detected at 5050 m MD associated with highuranium content (Figure 11). N-S oriented fracture exitsoccurred that were associated with a kind of large breakout.

DISCUSSIONThe average strike direction of fracture zones characterizedin the granite is N160E15, as shown in Figure 12. Twosecondary sets are present with N20E10 andN130E10 directions. The average dip is higher than 60in a dominantly westward direction (Figure 12). Theorientation of meso-scale fractures measured in the graniteon the borehole images and the cores was N175E30 withmore fractures dipping to the west (Figure 12).

If the orientation of the fracture zones is compared to themeso-scale fractures, a clockwise rotation of 10-15 can beobserved (Figure 12). However, the west dipping directiondominates in all cases.

Global fracture zone networkand mesofractures

Direction of the fracture zonesand mesofractures

A B

Figure 12: Synthesis of the orientation of fractures atdifferent scales in the Soultz granite. A- Poles offracture zones from the different wells (Green:EPS1, pink: GPK1, blue: GPK2, red: GPK3,


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purple: GPK4, black: 4550) superimposed on afrequency contouring diagram of themesofractures observed in the all Soultz wells (ingrey 7433 data). B- Rose diagram of the fracturezones in red (34 data) and mesofracturesobserved in the all Soultz wells in grey (7433data). (Schmidts projection, lower hemisphere,rose diagram: 10 class angle, contouring

diagram: 10%, 30%, 50%, 70%, 90% of themaximum frequency)

The fluid exit zones determined in the open hole sections ofGPK3 and GPK4 showed nearly vertical fracture zoneswith a dominating N-S set dipping westward as well as asecondary NW-SE set dipping eastward (Figure 13). Theseorientations fit with a slightly clockwise rotation with theglobal orientation of the meso-fractures measured in theopen hole sections (Figure 13). In this part of the granite,the main fracture set dips to the west.

In previous studies, these fracture zone concentrations wereinterpreted as the occurrence of large-scale normal faultzones related to the Rhine graben tectonics. However, the

directions of the fracture zone and the meso-scale fracturesare slightly different from the major fault orientationobserved in the sedimentary cover, which has a Rhenishdirection of N20E and corresponds to the graben openingat Oligocene. The granite contains numerous fractureorientations related to Hercynian and Alpine tectonics. Itseems that the N160E is an inherited direction, which wasreactivated during the graben tectonic activity. Thus, themain fracture zone orientation is rather different from thosenewly created in the sedimentary cover during theCenozoic.

Moreover, the average direction of the fracture zones israther consistent with the present-day stress field with H N169E14, one circular standard deviation (Klee andRummel, 1993; Valley and Evans, 2007). Some fracturezones which show flow anomalies or indications ofshearing appear as small fractures on the borehole imagelogs. This seems to indicate that critically oriented fracturesrelative to the stress state orientation could shear duringhydraulic stimulation (Evans, 2005).

GPK3 Open hole

A

GPK4 Open hole

B

Figure 13: A- Poles of the fluid exit zones superimposedon the frequency diagram of mesofractures (349data) in the open hole of GPK3. B- Poles of thefluid exit zones superimposed on the frequencydiagram of mesofractures (349 data) in the openhole of GPK4. (Schmidts projection, lowerhemisphere, rose diagram: 10 class angle,contouring diagram: 10%, 30%, 50%, 70%, 90%of the maximum frequency)

CONCLUSIONSThe characterization of fractures at different scales wasachieved along the Soultz wells based on direct (cores,cuttings) and indirect (borehole images, geophysical logs,

flow logs, temperature logs) methods. Among the thousandfractures detected and measured, a set of 39 fracture zoneswere highlighted intersecting the five boreholes of thegeothermal site as well as a peripheral well. This list is notextensive and could be completed using further dataacquisition or processing. In the open hole sections ofGPK3 and GPK4, 7 fluid exit zones were determined, and 9were characterized. In GPK2, some exit zones have been

detected, but no image logs are available to perform ageometrical characterization.

In a previous study, these fracture zone concentrations wereinterpreted as the occurrence of large-scale normal faultzones related to the Rhine graben tectonics, as shown inFigure 14. However, the direction of the fracture zone isslightly different from the major fault orientation observedin the sedimentary cover, which has a Rhenish direction ofN20E corresponding to the Oligocene graben formation.The granite contains fractures with numerous directionsrelated to Hercynian and Alpine tectonics. It seems that theN160E direction was an inherited and was reactivatedduring the graben tectonics. Thus, the main fracture zoneorientation is rather different from those purely created inthe sedimentary cover during the Tertiary. A 3D large-scalegeometrical model is being built to highlight therelationship between the major faults in the sedimentarycover and the fracture zone clusters. This is an on-goingwork based on an exhaustive geological database (Casteraet al. , 2008; Sausse et al. , 2008).

10km

0

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Baden-Baden

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Oligocene

Jurassic

Trias

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Fractured zonesin the granite

BA

Miocene,Pliocene et IV

Schematic cross-section from Le Carlier et al., 1994 Fractured zones in the granite from Dezayes et al., 2008

Figure 14: Schematic NW-SE cross-section (from LeCarlier et al., 1994) and location of the threefracture zone clusters in the Soultz well,represented with an average orientation

Knowledge and in situ characterization of fracture zonesalong the well have to be taken into account for a better

understanding of the lifetime of the geothermal reservoirduring early stages of hydraulic or chemical stimulationsand subsequent long-term hydraulic circulations. Due tosuch hydraulic tests, different thermo-hydro-mechanicaland chemical processes occur at depth in the reservoir andcould modify fluid pathways. In the vicinity of the re-injection well GPK3, a general cooling of the fractured rockmass associated with forced fluid flow provoking thermo-mechanical stresses is the dominant mechanism.Temperature reduction could also generate some chemicaleffects, inducing mineral dissolution or precipation, andthus strongly modifying the fluid pathway duringcirculation. Close to the production wells GPK2 and GPK4,pumping with submersible pumps could also induce somehydromechanical processes in the fractured rock mass. Allthese complex thermo-hydro-mechanical couplings willinteract with the major fracture zone system and must beseriously investigated using modelling to estimate reservoirlifetime (temperature drawdown, microseismicity risk,


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reservoir clogging). The fracture zone dataset studied in thispaper could be used to model the thermo-hydraulicmechanical behavior of the deep geothermal granitereservoir subject to hydraulic or chemical stimulations andhydraulic circulations (Rachez and Gentier, 2008).

ACKNOWLEDGEMENTS

This work was performed as a contribution to the EuropeanUnion's FP6 project 'Soultz EGS Pilot Plant' funded byADEME (EGS3D projet), BMU, EC and EEIG'Exploitation Minire de la Chaleur' and was supported bythe Swiss State Secretariat for Education and Researchunder project number 03.04.60. EHDRA working groupteams 6 and 7 are grateful for their contributions and thefruitful discussion.

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Baumgrtner J., Jung R., Grard A., Baria R., Garnish J.,(1996). The European HDR project at Soultz-sous-Forts: stimulation of the second deep well and firstcirculation experiments. Twenty-first WorkshopGeothermal Reservoir Engineering, StanfordUniversity, Stanford, California, USA, 267-274.

Baumgrtner, J., Hettkamp T; Teza D., Baria R; andMichelet S. (2004). Building of a Hot Dry Rockscientific power plant at Soultz-sous-Forts.Geothermal Resources Council Transactions, Vol. 28,201-206.

Bruel, D., (2002). Impact of induced thermal stress duringcirculation tests in an engineered fractured geothermalreservoir. Oil & Gas Science and Technology, RevueIFP, 57, no. 5, 459-470.

Castera J., Dezayes Ch., Calcagno Ph., (2008). Large-scale3D geological model around the Soultz site,Proceedings of the EHDRA scientific conference 24-25 September 2008, Soultz-sous-Forts, France.

Cocherie A., Guerrot C., Fanning C.M., Genter A., (2004).Datation U-Pb des deux facis du granite de Soultz(Foss Rhnan, France). Comptes Rendus Geoscience,336, 775-787.

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Dezayes Ch., Villemin Th., Genter A., Traineau H.,Angelier J., (1995). Analysis of fractures in boreholesof the Hot Dry Rock project at Soultz-sous-Forts(Rhine Graben, France). Scientific Drilling, vol. 5, 31-41.

Dezayes Ch., Villemin Th., Pcher A., (2000).Microfracture pattern compared to core scale fracturesin the borehole of Soultz-sous-Forts granite, Rhinegraben, France. Journal of Structural Geology, 22,723-733.

Dezayes Ch., Genter A., Gentier S., (2004). Fracturenetwork of the EGS Geothermal Reservoir at Soultz-sous-Forts (Rhine Graben, France). Geothermal

Resources Council Transactions, Palm Springs,California, USA, Vol. 28, 213-218.

Dezayes Ch., Valley B., Maqua E., Syren G., Genter A.,(2005). Natural fracture system of the Soultz granitebased on UBI data in the GPK3 and GPK4 wells.EHDRA Scientific Conference, Proceedings of theEHDRA scientific conference 17-18 March 2005,Soultz-sous-Forts, France.

Dezayes Ch., Genter A., (2008). Large-scale fracturenetwork based on Soultz borehole data. EHDRAScientific Conference, Proceedings of the EHDRAscientific conference 24-25 September 2008, Soultz-sous-Forts, France.

Dorbath L., Cuenot N., Genter A., Frogneux M., (2009).Seismic response of the fractured and faulted granite tomassive water injection at 5 km depth at Soultz-sous-Forts (France), Geophysical International Journal,doi: 10.1111/j.1365-246X.2009.04030.x

Evans K.F., (2000). The effect of the 1993 stimulations ofwell GPK1 at Soultz on the surrounding rock mass:

evidence for the existence of a connected network ofpermeable fractures. World Geothermal Congress2000, Kyushu - Tohoku, Japan.

Evans K.F., (2005). Permeability creation and damage dueto massive fluid injections into granite at 3.5 km atSoultz: Part 2 - Critical stress and fracture strength,Journal of Geophysical Research, 110, B04204, 14 pp.

Evans K.F., Genter A., Sausse J., (2005). Permeabilitycreation and damage due to massive fluid injectionsinto granite at 3.5 km at Soultz: Part 1 - Boreholeobservations, Journal of Geophysical Research, 110,B04203, 19 pp.

Foehn J.P., (1985). Interprtation des campagnes sismiques

1981 et 1984, concession de Pchelbronn, permis deHaguenau. Total Exploration internal report, October1985.

Genter A., (1989). Gothermie Roches Chaudes Sches : legranite de Soultz-sous-Forts (Bas Rhin, France).Fracturation naturelle, altrations hydrothermales etinteraction eau - roche. PhD thesis, Universitd'Orlans, France, 201 pp.

Genter A., Traineau H., Dezayes Ch., Elsass P., LedsertB., Meunier A., Villemin Th., (1995). Fractureanalysis and reservoir characterization of the graniticbasement in the HDR Soultz project (France).Geothermal Science & Technology, vol. 4 (3), 189-

214.Genter A., Castaing C., Dezayes Ch., Tenzer H., Traineau

H., Villemin Th., (1997). Comparative analysis ofdirect (core) and indirect (borehole imaging tools)collection of fracture data in the Hot Dry Rock Soultzreservoir (France), Journal of Geophysical Research,vol. 102, B7, 15419-15431.

Genter A., Castaing Ch., (1997). Effets d'chelle dans lafracturation des granites; Scale effects in the fracturingof granite. Comptes Rendus de l'Acadmie desSciences, Serie II. Sciences de la Terre et des Plantes325(6): 439-445.

Genter A., Homeier G., Chvremont Ph., Tenzer H., (1999).Deepening of GPK-2 HDR borehole, 3880-5090 m(Soultz-sous-Forts, France). Geological monitoring.Open file report BRGM/RR-40685-FR, 81 pp.


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Documents

Overview of the Fracture Network at Different Scales Within the Granite Reservoir of the ECS SOULTZ SITE