The significance of hydrothermal alteration zones for the mechanical behavior of a geothermal reservoir
© Meller and Kohl; licensee Springer. 2014
Received: 12 June 2014
Accepted: 11 September 2014
Published: 16 October 2014
The occurrence of hydrothermally altered zones is a commonly observed phenomenon in brittle rock. The dissolution and transformation of primary minerals and the precipitation of secondary minerals affect rocks in terms of mechanics, stress conditions, and induced seismicity.
The present study investigates commonly observed phenomena of hydrothermal alteration and observations at the geothermal site of Soultz-sous-Forêts, which are related to the occurrence of hydrothermal alteration. Geomechanical observations at Soultz are interpreted on the basis of synthetic clay content logs, which are created from borehole logging data, and which identify clay in hydrothermally altered zones.
It is shown that hydrothermal alteration results in a reduction of the frictional strength of the reservoir rock. Weak zones can act as stress-decoupling horizons, which locally perturb the stress field and affect the evolution of the microseismic cloud. For the first time, it is shown on a reservoir scale that large magnitude seismic events are restricted to unaltered granites, whereas in clay zones, only small magnitudes are observed. It is demonstrated that clay-rich zones foster the occurrence of aseismic movements on fractures.
Secondary mineral precipitation during hydrothermal alteration has a great effect on the geomechanical properties of a geothermal reservoir. The identification of such zones is a first step towards understanding the relation between alteration and mechanical processes inside a reservoir and can help in reducing induced seismicity during hydraulic stimulation of a reservoir.
The importance of clay zones for the geomechanical structure and the earthquake mechanics in brittle rock became an important issue in the framework of mitigation studies of natural and man-made disasters (Holmes et al. ). A strong focus was given to hydrothermal alteration in crystalline rock and its effect on mechanical friction. Recent studies on the San Andreas Fault revealed the significant impact of clay inside faults and fractures on their mechanical and hydraulic properties. Faults and fractures are target zones for enhanced geothermal systems (EGS), as they provide pathways for geothermal fluids. In terms of mitigation of induced seismicity, while increasing the permeability of the geothermal reservoir, detailed understanding of hydraulic and mechanical processes of fractured rock is the key for the success of an EGS project.
The significance of clay for geothermal projects
The development of EGS in low-enthalpy regions like the Upper Rhine Graben in central Europe involves the application of hydraulic stimulation for permeability enhancement in the geothermal reservoir. Mostly located near residential areas, there is a claim for safety and controllability of the geothermal technology from the public. In the past, people were concerned by the occurrence of small perceptible earthquakes, caused by stimulation activities or during operation of geothermal power plants like the magnitude ML = 3.4 earthquake in Basel in 2006 (e.g., Häring et al. ), the ML = 2.9, ML = 2.5, and ML = 2.3 in Soultz-sous-Forêts in 2003, 2000, and 2004 (Dorbath et al. ), respectively, or the ML = 2.4 and ML = 2.7 earthquakes near Landau in 2009 (Groos et al. ). The injection of fluid into the underground changes the effective stress, thus inducing slips on fractures and faults associated with seismic events in brittle rock. In order to predict or even control the seismic behavior of a geothermal reservoir, the geomechanical structures and the associated processes must be known.
In fresh and homogeneous rock, the relation between stress and mechanical failure is commonly described by the Mohr-Coulomb criterion (Scholz ) with flow through fractures to be characterized as sublaminar by the Darcy flow (Sausse ). In geothermal reservoirs, however, the percolation by geothermal brine promotes the formation of hydrothermally altered zones around fluid pathways. The dissolution of primary rock-forming minerals and the precipitation of secondary minerals like quartz, clay, or carbonates change the in situ conditions with respect to mechanical strength of the rock. In such zones, simple models might no longer apply, and the reservoir behavior is difficult to assess. Evidence, that simple rock mechanical models no longer account during and especially after the shut-in of hydraulic stimulation, has been only recently highlighted by Schoenball et al. () who demonstrated a change in the stress regime during stimulation.
Several studies demonstrate the relation between geomechanics, earthquake characteristics, and the weakness of rocks on a crustal and regional scale. For geothermal projects, however, the geomechanical properties of a reservoir are to be known on a very local scale in the order of several meters. The size of hydrothermal alteration zones can range from millimeters to several kilometers. In order to characterize a geothermal reservoir and to assess its geomechanics, it is important to understand the significance of such alteration zones. Therefore, it is necessary to know and to understand, if and how large-scale geomechanical rules and observations can be transferred to the reservoir scale.
The present paper conducts an investigation on the significance of hydrothermal alteration in the granite of the geothermal site in Soultz-sous-Forêts (France) and the change of its mechanical parameters. The basis of the analyses is synthetic clay logs, which are created from spectral gamma ray logs using a technique introduced by Meller et al. ([2014a]). These logs are indicative of the occurrence of clay-bearing fractures along the boreholes. The newly derived results are investigated under the light of the existing geomechanical interpretation, which is summarized in the 'Current state of research on the role of clay in fault zones' subsection.
Current state of research on the role of clay in fault zones
Clay minerals, which are a main product of hydrothermal alteration (e.g., Meunier ; Velde ), sometimes have very low friction coefficients of approximately 0.3 (e.g., Morrow et al.  and references herein, and c.f. Figure 1b). The frictional properties of clay minerals, however, strongly depend on their structure and water content. Therefore, it is not easy to estimate the frictional properties of clay-filled faults (Moore and Lockner ). Many studies have been conducted on the relationship between the nature of fracture fillings and fault mechanics. Zoback et al. () and Kohli and Zoback () investigated the relationship between clay content and the mechanical friction of shale gas reservoir samples under wet conditions. They observed a linear decrease of the friction coefficient with increasing clay content (Figure 1a) from 0.8 with 10 wt.% clay to 0.4 at approximately 50 wt.% clay. Similar results have been obtained by Tembe et al. () for artificial clay gouge samples of quartz and illite and for natural soil samples tested by Akayuli et al. (). The friction coefficients they measured for different clays vary and are much lower than those of other minerals like quartz or feldspars (Figure 1b).
The rupture behavior of a fault from the Dieterich-Ruina constitutive model (Ruina ; Dieterich ) describes the frictional evolution of a fault for different sliding velocities with the material parameter (a-b) representing the difference in steady-state friction. It indicates stable sliding of fault surfaces during slip if (a-b) > 0 or unstable sliding if (a-b) < 0. The synonyms for stable and unstable sliding are velocity-strengthening and velocity-weakening behavior, respectively. The effect of clay on the rupture behavior of faults has been studied by many laboratory experiments. Ikaris et al. () found experimental evidence for the relationship between the weakness of rocks and their frictional stability: rock samples with a low friction coefficient show velocity-strengthening behavior, whereas samples with high friction coefficients show velocity-weakening behavior (Figure 1). This indicates the occurrence of brittle failure only on rocks with high friction coefficients. Zoback et al. () observed experimentally on shale gas samples that faults with clay contents higher than 30% slide stable (i.e., (a-b) > 0), whereas faults with a lower clay content slip unstable (i.e., (a-b) < 0, Figure 1c). They reasoned that such clay-rich faults slide aseismically, whereas the faults with lower clay contents produce microseismic events. The dataset of Tembe illustrates a dependence of (a-b) of illite-quartz samples on the illite content. For these samples, no velocity-weakening behavior is observed. The reason for this is that quartz can behave both velocity strengthening and velocity weakening, and under the experimental conditions, it was velocity-strengthening (a-b) > 0 (Figure 1c), but nevertheless the effect of the clay proportion of the samples on (a-b) is significant.
As the frictional properties of rocks determine their slipping behavior, a correlation between the weakness of the rocks and the occurrence of large and small earthquakes is expected. The so-called b-value, which is derived from the Gutenberg-Richter law (Gutenberg and Richter ), describes the proportion of small relative earthquakes to large ones. A b-value of 1 represents a logarithmic relationship between the magnitude of events and their frequency, whereas b-values >1 reflect an increased number of small earthquakes. High b-values are expected in areas where no large differential stress can build up. Schorlemmer et al. () compared the results of numerous earthquakes from different settings and of laboratory data. They found that the b-value differs systematically with the faulting regimes. The highest b-values are found in normal faulting regimes (up to 1.2), whereas the lowest b-values occur in thrust events (as small as 0.6), and strike-slip events are in between. Based on the stress prevailing in the respective regimes, Schorlemmer et al. () concluded that the b-value inversely correlates with differential stress levels. This was also confirmed by laboratory experiments performed by Amitrano () who observed a decreasing b-value with increasing differential stress. Creeping fault sections show very high b-values of around 1.3 (Schorlemmer and Wiemer ). Based on these results, the occurrence of small events and aseismic movements in strongly altered and fractured areas is expected rather than large earthquakes. This assumption has also been proposed by Heinicke et al. () who investigated the correlation between hydrothermal alteration and the occurrence of earthquake swarms. They observed in the Vogtland region of northwestern Bohemia that in addition to increased pore pressure and shear stress, the mechanical weakening of the rocks and the dissolution of fracture walls play an important role for the evolution of earthquake swarms. Interestingly, the maximum magnitude of such earthquake swarms is limited to 5 (Heinicke et al. ), which supports the theory of only small earthquakes occurring in regions with rocks of low friction coefficients. When analyzing b-values, one has to consider that this value is affected by numerous parameters, not least by the way it is computed. Besides the strength of the rock, the main affecting parameters are the stress field, the focal mechanism of the earthquakes, and the presence of large geologic structures (Scholz  and references herein). In geothermal reservoirs, large variations of b-values in time and space have for example been observed by Bachmann et al. (). They calculated the b-value for the time period during injection and after injection. The b-values varied from 1.58 during injection to 1.15 after injection, which represents a larger proportion of small earthquakes during injection.
Dorbath et al. () calculated a b-value of >1.2 for the stimulations of the well GPK2 at Soultz, whereas for the well GPK3, which is a maximum 500 m away from GPK2, was determined to be 0.9. They related this behavior to the presence of large fault zones in the vicinity of the well, which dominate their seismic evolution.
The Soultz geothermal site
The sealing of fractures by secondary minerals and the transformation of silicates into clay minerals affected the hydraulic and mechanical properties of the rock (Valley and Evans ; Charléty et al. ), whereas the details of such processes are still subject to extensive research. Bartier et al. () highlighted for example the importance of clay mineralogy for the permeability of the Soultz granite, which is reduced by illite precipitation but enhanced by tosudite precipitation. Ledésert et al. () highlighted the complexity of processed linked to porosity/permeability formation and decrease by the dissolution and transformation of primary minerals and the formation of new minerals. The type and structure of clay minerals are not only important for the evolution of porosity and permeability but also for the shearing properties of a fault filled with clays.
The variation of hydro-mechanical properties of the rock with different alteration types and grades makes it important to first detect alteration zones and, second, to understand their significance for the performance of a reservoir (Figure 2).
The basis for the rock mechanical studies are neural network-derived synthetic clay content logs (SCCL), which present the clay content along the borehole in a semi-quantitative way with five groups of increasing clay content. In sedimentary rocks, clay minerals can be easily identified from peaks in spectral gamma ray (SGR) logs. In crystalline rock in contrast, apart from clays, numerous other minerals contain radioactive isotopes, which makes it difficult to identify clay minerals on SGR logs. Therefore, a neural network is used, which makes it possible to identify different signal patterns on logging data and to localize the clay-bearing zones. The resolution of the resulting SCCLs is on the scale of decimeters.
The SCCLs allow discriminating between zones of high and low clay contents. Whereas the upper parts of all wells are characterized by high SCCL values, representing the paleo-alteration surface, the lower parts are very different for the five wells. Intervals with high SCCL are mostly found around fractures, which have been identified as permeable on flow logs, but hydrothermal alteration also occurs away from such fractures. However, not all permeable fractures are located in altered zones. This might be due to the fact that extreme alteration leads to a clogging of fractures with clay minerals, thus reducing its permeability (Sausse ). The actually flowing fractures might not have been permeable in the past, which prevented the surrounding rock from being hydrothermally altered. Increased clay content is seen at the bottom of the wells below 4,600 m at the transition between the porphyritic and the two-mica granite. For details of this neural network method, the SCCLs, and the calibration of the logs by magnetic mineralogical investigations refer to Meller et al. ([2014a]; [2014b]) (Figure 3).
For the deep wells in Soultz, no core material is available. Therefore, petrophysical and geologic parameters can only be derived from borehole measurements and seismicity catalogs. This study is mainly based on breakout and fracture analyses conducted on borehole image logs and on a catalog of seismic events recorded during hydraulic stimulation. Borehole breakouts are enlargements and elongations of a borehole in a preferential direction and are formed by spalling of fragments of the wellbore during drilling. They generally form parallel to minimum horizontal stress, and their formation is facilitated in weak wall rocks (Babcock ). Their analysis can therefore provide information about the orientation of the stress field and on the mechanical properties of the penetrated rock. Seismic events induced during stimulation are an indication of structures in the geothermal reservoir. Their analysis provides indications about the stress state, fracture orientation, rock mechanics, and fluid pathways.
Results and discussion
Impacts of hydrothermal alteration on rock mechanics
These experimental results are in agreement with the breakout observations at Soultz, which indicate weakness of the hydrothermally altered zones, but which are in contrast to the high minimum friction coefficient of 0.81 determined by Cornet et al. () for the whole granitic rock mass. It is therefore assumed that hydrothermal alteration causes a variation in the frictional properties of the Soultz granite on a meter scale with higher frictional strength in unaltered rock and a lower frictional strength in altered rock.
Elastic properties of the Soultz granite have been experimentally studied by Valley and Evans (). They selected samples of different alteration grades from the EPS1 core and measured the uniaxial compressive strength (UCS) of the core pieces. Furthermore, they measured the S- and P-wave velocities of the samples in order to determine their E-moduli. They found an inverse correlation between alteration grade and UCS and the E-modulus of the samples (Figure 5). From the results of this study, it is expected that the highly altered clay zones affect the frictional properties in Soultz and the friction coefficient is not uniform but is lowered by hydrothermal alteration.
Recent researches showed that a characteristic of such weak zones is that they can fail at low stress levels, as it is for example observed on a large scale on the San Andreas Fault in a strike slip regime, whose slip direction deviates 70° from the maximum horizontal stress (e.g., Boness and Zoback ), the Zuccale normal fault on Elba (e.g., Smith et al. ) or some normal faults at the eastern side of the Sea of Japan (e.g., Sibson ; Faulkner et al. ). If such observations can be transferred to the reservoir scale, hydrothermally altered zones might fail at lower stress levels than the surrounding intact rock mass. This is especially important in terms of hydraulic stimulation, as weak faults could shear at much lower stimulation pressure than unaltered rock and influence the evolution of induced seismicity (Figure 4).
Impacts of hydrothermal alteration on the stress field
Clay layers inside rock masses give rise to large contrasts of mechanical properties. In contrast to intact crystalline or sedimentary rock masses, weak clay-rich zones cannot establish large differential stress (Zoback and Harjes ).
with Pp the pore pressure and z the depth in meters.
The SH orientation is approximately north-south, and the vertical stress SV is equivalent to the overburden. However, in inhomogeneous rock masses with changing mechanical properties, the magnitude and orientation of the stress field change at the transition between layers of different mechanical strength. The Soultz granite is very heterogeneous due to its porphyritic structure, its lithological variations, hydrothermally altered zones, and the profound fracturing. Borehole breakouts generally form in the direction of the minimum horizontal stress and are therefore useful indicators of the orientation of Sh and SH. An analysis of borehole breakouts can give evidence about local stress variations. The high resolution of the SCCLs in the order of decimeters for the first time allows a detailed analysis of the indications for stress field variations at Soultz on the basis of breakouts. In the following section, the occurrence of breakouts and their orientation is interpreted on the basis of the SCCLs.
Valley and Evans () analyzing breakouts in the well GPK1 between 2,840 and 3,510 m found an increased breakout concentration at the top of this interval. This agrees with the occurrence of a clay-rich interval in this section indicated by high SCCL (Figure 6). The mean SH orientation determined from breakouts is 0° ± 19°, which is in agreement with the mean orientation of the structures of the microseismic cloud. Excursions of the mean breakout orientation occur in the intervals 2,890 to 2,950 and 3,300 to 3,350 m, which are characterized by high SCCL values. The occurrence of breakouts in GPK1 between 2,960 and 3,500 m is not only restricted to high-clay zones but high breakout-densities as, for example, at 3,000 to 3,050 m or at 3,400 to 3,450 m depth also occur, when a depth interval without or with very little clay is followed by a very clay-rich interval. Here, the contrast of elastic moduli of the two depth intervals might cause a cumulating appearance of breakouts. This might also be represented in the different orientations of the microseismic cloud in the depth intervals 2,700 to 2,900 m, where it is oriented north-south, and 3,200 to 3,600 m, where its azimuth is 145° to 160° (Cornet et al. ). Cornet and his colleagues () linked this orientation deviation to the higher pore pressure above 2,900 m, but it could also be related to the presence of clay-rich zones. Such clay-rich zones could also lead to increased pore pressures (Wu ).
Similar analyses have been conducted by Langenbruch and Shapiro () who investigated stress states in boreholes from different regimes. Based on sonic logs, they created a model of the in situ elastic moduli to calculate the spatial distribution of in situ stress within a rock mass. Their large spatial variations of the stress regime suggest that linear stress models are not sufficient for Coulomb failure within a rock mass. Economides et al. () observed that within sedimentary formations, the vertical gradient of the minimum horizontal principal stress does not vary linearly with depth. The authors found that elastic heterogeneity has a significant influence on stress magnitudes, which vary by up to more than ±20% of the externally applied stresses. Cornet and Roeckel () observed this phenomenon in limestone layers of the Paris Basin and in the North German Basin. They saw that the local stress magnitudes are not linearly increasing with depth, and they saw variations of approximately 15° in the stress directions. In contrast to Langenbruch and Shapiro () and Economides et al. (), they assume that the stress magnitudes are controlled by the creeping characteristics of the various layers rather than by their elastic characteristics (Cornet and Roeckel ).
The change of the local stress field in magnitude and orientation has previously been described for large fracture zones (e.g., Brudy et al. ). In the San Andreas Fault, for example, a stress rotation of approximately 28° with respect to the stress field of the rigid crust has been measured (Chéry et al. ). Cornet and Roeckel () identified soft layers as decoupling layers introducing decoupling of stress fields in the layers above and below these layers. This was also observed by Meixner et al. () who documented a rotation of the maximum horizontal stress in different facies along the Bruchsal geothermal wells (c.f. Figure seven in his article).
However, in those studies, stress field variations are only observed on large scales of several kilometers. The analysis of breakouts on the basis of SCCLs provides indications that changes of the stress field both in magnitude and orientation of the principal stress can also be induced by small-scale soft alteration zones on the meter scale as observed in geothermal wells. Taking these observations into account, it is obvious that the estimation of mechanical properties on the basis of a linear stress field can only provide far field values, especially for zones, where the SCCL is high. So, in addition to the frictional parameters, the exact orientation of the stress field has to be constrained in hydrothermally altered zones in order to be able to assess their mechanical characteristics.
The impacts of hydrothermal alteration on induced seismicity
At Soultz, 20 hydraulic and chemical stimulations have been performed and large catalogs of seismic events are available (Genter et al. ). During hydraulic stimulation, large amounts of water are injected into the geothermal reservoir in order to increase the pore pressure prevailing in the reservoir rock. If the pressure increase is large enough to overcome the frictional stability of fractures, shear movements are induced, which can be observed by the occurrence of microseismic events. A detailed summary of the background of hydraulic stimulation can, for example, be found in Economides et al. () or Majer et al. ().
The parameters influencing the evolution of induced seismicity like the pressure of the fluid, the ambient stress field, the orientation of fractures, hydraulic properties, and the frictional characteristic of rock can be affected by hydrothermal alteration. Herein, the relation between hydrothermal alteration and induced seismicity at Soultz is investigated.
The correlation of aseismic movements with clay-rich intervals and their orientation at a significant angle to SH supports the assumption of clay acting as some kind of lubricant on the fault zones. This makes these fractures prone for aseismic shearing, although they are not optimally oriented in the present stress field. Aseismic movements are assumed to take a big share of the movements induced during hydraulic stimulation, and some authors even assume that the major part of shearing happens aseismically (e.g., Schoenball et al. ; Bourouis and Bernard ). Further evidence for aseismic movements in Soultz in GPK1 (Bourouis and Bernard ; Schmittbuhl et al. ; Schmittbuhl et al. ), GPK2 (Schoenball et al. ; Calò et al. ), and GPK3 (Calò et al. ; Nami et al. ) underlines the significance of clay on the structural reservoir evolution of the reservoir (Figure 8).
Therefore, the presence of large faults is most probably not the only reason for the different seismic behaviors of GPK2 and GPK3 as it was observed by Dorbath et al. (). The different b-values, which can be obtained from the seismic events induced during stimulation of these wells, could also be affected by the presence/absence of alteration zones (Figure 9).
The present SCCL method is an important basis to localize clay-rich zones as target zones for hydraulic stimulation and to identify fractures as candidates for aseismic movements. In order to optimally use the properties of hydrothermally altered zones, further effort has to be done on understanding of the processes affecting the geomechanical behavior of a geothermal reservoir. Once such processes are understood, it might become possible to exploit the properties of altered zones in order to increase the reservoir performance, while mitigating perceptible seismicity.
The occurrence of hydrothermally altered zones inside a geothermal reservoir can have large effects on many physical aspects, which are important for the performance of a geothermal system, and especially those related to induced seismicity. The observations at Soultz-sous-Forêts revealed that hydrothermal alteration lowers the mechanical strength of the Soultz granite and its fractures, which results in an inhomogeneously distributed friction coefficient. Geological units with low mechanical strength promote the occurrence of breakouts and can rotate the stress field as much as 90° from the mean orientation, which is indicated by high breakout-densities in clay-rich intervals and a deviation of their mean orientation.
A major result of this study is that hydrothermally altered zones can act as decoupling horizons, which change the local stress regime and thus significantly affect the seismicity induced during hydraulic stimulation at Soultz. It has been shown that large seismic events are restricted to fresh granite, whereas only small seismic events occur in clay-rich intervals. While this behavior has often been observed on the crustal scale, the present study for the first time confirms this effect on the scale of a geothermal reservoir.
Due to their low frictional strength and increased pore pressures, hydrothermally altered zones represent major target zones for hydraulic stimulation. In the future, EGS projects need to be structured in that prevention of large seismic events becomes a major achievement. Future stimulations could foster the creation of aseismic instead of seismic slip to increase the reservoir permeability, which requires knowledge on the location of such zones and advanced research towards the evolution of aseismic movements.
CM interpreted the SCCL logs on the basis of borehole breakout analyses by Sahara et al.  and of the thesis of Valley  and on borehole analyses mainly conducted by Evans , Evans et al. a; b] and Cornet et al. . Basis for the analysis were the seismic catalogues of GPK1 Jones et al.  and GPK3 Dorbath et al. . CM wrote the manuscript and TK conducted the final revision. All authors read and approved the final manuscript.
This research was conducted within the portfolio topic GEOENERGIE of the Helmholtz Association of German Research Centres and was funded by Energie Baden-Wuerttemberg (EnBW), Germany. Thanks are given to GEIE Exploitation minière de la chaleur for providing the Soultz borehole data.
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