Modelling heat extraction from forced fluid flow through stimulated fractured rock masses: evaluation of the soultz-sous-forets site potential

Geothermics Vol. 24, No. 3, pp. 439-450, 1995 Copyright © 1995 CNR Elsevier Science Ltd Printed in Great Britain. All rights reserved 0375-6505/95 $9.50 + 0.00

Pergamon

0375-6505(95)00019-4

MODELLING

HEAT EXTRACTION FROM FORCED FLUID FLOW

THROUGH STIMULATED FRACTURED ROCK M A S S E S : EVALUATION OF THE SOULTZ-SOUS-FORETS SITE POTENTIAL D. BRUEL Centre d'Informatique G~ologique, Ecole Nationale Superieure des Mines de Paris 35 rue St Honor& 77305 Fontainebleau Cedex, France Abstract - This paper presents the results of heat extraction modelling for a prototype Hot Dry Rock (HDR) system at the Soultz-sous-Forets site, France. The significant data from the 1993 experimental HDR programme at the Soultz site are summarised. Geological field evidence indicates that two major fault structures are superimposed on the four natural fracture sets used in the fracture network model. The generation of independent fracture networks allows the creation of a picture of the intersection of fractures within the borehole, which is reasonably realistic. Numerical modelling of the hydraulic tests carried out in the deepest well (GPK1) is extrapolated to predict the hydraulic behaviour in a two-well system, incorporating the present deep well, and a second deep well (GPK2) which is to be drilled in 1994/95. This allows the thermal performance of the planned system to be modelled, and indicates excessive thermal drawdown in the first six months of circulation, possibly as a result of preferential flow paths developed along a main fault. Confidence in the model will only be improved when experimental data from a circulating system are available to test the predictions, and a better understanding of the hydrogeologic system has been obtained. At this stage, the extrapolations to a two-well system should be considered only as demonstrative guidelines for further investigations. Key words: Geothermal reservoirs, modelling, hot dry rock, Soultz

INTRODUCTION: THE EXPERIMENTAL PROGRAMME AT SOULTZ Hot Dry Rock (HDR) investigations on a scale relevant to pre-industrial feasibility analyses are currently in progress at the Soultz-sous-Forets site, Alsace, France. A second deep borehole is to be drilled in this European HDR programme, and this paper is a contribution to the use of models to plan the location and inclination of this borehole with respect to the first deep borehole (GPK1). The model used in this study is of the realistic geometry type (Willis-Richards and Wallroth 1995) where numerous stochastically generated realisations of the fracture network are used to simulate the hydraulic and thermal behaviour during circulation. For a detailed explanation of the modelling approach discussed in this paper the reader is referred to Bruel (1995). This introduction summarises the most significant available data derived from the 1993 experimental programme performed at Soultz, concerning the fracture network, the stress regime, the hydraulic behaviour of the tested openhole sections and the recorded induced seismic activity. These are compiled from the latest Soultz field report edited by Baria et al. (1994), and will be used as the basis for the reservoir modelling presented. A brief summary of the Soultz project is given in Jupe et al. (1995). Given a geometrical description of the fracture network, the available well test data will be used to calculate head distributions in a forward modelling problem. This process involves some calibration 439

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effort, but it is hoped that the resulting model will be of value in predicting flow for the next phases of the Soultz programme. Fracture network

The existing fracture data base comes from the geophysical logs conducted in the deepened part of GPK1 borehole (c. 3500m depth), which is combined with the analysis of 810m of core, extracted during the drilling of borehole EPS1, a 2200 m deep borehole, approximately 500m South of GPK1. Borehole EPS1 was drilled on an inclined trajectory with regard to the vertical, thus allowing better sampling of near vertical joints. Genter et al. (1992) investigate the spatial distribution of the joints and point out, in both boreholes, correlations with hydrothermalized zones of higher porosity, in which joints are clustered. A similar discussion based on a fractal unidimensional analysis of the fracture locations is given by Ledesert et al. (1993), who conclude that there is a necessity to identify and include in any reservoir characterisation, the highly fractured zones that have developed wide alteration zones. Accordingly, the general fracture pattern consists of two main strike directions, respectively N 10-20E and N170. Two fractures of larger scale cross GPK1 at approximately 2800m and 3500m depth. Other minor altered zones have also been located. A third and a fourth directional fracture set striking N110 and N140 respectively have also been observed. In both wells, fractures appear sub-vertical, with dip angles of up to 30 ° from vertical. Fracture density can be very high (10m-l), but most of the joints are sealed. Flow logs and thermal logs do not indicate more than 10 hydraulically active fractures per kilometre of borehole length. Stress distribution In situ stress measurements pertormed on pre-existing fractures (Baria et al. 1994) indicate that the

amplitude of the stress components (MPa) for depths ranging between 2000m and 3600m can be approximated by the following linear relationships, where z is depth in metres. Minimum horizontal stress: Maximum horizontal stress: Vertical stress:

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= 48.0 + 0.014(z-3600) = 0.026z = 0,026z

The orientation of this stress tensor remains uncertain, but experimental evidence (Cornet et al., 1994) indicates that the maximum horizontal component to be oriented in the N170 direction. ttydraulic properties derived j?om stimulation tests

The rock mass surrounding the GPK1 borehole was extensively tested during the autumn of 1993 (Baria et al. 1994). These tests included a natural production test, as well as various injections (at injection rates of 31/s to 501/s) over different openhole sections. It should also be noted here that the injected fluid was fresh surface water, whereas the natural formation fluid is brine with an average density of around 1.07. The main part of the 1993 injection tests consisted of a three week injection at flowrates varying from 61/s to 381/s in a step wise manner. The corresponding pressure record is given in Figure 1. This experiment was conducted over a 550m long sanded off section of GPK1 (2850m to 3400m). An interesting outcome of this testing sequence was that a pressure level of approximately 9.5MPa was reached with a flow rate of 181/s, which was not significantly exceeded even with double the flowrate (Figure 1)~ Throughout the 1993 experiments injection pressure levels never exceeded 10.5MPa. After shut-in and venting phases, a second important injection test took place over the total openhole section of GPK1 (2850m to 3600m), in order to evaluate the relative hydraulic behaviour of the lower part which had been previously isolated. Although injection flowrates as large as 501/s were achieved the maximum injection pressure stabilised at around 10MPa.

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Fig. 1. Injection pressure (bars) and flow (I/s) history during the 1993 Phase 1 injection tests at Soultz-sous-Forets (September 1993). The peak injection pressure does not exceed 105 bars (10 5MPa), for a flowrate of approximately 381/s. The analysis of the flow logging carried out during the injections showed five different active levels, the most important of which was located at the top of the openhole section of GPK1 (c. 2850m). Moreover, this last zone appeared to become more and more active when the pressure level increased to the level of the minimum effective stress (jacking pressure) for sub-vertical fractures intersected at this depth range. As a consequence, around 60% of the injected flow left the GPK1 borehole in the first 100m to 150m of openhole below the casing shoe. The bottom section of openhole (i.e. a fractured zone located at 3500m) took about 10% of the injected fluid during the second series of injection tests. The contribution of other fractured hydrothermalised zones, at 3090m, 3200m and 3330m, to the flow venting at the final injection rate, was about 3 to 61/s each. The experimental programme is ongoing at the Soultz site, and in 1994 further hydraulic tests were performed along the total openhole section of GPK1 (Gerard et al., 1994) in order to evaluate the hydraulic improvements resulting from the 1993 test programme. The preliminary results of this 1994 work indicate that the natural productivity of the borehole has been drastically increased, as proven by a steady artesian flow in excess of 81/s, approximately 1 year after the initial 1993 injection tests. Further injection tests performed at the rate of 181/s only achieved an over pressure of 3.5MPa, compared to the 9.5MPa in 1993. However, injection flow profiles obtained from logging were nearly identical as those previously observed in 1993.

Induced microseismic activity Induced shear failure can be initiated along fracture planes when a shear stress exists on them and when the supported normal stress is sufficiently lowered, for instance by an increase of the fluid pressure. When such conditions are met, it is generally accepted that the failure and sliding motion generates seismic energy. The spatial distribution of the induced microseismic event hypocentres can be associated with the propagation of the fluid pressure front. It is also expected that hydraulic properties of sheared joints are improved, as a consequence of an irreversible dilation effect. Microseismic networks and accurate location algorithms are therefore vital in delineating geological structures and constraining estimates of the rock volume associated with the induced microseismicity. Whether or not this rock volume can be considered as the HDR reservoir remains open to some discussion, but taking into account the prevailing stress state, site specific jointing patterns, and assuming classical failure

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criteria, there seems little doubt that some microseismic activity can occur even with very limited overpressures. The general shape o f the recorded microseismic cloud at Soultz is illustrated by Figure 2. In the

vicinity of the upper part of the borehole, the events extend in a roughly NS direction, but with a predominant propagation to the North. Deeper and farther away from the borehole, the cloud is more diffuse and some subsets of events indicate NNW-SSE striking elongated features. During the second 1993 stimulation phase, events were generated in the bottom part of the well, migrating with a SSE tendency, while some seismicity was again generated inside the previously activated rock volume.

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GEOMETRICAL PARAMETERISATION OF THE GPK1 AREA The following section summarises how the data presented in previous sections is combined to produce a 3D modelled reservoir volume for the Soultz site. Details of the model and modelling methodology are presented elsewhere by Bruel (1995). The modelled volume of rock is a parallelepiped with dimensions of 1.8kin by 0.6km by 1.2km, along the North-South, East-West and vertical directions, respectively. The bottom of this working box is located at the real depth of 3600m. The hydraulically active fractures forming the modelled network combines randomly distributed joints of finite extension and some faults with a deterministic location (obtained from field observations). According to in situ measurements four fracture sets are considered, which are described in Table 1. As suggested by geological field evidence, two major structures must be superimposed on the fracture networks. Both strike in the N170 direction with a 70 ° dip to the west. The first one is represented by an effectively infinite plane cross-cutting the GPK1 borehole at 2800m depth, which is above the openhole section where the hydraulic tests were performed. The second one intersects the well at 3500m depth and its extension is fixed at 500m, so that its area remains bounded and of the order of lkm 2 (Baria et al., 1994). Table 1: Statistical parameters describing the four main fracture sets. The density is expressed by the number of fracture centres/unit volume, the size distribution follows a log normal law of mean ~t and standard deviation ~, the average fracture radius is then exp(~t + 1/2t~2) (approximately 25m) Set

Azimuth

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It is acknowledged that the figures given above are very poorly constrained, and this is particularly true for the size and extension parameters. Nevertheless, an posteriori verification can be carried out to check the relevance of this parameter distribution. To this end, we generate 50 equiprobable stochastic fracture networks, simulate a lkm long borehole passing through these networks, count the number of fractures intersected by the borehole and then calculate the separation between each pair of consecutive fractures intersecting this particular borehole. These can then be compared with field observation. The influence of the drilling inclination to vertical within such fracture networks is then investigated. The results provided by the 50 independent alternatives are as follows; the fracture density observed along the scan line is found to be 0.0104m -1 for vertical boreholes, and 0.0134m -1 and 0.0158m -1 for boreholes drilled in an N90 azimuth, with 20 ° and 30 ° deviation from vertical, respectively. A picture of the joints located along the 50 independent vertical boreholes is given in Figure 3. The statistics of separation distances for vertical and inclined boreholes are given by Figure 4. The fractures networks generated appear clustered, as is commonly observed in a real fractured rock mass. Histograms reporting the separation distances collected along the whole set of boreholes exhibit classical exponential shapes, with a decrease in the average separation distance for deviated wells. The simple conclusion from this first exercise is that reasonably realistic conducting fracture networks can be achieved, without the need to introduce sophisticated geometrical concepts. This should be sufficient for further investigations, given the limitations of the input data.

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Fig. 3. Distribution of simulated fracture spacing along 50 vertical boreholes, drilled into 50 stochastically generated alternatives of a fractured rock mass (based on data given in Table 1).

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Fig. 4. Separation between adjacent fractures (total population), a) Vertical boreholes, b) Inclined boreholes. N U M E R I C A L S I M U L A T I O N O F GPK1 W E L L TESTS This section describes the hydraulic calibration of the network model using 1993 injection test data prior to the simulation of the thermal performance of the proposed doublet. The various hydraulic parameters that are introduced into the model are of differing natures. The first set of parameters describes the hydraulic aperture of the joints, assuming they are in an unloaded situation (i.e. when the supported effective normal stress is reduced to zero). The second requirement is a non-linear flow rule, controlling the dependence of this initial aperture on the variations in normal stress• The purpose of the third set of parameters is to capture some of the hydraulic effects of stimulation treatments. It is our intention to approach this initial calibration phase on a global (reservoir) level, because of the limited number of imposed conditions and of the resulting non-uniqueness of the parameter estimates. The key measurements that the model must be compared with are; 1) The non-uniformity of the injected flow versus depth; this supports an argument for a high hydraulic conductivity fault, which crosscuts the whole simulated rock mass and passes close to the borehole. 2) The limited amount of fluid which is injectable with 10MPa over pressure in the bottom part of GPK1 borehole (limited injectivity); This is a relevant indicator of the permeability of the fractured rock mass away from the perturbation of a conductive fault• 3) The non linearity of the pressure versus flow record; This tells us that improved connections are created in the upper part of the GPK1 openhole section, when the pressure level is increased to the jacking threshold of sub vertical NS trending fractures. This observation justifies a non-linear relationship describing the dependence of the hydraulic conductivity on effective stress.

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4) The irreversible impact of the elevated injection pressures on the global system impedance; This is illustrated by the low pressure that was achieved in 1994 for 181/s injection flowrate. This information, combined with the extension of the microseismic cloud, contributes to a better estimate of the dilation effect. When performing the numerical simulations the hydraulic boundary conditions are necessarily highly idealised. The boundaries to the North, South and the upper edge of the parallelepiped are areas of fixed hydraulic head (0.2MPa). The remaining limits are zero flux boundaries. A vertical segment of variable length simulates the borehole, where a constant head is prescribed. After some initial numerical tests, reasonable results can be achieved using unloaded joint apertures of 0.7mm for the first two fracture sets, 0.5mm for the third and fourth sets, 1.25mm for the upper fault and 0.70mm for the lower. The non-linear relationship selected for the hydraulic properties is given by Figure 5. As an example, the hydraulic conductivity of a joint supporting an effective normal stress of 15MPa is enlarged by a factor of approximately 0.15/0.01, when the effective stress is reduced to 5MPa. According to the incremental form of the cubic law, this corresponds to an increase of the equivalent hydraulic aperture by a factor of approximately 2.5. For joints initially subjected to 10MPa, an over pressure of 10MPa leads to a zero effective stress and to a hydraulic conductivity approximately 1/0.025 (i.e. 40) times larger. ~ n t , l c n t y ictt.tol]

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10 15 20 25 30 35 normal loading [MPa] Fig. 5. Hydraulic efficiency factor (k/ko) which describes the hydraulic conductivity (k) at a given effective normal stress (~) as a fraction of the hydraulic conductivity at zero effective stress (ko). Stimulation is accounted for by releasing (at distances of up to 400m North and 300m South of the GPK1 borehole) the excess shear stress, supported by appropriate fractures, corresponding to a stimulation pressure of 9MPa. For this purpose, we assume that the Mohr-Coulomb failure criterion applies with standard values for the friction coefficient (Tanq)=0.8) and for the cohesion (S=0MPa). The dilation angle, which converts slippage into irreversible shear dilation aperture is taken to be 5 ° . The basis of these calculations is described in greater detail by Bruel (1995). An illustration of the combined effects of all the chosen parameters is given in Figure 6. This figure provides a representation of the actual hydraulic injection rate occurring in ten equally consistent geometrical networks. Quasi steady-state flow rates corresponding to a one week long simulation test are presented for a number of reservoir scenarios, results such as these enable calibration of the network model.

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Fig. 6. Results of the GPK1 injection test trial simulations, expressed in terms of injection flowrate for 10 equally consistent fracture networks (Column 1 ! in each bar-chart represents an average behaviour of the 10 networks), a) Case l, b) Case 2, c) Case 3, d) Case 4, e) Case 5, but see main text for each injection scenario. 1) An injection along a 550m long borehole, with a 9.5MPa prescribed injection head, but without prior stimulation. 2) An injection along a 750m long borehole, in a previously stimulated rock mass, with a 3.5MPa prescribed injection head. 3) An injection along a 350m long borehole, in a previously stimulated rock mass, with a 3.5MPa prescribed injection head.

Heat Extraction Modelling: Soultz-sous-Forets, France

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4) An injection along a 550m long borehole with a 10MPa prescribed injection head, within a stimulated rock mass. 5) An injection along a 750m long borehole with a 10MPa prescribed injection head, within a stimulated rock mass. EXTRAPOLATION TO A TWO WELL SYSTEM Some arguments have already been put forward for the siting of the second deep well (GPK2), at the Soultz-sous-Forets experimental site (Gerard et al. 1994, Jupe et al. 1995). These are essentially based on experimental knowledge that combines conclusions derived from the microseismic behaviour of the rock mass, geological evidence and the GPK1 injection tests. The suggested strategy would be to drill a sub vertical hole 400m to the South-East of GPK1, to approximately 3600m depth, but with the openhole section oriented in the N170 direction, directed towards GPK1. The purpose of this section of the paper is to perform scenario analyses to help in making the most valid engineering decisions for the design and operation of GPK2. This is based on the evaluation of the hydraulic and thermal performances of a stimulated doublet system, with GPK1 used as a production well. The separation of 400m between the two boreholes is essentially based on analytical 2D calculations for heat exchange in doublets with single or multiple parallel fractures (Jupe et al. 1995). The following 3D numerical simulations include some variations around this a priori assumption, and we estimate their respective hydraulic and thermal behaviour under given circulating conditions. For all the numerical simulations the injection head is set to 10MPa. Furthermore, it is assumed that the production borehole is equipped with a downhole pumping device, or an equivalent system, producing a drawdown (reduced head) of 1MPa. A thermal buoyancy effect is also considered along the 3km of borehole and is estimated to a result in a further 1.5MPa drawdown increment. Three different locations for the second vertical borehole are tested. The first one is placed, as already mentioned, on the N170 azimuth, at a distance of 400m to the south-east of GPK1. The second possibility is also located at a distance of 400m from GPK1, but approximately 70m to the west of the first GPK2 location. The third proposal is only 350m from GPK1, this time approximately 50m to the north of the second GPK2 location. The depth range of the openhole section in all three cases is 3100m to 3600m. Following our hypothesis of planarity and persistency for the fault crossed by GPK1 at 2815m, we may observe that the distance between the top of the openhole section of the future well (GPK2) and this fault is more important in the first case than in the latter two. In all three cases, we also assume that the rock mass surrounding the second borehole has been stimulated during preliminary injection tests, and that this treatment results in an irreversible improvement of the hydraulic conductivity of the fractures. According to what was observed in the GPK1 area (Figure 2), we use a stimulation pressure of 11MPa, which is roughly the jacking pressure for fractures favourably oriented to the minimum principal stress at 3100m depth, and we suppose that the perturbed zone is of similar extension to that of GPK1. The temperature field which is required for the calculations assumes a standard temperature gradient of 30°C/km in the depth range of interest and a temperature of 170°C at 3600m. Physical parameters, such as the thermal diffusivity (ct) of the rock and heat conductivity (~.) have typical values. We set cz= 1.1 x 10-6m2s -1 and ~=2.7Wm-IK -1. The fluid is entering the circulated fracture system at 50°C, and we assume that only a fraction of the geometrical area of the discs behaves as a heat exchanger. This ratio remains constant and equal to 40% throughout the simulations. Ten different networks are simulated, for each of the three different doublet scenarios. Average results are summarised in the Table 2. Despite the draining (sub-hydrostatic) conditions at the production boundary, large fluid losses are observed at the top of the geometrical models in both the second and third scenario. The sub vertical jointing pattern, associated with a better connection to the main fault, is responsible for such flow lines.

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Results for the second geometry, that offers the more attractive figures, are more extensively presented with the help of Figures 7 and 8. It can be observed that the production temperature at the beginning of the circulation is very near 153.5°C, which is explained by the fact that the fluid is mainly produced (in any one of the ten different realisations) in the upper part of the GPK1 welt. In general we find reservoirs with rather poor heat exchange capabilities, as denoted by the drawdown index value after six months of circulation. This may be a consequence of an overlap of the two stimulated areas creating short circuits in between the two wells, with preferential flow paths developed along the main fault that has been considered. Table 2: Average behaviour of ten equally consistent networks depicting scenarios 1, 2 and 3. The production temperature is a flux weighted average value. The drawdown index reflects the production temperature decrease with regard to the initial production temperature and is calculated as follows q(t) = (Tprod(t)-Tinj)/(Tprod(0)-Tinj) Geometry

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Network number Fig. 8. Thermal drawdown results for the 10 fracture networks presented in Figure 7. The top of each bar represents the initial production temperature, and the bottom, the value after 6 months. DISCUSSION AND C O N C L U S I O N The work presented in this paper has focused on the main results that can be learnt from the application of generic computer models (which solve equations describing the governing physical processes) using site-specific parameters representing the Soultz-sous-Forets site. The modelling of such a field problem requires (as any ground water model does) a calibration phase, where the parameters that are not directly attainable by field experiments have to be evaluated. The reliability of the estimated values depends on the confidence we have in the other measured parameters. Nevertheless, even when the calibration has been performed, accurate predictions can not be proposed at any other space/time scale without careful attention. Confidence in any model can only increase with time if new experiments are observed to agree with the previous predictions, thus achieving an increased understanding of the hydrogeologic system. At this stage, significant knowledge was gathered from a single deep borehole investigation programme. It must be emphasised that the main assumptions rely on the existence of a very conductive fault, that has been crossed by GPK1 and cased at a depth of 2815m, embedded in a stiffer fractured rock mass. The extrapolations to a two well circulating system (doublet) require many other assumptions and should at this time be considered only as demonstrative guidelines for further investigations. Some processes need to be further understood, for instance the complex problem of the stress perturbations due to thermal depletion. This is not properly accounted for by any field model and could potentially explain the improvement of the hydraulic properties of the fractures located under the casing shoe of GPK1 borehole. In summary, the conclusions of this work have been; 1) The resolution of this model is insufficient to properly constrain the position of the future borehole (GPK2) relative to GPK1, for example either north or south. As it stands the flow model reflects the governing effect of a major fault. However, because of the geometry of this fault and because fluids show a tendency to move upward, south appears to be a more attractive location. 2) In order to maintain a stable system during circulation, smaller flow rates should be injected in the second well (GPK2) than in GPK1, whatever its location (i.e. 301/s instead of 501/s). This is because

450

D. Bruel the openhole section is likely to be shorter ( < 500m) and because at present no major fracture is known to exist there.

3) A significant part of the injected fluid appears to move upward (25 %), and therefore has very little chance of being recovered. 4) At the present time natural fluid production has not been properly taken into account in the modelling, due to our poor knowledge about the potential contribution to the global mass balance. Only a draining (drawdown) effect at the production well has been considered. 5) According to our approach, attractive hydraulic properties (which are derived from the hydraulic stimulation tests performed in the upper levels of the GPK1 borehole) do not necessarily translate into interesting heat exchange capabilities. This is a direct consequence of the channelled aspect of the flow within the stimulated zones.

Acknowledgements - Financial assistance from the (CEC) European Communities Directorate General XII and Agence de l'Environnement et de la Maitrise de l'Energie of France are gratefully acknowledged by the author. REFERENCES

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