Estimate In situ Stresses from Borehole Breakout at Blanche 1 Geothermal Well in Australia

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1 Estimate In situ Stresses from Borehole Breakout at Blanche 1 Geothermal Well in Australia Dr. Baotang Shen, CSIRO, Australia, baotang.shen@csiro.au Prof. Mikael Rinne, Aalto University, Finland, mikael.rinne@tkk.fi Summary This paper describes a numerical study on in situ stress back-analysis in deep granite at a geothermal exploration well. The objective of this study was to estimate the magnitude of the principal horizontal stresses in deep granite using the borehole breakout data obtained from an acoustic scanner log. The ratio of the horizontal stresses to vertical stress is important to the generation and performance of a heat exchange reservoir as it influences the direction permeability enhancement resulting from injections for reservoir stimulation. The study was conducted using a numerical code FRACOD which simulates explicitly the rock fracturing process during breakout. The input parameters were based on the drilling operational data, laboratory test results and previous modelling experience. The study was focused on the granite section within a depth range from 1146m (where the breakouts started) to 14m (where the acoustic scan finished). The in situ stress results obtained from the borehole breakout analysis were compared with the World Stress Map data, stress measurement results in nearby Olympic Dam mine, and the recent hydrofrac testing results in this well. Keywords: geothermal, borehole breakout, in situ stress, fracture, FRACOD 1. Introduction A project has been undertaken by Green Rock Energy Ltd to explore and to ultimately develop a hot dry rock geothermal resource in South Australia. Blanche No. 1 well was drilled to a depth of 1935m to test for potential thermally anomalous granite near the Olympic Dam mining operation and only 5 kilometres from an existing 275kVa powerline [1]. The drilling was an initial test of the potential to find a suitable heat resource for a Hot Dry Rock geothermal electrical power generation project. The location was selected from a study of temperature and lithological data from nearby mineral exploration holes and from reprocessing a reflection seismic survey. The final location was selected based on a combination of favourable, thick thermally insulating cover rocks and a large, homogeneous, massive granite body that was at least 5,m deep. The knowledge of in situ stress state is crucial in predicting the path of rock fracturing and eventual water flow in underground geothermal heat-exchange reservoirs. Stress data will assist the layout design of the injection and production wells. There are no directly estimated or measured stress data at the depth of the granite in this area. Borehole breakout data can be used to reliably predict the direction of the maximum and minimum principal horizontal stresses. Breakout data can also provide an indication on the magnitude of the principal horizontal stresses. The granite section of the Blanche 1 well has been surveyed using an acoustic scanner log. It indicates that the maximum principal horizontal stress is in the East-West directions, resulting in the highest tangential stress and hence breakouts in the North-South directions at depths greater than 1146m. The breakout dimensions (width and depth) can be extracted from the acoustic scan data. It is therefore also possible to estimate the magnitude of the principal horizontal stresses. Although estimating the magnitudes of in situ horizontal stresses in a vertical wellbore from the breakout data is not as straight-forward as determining their directions, a successful analysis has

2 been conducted by Shen [2,3], where the stress magnitudes in a deep granite at the Habanero #1 well were estimated to consist of high horizontal stresses. This prediction was later proved to be correct as fracture stimulation of the reservoir created a predominately horizontal seismic cloud. This paper describes a study that estimate the magnitude of the in situ horizontal stresses at Blanche 1 well by using borehole breakout dimensions. The study consisted of the following steps: Conduct a brief literature review on previous borehole breakout studies; Establish the relationship between breakout dimensions and in situ stresses in granitic rock, by means of numerical modeling; Estimate the most likely in situ stress combinations by comparing the measured breakout dimensions with the numerical results; Validate the estimated in situ stress against other existing stress data in the region. 2. A brief literature review Knowledge of stress orientation is crucial for the understanding of many processes in the earth's crust such as tectonic development, earthquake occurrence, and fluid transport along faults. Analysis of borehole breakouts can yield an understanding of the in situ stress field. The geometry of the borehole breakout is governed by the stress state. The breakout angle can give an estimate of the horizontal stress by means of the Kirsch equations. Alternatively, the geometry of borehole breakouts can be understood from a numerical back-analysis. Previous observations and theoretical analyses of borehole breakouts indicated that failure of a borehole wall can be classified in two different modes where the fracturing process is governed by either tensile spalling or shear fracturing [4,5]. In the case of tensile spalling, the rock breakage starts in the vicinity of a borehole as a result of tensile crack initiation and propagation in the direction of the maximum compressive principal stress, i.e. normal to either maximum tensile principal stress or minimum compressive principal stress. A series of sub-parallel cracks are formed and the coalescence of these tensile cracks makes up a layer which may fall off from the borehole wall. This phenomenon is typical for hard crystalline rocks such as granite under compression with no or small lateral confinement (see e.g. [6,7,8,9,1,11]). In the case of shear fracturing, shear failure along one or more shear bands extends from the borehole wall into the rock. The shear fractures (or shear bands) can cause breakout when they intersect one another. This type of failure is often observed in soft and porous rocks, such as dolomite, limestone and sandstone [5,12,13]. Both failure modes can result in 'dog-ear' wedge shaped breakouts, i.e. breakouts with a wider area at the borehole wall and a sharp end in the rock. The borehole breakouts of a geothermal well in the Northeast German basin (north of Berlin) were analysed using the 2D fracture mechanics based software FRACOD. The objective of the study was to estimate the magnitude of the maximum horizontal stress around the vertical well at a depth of 41m [14]. Comparison of the numerical results and field observations revealed highly desirable results and agreements with the existing structural geology data. 3. Numerical modelling of borehole breakouts Borehole breakout in granitic rock is often dominated by explicit fracturing. The fracture propagation code FRACOD developed by Shen and Stephansson [15] and Fracom [16] has been shown to effectively simulate the breakout process [2,14,17]. FRACOD was used in this study to predict the breakout dimensions under various stress combinations. FRACOD is a 2D code, and hence only horizontal planes (perpendicular to the wellbore axis) are considered. The study is aimed to estimate the magnitude of the horizontal stresses only. The vertical stress is assumed to be the cover depth times the rock unit weight. The 2D approach is limited to the case where the principal stresses are in the vertical and horizontal planes. Wherever principal stresses are not in the vertical and horizontal planes, particularly near the major fracture

3 zone, errors are expected to arise from the 2D plane strain assumption. The thermal effects on rock stress during drilling were not considered in this numerical study. 3.1 Breakout observations in Blanche 1 well Based on the acoustic scan data from Blanche 1 well, borehole breakouts started to occur at the depth 1145m. The breakout, however, did not occur continuously below this depth, possibly because of the local variation of geology and rock strength. In general, the occurrence frequency and the dimensions of the borehole breakout increased with depth within the scanned depth range of 14m. Below this depth no acoustic scan data were conducted and hence this study is limited to this depth range. For the borehole sections where no breakout occurred, very little information is available and determining the in situ stresses is.25a impossible (except for their upper limits). Therefore, the study has been focused on the locations where breakouts had been observed For this reason, the stress results obtained from these locations are more likely to be in the higher a=37.7mm end than the lower end of the overall stress 71.1 regime..25a Figure 1. Geometry of breakouts measured by acoustic scan in Blanche 1 well at a depth of m. Three borehole cross-sections are chosen in this study. They are at depths of m, m and m. The general dimensions of breakouts are shown in Figure 1. No tensile fracturing in the borehole wall was observed from the acoustic scan data within a depth up to 14m. 3.2 Pore pressure and mud density The water level at Blanche 1 was found to be 36m below ground surface. The pore pressure in the granite section was not measured. For the purpose of this analysis, it is assumed that the pore pressure is equivalent to the hydraulic static pressure associated with a local water table. The mud density used during drilling ranges from kg/m 3. Because the mud pressure is higher than the pore pressure in the rock matrix, the difference between mud pressure and pore pressure acts as a net fluid pressure on the borehole wall, which improves the borehole stability. In this study, only the effective stresses (= total stress pore pressure) are used during the modelling because the mechanical response of the rock (including deformation and failure) is controlled by the effective stress rather than the total stress. The difference between the mud pressure and pore pressure can be considered as a net pressure on the borehole wall due to the presence of the mud. At the three depths investigated, the net borehole wall fluid pressure is:.6mpa (1146m), 2.2MPa (1247m), 5.75MPa (1392m), respectively. 3.3 Rock properties Four granite samples, collected from the depths of 15m, 1665m, 19m, and 1922m, were tested to determine the uniaxial compression strength (UCS) by Curtin University of Technology and Golder Associates [1]. The resultant UCS ranges from 132MPa to 211MPa. The sample from the depth of 15m is considered to be most relevant to this study, and therefore those results are

4 used in the numerical model. Other mechanical properties used in this study are mostly based on previous modelling experience for granitic rock properties of Habanaro #1 well [2,3]. Table 1 summarises the key mechanical properties used in this study. Table 1. Mechanical properties used in modelling Property Value Unit Uniaxial Compression Strength (UCS) 163 MPa Young s modulus 7.8 GPa Poisson's Ratio.31 Fracture Mode I toughness 1.35 MPa m 1/2 Fracture Mode II toughness 3.7 MPa m 1/2 3.4 Numerical models and modelling results Borehole breakouts are predicted numerically using the following steps: A numerical model is set up which includes borehole cross sectional geometry, rock properties and in situ stresses; FRACOD is run to calculate stresses in the borehole walls using solid mechanics principles; FRACOD determines if any failure (fracture initiation) occurs in the borehole wall based on the stresses and rock strength values used; If failure is detected to occur, new fractures will be generated in the model and FRACOD then determines if and how they propagate; Breakouts will be formed when fractures in the borehole wall propagate and coalesce; The dimensions of the final breakouts are obtained when there is no further failure and fracture propagation in the borehole wall. Over 4 cases with different in situ stress combinations were simulated and the key results are shown in Figure 2- Figure 4. At each of the three borehole depths, at least 12 cases were studied where the maximum principal horizontal stress (σ Hmax ) was varied from 2.5 to 3. times the vertical stress (σ v ) and the minimum principal horizontal stress (σ hmin ) changed from.75 to 2. times the vertical stress σ v. Some limited cases with σ Hmax /σ v =2. were also simulated at the shallowest depth of 1146m. Depth=1146m At the depth of 1146m, the measured breakouts are very small if any. The numerical modelling results indicate that, if the stress ratio σ Hmax /σ v is 2., no breakouts will occur in the borehole wall, regardless the ratio σ hmin /σ v. When σ hmin /σ v is.75 or less, tensile fractures will occur in the E-W sides of the borehole wall. No such tensile fractures were observed in the acoustic scan measurement in the hole. If the stress ratio σ Hmax /σ v is 2.5, no breakout will occur when σ hmin /σ v is 1.5 or higher. There are very limited breakouts when the σ hmin /σ v ratio is 1.25 or 1.. If the stress ratio σ Hmax /σ v is 2.75, small breakouts will occur when the σ hmin /σ v ratio is 1.25 or higher. When σ hmin /σ v is 1. or.75 the size of the breakouts increases and is noticeably larger than the actual breakout observed by the acoustic scan. If the stress ratio σ Hmax /σ v is 3., major breakouts will occur for all σ hmin /σ v values used. The dimensions of the breakouts are significantly larger than that observed from the acoustic scan. Judging from the dimension of the breakouts, it is apparent that a combination of σ Hmax /σ v = ( ) and σ hmin /σ v = ( ) will produce the breakouts close to those observed at this depth.

5 Green Rock Energy _ Borehole breakout (1146.5m) Green Rock Energy _ Borehole breakout (1146.5m) Pxx (Pa): -7.28E+7Pyy (Pa): E+7 Pxx (Pa): E+7Pyy (Pa): E+7 Pxy (Pa): E+.6 Max. Compres. Stress (Pa): E+8.6 Pxy (Pa): E+.6 Max. Compres. Stress (Pa): E+8.6 Max. Tensile Stress (Pa): E+7 Max. Tensile Stress (Pa): E+.8a Date: 19/5/27 9::12 Date: 19/5/27 8:53: a=37.7mm.8a (a) Measured breakout (b)σ Hmax = 2.75σ v ;σ hmin =1.5σ v (c)σ Hmax = 2.25σ v ;σ hmin =1.5σ v Figure 2. Measured and predicted borehole breakouts at depth of m for different σ Hmax - σ hmin combinations. Depth=1247m At this depth the measured breakouts are noticeable. The breakout angle (azimuth angle of the breakout area a measure of the breakout width) is about 51 and the breakout depth is about.16a (a is the borehole radius). Two selected numerical modelling results are shown in Figure 3. The cases where the predicted breakouts are close to the measurements include (1) σ Hmax /σ v =2.5 and σ hmin /σ v = 1.5; (2) σ Hmax /σ v =2.5 and σ hmin /σ v = 1.25 (3) σ Hmax /σ v =2.75 and σ hmin /σ v = 1.5; (4) σ Hmax /σ v =2.75 and σ hmin /σ v = None of the numerical results produced exactly the same breakout dimensions as the measurements. However, based on the trend, it is likely that the actual stress ratio σ Hmax /σ v is in the range between 2.5 and The ratio σ hmin /σ v has less effect on the breakout dimension than the σ Hmax /σ v ratio, and its range is expected to be within ( ). If the ratio is too low (e.g. 1. or.75), tensile fracturing may occur and the predicted results will deviate from those observed in the logs. Green Rock Energy _ Borehole breakout (1247.5m) Green Rock Energy _ Borehole breakout (1247.5m) Pxx (Pa): E+7Pyy (Pa): E+7 Pxx (Pa): E+7Pyy (Pa): E+7 Pxy (Pa): E+.6 Max. Compres. Stress (Pa): 2.373E+8.6 Pxy (Pa): E+.6 Max. Compres. Stress (Pa): E+8.6 Max. Tensile Stress (Pa): E+7 Max. Tensile Stress (Pa): E+7.16R Date: 19/5/27 9:13:7 Date: 19/5/27 1:15: a=37.7mm 51.16R (a) Measured breakout (b)σ Hmax = 2.75σ v ;σ hmin =1.5σ v (c)σ Hmax = 2.75σ v ;σ hmin =1.25σ v Figure 3. Measured and predicted borehole breakouts at depth of 1247m for different σ Hmax -σ hmin combinations. Depth=1392m At the depth of 1392m, the measured breakouts are significant. The average breakout angle is about 63 and the breakout depth is about.25a.

6 Two selected numerical modelling results are shown in Figure 4. Visual observation suggests that the stress combination of σ Hmax /σ v =2.75 and σ hmin /σ v =1.5 produces the breakout dimensions that matches best the acoustic scan measurements at this borehole depth. Other stress combinations produce either too small or too large breakout dimensions compared with the measurements. Green Rock Energy _ Borehole breakout (1392.5m) Green Rock Energy _ Borehole breakout (1392.5m) Pxx (Pa): -9.68E+7 Pyy (Pa): E+7 Pxx (Pa): -8.75E+7 Pyy (Pa): E+7 Pxy (Pa): E+.6 Max. Compres. Stress (Pa): 2.814E+8.6 Pxy (Pa): E+.6 Max. Compres. Stress (Pa): E+8.6 Max. Tensile Stress (Pa): E+7 Max. Tensile Stress (Pa): E a Date: 19/5/27 9:42:22 Date: 19/5/27 9:38: a=37.7mm.25a (a) Measured breakout (b)σ Hmax = 3.σ v ;σ hmin =1.5σ v (c)σ Hmax = 2.75σ v ;σ hmin =1.5σ v Figure 4. Measured and predicted borehole breakouts at depth of 1392m for different σ Hmax -σ hmin combinations. To summarise the modelling results at all the three depths, the most likely stress combinations in the granite section of the Blanche 1 well is believed to be approximately σ Hmax /σ v = and σ Hmin /σ v = No attempt was made to predict the exact stress ratios in decimal accuracy due to the likely variations in the input parameters (e.g. rock strength). An important finding from the modelling is that both the maximum principal horizontal stress (σ Hmax ) and the minimum principal horizontal stress (σ hmin ) are believed to be higher than the vertical stress in Blanche 1 granite. Any horizontal stresses lower than the vertical stress used in the numerical model not only produce breakout results which do not match the measurements but also create tensile fractures in the E-W sides of the borehole wall that were not observed in the acoustic scan. 4. Validation of estimated stress magnitude The numerically estimated principal horizontal stress magnitudes are compared with the existing knowledge about the stress field in this region. 4.1 Overall stress field in this region Based on the Australia Stress Map [18], the maximum principal horizontal stress in Flinders Ranges is predominately in the E-W direction with strong local variations, see Figure 5. The stress regime is either the Strike Slip Stress Regime (SS) (i.e. σ Hmax >σ v >σ hmin ) or Thrust Faulting Stress Regime (TF) (i.e. σ Hmax >σ hmin >σ v ). Using a rating convention (Normal Faulting =1.; Strike Slip =.5; Thrust Faulting = ), Hillis and Reynolds [18] suggested that the stress regime rating in Flinders Ranges is.17, which means that the regional stress regime is close to the Thrust Faulting condition. In Cooper Basin, north of the Blanche 1 site, the stress regime in the deep granite is known to be the Thrust Faulting condition as determined during the recent HDR operations by Geodynamics Ltd based on both borehole breakout analysis [2,3] and reservoir stimulation tests [19].

7 These existing data about the stress regime in the regions close to Blanche 1 site appear to support the numerical results, i.e. a thrust faulting stress condition (σ Hmax >σ hmin >σ v ) is likely to exist in Blanche 1 granite section. the Blanche 1 deep granite. Blanche 1 Figure 5. Australia Stress Map (after [18]) 4.2 Stress measurement results in Olympic Dam Mine The Olympic Dam Mine is only several kilometres away from the Blanche 1 well. Although the mine operation is at a much shallower depth (e.g. <6m), the existing stress measurement results at Olympic Dam Mine can still provide a good reference for Measurements of the Olympic Dam Mine stress field have been undertaken since underground operations commenced at the mine in 1982 during the Whenan Shaft sinking operations [2]. The stress measurement database at the Mine consists of a total of 2 measurements using the CSIRO HI cell, JCUNQ Borehole slotter, and the Acoustic Emission (AE) method. To determine the trend of in situ stresses in granite, Pascoe [2] only used the 13 measurements by CSIRO HI cell. Other measurements results were regarded as unreliable. The estimated directions of the three principal stresses are Major principal stress 133 /2 (bearing/plunge) Intermediate principal stress 224 /18 Minor principal stress 37 /72 The magnitudes of the three principal stresses are given as: Major principal stress [MPa] = x D [m] Middle principal stress [MPa] = x D [m] Minor principal stress [MPa] = 3 x D [m] where D is depth below surface in metres. Overall, the major and intermediate principal stresses are both in nearly horizontal directions and minor principal stress is in nearly vertical direction. In other words, the horizontal stresses at Olympic Dam Mine are both higher than the vertical stress. This measurement results are consistent with the numerical results from this study. The measured ratio of the horizontal stresses to vertical stress at the shallow granite in the mine appears to be lower than the numerical results at the deep granite in Blanche 1 Well. Also noticed is that the major principal stress at the Mine is in the NE-SW direction, differing from the E-W direction in Blanche 1 Well. It is believed that stress field at the shallow depth of the mine is likely to be affected by the geological structures etc and hence deviate from the main trend of the regional stress field.

8 4.3 Hydraulic fracture testing at Blanche 1 well Hydraulic fracture (hydrofrac) testing was carried out in the open section of the granite rocks at Blanche 1 well [21][22]. The objective of the testing was to derive reliable data on the magnitude and orientation of the in situ stress regime. During March 28 a total of 12 hydraulic fracture stress measurement tests were carried out between depths of 881m and 1,739m using a 71mm diameter inflatable straddle packer system. Stress magnitudes and orientations were calculated by MeSy GmbH [22]. The direction of the maximum horizontal stress SHmax was determined as 97 ±3. The magnitudes of S Hmax and Shmin vary with depth according to the following equations: Shmin [MPa] = (12.4±1.2) + (8±.3) * (z[m]-88) SHmax [MPa] = (35.8±2.8) + (.6±.1) * (z[m]-88) These equations were valid for a depth (z) interval between 88 and 174m. Above approximately 174m the minimum principal stress was horizontal, resulting in vertical hydraulic fractures. Below this depth the trends are expected to continue, owing to the fact that seismic surveys show that the granite fabric continues unchanged down to 6km. This stress trend needs to be verified by deeper stress measurements in any wells drilled to access deeper heat reservoirs. Horizontal stresses ranged from 15 to 45MPa for the minor and 35 to 9MPa for the major horizontal principal stress. Results confirm a reverse thrust stress regime prevails at Blanche 1. Current active faults are expected to strike at an orientation perpendicular to SHmax and to dip at a critical angle to the horizontal. Open tensional joints are expected to lie on a horizontal plane. Figure 6. In situ stress magnitude estimated from hydrofrac testing at Blanche 1 (after [21]) (red=s hmin ; blue = S Hmax ) Based on the hydrofrac testing results, the stress ratios at a depth of 13m (in the range of the borehole analysis) are: SHmax/Shmin/Sv = 1.8/.8/1. The SHmax/Sv and Shmin/Sv ratios are smaller than the borehole breakout results. The ratio however is expected to increase with depth. 5. Discussion and Conclusions This study attempts to estimate the magnitude of the horizontal stresses using borehole breakout dimensions at the Blanche 1 well site. The study was based on a number of assumptions and simplifications. Only a simple mechanical process was studied; without considering the complex coupled mechanical-thermal-fluid process. Some key input parameters (e.g. rock strength) were based on a limited number of laboratory test results whereas other mechanical parameters were assumed from the previous modelling experience. The results of this study highlight the following two key conclusions: It is likely that the maximum and minimum principal horizontal stresses are both higher than the vertical stress in the granitic rock within the depth range of m at the Blanche

9 1 well site. The maximum horizontal stress is known to be in the East-West direction based on the borehole breakout data. The ratio of the horizontal stresses to vertical stress is estimated to fall into the range of σ Hmax /σ hmin /σ v = ( )/( )/1.. There is a degree of uncertainty particularly in the ratio of σ hmin /σ v. The predicted stress magnitudes only represent the likely overall stress state in the Blanche 1 granite. Local stress release may exist and can be observed, since there are no breakouts in many sections of the well bore between 1146m and 14m. The predicted stress magnitudes agree well with the stress measurement results from the nearby Olympic Dam Mine and both indicate that the vertical stress is the minimum principal stress in this area. The predicted results are generally consistent with the regional stress pattern in both Flinders Ranges and Cooper Basin where thrust faulting stress condition has been reported. However, significant variations exist in the σ hmin /σ v ratio predicted by different methods. The uncertainty in the calculated σ hmin /σ v ratio is higher than the σ Hmax /σ v ratio. Recent hydrofrac testing results in Blanche 1 well have confirmed that σ Hmax is much higher than σ v, but σ hmin is smaller than σ v at depth less than 17m, differing from the breakout analysis results. Below this depth, however, σ hmin is expected to be greater than σ v, which is consistent with the breakout analysis results. The stress ratios obtained from this study are important to the design of the heat exchange reservoir. The values of the vertical stress being the minimum stress implies that fracture movement and fluid flow are most likely to extend in a subhorizontal direction. This is an ideal situation to achieve an optimal heat exchange conductivity distribution and will allow using a maximum distance between the injection and production wells. This study demonstrates that it is possible to use the borehole breakout dimension to estimate the magnitude of the principal horizontal stresses. The results, however, are sensitive to a number of parameters, particularly the rock strength. To refine the results, it is necessary to have an accurate rock strength data. It is also desirable in the future studies to investigate the effect of the fluid flow and rock temperature change in the vicinity of the wellbore, because the pore pressure and temperature gradient may change the rock effective stress distribution and hence affect the borehole breakout dimensions. 6. Acknowledgements The study was sponsored by Green Rock Energy Ltd. The authors wish to thank Mr Adrian Larking and Mr Gary Meyer for initiating this study and providing site data and information. We would also like to thank Dr Rob Jeffrey of CSIRO for his technical contributions to this study, and Dr Habib Alehossein of CSIRO for reviewing the paper. 7. References [1] GREEN ROCK ENERGY LTD., Blanche 1 Geothermal Exploration Hole Completion Report. Green Rock Energy Ltd Report, Prepared by Gary Meyer, [2] SHEN B., Using borehole breakout data to estimate in situ stresses in deep granite at Habanero #1 Hot Dry Rock well, CSIRO Exploration and Mining Report 1166C, 24. [3] SHEN B., Borehole Breakout and In situ Stresses. Proceedings of the 1st Southern Hemisphere International Rock Mechanics Symposium, SHIRMS 28. Vol.2, pp [4] VARDOULAKIS J., SULEM J. and GUENOT A., Borehole instabilities as bifurcation phenomena, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1988, Vol.25, pp [5] GUENOT A., Borehole breakouts and stress fields, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1989, Vol.26, pp [6] EWY R.T. and COOK N.G.W., Deformation and failure around cylindrical openings in rock*i.

10 Observations and analysis of deformations, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 199. Vol.27, pp [7] EWY R.T. and COOK N.G.W., Deformation and failure around cylindrical openings in rock*ii. Initiation, growth and interaction of fractures, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 199, Vol.27, pp [8] LEE M.Y. and HAIMSON B.C., Laboratory study of borehole breakouts in Lac du Bonnet granite: a case of extensile failure mechanism, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr., 1993, Vol.3, pp [9] MARTIN C.D., MARTINO J.B. and DZIK E.J., Comparison of borehole breakouts from laboratory and field tests, in: Proceeding on Rock Mechanics in Petroleum Engineering. Delft, Balkema, Rotterdam, 1994, pp [1] HAIMSON B.C., Fracture-like borehole breakouts in high-porisity sandstone: Are they caused by compaction bands? Phys. Chem. Earth (A), 21, Vol.26(1-2), pp [11] AMADEI B. and STEPHANSSON O., Rock Stress and Its Measurement, Chapman & Hall, London, 1997, 49 p. [12] ZOBACK M. D., MOOSS, D., MASTIN, L. and ANDERSON R., Wellbore breakout and in situ stress. J. Geophys. Res., 1985, Vol. 9(B7), pp [13] ZOBACK M D, BARTON C A, BRUDY M, CASTILLO D A, FINKBEINER T, GROLLIMUND B R, MOOS D B, PESKA P, WARD C D, and WIPRUT D J, Determination of stress orientation and magnitude in deep wells. Int. J. Rock Mech. Min. Sci, 23, Vol.4, pp [14] BACKERS T, STEPHANSSON O, MOECK I, HOLL H-G, and HUENGES E, Numerical borehole breakout analysis using FRACOD2D. EUROCK 26-Multiphysics Coupling and Long Term Behaviour in Rock Mechanics. Taylor & Francis Group, London, ISBN , London, 26. [15] SHEN B. and STEPHANSSON O. Modification of the G-criterion of crack propagation in compression. Int. J. of Engineering Fracture Mechanics. 1994, Vol. 47(2), pp [16] FRACOM, FRACOD Version 1.1, User s Manual, FRACOM Ltd. 22. [17] SHEN B., STEPHANSSON O. and RINNE M., Simulation of Borehole Breakouts Using FRACOD2D, Oil & Gas Science and Technology Rev. IFP, 22. Vol. 57 (5), pp [18] HILL R.R. and REYNOLDS S.D., In situ stress field of Australia. Geological Society of Australia Special Publication, 22, Vol.22, pp [19] GEODYANMICS LTD., Habanero #1 Stimulation Operations Review, Geodynamics Ltd Report, Prepared by Stephen C Davidson, Brian Assels, Eugene Iliescu, Doone Wyborn, 24. [2] PASCOE M. J. Olympic Dam In situ Stress Field. BHP Billiton Internal Communication Document, 27. [21] MEYER G., LARKING A., JEFFREY R., and BUNGER A., Olympic Dam EGS Project Proceedings World Geothermal Congress, Bali, Indonesia, April, 21 [22] KLEE G., BUNGER A., MEYER G., RUMMEL F., JEFFREY R., and SHEN B., High horizontal stress in South Australia derived from breakouts, discing, and hydraulic fracturing to 2 km depth. In: 3rd World Stress Map Conference, Potsdam, 28