The development of Hot-Dry Rock (HDR) geothermal energy in Australia with drillings to some kilometres depth yields an impetus for deep stress logging. For the Olympic Dam HDR-project, borehole Blanche-1 was drilled to almost 2 km depth and provided the possibility to estimate the in situ stresses within the granitic borehole section by the analysis of borehole breakouts and core discing, as well as by hydraulic fracturing combined with acoustic borehole televiewer logging for fracture orientation determination. Although the stress magnitudes derived by the different methods deviate significantly, they clearly indicate for the depth range between 800 and 1,740 m a compressional stress regime of S v ≤ S h < S H and a consistent East–West orientation of maximum horizontal compression in agreement with existing stress data for Australia. The minor horizontal stress S h derived from the hydraulic fracturing closure pressure values is about equal to the overburden stress and may be regarded as most reliable.

Incoherent results of in-situ stress measurements and their implications in designing a crude oil storage caverns project are presented in this paper. Core-discing at depths corresponding to abnormally higher stress magnitude values were observed at the project site. It is brought to the notice that an optimistic assessment of in-situ stresses may result into costly design surprises during the excavation stage. Consideration of incoherency in the computed in-situ stress magnitudes and orientation resulted into lowering of the caverns level farther 5m from the surface than the design requirements as per the storage pressure. The paper also briefly outlines the project settings, fundamentals of crude oil storage projects in unlined mined caverns and in-situ stress measurements using hydro-frac technique. Impacts of in-situ stress regime on the caverns design is evaluated by using numerical modeling for which plain strain numerical models are prepared using a general purpose finite difference code, FLAC3D.

Predictions on interplate coupling and shear heating in the deeper levels of subduction zones require an understanding of the rheology of eclogite. The strength of eclogite is probably limited by the flow strength of its major constituent omphacite, representing a solid solution between the clinopyroxene end‐members diopside and jadeite. Here we report the results of deformation experiments on the end‐member jadeite (NaAlSi2O6), carried out on fine‐grained synthetic aggregates crystallized in the stability field of jadeite from a synthetic glass precursor. A standard procedure was established to yield samples with a uniform microstructure and an average grain size of approximately 10 μm. The presence of micropores and the detection of small amounts of freezable water by Fourier transform infrared spectroscopy (FTIR) in both undeformed and deformed samples indicates the presence of a free aqueous fluid, hence wet conditions. Deformation experiments were carried out in a modified Griggs‐type apparatus at a confining pressure of 2.5 GPa, temperatures between 800°C and 1100°C and at strain rates of 4 × 10−4 s−1 to 2 × 10−6 s−1. A molten eutectic CsCl/NaCl mixture was used as pressure medium. The microfabrics of the deformed samples and the mechanical data indicate deformation in the dislocation creep regime. The mechanical data are fit by a power law using a global inversion method, yielding an activation energy of Q = 326 ± 27 kJ mol−1, a stress exponent n = 3.7 ± 0.4, and a preexponential factor of ln A = −3.3 ± 2.0. Extrapolation of this flow law for synthetic jadeitite to low geological strain rates, and comparison with available flow laws for diopside, indicates jadeite to be significantly weaker than diopside in the dislocation creep regime.

We deformed synthetic polycrystalline aragonite aggregates in a Griggs‐type apparatus using a molten salt cell at temperatures between 600 and 900°C, confining pressures between 1.6 and 2.8 GPa, and strain rates between 3 × 10−6 and 5 × 10−3 s−1. At temperatures above 600°C, triaxial compression tests to ∼15% axial strain are characterized by a constant flow stress. The strength of the aragonite marble is comparable to strength data previously published for calcite rocks of comparable grain size, i.e., 50 to 100 μm. The mechanical data and microstructures of the deformed aragonite specimens indicate dislocation creep as the dominant deformation mechanism. A power law flow law, = Aexp(−Q/RT)σn, fits the mechanical data with a preexponential factor ln(A[MPa−5.2 s−1]) = −0.6 ± 2.5, an activation energy Q = 249 ± 29 kJ mol−1, and a stress exponent n = 5.2 ± 0.6. Extrapolated to natural strain rates, the flow law provides an upper bound to the strength of carbonate rocks at (ultra‐) high‐pressure metamorphic conditions.

Coesite has been found as a relic in ultrahigh pressure metamorphic (UHPM) crust worldwide and is expected to play a major role in the mechanical behavior of continental crust at UHPM conditions. We performed triaxial compression tests on synthetic polycrystalline coesitite in a solid medium apparatus at confining pressures of 3.1 to 3.7 GPa, temperatures of 700° to ∼1160°C, and strain rates betweeen 6×10−7 and 1×10−3 s−1. The problem of the limited stress resolution of the solid medium apparatus was addressed by applying two extreme friction corrections that yield lower and upper bounds to the differential stress. The correlation between the mechanical data and the microstructural record of the deformed samples, as a function of temperature and imposed strain rate, is consistent with deformation by dislocation creep. We deduced parameters of a power law as n ≈ 3±1 and Q ≈ 275±50 kJ mol−1. Extrapolation of the experimental data to natural conditions cannot be constrained by comparison with natural microstructures, due to the lack of preserved coesite other than as single crystal inclusions. Nevertheless, the extrapolation indicates a low strength (of order 10 MPa) for natural strain rates at typical UHPM conditions. Absent deformation of the UHPM Brossasco granite (Dora Maira Massif, Western Alps) thus implies low stresses; deformation must have been localized in very weak shear zones during burial and exhumation.

The physical properties of foliated marls and shales from the Swiss Central Alps were investigated in the laboratory as part of the site characterization of a potential low and intermediate level radioactive waste repository. The rocks were first characterized by density, porosity and water content, then P-wave velocity measurements and uniaxial compression tests were conducted on samples oriented parallel, perpendicular and ∼45° inclined to the foliation. Correlations between P-wave velocities, Young's moduli, and uniaxial strengths and directional dependence of compressibility and permeability (determined at elevated hydrostatic pressures) revealed that two distinct families of microcracks cause the significant anisotropy. The pressure and time dependence of strength was then measured in triaxial compression, creep, and relaxation tests. In triaxial compression at room temperature, confining pressures up to 90 MPa and strain-rates as low as 10−8 s−1 failure occured after small inelastic strains. Failure strength exhibits a strong pressure and a weak strain-rate dependence. The low friction coefficient of clay minerals appears to dominate the bulk frictional properties. Micromechanical modeling of triaxial compression, creep, and relaxation tests suggests that inelastic deformation is accommodated by subcritical crack growth.

In situ stress measurements by hydraulic fracturing were carried out in the 617 m deep borehole specially drilled in the epicentral zone of the 1993 Latur earthquake for the purpose of research. The stress measurements carried out at 592 m depth in this borehole are the deepest of all such measurements made so far in the Indian shield. The maximum and minimum principal horizontal stresses (S H max andS h min) have been derived from the hydrofracture data using the classical method. TheS H max andS h min are found to be 16.5 and 9.6 MPa at 373 m depth, and 25.0 and 14.1 MPa at 592 m depth, indicating that the vertical gradients ofS hmax andS hmin in the epicentral zone are 39 MPa/km and 21 MPa/km respectively. The principal horizontal stresses in the epicentral zone are comparable with those at Hyderabad and 30% higher than in most other comparable intra-continental regions. Analysis of the results indicate that the stresses in the focal region of the 1993 Latur earthquake have not undergone any significant change following its occurrence and this is in agreement with a similar inference drawn from the seismic data analysis. It appears that the Latur earthquake was caused due to rupturing of the overpressured fault segment at the base of the seismogenic zone.

Hot Dry Rock (HDR) technology started from an idea to help fulfill future energy needs as the availability of cheap fossil and other known fuels slowly reduces. The HDR concept itself is very simple but the development of the associated technology has taken significantly longer than anticipated. Anyone with experience of natural materials such as rocks knows that there are always imponderables that have not been really understood and indeed cannot at present be dealt with in a fully satisfactory manner. Furthermore, geology always has a habit of presenting us with new problems. Under these auspices and considering the limited funds that have been made available, it is encouraging to note that at long last light is visible at the end of the long tunnel of uncertainty. The results of the 1997 circulation test at Soultz-sous-Forets (France) certainly seem to show that we may have come up with a type of concept and an appropriate set of background site conditions to advance the technology.The concept of an HDR reservoir has evolved from that of a single penny-shaped fracture borrowed from the oil industry to the present graben or HWR (Hot Wet Rock) concept. International co-operation has been a key issue so far, and the expensive nature of this research demands that co-operation of this type continues to break new ground in the future. The necessary supporting technology has also evolved and the time appears to be ripe for taking advantage of this new and exciting development. Not all the answers are known, but at least we know now which questions to ask. It is worth remembering that there is still no commercial HDR plant in existence to provide real data on building, operating and maintenance costs for planning a new unit. This should not be regarded as an insuperable problem. If such were the case, then we would not have any aircraft, steel, shipping, telecommunication or nuclear industries.Many people fed that HDR technology will be needed sooner or later and the important question now is how quickly it can be put into practice when the need does arise! Recent moves to form a consortium for this next step at the Soultz site are very encouraging and show the promise and confidence that commercial and industrial interests have in its future. (C) 1999 CNR. Published by Elsevier Science Ltd. All rights reserved.

Since 1990, a total of 149 hydrofrac stress measurements to about 200 m depth were conducted in 18 boreholes as part of several geotechnical site investigation programs in the Hong Kong area. The in situ tests were carried out by using the wireline hydrofrac technique to move the straddle packer tool within the 76 mm or 101 mm diameter boreholes. Although the tests were performed both in fractured and unfractured crystalline rocks and the boreholes are located in areas of pronounced topographic relief, the results yield a consistent orientation of the maximum horizontal stress of N 108 degrees +/- 28 degrees Above 150 m depth, the vertical stress S-v due to the weight of the overburden with given rock density is the minimum principal stress, while the few deeper data available suggest that the minimum horizontal principal stress is the least principal stress. The derived stress magnitudes can be reasonably summarized by the following normalized stress-ratios:k(h) = S-h/S-v = 72.3/z + 0.66k(H) = S-H/S-v = 110.0/z + 1.29where z is the depth in meters and S-h and S-H are the minimum and maximum horizontal principal stresses. Due to the considerable scatter of the stress data at shallow depth above 100 m, it is suggested that further detailed in situ stress measurements be undertaken in areas where large-scale underground excavations are planned. (C) 1999 Elsevier Science Ltd. All rights reserved.

—A deformation apparatus has been developed to study the mechanical behaviour of high pressure and ultra-high pressure metamorphic rocks. It is based on the conventional Griggs design and the molten salt cell concept introduced by H. E. Green II. Both, the axial loading and the confining pressure are servohydraulically controlled. Alternatively, a self-made multilayer pressure vessel or a commercial stripwound construction are used. The pressure cell is improved with respect to systems described previously by the use of different salt mixtures with low eutectic temperatures, by a mechanically stable arrangement of the thermocouples, and by an optimization of the frictional characteristics of the axial loading system. The apparatus has been successfully used in deformation experiments on cylindrical aragonite and coesite samples 3 to 4 mm in diameter and 6 to 10 mm in length at confining pressures up to 3.7 GPa and temperatures up to 1170°C.

For many years, in situ stress in the brittle crust has been measured at relatively shallow depth and related to the mechanical behavior of the crust as inferred from laboratory studies and faulting theory. A continuous profile of the magnitudes and orientations of the three principal stresses has been estimated to depths of 7.7 km and 8.6 km in the German Continental Deep Drilling Program (KTB). This was achieved by hydraulic fracturing tests at relatively shallow depth (1–3 km), estimates of the magnitude of the least horizontal principal stress provided by modified hydraulic fracturing experiments at 6 km and 9 km depths, and analysis of compressional (breakouts) and tensile (drilling‐induced tensile wall fractures) failures of the borehole wall over nearly the entire depth of the KTB borehole. The orientation of the maximum horizontal principal stress was found to be uniform with depth with an orientation of N160°±10°E, which is consistent with the average orientation found throughout western Europe. The only significant change in stress orientation was observed directly below a major fault zone crosscutting the borehole. The profile of stress magnitudes we have obtained demonstrates that to a depth of 8 km, the state of stress in the brittle crust in southern Germany is in frictional equilibrium. That is, the ratio of shear to normal stress as resolved on preexisting faults which are well‐oriented to the in situ stress field is comparable to their frictional strength based on predictions of Coulomb faulting theory for a coefficient of friction of about 0.7 and near‐hydrostatic pore pressure.

Triaxial compression tests were conducted on cold-pressed calcite, aragonite and limestone aggregates and on Solnhofen limestone specimens to study the effect of experimental and microstructural parameters on the transition from brittle failure to cataclastic flow. The tests were pel-formed at confining pressures up to 195 MPa and at strain rates between 5 . 10(-4) s(-1) and 5 . 10(-6) s(-1). Axial as well as volumetric strain were measured. Samples were produced by cold-pressing powders of crushed calcite and aragonite crystals and of crushed Solnhofen limestone. Sample porosity ranged between 5 and 25% and the average grain size varied between 5 and 400 mu m.For both the cold-pressed aggregates and the intact limestone specimens, the confining pressure at the transition from localized brittle failure to non-localized cataclastic flow decreases with increasing porosity and grain size. The transition is characterized by a zero work-hardening coefficient, by dilation for low porosity and compaction for high porosity rocks, by a constant ratio between axial stress and confining pressure, and by decreasing yield strength for increasing confining pressure. The experimental results disagree with the critical state concept over most of the porosity range investigated, and indicate non-associated material behaviour. These properties of the brittle-ductile transition are addressed on the basis of continuum mechanics or by models suggested for granular materials. The problems discussed and the results obtained are of fundamental interest to rock deformation and structural geology.

IT has been suggested1–6that in many cases the average strength of the continental crust is quite low (tens of megapascals), so that the crust has little effect on the large-scale deformation of the lithosphere. But laboratory friction studies7,8, combined with simple faulting theory9,10 (as well as extrapolation ofin situ stress measurements from the upper 3 km of the crust11), imply that if pore pressure is approximately hydrostatic at mid-crustal depth, crustal strength is appreciable (hundreds of megapascals) and would markedly constrain the nature of lithospheric deformation12–15. Here we report estimates of the magnitude of in situstresses to 6 km depth in the KTB borehole in southern Germany. Our results indicate a high-strength upper crust, in which the state of stress is in equilibrium with its frictional strength. We suggest that plate-driving forces in the continental lithosphere in this part of western Europe are transmitted principally through the upper crust, and that this may also be the case in other continental areas of moderate to elevated heat flow.

Obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling. The borehole cavity, however, disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences. At sufficiently low stress magnitudes, the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress. At higher levels, the stresses damage then fracture the rock near the wellbore wall or within a growing core stub. Indeed, almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses. This contribution reviews, at a fundamental level, the concentration of stresses by the borehole, the effects on the materials in the borehole's vicinity, and how these are used to infer stress states. Stress concentrations applied to rocks, which are generally nonlinear elastic materials, induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods. Hydraulic fractures initiate, propagate, and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes. At higher stress levels, the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes. Similarly, the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered. Unfortunately, no method currently allows for complete determination of the stress tensor; using a number of different but complementary techniques is necessary to best constrain in situ stress states.