Bezalel Haimson

• PhD Rock Mechanics
• Professor Emeritus at University of Wisconsin–Madison

Experiment 2 of the EGS Collab project is aimed at testing stimulation by hydro-shearing of existing natural fractures, versus Experiment 1, which is focused on hydraulic fracturing a rock mass to enhance permeability. The main criterion for the testbed selection in Experiment 2 is the presence of an interconnected network of fractures, at least 10...

Tracer tests are standard methods for characterizing flow paths along connected high-permeability zones in the subsurface. In this study, we propose a fast and simple method to analyze tracer tests from the DOE's EGS Collab project, and feed the characterized flow paths back into thermal breakthrough predictions. In this project, two phases of cont...

The objective of the EGS Collab project is to establish a suite of intermediate-scale (~10-20 m) field test beds coupled with stimulation and interwell flow tests to provide a basis to better understand fracture stimulation methods, resulting fracture geometries, and processes that control heat transfer between rock and stimulated fractures. Experi...

A comprehensive laboratory study was conducted on the mechanics of failure in two compactive, porous, quartz-rich sandstones, subjected to true triaxial compressive stresses (σ1 ≥ σ2 ≥ σ3). True triaxial experiments using the polyaxial testing apparatus were performed in cuboidal specimens of Coconino (17% porosity) and Bentheim sandstone (24%) as...

We performed an extensive suite of true triaxial experiments in two porous sandstones, Bentheim (porosity ≈ 24%) and Coconino (17%). The experiments were conducted using a novel loading path, which maintains constant Lode angle (Θ) throughout the test. This path enabled the examination of the effects of Lode angle and mean stress on failure. Our te...

We summarize two suites of recent true triaxial tests in two porous sandstones, Coconino and Bentheim. One suite of tests conducted under constant σ 2 and σ 3 , which revealed the effect of σ 2 and σ 3 on rock failure. The other suite of tests, performed under constant Lode angle, determined the failure dependencies on mean stress and Lode angle. B...

A series of tests have been completed to determine the failure of Sierra White Granite under a range of true triaxial stress conditions ranging from axisymmetric compression to axisymmetric extension. Tests were performed under constant mean stress conditions. Results show a significant difference in failure due to the intermediate principal stress...

We conducted an extensive suite of true triaxial experiments in two porous sandstones, Bentheim (porosity ≈ 24%) and Coconino (17.5%). Our experiments demonstrate that failure of both sandstones is not only a function of σ3, but also of σ2. For a given σ3, σ1 at failure (σ1,peak) increases as σ2 is raised above σ3 between tests. σ1,peak reaches a p...

A series of laboratory drilling experiments were conducted on two arkosic sandstones (Tenino and Tablerock) under polyaxial far-field stress conditions (σ h ≠ σ H ≠ σ v ). V-shaped breakouts, aligned with the σ h direction and revealing stress-dependent dimensions (width and length), were observed in the sandstones. The microscale damage pattern le...

True triaxial tests have been carried out in two quartz-rich, high porosity, sandstones, Coconino (n = 17.5%) and Bentheim (n = 24%) by maintaining constant but different σ2 and σ3 and raising σ1 until failure occurred (at σ1,peak). For each constant σ3 level, σ2 was varied from test to test between σ2 - σ3 and σ2 = σ1, σ1,peak for a given σ3 incre...

True triaxial tests have been carried out on two porous sandstones, Coconino (n = 17.5%) and Bentheim (n = 24%) to investigate the effect of the intermediate principal stress (σ2) on compressive strength, failure-plane angle and failure mode. These rocks were selected because they are similar to sandstones forming hydrocarbon reservoirs, as well as...

Porous sedimentary rocks such as sandstones are typical oil-bearing formations in which failure due to high stress concentration is likely to occur during wellbore drilling and subsequent operations. The objective of this research was to investigate the effect of ?2 on strength, failure-plane angle, and failure mode under realistically simulated fi...

A series of true triaxial tests has been carried out on the quartz-rich, high-porosity Bentheim sandstone (n = 24%) using a novel loading path that maintains constant the deviatoric stress state parameter (N). We applied six different N's, spanning from axisymmetric compression (σ2 = σ3) to axisymmetric extension (σ2 = σ1). Tests reveal the effect...

The failure criterion based on true triaxial testing considers the effect of all three principal stresses on rock compressive strength, and is entirely based on true triaxial tests conducted on rectangular prismatic specimens subjected to three independent principal stresses. The failure criterion is commonly expressed in terms of the octahedral sh...

Failure criteria are often expressed in terms of the major principal compressive stress that rocks can sustain for given values of the other two principal stresses. Triaxial tests have been widely used for the study of mechanical characteristics of rocks because of equipment simplicity and convenient specimen preparation and testing procedures. The...

The introduction of the "true triaxial" testing machine by Mogi (JGR, 1971) and later by Haimson and Chang (JGR, 2000; IJRMMS, 2000) was instrumental in discovering the hitherto largely unrecognized effect of the intermediate principal stress (σ2) on rock brittle failure (or faulting) and fault angle (angle between fault-normal and direction of the...

True triaxial tests have been carried out on the quartz-rich Coconino sandstone, (porosity 17.5%) to investigate the effect of the intermediate principal stress (σ2) on strength and fault angle. In all tests the least and intermediate principal stresses (σ3 and σ2) were maintained constant, while the largest stress (σ1) was raised until failure (at...

We conducted two series of true triaxial tests in Coconino Sandstone (17.5% porosity; 96% quartz), in which square cuboidal rock specimens (19 x 19 x 38mm) were independently compressed in three principal directions, using the UW-Madison true triaxial testing apparatus. In the conventional series of true triaxial tests the least and intermediate pr...

Ma and Haimson (see accompanying Abstract) have conducted true triaxial tests on a compactive porous sandstone (Coconino) using a novel loading path consisting of keeping the minimum (least compressive) principal stress (σ3) constant but increasing the other two stresses in a fixed ratio. For stress ratios Δσ2/Δσ1 = 1:1; 1:2; 1:3; 1:6; 0 (where Δσi...

Localized volume reduction (LVR) in rocks plays an important role in determining rock mechanical and transport properties. In contrast to reversible LVR (associated with elastic strain), irreversible LVR (associated with plastic strain) produced mainly by mechanical and chemical compaction is retained even upon removal of the external load. Despite...

Highlights ► Two series of true triaxial tests conducted on the SAFOD granodiorite. ► In both tests rock strength increases with raising σ2 for a given σ3. But unjacketed rock strength is 50% lower than jacketed one for the same σ2 and σ3. ► Jacketed sample failed typically by forming a through-going shear fracture. But unjacketed sample failed by...

An extensive true triaxial testing program was carried out on core samples from three ICDP-sponsored deep scientific boreholes, KTB (Germany), SAFOD (United States), and TCDP (Taiwan). The three rocks differ in the processes that formed them and in many of their mechanical properties. However, all three rocks exhibited similar failure mechanism, in...

Drilling into rock blocks subjected to critical far-field stresses can result in localized failure around the created borehole, manifested by the formation of two diametrically opposed breakouts. In weakly bonded quartz-rich porous sandstones breakouts take the form of narrow tabular (slot-like) openings extending along a plane perpendicular to the...

As part of the Taiwan Chelungpu-fault Drilling Project (TCDP) two scientific holes were drilled 40 m apart, each penetrating the fault, at 1111 m (hole A) and 1125 m (hole B) at a point some 100 km north of the Chi-Chi earthquake epicenter. The only direct stress measurements were conducted at 4 depths in hole B using hydraulic fracturing (HF). The...

Hydraulic fracturing (HF) has been the method of choice for estimating the state of stress around deep vertical boreholes for the last four decades, starting with the Rangely, Colorado and the Michigan Basin measurements (Haimson, Tectonophysics, 1975; JGR, 1977). However, recently considerable controversy has developed regarding the reliability of...

a b s t r a c t We conducted true triaxial compression tests on specimens prepared from two siltstone core sections, one above and one below the Chelungpu Fault, Taiwan. For different constant s 2 and s 3 magnitudes, the maximum principal stress (s 1) was raised until a post failure stage was reached, and a through-going fault had developed. Despit...

This is not a scientific paper in the normal sense. It is more like a lecture in which I try to point out that in conducting in situ stress measurements by hydraulic fracturing the exception is often the rule. Results of measurements can often be bewildering, requiring thorough investigation of the reasons for the unexpected stress conditions. I fo...

Wellbore instability often leads to failure in the form of breakouts, which are a major concern in the petroleum industry because they interfere with production and facilitate unwanted sanding. We report on two sets of experiments in which a high-porosity quartz sandstone was subjected to conditions inducing breakouts around boreholes. As in other...

We conducted true triaxial compression tests on rectangular prismatic specimens (19×19×38 mm) of siltstone core extracted from a depth of 1252 m, some 140 m below the borehole intersection with the Chelungpu Fault, Taiwan. Experiments consisted of four series of tests in each of which sigma3 was kept constant and sigma2 was varied from test to test...

To reveal details of stress perturbations associated with faults and fractures, we investigated the faults and large fractures accompanied by stress-induced borehole breakouts or drilling-induced tensile fractures in hole B of the Taiwan Chelungpu-fault Drilling Project (TCDP). Then, we determined the relationship between the faults and fractures a...

The paper describes the computation of the maximum horizontal stress (σH) magnitude in the vicinity of the Chelungpu Fault, Taiwan, host of the slip zone during the Chi-Chi earthquake (Mw 7.6; 1999). The scientific hole B intercepts the Chelungpu Fault at 1136 m. At the depths of logged breakouts (940–1310 m), the vertical stress (σv) as estimated...

We conducted two series of tests on boreholes drilled into St. Meinrad sandstone (quartz-rich, 28% porosity, very fine-grained) subjected to critical far-field stresses that resulted in breakout formation. In one series, 50 mm dia. samples with 5 mm through-going central boreholes were loaded hydrostatically at a constant rate of 1MPa/min until acc...

Drilling experiments in rock blocks subjected to pre-existing true triaxial far-field stresses simulating real in situ conditions often result in localized failure around the created borehole, which brings about the formation of borehole breakouts. In weakly bonded quartz-rich porous sandstones breakouts take the form of narrow tabular (slot-like)...

Conventional triaxial tests (in which σ1 > σ2 = σ3)suggest that the process of brittle failure in rock begins with microcracking upon dilatancy onset, followed by localization along an eventual shear band, ending in faulting or brittle failure. True triaxial tests (σ1 > σ2 > σ3) reveal additional details of the mechanical behavior not otherwise obs...

The Taiwan Chelungpu-fault Drilling Project (TCDP) is located some 100 km north of the Chi-Chi earthquake (1999) epicenter. Scientific hole B intercepts the active fault at 1136 m. Borehole breakouts were logged between 930 and 1300 m, thus straddling the fault. The vertical stress (sigmav) within this depth range, as estimated from density logs, v...

This paper reports on a series of hydraulic fracturing (HF) tests conducted near a headrace tunnel for the purpose of estimating the stability of the excavations. It is shown that the results although defendable, could have benefited from proposed experiments in the future Deep Underground Scientific and Engineering laboratory (DUSEL) at Homestake...

Salinian granodiorite core from the 1462-1470m segment of the SAFOD drillhole was used to derive its critical mechanical properties under true triaxial stress conditions, analyze shear localization and brittle fracture characteristics, and establish the strength criterion under dry conditions (Eos Trans. AGU, 87/52, Abstract T32C- 03). Here we repo...

We report on two sites in Iceland Blanda and Kárahnjúkar, where hydraulic fracturing stress measurements were performed in the initial stages of pre-excavation design of two underground hydroelectric projects. The dominant rock is young basalt, naturally fractured and blocky. The measured stresses were used in both cases to define the boundary cond...

We conducted true triaxial compression tests to examine the effect of the intermediate principal stress (sigma 2) on brittle failure and deformability of the siltstone overlying the Chelungpu fault, Taiwan. For constant minimum stress (sigma 3) a pattern of increased strength (sigma 1) as a function of sigma 2, unpredicted by common strength criter...

This paper reviews the different borehole breakout failure micromechanisms observed during a multiyear laboratory research effort at the University of Wisconsin. Vertical borehole drilling experiments were conducted in a variety of granites, limestones, and sandstones under a wide range of pre-existing stress fields. Test samples that developed bre...

We conducted laboratory true triaxial experiments in the nearly impermeable Pohang rhyolite to investigate failure mechanisms under ‘dry’ and ‘wet’ rock conditions. Under ‘dry’ conditions prismatic specimens were jacketed all around to prevent confining fluid penetration. Under ‘wet’ conditions one pair of the specimen faces was left unjacketed and...

Drilling of vertical wellbores in sandstone often results in stress-induced failed zones called breakouts. In the laboratory, miniature drilling under simulated far-field crustal stress conditions above a certain threshold produces breakouts that can be studied to enhance our understanding of deformation and failure mechanisms in rock. We report on...

A limited amount of core from the 1462-1470m segment of the SAFOD (San Andreas Fault Observatory at Depth) drillhole, near Parkfield, California, was made available to us for mechanical testing under true triaxial stress conditions. The objective of this project is to derive critical mechanical properties, analyze brittle fracture characteristics,...

We conducted laboratory vertical drilling experiments to study the relationship between in-situ stress and borehole breakouts in two types of sandstones, arkosic and quartz-rich, varying in porosity from 16% to 28%. Arkosic sandstones (Tablerock and Tenino) developed V-shaped breakouts along the σh springline. Breakout dimensions (length and angula...

Laboratory drilling experiments were conducted to determine the effects of several drilling variables on the development of breakouts in the quartz-rich, 28% porosity, fine-grained, St. Meinrad sandstone. Resulting breakouts were all fracture-like, i.e. tabular and very long and narrow throughout. Tests showed that breakout depth (or length) was di...

We conducted laboratory rock strength experiments in two ultra-fine-grained brittle rocks, hornfels and metapelite, which together are the major constituent of the Long Valley Caldera (California, USA) basement in the 2025–2996 m depth range. Both rocks are banded, and have very low porosity. Uniaxial compression tests at different orientations wit...

We employed our new polyaxial cell to carry out true triaxial compression tests on dry (jacketed) rectangular prisms of two crystalline rocks, in which different magnitudes of the least and intermediate principal stresses sigma(3) and sigma(2) were maintained constant, and the maximum stress sigma(1) was increased to its peak level in strain contro...

A research project is underway at the University of Wisconsin-Madison, the principal objective of which is to investigate deformability, brittle fracture, and rock strength criteria adjacent to and inside the San Andreas Fault Zone. This laboratory study is being carried out on core extracted during the drilling of SAFOD. The rock mechanical proper...

We conducted laboratory-drilling experiments in Coconino sandstone in which vertical boreholes were drilled into prismatic samples subjected to true-triaxial far-field stresses (\sigmaH > \sigmav > \sigmah) simulating field conditions. This early Permian aeolian sandstone comes from Arizona, and was selected because of its medium porosity (16-20%)...

Two pre-excavation hydraulic fracturing (HF) stress measurement campaigns are reported, emphasizing their usefulness for rock mass characterization and the process of rational design of respective engineering projects. In one campaign the successful design of a hydroelectric scheme has been attributed in part to the HF measurements conducted in two...

We compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high-porosity sandstones subjected to unequal far-field principal stresses. Tablerock sandstone has a porosity of 28%, and is composed of 55% quartz and 37% weaker feldspar grains. Grain cementation is substantial through microcrystalline quartz. Critic...

We compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high-porosity sandstones subjected to unequal far-field principal stresses. Tablerock sandstone has a porosity of 28%, and is composed of 55% quartz and 37% weaker feldspar grains. Grain cementation is substantial through microcrystalline quartz. Critic...

We have conducted five hydraulic fracturing stress measurement campaigns in Korea, involving 13 test holes ranging in depth from 30 to 250 m, at locations from North Seoul to the southern coast of the peninsula. The measurements reveal consistent crustal stress magnitudes and directions that suggest persistence throughout western and southern Korea...

Stress-induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far-field stress. Wellbores are sometimes drilled into aquifers or oil reservoirs that are weak, poorly consolidated, and highly porous sandstone formations, which ar...

Vertical drilling experiments in high-porosity (22% and 25%) Berea sandstone subjected to critical true triaxial far-field stresses, in which ay (maximum horizontal stress) > a, (vertical stress) > ah (least horizontal stress), revealed a new and non-dilatant failure mechanism that results in thin and very long tabular borehole breakouts that have...

We conducted laboratory-drilling tests under true triaxial stress conditions (sigma_Hneqsigma_hneqsigma_v) on seven porous sandstones, seeking an understanding of the mechanics of failure that results in and propagates borehole breakouts. Three of the sandstones (Kyune, n=16%; Berea, n=17%; and Tablerock, n=30%) contain only 55 to 75% quartz grains...

The Aztec sandstone used in this research is from the Valley of Fire State Park area, Nevada. This Jurassic aeolian sandstone is extremely weak (uniaxial compressive strength of 1-2 MPa); porosity averages 26%; grains are subrounded and have a bimodal size distribution (0.1 mm and 0.5 mm); its mineral composition (K. Sternlof, personal comm.) is 93...

Estimation of the maximum horizontal in situ stress from logged borehole-breakout dimensions in brittle elastic rocks requires knowledge of the true triaxial compressive strength of the medium. We employed our true triaxial loading apparatus to determine the strength criterion of the nearly impermeable amphibolite penetrated by the German Continent...

We induced borehole breakouts in a 25%-porosity Berea sandstone by drilling 23 mm diameter holes into 152×152×229 mm blocks subjected to constant true triaxial far-field stresses. BSen5 consists of large quartz grains (0.5 mm) cemented mainly by sutured grain contacts. Breakouts in BSen5 are demonstratively different from those observed in granite,...

We conducted true triaxial compression tests in two metamorphic rocks, hornfels and metapelite, recovered from the Long Valley Exploratory Well, California, in order to determine their deformation and failure characteristics. The two rock types constitute the major lithology of the Long Valley caldera basement in the 2025-2859 m depth range. Both r...

We are conducting tests in St. Peter sandstone (average grain size 0.25 mm, cementation by sutured grain contact) of various porosities (ranging from 11 to 19%), in which drilling of vertical holes into 130 x 130 x 180 mm blocks subjected to pre-determined true triaxial far-field stresses (sigma H!=qsigma h!=qsigma v) induce borehole instability re...

We carried out hydraulic fracturing tests in hollow cylinders of Tablerock sandstone subjected to vertical (σ ν ), confining (σ h ) and pore (P o ) pressures. Borehole fluid was injected at a constant flow rate until a peak pressure was reached, and vertical fracture was observed. Based on the analysis of pressure-time records, we submit that brea...

It has long been recognized that the intermediate principal stress \sigma2 is an important contributor to rock strength, but this information has largely gone unnoticed, in part due to the difficulties of multi-axial testing and because of the simple elegance of the Mohr failure criteria. Yet, it is well documented that the state of stress in the e...

We designed and fabricated a true triaxial loading system and used it to determine deformational and strength characteristics of the amphibolite penetrated by the superdeep hole drilled in the Bohemian massif of southeastern Germany under the German Continental Deep Drilling Program (KTB). Amphibolite is found between 3200 and 7300 m and thus the d...

A new true triaxial cell has been designed, fabricated, calibrated, and successfully tested. Its main feature is very high loading capability in all three orthogonal directions, enabling the testing to failure of hard crystalline rocks subjected to large least and intermediate principal stresses. All three principal stresses applied to rectangular...

Laboratory-drilling experiments were carried out in Berea sandstone blocks of two different porosities in which borehole breakouts were induced under several truly triaxial far-field stress conditions. A petrographic microscope was used to study thins sections of the drilled boreholes. The mechanism of failure that ultimately resulted in breakout f...

We conducted laboratory simulation tests of borehole breakouts and investigated their potential use as an indicator of in situ stress magnitudes in Westerly granite and Berea sandstone. We also carried out simple triaxial tests and used the results to derive several strength criteria for these rocks. Using the assumption that the state of stress at...

A comprehensive laboratory investigation of the mechanical properties of pink Lac du Bonnet granite was carried out. Specimens were tested in three orthogonal directions. In uniaxial and triaxial compression the strength and deformability were practically isotropic. Sonic velocity tests yielded minor directional differences on the order of 10%. Bra...

A set of 10 complete hydraulic fracturing stress measurements in an anorthosite was conducted as part of the site investigation of the SM3 hydroelectric project, located some 800 km northeast of Montreal, Quebec. These were the first such measurements in that region of Canada. The vertical test hole was N-size, 260 m deep and adjacent to the future...

The determination of material parameters for rocks which display anisotropic behaviour has become more important in light of the development of powerful and economical analytical modeling techniques. The study presented herein is based on a testing framework aimed at determining the material parameters required to describe the behaviour of a transv...

Hydraulic fracturing (hydrofracturing) is the best method of assessing in situ stress at great depths. After a brief history of the method, the test equipment and set up are described. Data interpretation from the key pressures and fracture characteristics are given. In situ stress derivation from test data is analysed in terms of axial hydraulic f...

We subjected vertical boreholes drilled in cubical specimens of Cordova Cream to a 3-dimensional stress field, and increased one or both of the horizontal principal stresses until breakout failure occurred. Acoustic emission, borehole strain, and visual monitoring of the borehole wall during testing revealed four stages of breakout development: cra...

We conducted 9 complete hydraulic fracturing in situ stress measurements in vertical borehole HF1 at the 420 Level of AECL's Underground Research Laboratory (UPL), near Pinawa, Manitoba. The tests did not result in vertical fractures. Thus, a generalized least-squares criterion was employed to compute the estimated principal horizontal in situ stre...

We subjected boreholes drilled vertically in the center of cubical specimens of Lac du Bonnet granite to constant vertical (S(v)) and least horizontal (S(h)) far-field stresses, and increased the maximum horizontal stress (S(H)) until breakouts occurred. A thin section study revealed that extensile cracking behind the borehole in two zones aligned...

In this paper, which is the first of a two-part series, a laboratory investigation of the mechanical behavior of initially closely mated joints in rock undergoing small sliding displacements is undertaken. The goal is to elucidate important features of joint behavior such as dilatancy, damage of surface roughness, cyclic sliding behavior and the de...

In this paper, which is the second of a two-part series, an analytical investigation of the mechanical behavior of initially closely mated joints in rock undergoing small sliding displacements is undertaken. Guided by the experimental observations reported in Part I, the goal is to develop a quantitative theory for joint behavior including phenomen...

This special issue contains 166 papers arranged in the following sections: experimental techniques and laboratory investigations; geophysical properties of rocks; rock fractures and joints; acoustic emission and failure prediction; in situ stress measurements; borehole instability and breakouts; coupled mechanical and hydrologic processes; rock mec...

These papers were originally presented at the 34th US Symposium on Rock Mechanics, June 1993. Fifteen papers are presented and deal with in situ stress determination by hydrofracturing techniques, stress heterogeneity, gravitational stress effects, anelastic strain recovery measurements, and differential strain curve analysis. -R.Gower

The First International Workshop on Scale Effects in Rock Masses was held in Loen, Norway in 1990. The current state of knowledge on scale effects in rock stress measurements, as reflected in the workshop proceedings is summarized in this paper. One type of scale effect is inherent in the rock structure due to heterogeneities ranging from micro- to...