1. Borehole Breakouts and In-situ Rock Stress--A Review
Stephen Prensky, U.S. Geological Survey, Denver
[Originally Published in 1992, The Log Analyst, v. 33, no. 3, p. 304-312.]
Introduction
Borehole breakouts, so named by Babcock (1978), are enlargements and elongation of a borehole in a
preferential direction and are formed by spalling of fragments of the wellbore in a direction parallel to the
minimum (least) horizontal stress (Sh). Borehole spalling occurs along intersecting shear fractures
generated soon during drilling and progresses with time (Bell, 1990). The identification and analysis of
borehole breakouts as a technique for in-situ measurement of stress orientation and magnitude, and for
identifying orientation (azimuth) of both naturally occurring and induced fractures (hydrofrac), has
received a great deal of attention during the past ten years. Knowledge of the orientation of horizontal
earth stresses derived from analysis of borehole breakouts is important to the following areas of study:
Reservoir Applications
Planning Hydrocarbon Exploration Strategies. Locating fracture porosity and permeability in specific
rock formations to maximize recovery (Babcock, 1978; Schafer, 1979; Baumgardner and Laubach, 1987).
Developing Production Strategies and Reservoir Engineering. In contrast to breakouts, hydraulic
(induced) fractures form perpendicular to the least principal stress. Knowledge of the orientation of
borehole breakouts can be used for predicting hydraulic fracture propagation. This information is essential
to (a) optimal placement of production and injector wells when designing and analyzing effective well
stimulation, waterflooding, and enhanced oil recovery (EOR) programs, especially in fractured and/or
low-permeability reservoirs (Hassan, 1982; Bell and Babcock, 1986; Hansen and Purcell, 1986; Guenot,
1989; Lacy and Smith, 1989).
Drilling and Wellbore Mechanics
Avoiding problems associated with drilling and borehole instability stemming from in-situ rock stress
(Hottman et al., 1979; Maury and Sauzay, 1987).
Studies of Crustal Stress
The orientation of stress within a tectonic plate reflects the forces acting on that plate, e.g. extension,
compression, or strike-slip (Gough et al., 1983; Suter, 1987; Dart and Zoback, 1988; Zoback et al., 1989;
Zoback and Zoback, 1991). Stress data for many boreholes are used to examine regional stresses patterns
and this information is in turn used to constrain plate tectonics models (Solomon et al., 1980; Mount and
Suppe, 1987; Moos and Zoback, 1990; Harper and Szymanski, 1991; Zoback, 1991; Zoback and Magee,
1991), regional tectonic processes (such as volcanism and faulting), and potential seismic hazards in
zones of crustal weakness (Zoback and Zoback, 1980; see discussion below).
Rock Mechanics
Understanding rock mechanics for the safe design and construction of cylindrical openings in stressed
rock, e.g. tunnels, mine shafts, and caverns (for waste storage). Sidewall failure (or slabbing), similar to

2. borehole breakouts, occurs often, and on a large scale during these projects (Kaiser et al., 1985; Ewy and
Cook, 1990a, 1990b).
Background
The advent of the four-arm dipmeter with its opposed pairs of calipers permitted a more accurate
description and measurement of borehole shape than the earlier three-arm version, specifically, borehole
asymmetry or ellipticity. Leeman (1964) reported fracturing of the borehole wall in zones of high stress
and Cox (1970), in a study in Alberta, Canada, was the first to observe a preferential elongation of
borehole direction, and he further observed that this elongation direction was independent of geologic age
and the magnitude of dip. Babcock (1978) also noted that depth, lithology, hole deviation, and breakout
azimuth are independent elements; that breakouts are associated with a slowing or cessation of dipmeter-tool
rotation since the calipers lock into a preferred azimuth (Figure 1); and that the azimuths of borehole
elongation and jointing in outcrop are parallel. While noting that the minimum tectonic stress direction is
parallel to the dominant azimuth of borehole elongation, Babcock (1978) and Schafer (1979), ascribed
breakouts to the intersection of the borehole with preexisting joints (as seen in outcrop).
One of the early selling points for Schlumberger's Fracture Identification Log, based on the 4-arm
dipmeter, was that breakouts (hole ellipticity), particularly in the fractured chalks of Louisiana and south
Texas, could be caused by fracturing, and breakouts could be used as an indicator of fracturing in these
rocks (Beck et al., 1977; Babcock, 1978; Schafer, 1979). Cox (1982) did not find a correlation between
fractures and breakouts except for the Cotton Valley and Austin Chalk. Baumgardner and Laubach (1987)
suggested that the same borehole elongation in the Travis Peak Formation of east Texas may be caused by
fractures and Baumgartner et al. (1989) found breakouts associated with natural fractures in crystalline
rock.
Bell and Gough (1979, 1982) noted that the conclusions of Babcock (1978) and Schafer (1979) regarding
breakouts and jointing did not account for a second, equally prominent and perpendicular joint set also
seen in outcrop. They argued that breakouts are related to unequal horizontal stresses. Hottman et al.
(1979) independently arrived at the same conclusion.
Breakouts as a Stress Indicator
Drilling a wellbore in stressed rock causes these stresses to be redistributed and a zone of yielded rock, a
breakout, results (Maloney and Kaiser, 1989). Bell and Gough (1979, 1981, 1982) and Gough and Bell
(1981, 1982) using data from in-situ stress measurements demonstrated that breakouts both in Canada and
Texas are formed by brittle shear fracture around the borehole and that breakout azimuth is related to the
compressive forces of unequal horizontal principal stresses near the borehole (Figure 2). Breakouts form
in the direction perpendicular to the principle horizontal compressive stress. In addition to conclusions
based on empirical observation, formation of borehole breakouts has been analyzed based on rock
mechanics theory (Bell and Gough, 1982; Gough and Bell, 1982; Zoback et al., 1985; Papanastasiou et
al., 1989; Plumb, 1989; Zheng et al., 1989; Qian and Pedersen, 1991; Fjaer et al., 1992) and laboratory
experiments (Mastin, 1984; Haimson and Herrick, 1985, 1986, 1989; Ewy et al., 1990; Onaisi et al.,
1990; Hansen, 1991).
McGarr and Gay (1978), Zoback and Zoback (1980), Zoback and Haimson (1982), and Gough and
Gough (1987) reviewed the available methods used for in-situ measurement of stress: overcoring (stress-relief),
induced hydrofracturing (microfracturing), strain/stress gauge, earthquake fault-plane solutions.
Stress orientations inferred from breakout azimuths are consistent with data obtained by these other,
independent measurements of in-situ stress (Blumling et al., 1983; Fordjor et al., 1983; Newmark, et al.,

3. 1984; Dart, 1985; Hickman et al., 1985; Plumb and Hickman, 1985; Teufel, 1985; Zoback et al., 1985;
Bell and Babcock, 1986; Plumb and Cox, 1987; Mount, 1989).
Identifying Breakouts
Not all elliptical borehole enlargements are stress-induced breakouts: Dart and Zoback (1988) described
six types of borehole enlargement, including breakouts; Fordjor et al. (1983), Plumb and Hickman (1985),
and Springer (1987) proposed criteria for recognizing breakouts from 4-arm dipmeter logs and
distinguishing them from other causes of borehole ellipticity (Figure 3). Plumb and Cox (1987) discussed
four assumptions involved in inferring stress directions from dipmeter data: (1) failure and elongation of
the borehole is due to brittle fracture and not to plastic deformation; (2) elongation is not due to the
intersection of natural fractures; (3) the well is drilled parallel to one of the principal stresses; (4) borehole
elongation is symmetric.
Besides the dipmeter several other downhole devices have been used for examining borehole breakouts;
these include motion pictures (Springer and Thorpe, 1981; Springer et al., 1984) and both acoustic
(BHTV) and electrical (FMS) borehole-imaging devices (Healy et al., 1984; Newmark et al., 1984; Paillet
and Kim, 1985; Plumb and Hickman, 1985; Zoback et al., 1985; Barton, 1988; Barton et al., 1988; Burns,
1988; Shamir et al., 1988; Morin et al., 1989; Shamir and Zoback, 1989) (Figure 4). While dipmeter data
are most often used in regional and field studies because they are widely available in areas of
hydrocarbon exploration, imaging tools are considered the best devices for identifying breakouts and
distinguishing them from other types of borehole elongation (Springer, 1987; Bell, 1990). Plumb (1989)
used digital BHTV data for establishing criteria to distinguish breakouts caused by natural fractures
versus drilling-induced fractures.
Measurements of Stress Magnitudes from Breakouts
The reliability of hydraulic fracturing for measurement of in-situ stress in the hostile environments of
high pressure and high temperature (deep wells, geothermal wells, naturally fractured rock) is
questionable and an alternate method for estimating stress magnitudes is needed, i.e., the quantitative
analysis of breakouts (Haimson and Herrick, 1986; Zoback et al., 1986). Theoretical and laboratory
studies conclude that in quasi-isotropic (e.g., sedimentary) rocks, breakout geometry (depth and width,
shape) are related to the magnitude of Sh. Haimson (1987) declareed that the potential exists for using
breakouts to estimate stress magnitudes if the dimensions of the failed zone can be determined. Barton et
al. (1988) proposed a method for using breakout width, obtained from BHTV images, to estimate stress
magnitudes. There is, however, disagreement as to the extent to which this geometry can be used and Bell
(1990) pointed out the difficulty in obtaining reliable measurements needed to arrive at these values and
as well as the need to better understand the mechanism of rock failure. Vernik and Zoback (1992)
reported that Shmax profiles estimated from breakouts compares "fairly well" with those from hydraulic
fracturing. Additional work is being carried out to better understand implications for in-situ stress
evaluation from breakouts in anisotropic (e.g., igneous and metamorphic) rocks where the failure
mechanism may not be the same as in isotropic rocks (Paillet and Kim, 1985; Plumb, 1989; Vernik and
Zoback, 1989, 1990).
Recent Developments
Mastin (1988) discussed the effect of borehole deviation on breakouts in different faulting regimes
(normal, strike-slip, thrust). Lacy and Smith (1989), Avasthi et al. (1990), and Bell (1990) reviewed the
methods used for measuring in-situ stress and fracture orientation, including breakout data, and the
applications of this information to well stimulation. Allison and Nielson (1988) suggested an additional

4. application of breakout data: to guide directional drilling in geothermal wells to increase the probability
of intersecting the greatest number of active or open fractures. A primary objective of deep scientific
drilling is the determination of in-situ stress; however, the high pressures required to initiate induced
fractures for measuring in-situ stress, combined with the high bottomhole temperatures encountered in
theses wells, may exceed limits of current packer technology (Zoback et al., 1986). Thus, borehole
breakouts may become the primary method for evaluating in-situ stress orientation.
Regional Stress Regimes
The unequal stresses around a borehole are representative of regional stress fields that are related to
compressional, extensional, and strike-slip tectonic forces that produce regional faulting. An improved
understanding of the orientation and magnitude of earth stress, in part obtained through analysis of
borehole breakouts, can contribute to understanding earthquake mechanisms and future prediction/control
(McGarr and Gay, 1978; Zoback and Zoback, 1980; Zoback and Zoback, 1981; Newmark et al., 1984;
Zoback and Healy, 1984; Springer, 1987; Zoback et al., 1987; Shamir et al., 1988; Zoback, 1991; Vernik
and Zoback, 1992; Zoback and Healy, 1992). Under normal conditions, breakout orientation is constant
(homogeneous) with depth. In seismically active areas, where the stress regime has been disturbed by
faulting, breakout orientations are heterogeneous and this heterogeneity may serve as an indicator of
geologically recent fault movement (Nielson, 1989; Allison, 1990; Shamir et al., 1990; Hansen, 1991,
Zoback and Magee, 1991).
Recent studies of regional stress regimes and stress provinces that incorporate data from borehole
breakouts include: global patterns (Zoback et al., 1989); the Western Canada basin (Bell and Babcock,
1986; Gough and Gough, 1987; Bell, 1990); central and eastern U.S. (Dart and Zoback, 1987); Oklahoma
and Texas Panhandle (Dart, 1989); California (Mount, 1989); Continental U.S. and North America
(Zoback and Zoback, 1989; 1991); Alaska (Estabrook and Jacob (1991); Canada (Adams and Bell, 1991;
Yassir and Dusseault, 1991); Mexico and Central America (Suter, 1987; 1991); Europe (Becker et al.,
1987; Brereton and Evans, 1989; Brereton and Mueller, 1991; Muller et al., 1992); United Kingdom
(Evans and Brereton, 1990); North Sea and Norwegian shelf (Clauss et al., 1989; Spann et al., 1991).
Summary
Measurement of in-situ rock stress is important to hydrocarbon exploration and exploitation, rock
mechanics, and scientific research. Data on borehole breakouts acquired by dipmeter and, more recently,
from borehole-imaging tools, provide a readily available, inexpensive, and worldwide database for the
determination of in-stress orientation. Ongoing research on the physics of breakout formation under
different stress conditions may eventually permit determination of in-situ stress magnitude directly from
breakout geometry.
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California and a seismic structural analysis of the Perdido fold belt, northwest Gulf of Mexico:
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Mount, V.S., and Suppe, J., 1987, State of stress near the San Andreas fault--implications for
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10. Muller, B., et al., 1992, Regional patterns of stress in Europe: Journal of Geophysical Research,
v. 97, no. B7, June 10, [in press].
Newmark, R.L., Zoback, M.D., and Anderson, R.N., 1984, Orientation of in situ stresses in the
oceanic crust: Nature, v. 311, October 4, p. 424-429. Also published in 1985, as, Orientation of
the in situ stresses near the Coast Rica rift and Peru-Chile trench--Deep Sea Drilling Project Hole
504B, in Initial reports of the Deep Sea Drilling Project, v. 83: U.S. Government Printing Office,
p. 511-515.
Nielson, D.L., 1989, Stress in geothermal systems: Geothermal Resources Council Transactions,
v. 13, p. 271-276.
Onaisi, A., Sarda, J.P., and Bouteca, M., 1990, Experimental and theoretical investigation of
borehole breakouts, in Hustrulid, W.A., and Johnson, G.A., eds., Rock mechanics contributions
and challenges [proceedings of the 31st U.S. symposium on rock mechanics]: A.A. Balkema,
Rotterdam, p. 703-710.
Paillet, F.L., and Kim, K., 1985, The character and distribution of borehole breakouts and their
relationship to in situ stresses in deep Columbia River basalts: Rockwell Hanford, Richland,
Washington, Operations Report RHO-BW-CR-155, December, 27 p. Later published in
1987, Journal of Geophysical Research, v. 92, no. B7, p. 6,223-6,234. Later reprinted in
1990, in Borehole imaging reprint volume: Society of Professional Well Log Analysts, p. 387-
398.
Papanastasiou, Vardoulakis, I.G., and Santarelli, F.J., 1989, Modeling borehole
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Engineers, PD-Vol. 22, p. 49-55.
Plumb, R.A., 1989, Fracture patterns associated with incipient wellbore breakouts, in Maury, V.,
and Fourmaintraux, D., eds., Rock at great depth: A.A. Balkema, Rotterdam, v. 2, p. 761-768.
Plumb, R.A., and Cox, J.W., 1987, Stress directions in eastern North America determined to 4.5
km from borehole elongation measurements: Journal of Geophysical Research, v. 92, no. B6,
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Plumb, R.A., and Hickman, S.H., 1985, Stress-induced borehole elongation--A comparison
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11. Engineers, New York, PD-Vol. 22, p. 11-15. A similar paper was published in 1989 as, The stress
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stress induced borehole breakouts, in Maury, V., and Fourmaintraux, D., eds., Rock at great
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Springer, J.E., 1987, Stress orientations from well bore breakouts in the Coalinga
region: Tectonics, v. 6, no. 5, p. 667-676.
Springer, J.E., and Thorpe, R.K., 1981, Borehole elongation versus in-situ stress orientation:
Lawrence Livermore National Laboratory, Report No. UCRL-87018, 15 p.
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to tectonic stress at the Nevada Test Site: Lawrence Livermore National Laboratory, Report No.
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chapter 22, in Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Blackwell, D.D.,
eds., Neotectonics of North America: Geological Society of America, Decade Map Volume 1, p.
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masses [26th U.S. symposium on rock mechanics, South Dakota School of Mines and
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of the state of stress at depth, in Maury, V., and Fourmaintraux, eds., Rock at great depth: A.A.
Balkema, Rotterdam, v. 3, p. 1041-1048.

12. Vernik, L., and Zoback, M.D., 1990, Strength anisotropy in crystalline rock--implications for
assessment of in situ stresses from wellbore breakouts, in Hustrulid, W.A., and Johnson, G.A.,
eds., Rock mechanics contributions and challenges [31st U.S. symposium on rock mechanics,
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1983, Hydraulic fracturing and stress measurements: National Academy Press, 270 p.
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v. 2, no. 6, p. 689-698.
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Zoback, M.D., and Zoback, M.L., 1991, Tectonic stress field of North America and relative plate
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13. Zoback, M.D., Zoback, M.L., Mount, V.S., et al., 1987, New evidence on the state of stress of the
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Zoback, M.L., and Magee, M., 1991, Stress magnitudes in the crust--constraints from stress
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About the Author
Stephen Prensky is a research geologist with the U.S. Geological Survey, Branch of Petroleum Geology,
Denver, Colorado. He has been with the USGS since 1975, working as well-log specialist in reservoir
characterization. Previous experience includes exploration and production geology with Texaco's
Offshore Division. He holds a B.A. and M.S. in geology from SUNY Binghamton, and the University of
Southern California. Stephen has been a member of SPWLA since 1978, and is currently serving on the
Board of Directors as Vice-President of Publications. His "Bibliography of Well-Log Applications," has
been published annually in The Log Analyst since 1987. Stephen is also a member of AAPG, MGLS,
SCA, and SPE.
Figure Captions
Figure 1. Annotated dipmeter log record with zones of borehole breakout. Breakout intervals are
indicated by caliper trace separation and interruption in the normal diagonal pattern of the
azimuthal trace. Fluctuation in conductivity values also delineate, to some extent, breakout
intervals (Figure and caption from Dart and Zoback, 1988).
Figure 2. Cross-sectional schematics of wellbores showing original borehole shape (dashed line)
and enlarged borehole shape (solid line). Orientation of borehole elongation relative to the
directions of the principal horizontal stresses for breakout (A) and fracture (B) is shown (Figure
and caption from Dart and Zoback, 1988).
Figure 3. Examples of dipmeter caliper logs and common interpretations of the borehole
geometry. Cal 1-3 and cal 2-4 indicate borehole diameter as measured between opposing
dipmeter arms. (a) An in-gauge hole. (b) The geometry resulting from stress-induced wellbore
breakouts. (c) A minor "washout" with superimposed elongations. (d) A key seat where the sonde
is not centered in borehole resulting in one caliper reading being less than bit size. The shaded
regions in the direction of elongation represent local zones of slightly higher conductivity when
compared with orthogonal direction (figure and caption from Plumb and Hickman, 1985.
Copyright by AGU, reprinted by permission).

14. Figure 4. Examples of borehole breakouts seen in borehole televiewer images. (a) and (b) are
continuous and (c) discontinuous breakouts (from Paillet and Kim, 1987. Copyright by AGU,
reprinted by permission).
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