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.]
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:
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).
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 borehole breakouts, occurs often, and on a large scale during these projects (Kaiser et al., 1985; Ewy and Cook, 1990a, 1990b).
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., 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).
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).
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 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).
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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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 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).
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).