The KTB borehole in the Oberpfalz region of West Germany will eventually be one of the deepest boreholes in the world and will offer a unique opportunity to study stress orientations at depth. In the KTB pilot hole (4,000 m in depth) information has been obtained from stress induced wellbore breakout analysis utilizing thirteen four-arm caliper logs and five digitized borehole televiewer logs. Caliper and televiewer data show that elongations are concentrated in two regions:
at 500–1200 m depth, and
at approximately 1500–2500 m depth.
In the upper depth interval, the smooth cross sectional shape of the borehole and the coincidence of the elongation direction to the borehole deviation direction suggest that elongations were caused by tool drag rather than stress-induced failure. In the lower depth interval the direction of borehole elongation doesn't agree with that of borehole deviation and elongations have cross sections that are typical of true stress-induced breakouts. From caliper log measurements, the average breakout orientation below 1500 m depth, weighted according to the number of meters of breakout, is N215±150E. This is within the standard deviation of the least compressive horizontal stress (Sh) orientations determined in hydraulic fracturing tests from holes at Falkenberg, West Germany, 30 km NE of KTB. However it also agrees with the dip direction of foliations in the gneissic rock through which much of the hole was drilled. Further monitoring of breakout orientation at KTB will help resolve the relative importance of these two factors (Sh direction and rock foliation) in controlling breakout orientation.
One of the most widespread methods of determining stress orientation in the earth is from wellbore breakouts. Breakouts are zones of stress-induced spalling and failure on opposite sides of a borehole wall that cause the borehole diameter to enlarge in a direction parallel to that of the least compressive horizontal stress (Sh; e.g. Bell and Gough, 1979, 1983; Gough and Bell, 1981, 1982; Zoback et al., 1985; Plumb and Hickman, 1985). Breakouts can provide two key pieces of information that are generally not provided by other stress indicators. First, they indicate principal stress directions in a depth range that lies between that of most earthquake focal mechanisms (commonly several kilometers to several tens of kilometers) and that of most other indicators, whose depths are usually within a few hundred meters of the surface. Second, they can provide nearly continuous information on changes in stress orientation with depth (Shamir et al., 1988). In addition, under ideal circumstances they provide additional information on the magnitudes of in situ stresses (e.g. Barton and Zoback, 1988; Moos and Zoback, in review). Numerous regional studies since the early 1980's (e.g. Bell and Gough, 1979; Zoback and Zoback, 1980, in press; Hickman et al., 1985; Janot et al., 1988; Zoback et al., 1988) have demonstrated that breakouts are regionally consistent in orientation, are generally not greatly affected by rock type and structure, and agree in orientation with other regional stress indicators.
