Data from the Cajon Pass (USA), KTB (West Germany), and Kola (USSR) scientific drillholes suggest that intrinsic strength anisotropy of crystalline rocks may be a major factor affecting wellbore breakout-derived orientation and magnitude of in situ stresses. Intrinsic strength anisotropy in crystalline rocks is primarily fabric-related and can reach as high as 50%. The amount of anisotropy mostly depends on content and degree of preferred orientation of mica and can be inferred from petrographic and microstructural analysis of rocks. Along with the amount of anisotropy, its symmetry and attitude to the borehole axis is critical when analyzing stress-induced breakouts. In the case of transversely-isotropic symmetry with plane of symmetry parallel to foliation, the rock failure at the wellbore wall is strongly affected by the dip of foliation. Estimation of both orientation and magnitude of maximum in situ principal stress from breakouts in crystalline rocks with high effective strength anisotropy (i.e. anisotropy in horizontal plain around the hole) requires detailed knowledge of the shape of the breakouts and strength anisotropy of the rock.
For the last decade wellbore breakout analysis has developed into a promising technique for estimation of in situ stress orientation and magnitude (Bell and Gough, 1979; Plumb and Hickman 1985; Zoback et al., 1985; Zheng et al., 1989, Maloney and Kaiser, 1989). In mechanically quasiisotropic rock breakouts occur at the azimuth of Shatin, the least horizontal principal stress, as a brittle failure of the rock matrix in response to stress concentration around the hole. However, plutonit and metamorphicrystalline rocks rarely correspond to the isotropic model, especially with respect to their strength properties (Borg and Handin, 1966; Gottschalk and Kronenberg, 1988; Vemik and Zoback, 1989; Gottschalk et al., in press). Vemik and Zoback (1989) pointed out that strength anisotropy can significantly affect both occurrence and localization of failure of the wellbore wall and proposed a simple conceptual model of breakout mechanism in anisotropic media based on the data from the Kola super deep well. The authors have stressed that neither orientation nor magnitude of the maximum in situ principal stress SH can be reliably assessed without preliminary thorough investigation of core strength and its directional distribution. In this paper we further expand on this problem and provide detailed data and observations from the Cajon Pass, California, and KTB, West Germany, scientific drillholes.