ABSTRACT
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.
