6.8 Strength anisotropy

Sedimentary rocks are anisotropic. Rock lamination contributes to stiffness and strength anisotropy. Consider a deviated wellbore as in Fig. 6.31. Far field stress anisotropy will result in high stress anisotropy regions (red zone perpendicular to the direction of maximum stress) and potential breakouts if the strength of intact rock is overcome. Yet, there will be other areas around the wellbore without such high stress anisotropy but high enough and aligned optimally to induce shear failure in regions weakened by the presence of rock lamination (blue zone). Such areas may also be prone to develop a second family of breakouts.

Figure 6.31: Implications of rock strength anisotropy on wellbore breakouts.
Image 7-RockAnisotropy-Breakouts-v2