What Does Anisotropic Stress Mean?
Anisotropic stress in the context of in-situ stress refers to the stresses in a formation that are not isotropic or are directionally dependent. Confined underground formations experience vertical stress, minimum horizontal stress and maximum horizontal (also called principal stresses.)
The magnitude and direction of these stresses depend on tectonic conditions and influence rock failure. Stresses in underground formations are not uniform and change in magnitude based on direction. The stresses are generally anisotropic, non-homogenous, and compressive.
Trenchlesspedia Explains Anisotropic Stress
Formation rock anisotropy and tectonic influence produces horizontal stress anisotropy and results in a difference between the two principal horizontal stresses. The anisotropic and non-homogenous nature of rock formations makes the actual state of stress in the rock formation very complex.
In formations with low permeability, low Poisson's ratio and high stiffness, the stress anisotropy caused by the principal stresses may be more severe than in conventional reservoirs.
The possibility of wellbore failure increases with increase in the difference between the principal stresses. For high anisotropic stress, wellbore instability during drilling can cause significant breakouts in the wellbore in the direction of minimum principal stress, especially in tight sand formations.
The minimum horizontal stress can be estimated using acoustic log data analysis or by diagnostic fracture injection testing (DFIT).