The Implications of Using Anisotropic Elasticity and Fully-Triaxial Failure Criteria for Borehole Stability Analysis in Shales

Setiawan and Zimmerman (S, 2018) recently developed a computational toolkit for analyzing the stability of boreholes drilled at an arbitrary angle through an anisotropic formation. The borehole stresses can be computed using either the Hiramatsu-Oka solution based on isotropic elasticity, or the Lekhnitskii-Amadei anisotropic elasticity solution. Shear failure along the borehole wall can be modeled using two variants of the Jaeger plane of weakness model, using either the Mohr-Coulomb or the Mogi-Coulomb failure criterion for failure along planes other than a bedding plane. In this paper, the implications of using these various models are investigated. All four variants of the above-mentioned models have been evaluated over a wide range of the relevant parameter space, including the elastic anisotropy ratio, the well deviation angle, the u stress anisotropy, c. The breakout pressure will be influenced by the elastic anisotropy when the ratio E_v/E_h exceeds about 2.3, even in a nearly-vertical wellbore. This effect is further amplified as the degree of elastic anisotropy increases, or the wellbore becomes more deviated. Due to its incorporation of the strengthening effect of the intermediate principal stress, the Mogi-Coulomb criterion predicts lower values of the minimum required mud weight than does the Mohr-Coulomb criterion.