Risk-controlled wellbore stability analysis in anisotropic formations

• A semi-analytical solution for the stability of inclined boreholes.
• The model incorporates full material elastic anisotropy and undrained effect.
• Strength anisotropy analysis that incorporates matrix and slip failures.
• Different failure criteria are applied and contrasted.
• A proposed model for risk-controlled wellbore stability in highly stressed rocks.

The paper presents a semi-analytical solution for the stability of inclined boreholes drilled in isotropic and anisotropic formations. Conventional analytical or semi-analytical models are designed to yield no borehole collapse failure along the borehole wall. However, in deep wells, where there are high hoop stresses acting on the borehole wall, it is difficult to apply the safe mud weight produced by these ubiquitous models. The proposed solution in this work imposes a constraint on how much failure will occur along the borehole wall. This risk-controlled stability analysis will produce a safe mud window that can be realistically achieved during drilling operations. Analytical solutions for stress distribution for isotropic and anisotropic rocks are presented. In addition, a solution for the upper limit for the mud window to prevent tensile failure is developed. The initial poroelastic or “undrained” drilling effect on pore pressure is also incorporated in the discussed models.