Collapse pressure prediction for shale formation with weak strength and polyaxial-wellbore failure in intact rock

• Intact rock collapse and weak bedding plane were analyzed in strike-slip regime. • Wellbore failure considered 3D radial, axial, and tangential stress components. • Quartic model and an approximate quadratic model were derived. • Collapse pressures varied with inclination, azimuth, and horizontal stresses. • Azimuths between 250°–40° and inclinations from 10°–43° were most unstable. Wellbore instability in anisotropic shale with weak bedding planes (WBPs) remains a major operational challenge in non-vertical wells, requiring more accurate analyses. This study develops an improved geomechanical modeling framework for predicting collapse pressure in both intact rock and WBPs, integrating full 3D wellbore stresses and a polyaxial failure model, unlike previous studies. Using secondary data from a well in a strike-slip stress regime, Jæger’s criterion was applied to characterize WBP failure. A quartic closed-form collapse-pressure solution was derived using Ferrari’s method, along with a simplified quadratic model for rapid field application. For intact rock, the predicted value (0.501 psi/ft) closely matched the graphical result (0.496 psi/ft). The quadratic model produces predictions approximating the quartic solution (difference of only 0.0006 psi/ft), offering faster computations without loss of accuracy. For WBPs, the prediction (0.63 psi/ft) aligns excellently with other shear-failure models (0.6312 psi/ft). Sensitivity analysis using Monte Carlo simulations showed that wellbore azimuth is the dominant variable in intact rock, whereas dip-trace angle and vertical stress exert the greatest influence in WBPs. Furthermore, the wellbore becomes most unstable for inclinations with azimuth of 250°– 40 ° , due to unfavorable stresses and alignment with WBPs. Horizontal wells aligned with the direction of maximum horizontal stress were most stable. Collapse pressures vary significantly with inclination, azimuth, bedding dip, and attack angle, with the weakest WBP response occurring at attack angles near 60° Overall, the proposed workflow improves safe mud-weight prediction in complex strike-slip regimes and provides a practical tool for drilling through anisotropic formations.