Study on Wellbore Instability Considering the Weak Plane- Seepage-Stress Coupled Effect

Wellbore instability is a critical challenge in deep and unconventional oil and gas drilling, with mechanisms particularly complex in fractured reservoirs due to the coupling effects of weak planes, seepage, and stress. Conventional models struggle to accurately describe wellbore instability under conditions involving multiple weak planes and anisotropic seepage. This study establishes a weak plane-seepage-stress coupling model, integrating in-situ stress, wellbore trajectory, and fracture orientation through coordinate system transformation. The weak plane strength criterion and Newton's iterative method are employed to predict wellbore collapse pressure. Results indicate that the presence of weak planes significantly disrupts the symmetry of stress distribution around the wellbore, leading to highly heterogeneous collapse pressure contours. Wellbore stability is primarily controlled by weak plane orientation, with the optimal drilling direction ranging from 200° to 280°. Meanwhile, the sealing efficiency of drilling fluid, characterized by the seepage coefficient δ, exerts a notable influence on wellbore stability: as δ increases from 0 (no sealing) to 1 (complete sealing), the collapse pressure decreases significantly, with the improvement being particularly pronounced under high sealing efficiency. Changes in the seepage field interact synergistically with the geometric attributes of weak planes, affecting the spatial distribution of collapse pressure. This study clearly demonstrates that enhancing wellbore sealing is the most effective engineering measure to control wellbore instability, providing an important theoretical basis for safe drilling design in fractured formations.