Finite Difference Method‐Based Wellbore Stability Evaluation of Actual Horizontal Trajectories in Interbedded Coal

ABSTRACT Coal‐bed methane (CBM), as a key component of unconventional natural gas production, often encounters wellbore stability challenges during development. The inherent fragmentation and weak cementation of coal contribute to wellbore instability, while horizontal drilling challenges mainly arise from thin interlayers and fractured zones. However, many existing studies rely on simplifying assumptions, making it increasingly important to accurately model the stress state around wellbores with complex trajectories. In this paper, a hybrid‐grid geological model incorporating stress‐fluid coupling is developed using the finite difference method (FDM) to more realistically represent subsurface conditions, especially at lithologic interfaces. This study investigated stress distributions around a horizontal section under varying drilling fluid densities using the established model that accounts for lithological variations, and analyzed mechanical behavior under various dogleg severity, drilling fluid soaking times, and trajectory extension conditions. The results indicate that a wellbore located near a fault or interface exhibit more uneven stress distributions and face a higher risk of instability. Increasing the density of the drilling fluid enhances wellbore support, significantly reducing damage. Once a critical density (1.70 g/cm 3 ) is reached, the effect stabilizes, ensuring wellbore stability. Moreover, larger dogleg severity expands the damage zone. As soaking time increases, the influence of shear stress on wellbore instability grows progressively stronger. Overall, the geological model provides a new approach for designing well trajectories during the pre‐drilling phase and for making real‐time adjustments during drilling.