The Influence of Fractures Induced by Drill Bit during Rock Fragmentation on Wellbore Stability

Summary Wellbore stability is a critical issue in drilling engineering, with most studies focusing on the interactions between drilling fluids and surrounding rock formations. However, the fractures induced by the roller bit during the rock fragmentation process have a significant yet often overlooked impact on wellbore stability. In this study, we investigate the role of drill-bit-induced fractures using a coupled numerical approach. First, a rock fragmentation model based on the discrete element method (DEM) is developed to analyze the distribution and length of fractures under varying wellbore pressures (Pw) and rotational speed (n). Then, these fractures are incorporated into a hydromechanical wellbore stability model using a coupled discrete fracture network-discrete element method (DFN-DEM) approach. Wellbore stability is quantitatively evaluated based on rock shear failure, rock radial displacement, and fracture shear failure. The results indicate that increasing wellbore pressure reduces tangential stress around the wellbore, effectively lowering confining pressure during rock fragmentation, which, in turn, promotes radial fracture propagation. Additionally, when wellbore pressure differs from formation pressure (Pw≠Pp), higher drill-bit rotational speeds lead to longer fractures. These fractures serve as pathways for drilling fluid infiltration, potentially triggering shear failure and compromising wellbore stability. Notably, under overbalanced drilling conditions, increasing wellbore pressure at lower rotational speeds proves beneficial in maintaining wellbore stability. Our findings suggest that adjusting drill-bit rotational speed during underbalanced and overbalanced drilling can effectively mitigate fracture propagation, thereby improving wellbore stability. With this study, we provide valuable insights for petroleum engineers by revealing the impact of drill-bit-induced fractures on wellbore stability.