Peridynamics modeling of underground borehole instability: Collapse mechanism and stabilization strategy

Abstract

Borehole instability is a critical challenge that affects safety and efficiency in deep drilling. Classical continuum mechanics struggles to accurately capture the discontinuous processes of crack initiation and propagation around boreholes. This paper develops and validates a nonlocal borehole damage model based on peridynamics. The borehole collapse mechanism is explored, and the regulatory role of drilling fluid pressure in maintaining borehole stability is evaluated. The results show that borehole collapse initiates along the direction of minimum horizontal pressure, characterized by a crescent-shaped shear failure accompanied by tensile fractures. The borehole eventually evolves into a butterfly-shaped damage pattern with a central fragmented zone. Increasing the elastic modulus of the surrounding rock and reducing the borehole radius effectively inhibits damage propagation. As the elastic modulus increases from 6 to 30 GPa, the areas of the collapse zones are reduced by 87.50%, indicating a substantial enhancement in the material’s resistance to both microcrack initiation and macroscopic instability. Low-modulus rocks are more prone to form continuous shear-fracture zones. In contrast, the horizontal pressure difference emerges as the primary driver of damage evolution; once it exceeds 30 MPa, the crack growth resistance deteriorates rapidly, leading to accelerated crack coalescence and the formation of a connected fracture network. An optimal drilling fluid pressure window can suppress up to 85.05% of the damaged area. However, excessive pressure may induce radial tensile fractures. The findings revealed the mechanisms of borehole collapse and confirmed the superiority of the peridynamics model in predicting borehole instability. This study provides theoretical insight and methodological support for stability control in deep drilling operations.