An elasto-plastic damage model to investigate the wellbore failure under cyclic load of drill string

Abstract

During drilling operations, the drill string often undergoes vibrations, imposing cyclic loading on the wellbore walls. Such cyclic loads can potentially lead to wellbore failure due to fatigue, i.e. the gradual reduction of rock strength due to repeated loading at stresses below failure strength. While instances of wellbore failure due to vibration-induced loads from the drill string have been reported, research in this area remains limited. This paper introduces an elasto-plastic damage model designed to assess rock failure under cyclic loading. The Drucker-Prager elasto-plastic model with nonlinear hardening served as the foundational framework for the computational analysis. Additionally, a coupled damage model was proposed to simulate material strength degradation under cyclic loading. This customized elasto-plastic model, along with the coupled damage model, was implemented as a user-defined subroutine in Abaqus. The accuracy of the model is verified using existing experimental fatigue test data from the literature, demonstrating a good agreement between the predictions of the proposed model and the experimental findings. Utilizing this validated model, the impact of drill string vibrations on wellbore stability is further investigated. The findings underscore the significance of accounting for the repetitive impacts of the drill string, as they weaken the material strength. Relying solely on assessments of wellbore stability without considering the drill string impact seems to be overly optimistic. Notably, drill string vibrations result in damage localization around the wellbore, leading to the formation of localized bands. These bands eventually intersect, culminating in wellbore failure. The introduced elasto-plastic damage model offers a robust tool for evaluating rock fatigue failure.