Numerical study of material contrast effect on damage and instability in wellbores under repeated drill string impacts

Drill string vibration during drilling plays a vital and potentially decisive role in maintaining wellbore stability, as repeated impacts may lead to fatigue and borehole collapse. While drilling through geological layers, a material contrast may act as a localization point for wellbore damage. The hypothesis tested in this paper is that wellbore instability is focused on the boundary between the layers and that mechanical contrasts accelerate the wellbore collapse. In this study, an elastic-plastic damage model was employed to investigate the effects of repeated mechanical impacts on wellbore stability. A 2-dimensional (2D) model of a wellbore surrounded by contrasting materials was developed, and the accumulated damage caused by repeated lateral impacts was monitored. It was found that damage develops not only around the wall of the wellbore but also along the material boundaries. A sensitivity analysis was carried out to identify the impact of contrasts in both elastic (Young's modulus and Poisson's ratio) and plastic (cohesion, friction angle, and dilation angle) parameters between layers. Four damage patterns were identified in the simulated models. The results also suggested that the number of impacts required to reach the critical damage was highly affected by the contrast in elastic parameters, while cohesion and friction angle contrasts had a lesser effect. Additionally, increasing the contrast in the dilation angle localized the damage, thus reducing the number of impacts required to trigger wellbore failure.