The effects of drill string impacts on wellbore stability

• To address the problem of wellbore instability, the effect of drill string impacts on wellbore instability or enlargement is investigated systematically.
• Wellbore instability or enlargement cannot be attributed only to the physical–chemical interactions between the rock and the drilling fluid.
• Formulae for the drill string′s critical speed and the wellbore rock fragmentation volume are derived.
• Shallow formations are affected by drill string impacts more than deeper formations.
• The contact length of the drill string also greatly affects wellbore instability or enlargement.

Numerous studies have focused on the problem of wellbore instability or enlargement; however, most have focused on the physical–chemical interactions between the rock (especially shales) and the drilling fluid and have ignored the effects of drill string impacts. Drill string impacts on the wellbore wall often have important effects on wellbore instability. This paper presents a model for calculating drill string impacts on a wellbore wall based on an experimental study, principles of rock mechanics and rock damage mechanics and the Drucker–Prager yield criterion. The law of rock fragmentation during drill string impacts is investigated using simulations; later, relationships between the rock fragmentation volume and the drill string impact are developed under different conditions (contact lengths of the drill string, impact speeds, rotation speeds, well depths and rock softening). Moreover, formulae describing the drill string′s critical speed and the rock fragmentation volume when the wellbore rock fails are developed based on laboratory experiments and simulations. The results show that drill string impacts greatly influence wellbore instability or enlargement. In particular, shallower formations are affected more by drill string impacts than deeper ones. After rock softening, the drill string′s critical speed decreases, whereas the wellbore rock fragmentation volume increases; compared to a radial impact by the drill string, the drill string′s rotation speed plays only a secondary role in rock failure. The two formulae can be used directly in engineering problems because they provide relevant preventive measures during drilling. The conclusions of this paper will help drilling engineers understand the relationship between drill string vibrations and wellbore instability.