Due to the low density and small damping factor of gas, drill string vibration in air drilling is much more violent than that in mud. Drill string impact on wellbore wall has a vital effect on wellbore stability in many cases. However, there are few researches about drill string impact on wellbore wall so far. In this paper, the calculation model of drill string impact on wellbore wall was established based on the experimental study. The instability law of wellbore was researched through numerical simulation under different conditions. In addition, authors researched the fragmentation mechanism of wellbore rock and derived the formulas of maximum pressure wellbore rock suffered when it began to fail. The results obtained in this paper can offer a great help on understanding both the law and the effect of drill string impact on wellbore wall for drilling engineers.
Keywords: wellbore instability, impact, air drilling, rock fragmentation, drill string vibration