关键词: 井下冲击扭矩, 二次上扣, 钻柱动力学, 粘滑振动, 涡动

Abstract: The measurement results show that during the drilling operation of China's first 10 000-meter ultra-deep well-the SDTK-1 well, the phenomenon of secondary makeup of threaded joint occurs frequently, indicating the presence of downhole impact torque that exceeds the make-up torque. The impact torque can not only cause difficulties in tool breakout but may also lead to failure of downhole drill string connections, severely compromising drilling safety. Therefore, it is crucial to understand the dynamic mechanism of impact torque and form effective control methods. This paper studies the formation mechanism of downhole impact torque endured by the drill tool of the SDTK-1 well by combining the self-developed drill string dynamics simulation program and ABAQUS software. Firstly, a finite element dynamic model of the drill string was established based on Hamilton’s principle, and solved by using the Newmark method and the SOR node iteration method. Then a model for calculating downhole impact torque was developed based on the analysis of the drill string dynamics under different operating conditions. Subsequently, taking the drill string used in the hole section of 8 269.0-8 503.0 m of the SDTK-1 well as an example, the whirl characteristics and the formed impact torque of the downhole drill string when stick-slip vibration occurs were analyzed. The results show that the impact torque caused by the self-rotation of the drill string is relatively small and insufficient to cause the occurrence of the "secondary makeup" phenomenon. When the drill string undergoes stick-slip vibration, it is very likely to form an underground impact torque. In this case, the intense whirl motion accompanying the slippage stage of stick-slip vibration generates a large impact torque, which can reach 87.4 kN ·m, much greater than the makeup torque. It is precisely this impact torque that leads to secondary makeup of thread connection. The deviation between the calculated result and the actual breakout torque is only 2.8 %, demonstrating the reliability of the model. Theoretical analysis results show that reducing stick-slip vibration is an effective method to control impact torque, but it is rather difficult for ultra-deep well drilling operations. However, using double-shoulder threaded connections can effectively enhance the drill string’s resistance to impact torque. In the drilling operation of the SDTK-1 well at a depth of nearly 10 000 meters, the drill string with double-shoulder threaded connections was adopted, which basically eliminated the formation of micro-cracks in the threaded connections,although the phenomenon of secondary make-up still occurred, proving the effectiveness of this method.

Key words: downhole impact torque, secondary makeup, drill string dynamic, stick-slip vibration, whirl motion