Effect of Window Sidetrack Drilling on Thread Stress and Sealing Performance of Drill Strings

In order to analyze the influence of the window sidetrack drilling process on the strength and sealing performance of the threaded joints of the drill strings, a three-dimensional finite element model of the threaded joints of the drill strings was established by considering the working state of the threads in the inverted hole redressing and the window sidetrack drilling. The distribution laws for the stresses and contact pressures of the threaded joints under the complex loads were analyzed, and the stress and sealing performance of the threaded joints with different materials and under different tolerance fits were discussed. The accuracy of the model was verified by comparison with the calculation results in a literature. The study shows that during window sidetrack drilling, the threaded joint is subjected to the maximum bending moment, and the sealing performance of the tension side of the main and auxiliary shoulders is only 52% of that of the compression side. The drill string is subjected to the maximum axial load at the wellhead during inverted hole redressing, and the stresses of the root of the teeth of the first two turns of the external threads exceed the yield strength, so there is a risk of fracture. The main shoulder surface is detached, and the sealing of the threaded joint is not effective. As the elasticity modulus of the material decreases, the bending moment of the threaded joint during window sidetrack drilling is smaller. When the elasticity modulus of the material decreases to 50%, the maximum stress of the thread is reduced by 17%, and the minimum contact pressure is reduced by 19.6%. In the limiting tolerance, the threaded joint is changed from the auxiliary shoulder bearing the main contact pressure to the main shoulder bearing the main contact pressure, and the stress on the auxiliary shoulder is improved, but the sealing performance is completely invalidated. According to the study on the stress state of thread structure during window sidetrack drilling in this paper, the safety and sealing performance of the drill string joint can be improved by changing the thread material and tolerance fit.