Research on Pipe String Failure Based on Fluid-Structure Coupling Vibration of Pipe String During Instantaneous Shut-in

In high-production gas wells, fluid-structure coupling vibration often occurs in the pipe string during instantaneous well switching operations. Severe vibration of the pipe string can lead to fatigue failure of the pipe string or thread leakage. This article focuses on on-site gas well pipe string failure cases and thread sealing failure problems, and studies the fluid-structure coupling vibration and pressure fluctuation of the pipe string during instantaneous shut-in. The model is verified using the measured wellhead data in the reference literature, and the pipe string leakage points are detailed. On this basis, the easy failure points on the pipe string were analyzed based on the three key factors of valve closing time, well depth and production, and the fluid-structure coupling vibration trend and pressure fluctuation trend of the pipe string were studied. The following conclusions were drawn: (1) As the shut-in time increases, the wellhead pressure fluctuation advances, the fluctuation peak value decreases, and the axial stress decreases; (2) As the well depth increases, the wellbore pressure increases, but the fluctuation trend is gentle. At the same time, the axial stress of the pipe string decreases in an arithmetic sequence, and the axial vibration speed of the middle section of the pipe string is the largest; (3) The greater the production, the peak wellhead pressure will decrease to a certain extent, but the fluctuation frequency will greatly increase. At the same time, the axial stress fluctuation amplitude of the pipe string increased by 2.2 times, and the axial vibration speed fluctuation amplitude increased by 3.59 times. The research results can provide technical reference and theoretical support for the prediction of easy failure points on the test string during high-production gas well testing.