Fatigue investigation of tubing string in high-yield curved wells

In view of the fatigue failure of tubing string caused by fluid-induced vibration (FIV) in high-yield curved well, a nonlinear vibration model of tubing string is established using energy method and Hamilton principle, which considers the longitudinal/lateral coupling effect of tubing string and the nonlinear contact collision effect of tubing-casing and verified by a similar experiment of tubing vibration. The cumulative damage theory (Miner's Law) is used to establish the fatigue life prediction method of the tubing string in high-yield curved well combined with the stress response which was determined by the proposed vibration model and the S-N curve of the tubing material (13Cr-L80) which was measured by fatigue experiment. Based on that, the fatigue life of the tubing in two typical horizontal well and directional well was analyzed under different production rate (0.4-1.6 million square/day) and different tubing diameter (3.5-in and 4.5-in). The result shows that the most dangerous position of the tubing string appears between 3/4 position of the tubing string and the packer. The critical production rate of 4.5-in tubing string in horizontal well is 1.08 million square/day to satisfy the life requirements of gas reservoir tubing (20 years), and which is 0.45 million square/day in directional well. The fatigue life of 4.5-in tubing string is higher than that of 3.5-in tubing string in both horizontal and directional wells. The research results provide a theoretically sound guidance for designing and practically sound approach for effectively improving the service life of tubing string in high-yield curved wells.