Analysis Of Sealing Evaluation And Fatigue Life To Multistage Fracturing String Joints In Cluster Wells

In the process of multi-stage fracturing in cluster wells,the joint of string is affected by complex alternating loads such as temperature and pressure,which leads to serious failure of the joint and affects the normal production of oil and gas fields.In order to alleviate the failure of string joints,this paper studies the sealing performance and fatigue life of fracturing string joints under alternating load.In this paper,three kinds of premium thread models of sealing structure(cone-cone,cone-sphere,cylinder-sphere)are established by APDL parametric language of ANSYS.First,three kinds of joints are analyzed for make-up.Then,the forces caused by temperature,pressure and fluid in the pipe are simplified into axial loads,the sealing performance of three kinds of joints under the cyclic load of tension tension and tension compression is analyzed.Finally,based on nCode DesignLife advanced fatigue analysis software,miner damage criterion and local stress-strain method,the fatigue life of premium thread is analyzed.The results show that the stress of the premium thread bearing surface of the three sealing structures is greater than that of the guide surface,and the contact length of the cone-cone structure is the longest but the average contact pressure is the smallest after make-up.During the tension-tension cycle load,In the tension stage,the contact pressure of the premium thread of the cone-sphere sealing structure drops the most seriously,the contact length of the cone-cone decreases the most,the contact pressure of the sealing surface fluctuates greatly in the first six cycles,the contact length of the sealing surface recovers in the unloading stage,and the maximum contact pressure decreases in a stepped manner with the increase of the cycle.During the tension-compression cycle,when the tensile load changes to the compression load,the contact pressure and contact length of the sealing surface increase,and the contact pressure in the first period is significantly higher than that in other periods,In the unloading stage,the maximum contact pressure of the three sealing structures varies.Except for the first cycle,the contact pressure of the unloading sealing surface after compression is greater than that after tension,and the pull pressure cycle load has a greater impact on the sealing surface.Through nCode DesignLife analyzes the fatigue life of premium thread under X and Y load spectrum.It is found that the failure position of thread under the two load spectrum is the same.The two ends of coupling and incomplete thread section of pipe body are easy to be damaged.The larger the stress amplitude,the less sensitive of fatigue life is to the roughness,residual stress and load size.Under high load level,the load size is the main factor affecting the fatigue life of premium thread.Considering all kinds of analysis,the performance of three premium thread is the best from cone-cone and the worst from cylinder-sphere.