A Study On Collapsing Property Of The Tube Of High Pressure And Output Gas Wells

In recent years, in the high pressure and output gas wells, the tube, which meets APIstandard, was collapsed duirng reverse circulation flushing or blow ofifng. It’s necessary to carryout failure analysis, find failure factors, and proposed effective measures for improvement.Compairng and analyzing the mateiral properties of failed tube of case#1and case#2separately. All properties of failed tube of case#1meet the standard requirements except tensilestrength, which is lower than standard. And regarding to failed tube of case#2, all propertiesmeet standard requirements. Atfer analyzing, the reason of collapsed of oil tube, is not completerelated to the lower strength of material.The model of tube is established by using ANSYS ifnite element analysis sotfware, and theinfluence of anti-collapsed properties of tube was analyzed by yield strength and axial load,physical dimension, which included eccentricity and ovality.Five models of gas column shitfing are simulated by using Fluent software. Based oncalculating, it shows the gas volume is expanded and the pressure is decreased rapidly duirng theprocedure of gas column shitfing and jamming occurred at bottom of oil tube, which leads toincreasing difference between internal pressure and external pressure of tube.Meanwhile, the working load of tube is detailed calculated, which includes external pressure,internal pressure and axial load, and the collapse strength value was calculated by using tiraxialcollapse strength formula. Another collapse strength value of tube was calculated based on sameworkload by using ANSYS. The two different calculating result shows, the tube is easy to becollapsed when the bottom channel is jammed, which makes the annular pressure reaches49MPa,and the external pressure of tube (broken area) reaches61MPa.Based on the failure analysis of object and the theory of calculation, several followingconclusions are achieved: the collapsed of tube is not entirely caused by lower mateiral strength,but also the complex working load in gas well including pressure difference of internal andexternal of tube. Several measures, improving the strength of mateiral, enlarging the diameter oftube for discharging, increasing the thickness of tube, and controlling the wellhead pressure ofblowing off, are effective to keep from collapsing of the tube.