Reasons Analysis on Thread-off of Polyethylene Lined Tubing in Coal-rock Gas Well
In order to avoid the influence of directional well on rod pump lifting system caused by eccentric wear and scale formation in a coal-rock gas development block,HDPE tubing is lined inside the tubing to form tubing string.During the service of a well in this block,when the 40th tubing was running into the well,the tubing string fell to the bottom of the well.Then the methods of salvage operation,mac-roscopic measurement,thread observation,material analysis,mechanical property test,microstructure analysis,water quality test,and XRD analysis of corrosion products were adopted.Based on the inspection results and manufacturing process of the same batch of tubing,the failure reasons of the thread end was analyzed.The results show that during manufacturing,insufficient torque applied to the coupling and tubing body resulted in gaps and crevice corrosion between the threads.Under the action of corrosive media,severe corrosion occurred at the threaded connection,leading to loss of connection function and inadequate bearing capacity,which caused the tubing string to fall into the wellbore.It is recommended to refer to the GB/T 17745-2011 standard to control the torque of the factory end and field end during the manufacturing and well entry of the liner composite tubing.At the same time,the thread compound with corrosion retarding function should be used correctly to ensure the connection effect of the factory end and field end.
rod pump lifting systemcrevice corrosionpolyethylene lined tubingcoal-rock gas
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