某煤岩气井聚乙烯内衬复合油管螺纹脱扣原因分析

Reasons Analysis on Thread-off of Polyethylene Lined Tubing in Coal-rock Gas Well

某煤岩气开发区块为了避免“定向井”对有杆举升系统造成偏磨及结垢影响,采用高密度聚乙烯(HDPE)管材内衬于油管内部组成复合油管柱。该区块某井“二次”油管入井作业,第40支油管入井时,油管柱坠落井底。随即采用打捞作业、宏观测量、螺纹检查、材质分析、力学性能测试、显微组织分析、水质测试、腐蚀产物XRD分析等方法,综合同批次该油管质量抽检结果及制造工艺,对失效螺纹端进行分析。结果表明:制造时接箍与管体上扣扭矩不足,导致螺纹间产生缝隙腐蚀,在腐蚀性介质作用下螺纹连接处腐蚀严重,失去连接作用,致使承载力不足,油管柱落入井底。建议内衬复合油管制造及入井作业时,参考GB/T 17745—2011标准对工厂端及现场端上扣扭矩进行控制,同时使用专用螺纹脂保证工厂端和现场端的螺纹连接效果。

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,macroscopic 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.