西部某凝析气井P110钢级油管腐蚀断裂的失效分析

Failure Analysis of Corrosion and Fracture of P110 Steel Tubing in a Condensate Gas Well in West China

西部油田某凝析气井在修井作业时,发现油管内壁存在局部腐蚀结垢现象,部分油管发生断裂。为得出油管腐蚀失效的原因,以提出针对性措施,对失效油管进行了理化性能及力学性能测试,利用扫描电子显微镜(SEM)与能谱仪(EDS)对油管断口形貌和腐蚀产物进行了分析。结果表明:失效油管的化学成分、硬度及金相组织均符合标准要求;油管内壁发生结垢形成了垢下闭塞环境,导致垢下酸性介质聚集、pH值降低,地层水中Cl-含量较高导致了垢下Cl-腐蚀,同时垢下Cl-不断的自催化作用加剧了点蚀的发展,油管点蚀处形成应力集中,在上提管柱时发生准解理断裂。建议在井下添加缓蚀阻垢剂进行防护,减少结垢及腐蚀的发生。

During workover operation in a condensate gas well in Western oilfield,it was found that local corrosion and scaling occurred on the inner wall of tubing,and some tubing was broken.In order to find out the causes of corrosion failure of tubing and put forward corresponding measures,the physical/chemical properties and mechanical properties of the failed tubing were tested.Moreover,the corrosion fracture morphology and products of tubing were analyzed by scanning electron microscopy(SEM)and energy dispersive spectrometer(EDS).Results showed that the chemical composition,hardness and metallographic structure of the failed tubing met the requirements of the standard.The scale on the inner wall of the oil pipe formed a blocked environment under the scale,which led to the accumulation of acid medium under the scale and the reduction of pH value.Meanwhile,the high content of Cl-ion in the formation water caused the corrosion of Cl-ion under the scale.The continuous autocatalytic action of Cl-ion intensified the formation of pitting corrosion,forming the stress concentration at the pitting corrosion of the oil pipe.Hence,a quasi-cleavage fracture occurred when the pipe string was lifted up.It was suggested that corrosion and scale inhibitor should be added in the well to prevent scaling and corrosion.