高Cl^(-)环空保护液中超级13Cr油管点腐蚀行为研究

Pitting Corrosion Behavior of Super 13Cr Tubing in High Chloride-containing Annulus Protection Fluid

目的 研究超级13Cr管材在油气井服役环境中的点腐蚀失效机制,分析超级13Cr马氏体不锈钢在高温、高Cl^(-)环空保护液、超临界H_(2)S/CO_(2)环境中的点腐蚀失效行为,明确其适用性,并提出相应的腐蚀控制措施。方法 通过分析失效油管的宏观形貌、显微组织、腐蚀形貌及腐蚀产物,判断超级13Cr油管现场失效的原因,结合高温高压反应釜模拟井下腐蚀环境,从平均腐蚀速率、点腐蚀速率等方面揭示超级13Cr油管的点腐蚀失效机理。结果 该超级13Cr材质管柱在受到H_(2)S/CO_(2)污染的环空保护液环境下会发生点腐蚀穿孔失效;通过观察现场失效油管发现,在受到腐蚀性气体污染的高Cl^(-)环空保护液环境中,油管外壁发生了明显的局部腐蚀,油管腐蚀由外壁向内壁扩展,发生了严重的点腐蚀穿孔,并具有一定的H_(2)S应力腐蚀开裂(SCC)特征;在环空保护液环境下,失效油管表面有Cr、O、Cl、S离子聚集,腐蚀受到CO_(2)-H_(2)S共同影响;模拟腐蚀实验结果显示,超级13Cr油管在腐蚀性气体污染的海水基环空保护液环境下具有点腐蚀敏感性,蚀坑深度为80.346μm,点腐蚀速率达到10.34 mm/a。结论 超级13Cr油管在环空保护液中具有优异的抗均匀腐蚀能力,但在受到H_(2)S/CO_(2)污染的高Cl^(-)环空保护液环境中具有明显点腐蚀倾向,建议环空保护液用淡水配制,并进行除氧处理。

In recent years,the research on super 13Cr corrosion at home and abroad mainly focuses on uniform corrosion in formation fluid environment and H_(2S) stress corrosion cracking in annulus protection fluid,but rarely considers pitting corrosion failure in annulus protection fluid.At home and abroad,there are few cases of short-term pitting corrosion perforation failures of super 13Cr pipes in annulus protection fluid.This paper aims to study the pitting corrosion failure mechanism of super 13Cr pipes in the service environment of oil and gas wells,and analyze the pitting corrosion failure behaviors of super 13Cr martensitic stainless steel in the environment of high temperature,high chloride-containing annulus protection fluid and supercritical H_(7) S/CO_(2) to make clear the applicability of super 13Cr tubing and put forward corrosion protection measures.Based on the analysis of the macro and micro structure,corrosion morphology and corrosion products of the failed tubing,the failure reason of super 13Cr tubing was determined.Combined with high temperature and high pressure reactors,the downhole corrosion environment was simulated,the mechanism of pitting corrosion loss of super 13Cr tubing was revealed from the aspects of average corrosion rate and pitting corrosion rate.The results showed that the super 13Cr tubing in the failed well had pitting perforation failure in the environment of H_(2)S/CO_(2) polluted annulus protection fluid.Field failure tubing showed that in the environment of high chloride-containing annulus protection fluid polluted by corrosive gas,obvious local corrosion occurred on the outer wall of tubing,and the corrosion expanded from the outer wall of tubing to the inner wall,resulting in pitting corrosion perforation.The failed tubing had certain H_(2)S stress corrosion cracking(SCC) characteristics.There were Cr,O,Cl and S ions on the surface of the failed tubing,and the corrosion was jointly controlled by CO_(2)-H_(2)S.The simulated corrosion test results showed that the super 13Cr tubing was sensitive to pitting corrosion under the environment of seawater annulus protection fluid polluted by corrosive gas.The pit depth was 80.346 μm and the pitting corrosion rate was 10.34 mm/a.It was considered that the corrosion failure of super 13Cr tubing in this well was controlled by CO_(2)-H_(2S) and extended from annulus to tubing.Serious pitting corrosion failure occurred under high chloride-containing annulus protection fluid containing CO_(2)/H_(2S) corrosive gas.Super 13Cr tubing had good uniform corrosion resistance in the annulus protection fluid,but it had obvious tendency of pitting corrosion in the environment of high chloride-containing annulus protection fluid polluted by H_(2)S/CO_(2).The super 13Cr tubing without removing the oxide skin would aggravate the damage of the passivation film,form local corrosion sensitive areas,induce local pitting corrosion initiation,and increase the diffusion rate of medium and corrosion rate in the environment of seawater base annulus protection fluid polluted by corrosive gas.It is recommended that in the process of offshore oilfield development and production,attention should be paid to the deoxygenation process of annulus protection fluid,and the base fluid configured with annulus protection fluid should be changed from seawater filtration to fresh water.At the same time,the match between annulus protection fluid and the material of the used pipe string should be fully considered in the design stage,so as to fundamentally solve the pitting corrosion failure of super 13Cr tubing in the downhole.The research in this paper can provide help for casing material selection and corrosion control in the development of such offshore oilfield.