Research On In-situ Corrosion Failure Of Surface Pipelines In An Offshore Oilfield And Adaptability Of Different Pipe Materials

The oil and gas industry is the lifeblood of the national economy,and its exploration,exploitation,and transportation have a vital impact on the development of social economy.Oilfield mining equipment and surface pipelines will inevitably undergo corrosion under the influence of a variety of factors,and the service environment has a very complicated influence on the corrosion process of surface pipelines.Corrosion mechanisms under different environments have always been research hotspots.Therefore,the targeted failure analysis of the surface pipeline failure section and the comparison of the corrosion behavior of different pipe materials under working conditions have important practical value and theoretical significance for the safe and effective production of oilfields.In this paper,the failure mechanism of perforation failure is clarified by analyzing the macro and microscopic corrosion form,composition and material composition of the corrosion products of the failed pipe section on the surface of a failed pipe section in an offshore oil field,combined with the on-site service conditions.After that,the corrosion simulation experiment of commonly used surface pipeline steel under this service condition is carried out,and modern surface analysis techniques are used to compare and study the influence of different structures,pearlite content and Cr content on metal corrosion behavior.The main findings are as follows:（1）The on-site surface pipeline is made of 20#steel,and its service conditions are:the temperature is between 53°C and 58°C,the produced gas mainly contains CH₄and a small amount of CO₂,and the produced liquid has a higher degree of salinity and the highest content.The ion is Cl^-,and the flow rate is between 1.0 m/s-1.2 m/s.Surface pipelines mainly undergo CO₂corrosion under this working condition.With the prolonged service time,a layer of Fe CO₃corrosion product is formed on the metal surface.Since the corrosive medium contains a large amount of Cl^-with a small radius,it is easy to pass through the pores in the corrosion product.It reaches the metal surface,and promotes the rapid dissolution of the metal at the location through the local catalysis-acidification mechanism,which causes local corrosion and perforation in the area in a short time.（2）Corrosion behavior comparison of bainite steel（X56）and pearlitic steel（Q345）:Fe₃C in the bainite structure of ferritic-bainite steel（X56）is granular,scattered in Fe CO₃during the corrosion process Among the corrosion products,the anchoring effect on the corrosion products is weak.The Fe CO₃corrosion product formed on the surface of X56steel has a large number of pores which become a fast channel for the corrosive medium to reach the substrate/corrosion product interface,and its corrosion form is localized corrosion.The Fe₃C in the pearlite structure of the ferrite-pearlite steel（Q345）is in the form of flakes,which has a certain anchoring effect on Fe CO₃during the corrosion process,which can hinder ion exchange to a certain extent and prevent the occurrence of local corrosion.Moreover,the Fe CO₃corrosion product formed on the surface of Q345 steel is relatively dense,and its corrosion form is mainly full-scale corrosion.（3）The influence of pearlite content in steel on the corrosion behavior of ferrite-pearlite steel:as the carbon content decreases,the galvanic potential difference between ferrite and pearlite decreases,resulting in a decrease in its corrosion rate,but As the content of pearlite decreases,the uniformity of corrosion products formed on the surface of ferrite-pearlite steel decreases,the content of anchoring Fe₃C in the inner corrosion product film decreases,and the protection of the corrosion product film becomes weaker,which eventually leads to local corrosion.（4）The effect of Cr content in steel on the corrosion behavior of X56 steel:the corrosion product films formed on the surface of X56 steel with three different Cr content are all in a double-layer structure,and the outer corrosion products are formed by stacking of Fe CO₃grains.The corrosion products of the inner layer are Fe CO₃,and the corrosion products of the inner layer on the surface of 2Cr-X56 and 3Cr-X56 steel are Fe CO₃and Cr（OH）₃.With the increase of Cr content,Cr is enriched in the inner film during the corrosion process,which makes the corrosion form of X56 steel change from local corrosion to general corrosion.However,since the activity of Cr element is higher than Fe,the content of Cr in steel is less than 2%.At this time,a complete Cr-rich layer is not formed on the surface,which causes the corrosion rate to increase with the increase of Cr content.When the Cr content continues to increase to 3%,a complete Cr-rich layer is formed in the corrosion products of the metal surface,which enhances the protection of the corrosion products and reduces the corrosion rate.