Study On Coiled Tubing CO₂ Corrosion Under High Temperature And Pressure

With the increasing demand for oil and gas,Coiled Tubing(CT)has been widely used in deep and ultra-deep wells because of its advantages of short operation time and high efficiency.CT is easy to be corroded and fail in the service of H2 S,CO₂ gas,minerals and other corrosive media and high temperature and high pressure environment,which will lead to serious accidents.Therefore,it is of great significance for the safe service of CT to study the CO₂ corrosion behavior of CT under high temperature and pressure environment and to accurately evaluate corrosion residual life.In this paper,CT110 was taken as the research object for CT CO₂ corrosion under high temperature and high pressure environment.Through CT corrosion experiment,the corrosion resistance of base metal(BM)and weld metal(WM)was analyzed,and electrochemical corrosion parameters of CT were obtained.In order to further analyze the corrosion law of CT under multiple working conditions,the finite element method was used to analyze the influence of welding parameters and environmental parameters on the corrosion of CT,and the influence of defect parameters on stress corrosion of CT.Based on the results of finite element analysis,the corrosion life prediction model of CT was established based on BP neural network method,which provides reference for corrosion protection and safe service of CT in field.The main work is as follows:(1)Through CT corrosion mass loss experiment under high temperature and high pressure,it can be obtained that the corrosion rate of BM and WM under different temperature and CO₂ partial.Then through the electrochemical corrosion experiment,it can be obtained that the electrochemical corrosion parameters such as corrosion potential,corrosion current density and electrochemical impedance of BM and WM.The comparative study showed that WM had poor thermodynamic stability,weaker corrosion resistance and more prone to corrosion.The mechanical tensile test results showed that corrosion will reduce the mechanical properties of CT,and had a greater impact on the mechanical properties of WM.(2)On the basis of experiment,the physical coupling analysis software COMSOL to carry on the numerical simulation of the galvanic corrosion and CO₂ corrosion of CT,study the straight pipe welding parameters(the area ratio of BM and,the weld residual high)and environmental parameters(temperature,CO₂ partial pressure,p H,and flow velocity)on the influence law of corrosion.(3)The CT stress corrosion model with defects was established by COMSOL based on the principle of force-electrochemical coupling interaction considering stress.Take mises stress,corrosion potential and corrosion current density to characterize the stress corrosion degree of the CT,study the effects of defect types,defect length,defect depth and defect number on stress corrosion.The results showed that the smaller the defect length was,the greater the defect depth was,and the more serious the stress corrosion was.The influence of rectangular defects on stress corrosion was basically the same as that of elliptical defects,and the increase of the number of defects had a certain mitigating effect on stress corrosion.(4)Based on machine learning method,BP neural network was used to build the corrosin rate prediction model of CT.The model was trained and tested by the CO₂ corrosion of CT and the finite element analysis results of stress corrosion of CT with defects,and the accuracy of the model was verified.