Research On Performance Of Corrosion Fatigue For Octg Material InEnvironment Containing CO₂

With the development of petroleum industry, the well starts to the deep and ultra-deep wells continuously, the environment of oil exploitation has become increasingly harsh, service condition is more and more severe on Oil Country Tubular Goods(OCTG). OCTG is not only undermined by corrosive medium containing Cl-, H2 S, CO2,O2 and so on, but also by alternating loads. OCTG occur corrosion fatigue failure when corrosive medium and alternating loads damage together. Crack initiation life accounts for 10% of the total life, crack propagation life accounts for 90% of the total life in the process of corrosion fatigue, therefore, the research on corrosion fatigue crack propagation rate of OCTG, which can effectively predict the remaining life in service conditions. In this paper, corrosion fatigue crack propagation rate of OCTG in environment containing CO2 was researched and relevant life prediction models were established.The influence factors of corrosion fatigue crack propagation include environmental factors, stress state and material properties, to research characteristics on corrosion fatigue crack propagation of P110 and 13 Cr tubing by changing corrosive environment and stress state during the experiment. The corrosive media include air environment(as a comparative experiment), 3.5wt% Na Cl environment and CO2-Cl- environment. Frequency of alternating load is 10 Hz in air environment, frequency contains 10 Hz,5Hz,1Hz in corrosive environment. Corrosion fatigue crack propagation rates of tubing are varying under difficult conditions and stress intensity factor. Finally, fracture appearance observation and EDS analysis by scanning electron microscopy, the following conclusions:(1) Fatigue crack propagation rate of tubing is only related to stress state in air condition, which can be used as pure fatigue.(2) Because corrosion damage time is more longer during every cycle when frequency decreases in 3.5wt% Na Cl and CO2-Cl- environment, fatigue crack propagation rate of tubing is accelerated.(3) It is obvious that corrosion fatigue crack propagation rate of P110 tubing is affected by the frequency, increasing corrosion damage accelerates the tubing failure because of its poor corrosion resistance. It is not obvious that corrosion fatigue crack propagation rate of 13 Cr tubing is affected by the frequency. Crack propagation rate changes little when the frequency is higher, and crack propagation rate increases when the frequency is only lower.(4) According to the fracture morphology and energy spectrum analysis, the damage of P110 tubing manifests pitting failure in Cl- environment. Corrosion fatigue crack propagation rate of P110 tubing changes little in CO2-Cl- environment comparing with Cl- environment. 13 Cr tubing is the intergranular fracture failure in Cl- environment, and CO2 environment can promote the corrosion fatigue failure of 13 Cr tubing in CO2-Cl- environment.