Investigation On Stress Corrosion Cracking Mechanism And Monitoring Method Of P110 Tubing Steel In Simulated Annulus Fluid

In recent years,stress corrosion cracking?SCC?is a major concern in oil-gas fields,the downhole tubing has suffered severe CO₂ corrosion with the promotion of CO₂ flooding technology,which hinders the safe production of oil field seriously.The safety accidents due to the fracture of oil tubing and sucker rod can be greatly reduced if early detection of SCC can be achieved by non-destructive testing techniques?NDT?.Therefore,it is of great significance to early monitor the SCC behavior of tubing steels for the safe production of oil and gas fields.In this paper,slow strain rate test?SSRT?is used to study the effect of strain rate on the SCC sensitivity of P110 tubing steel,the results show that the SCC sensitivity of P110 steel is the largest when the strain rate is 1×10^-6s^-1or 2×10^-6s^-1,in order to shorten the experimental period,this study selects the strain rate of 2×10^-6s^-1for SSRT.Then,the SCC behavior of P110 high-strength tubing steel in simulating downhole annular fluid?SAF?and the effect of CO₂ on the initiation and propagation of crack are investigated by SSRT,electrochemical noise?ECN?,scanning electron microscope?SEM?and electrochemical impedance spectroscopy?EIS?techniques.Several statistical parameters of ECN spectrum,such as charge,lifespan and amplitude of current transients,are defined to distinguish different corrosion processes.The results indicate that the spikes featured by short duration?3⁵ s?and low amplitude?0.01¹?A?,are often associated with metastable pitting events,the spikes featured by long duration?>30 s?and high amplitude?>1?A?,are related to the initiation of cracks.During the elastic stress stage,the surface of the tensile specimen keeps passive with no significant fluctuation in the current/potential signal.The ECN spectrum is dominated by metastable pitting peaks during the early yield stage,and the nucleation rate of metastable pits increases with the increasing concentration of CO₂ in the solution.At the later yield stage,the cracks gradually initiates from the stable pits under the action of tensile stress and Cl^-,and the crack characteristic peaks appear obviously on the ECN spectra,the lifespan,amplitude and charge of peaks increase with CO₂ concentration.The increasing of tensile stress promotes the continuous growth of cracks at the hardening stage,the crack characteristic peaks are reduced or even disappear due to the rapid active dissolution of crack tips.When reaching necking stage,the entire tensile specimen is in uniform corrosion state,the current/potential fluctuates by high frequency and low amplitude,and there is no apparent transient peak on the ECN spectrum.The increasing SCC susceptibility of P110 steel is mainly attributed to the decreasing pH of the solution by the addition of CO₂,which not only accelerates the anodic dissolution of crack tips but also prolongs the expansion period of cracks.Taking into account that sulfate reducing bacteria?SRB?can reduce SO₄^2-to S^2-in annular solution,which may bring the risk of sulfide stress corrosion cracking?SSCC?to tubing steel.And the effect of S^2-on the SCC behavior of P110 steel is also studied.It is found that the addition of S^2-accelerates the breakdown of passivation film on the surface of P110 steel tensile specimen,and the transformation time of metastable to stable pits is shortened significantly,which not only promotes the growth of pits but the initiation of cracks from the stable pits under the action of tensile stress.The SCC mechanism is a mixed mechanism of anodic dissolution and hydrogen induced cracking.