Stress Corrosion Cracking Behavior And Prevention Of High Strength Tubing Steels In Typical H₂S/CO₂Annulus Environments

Most of oil-gas fields in our country have entered development phase of high H2S, CO2and water containing; and difficulties of low recovery efficiency and water injection difficulty commonly exist. In actual production special oil displacement technique, such as injection of CO2flooding, is set up and adopted to increase the efficiency of oil field recovery. However, due to the leakage of CO2injection, annulus environment becomes the low-pH, high-pressure and low-temperature corrosion environment, containing which is of high CO2, Cl-, SO42-and contains a small amount of sulfide. It’s easy to cause the stress corrosion cracking of steel tubing and threatens the safety materials service seriously. Strong corrosive annulus environment caused by the leakage or flow of corrosive midium which are used in high H2S/CO2oil-gas fields, as well, which could cause serious corrosion, especially stress corrosion problems, eventually leading to safety accidents or scraps of oil-gas wells. The mechanism and main influence factors of stress corrosion of the typical high strength steel in the typical annulus environment of H2S/CO2were systematically studied. The main conclusions are as follows:(1)By analyzing a tubing failure case of P110steel in H2S/CO2annulus environments of CO2injection wells, it was found that the microstructural, chemical composition and mechanical properties all meet the standards; and there was no material quality problem. The main cause of tubing failure were high-pressure, low-temperature and partial acid annulus water environment, which containing CO2, Cl-, SO42-and a small amount of sulfide, the surface residual stress and structural stress caused by stress corrosion cracking.(2) The SCC mechanism of P110tubing steel in H2S/CO2annulus environment is synergism of hydrogen-induced embrittlement and anodic dissolution. The stress corrosion can be suppressed by increasing the dosage of inhibitors, decreasing the content of sulfide and increasing pH value in simulated annulus solution.(3) In the typical H2S/CO2annulus environment, the imidazoline inhibitors pose a great suppression on stress corrosion of tubing steels. However, it is worthy noting that insufficient concerntration of inhibitors will promote SCC of tubing steels on the contrary. A sufficient concerntration of inhibitors can inhibit the uniform corrosion and SCC both very well. Addition of desulfurizer can greatly suppress the occurrence of stress corrosion, but the concentration of desulfurizer should not exceed500ppm. Addition of the right amount of HG-B1or HG-B3fungicides has no adverse effects on effect of corrosion inhibitor and desulfurizer.(4) L80oil tubing steel exhibits a certain extent of SCC susceptibility in the the typical H2S/CO2annulus environment. Water content, stress level, H2S partial pressure, CO2partial pressure and temperature etc. pose different degrees of effects on it respectively. With the increase of water content, corrosion rate of L80oil tubing steel showed a gradually increasing trend, but there was no obvious degradation of its SCC resistance. With increasing of tensile stress, the corrosion rate of L80steel was significantly increased, showing a significant stress corrosion tendency. With the increase in the partial pressure of H2S, corrosion rate of L80steel increased correspondingly; and showed an increasing sensitivity to SCC. With the CO2partial pressure increases, the corrosion rate of L80steel bottoms at about1.1MPa, but unconspicuous variation was observed on its SCC susceptibility. As the temperature increased, the corrosion rate of L80steel first to increased considerably after with smaller amplitude decreases with the maximum in the vicinity of80℃corrosion rate, while its tensile properties did not show significant degradation.(5)On the basis of laboratory research, the prediction model of corrosion life of steel tubing in the annulus environment of the typical H2S/CO2annulus environment was established. This model gives an overall consideration on the evolution of environment medium with working condition, effect of variation of stress level with service time on L80corrosion behaviors and the critical condition of SCC. It conforms well to engineering practice. Service life of L80oil tubing steel is forecasted by this model. The results showed that, in the typical H2S/CO2annulus environment, L80oil tubing steel served37years for the absence of the initial service stress and16years for the service stress of0.1as, respectively.