Effect of CO₂ partial pressure on the stress corrosion cracking behavior of N80 tubing steel in the annulus environment of CO₂ injection well

CO₂-enhanced oil recovery (CO₂-EOR) technology is the process of capturing CO₂, transporting the captured CO₂ to a storage site, and injecting the captured CO₂ into an oil field to enhance oil recovery. CO₂-EOR technology can greatly increase the profitability of oil fields. It is also a promising method for reducing CO₂ emission and improving the environment. For these reasons, this technology has become increasingly important for the development of the global oil industry and has been widely explored. However, CO₂ injection significantly increases the risk of corrosion failure of tubing steel. As such, the effect of CO₂ on the stress corrosion behavior of tubing steel should be investigated. In this study, the effect of CO₂ partial pressure (P_\rmCO_2 ) on the stress corrosion behavior of N80 steel was examined using an immersion test, a surface analysis technique, and an electrochemical technology. Results reveal that the influence of P_\rmCO_2 on the corrosion rate has an inflection point of approximately 1 MPa. When P_\rmCO_2 is <1 MPa, a corrosion product film (FeCO₃) forms slowly, and the coverage rate is low. As P_\rmCO_2 increases, the corrosion current density of N80 steel increases. When P_\rmCO_2 is >1 MPa, the corrosion product film can form at a faster rate, and the corrosion current density of N80 steel decreases as P_\rmCO_2 increases. The pH of the solution decreases continuously when CO₂ is dissolved in solution. Consequently, the stress corrosion cracking (SCC) of N80 tubing steel occurs in an annulus environment. The SCC mechanism of N80 steel in the annulus environment of CO₂ injection wells is the combination of anodic dissolution (AD) and hydrogen embrittlement (HE). Localized AD (pitting) is dominant in SCC at the initiation stage, and SCC is most likely initiated at P_\rmCO_2 of 1 MPa. At the crack growth stage, HE has a stronger effect on SCC than AD, the SCC easily grows with a high P_\rmCO_2 , and SCC sensitivity further improves.