Research on Corrosion Mechanism and Application in the Yingzhong Block of Qaidam Basin

Abstract The Yingzhong block in the Qaidam Basin belongs to a high-temperature, high-pressure, high-sulfur, high-mineralization region with H2S content up to 20,000 ppm, CO2 content up to 20,000 ppm, and an average pH value of 6.5 in the formation water. The average chloride content is 153,785.5 ppm, and the average total mineralization is 2,987,079 ppm. Overall, the Yingzhong block wellbore produced fluid has strong corrosivity, and the corrosion mechanism is extremely complex. The selection of oil tubing lacks scientific basis. To address these issues, we conducted an assessment of corrosion damage and stress corrosion cracking sensitivity (SSC) of oil tubing in H2S/CO2/high-mineralization formation water medium, an assessment of corrosion damage of oil tubing in high-temperature fresh acid, an assessment of corrosion damage of oil tubing in high-temperature spent acid, and a full-scale corrosion test of oil tubing in a typical H2S/CO2/high-mineralization formation water medium to study the corrosion grade and characteristics of the tubing material under different service conditions in the Yingzhong block and clarify the corrosion mechanism and law of oil well tubing in service environment. The experimental results show that in the fresh acid condition, as the acidizing temperature increases gradually, the average corrosion rate of the material increases and the corrosion worsens. In the spent acid condition, the temperature reaches 180 ℃, and N80, P110, C110, and P110SS are all severe corrosion or more severe corrosion. In the simulated formation water environment under two CO2/H2S partial pressure ratios, both H2S and CO2 promote corrosion. In the high-temperature corrosion system, temperature and H2S synergistically promote the aggravation of corrosion. As the temperature increased gradually, the average corrosion rate of the oil pipe material showed an increasing trend; with the increase of PH2S, the degree of corrosion increased, and H2S played the main control role in the corrosion system; the initial experimental assessment of the corrosion damage performance and stress corrosion cracking sensitivity of the completion fluid was carried out using a simulated saltwater completion fluid with a density of 1.23g/cm3 and a temperature of 180 ℃. P110, C110, and P110SS oil pipe materials were all severely corroded. In the geological environment of the Yingzhong block where the water temperature is ≤ 120 ℃, C110 and P110SS oil pipe materials are recommended to be used. In the geological environment where the water temperature is 180℃, BG2532 oil pipe material is recommended to be used. The selection standard is Vcorr ≤ 0.81 mm/a.