Influence of H2O phase state on corrosion behavior of Q125 in high-temperature H2O-CO2-H2S-H2 environment

Abstract

The wellhead of the production well for in-situ shale oil extraction is in an environment where H₂O, CO₂, H₂S and H₂ coexist at 150 °C, with H₂O existing in gaseous and liquid forms. Advanced methods and thermodynamic calculations were used to study the differences and mechanisms of the corrosion behavior on the surface of Q125 steel in different H₂O phase state environments. Results show that the average corrosion rate of Q125 in the gaseous water environment (0.0788 mm/a) is much lower than that in the liquid one (0.3625 mm/a). Part of the reason is that the presence of H₂ can slow down the cathodic reaction and thus reduce the corrosion rate in the gaseous water environment. Localized corrosion occurs in the liquid water environment mainly because it contains a large number of Cl⁻, whereas such a phenomenon is absent in the gaseous water environment. The corrosion products on the surface of the specimens in both environments are composed of sulfides on the outer layer and oxides on the inner layer. The corrosion products in the gaseous water are looser than in the liquid water, mainly because of the inward diffusion of condensed H₂O on the specimen surface in the gaseous water.