Impact of NaHCO3/Na2CO3 Buffer Reagent on Mitigating the Corrosion of C110 Steel in Water-Based Annulus Protection Fluid at Ultrahigh Temperature

The drilling of ultradeep oil wells brings many challenges to the downhole tubular materials, where corrosion induced by halide annulus protection fluid is one major problem. In this work, the Na₂CO₃/NaHCO₃ buffer system is employed to mitigate the corrosion of C110 steel in NaBr annulus protection fluid at 220 °C. Weight loss tests, corrosion morphologies characterizations, and electrochemical measurements were used to investigate the inhibition effect. X-ray diffraction and X-ray photo-electron spectroscopy were employed to analyze the surface phase compositions. It is found that the Na₂CO₃/NaHCO₃ buffer reagents effectively inhibit the corrosion of C110 steel, and the inhibition efficiency can reach 96.1%. The higher pH leads to the better inhibition performance, and, particularly, the buffer system is more effective in the corrosion environment of greater aggressivity. Without buffer reagents, the steel substrate is subjected to higher degree of uniform etching and pitting corrosion due to the formation of loose and porous corrosion products. In contrast, the addition of buffer reagents facilitates the formation of thinner but denser and more protective Fe₃O₄ passive film, contributing the high corrosion inhibition efficiency. Our work paves the way for the safe service of NaBr annulus protection fluid at 220 °C in ultradeep oil wells.