The Inhibitive Performance and Mechanisms of a Compound Corrosion Inhibitor in Inhibiting P110 Steel in a High Temperature High Density Inorganic Salt Completion Fluid

A high density low-corrosion inorganic completion fluid was developed to try to satisfy the needs of developing petroleum from high temperature deep wells (bottom hole temperature > 160 ℃ and well depth > 5 000 m). These needs include formation pressure balancing, formation damage mitigation and corrosion inhibition. Corrosion of this completion fluid to the P110 steel was evaluated with a high temperature high pressure (HTHP) corrosion tester and weight-loss method at different temperatures (160 – 180 ℃), completion fluid densities (1.80 – 2.30 g/cm³) and corrosion inhibitor concentrations (1% – 5%). The study shows that the average rate of corrosion and the rate of pitting corrosion increase with temperature, and decrease with the concentration of the corrosion inhibitor. The corrosion inhibitor in the completion fluid has good corrosion inhibition effect. The dense film of the corrosion products on the surface of the P110 steel plate tested was observed using SEM. The mechanisms of corrosion inhibition of the corrosion inhibitor in the completion fluid were revealed through X-ray energy spectrometer (EDS) and X-ray photoelectron spectrometer (XPS). It was found that there were Fe₃O₄, Fe₂O₃ and an organic iron complex between the trivalent iron and the organic components of the corrosion inhibitor in the completion fluid formed on the surface of the P110 steel plate. The corrosion inhibitor not only takes part in the formation of the product film, it also accelerates the oxidation passivation of the iron, thereby synergistically achieving the inhibition of the corrosion to the P110 steel by the high density inorganic salt completion fluid at elevated temperatures. This high density low-corrosion inorganic salt completion fluid was tried on a well drilled in Wuwei county, Anhui province. The completion fluid not only balanced the formation pressure, it also mitigated the corrosion of the completion fluid to the completion string. This high density low-corrosion completion fluid has broad application prospect.