Corrosion analysis, and use of an inhibitor in oil wells

Abstract

When oil fields enter a period of high water content the downhole string becomes severely corroded, which not only affects normal oil field production but also necessitates high maintenance and replacement costs. The corrosion is mainly caused by such substances as H2S, CO2, O2, and Cl−, and by bacteria. According to test results from analysis of well fluid, average CO2, H2S, dissolved O2, and Cl− content were 28.4, 29.33, 0.43, and 21162.8 mg/L, respectively. Corrosion mechanisms involve an autocatalytic effect caused by pitting of, or formation of films on, the metal surface, destruction of the passivation film by Cl−, and the synergetic effect of O2 on H2S and CO2 corrosion; all of these combine to increase the rate of corrosion of metallic materials. On the basis of the mechanism and causes of serious corrosion of the oil well downhole string during production, the corrosion inhibitor YC-JTHSJ, suitable for high water-content oil wells, was developed. The weight-loss method showed that corrosion was reduced by 87.04 % by this inhibitor, and the average iron content of oil well liquid dropped by more than 85 % when 80 ppm YC-JTHSJ was added. This result showed that application of this well-specific inhibitor can reduce the rate of wellbore corrosion, the time required for pump inspection and maintenance, replacement of tubing and sucker rod, and thus production costs.