Study on Anti-Corrosion Performance and Applicability of Self-Healing Coating for Tubulars in CO2 /H2S Environment

Abstract

In a Middle East oilfield, reservoirs are characterized by high temperature, high salinity, high CO2 / H2S content, and low pH, which leads to harsh corrosive environment. With development of the field, the increasing water cut and application of CO2-EOR technology have made tubulars face greater corrosion risk. Therefore, employing feasible anti-corrosion coating to corresponding part is one of the most effective technologies to mitigate downhole corrosion risk, reduce workover, and avoid potential HSE risk. Corrosion environment characteristics was thoroughly studied by reviewing production history, water chemistry, gas composition, downhole temperature and pressure, logging data, etc. Downhole corrosion condition was classified based on NACE standard RP0775-2005. Based on this, the field simulated corrosion tests were carried out on the self-healing coating in high-temperature and pressure autoclave. The self-healing coating was fabricated by loading slow-released MBT-LDH nanocomposites to the phenolic epoxy resin. For comparison, the similar experiment was also conducted on commercial phenolic epoxy coating and heavy-duty coating. The anti-corrosion performance and applicability of the coatings were characterized by SEM/EDS, FT-IR and EIS. Two main factors have been considered while evaluating the coating options, the first consideration is reliability and durability of the coating. If the coating is easily damaged during operation and transportation, its protective performance decreases after damage, which cannot be easily repaired again. The second consideration is the compatibility of the coating in a harsh downhole environment. Considering the above aspects, the self-healing coating, commercial phenolic epoxy coating and commercial heavy-duty were selected. After corrosion tests, a small amount of corrosion products can be observed on the commercial phenolic epoxy coating surface both in the simulated well head and bottom condition. There were no obvious morphology changes on the heavy-duty coating surface in both condition, however, chemical degradation of the coating was observed in well bottom condition. Notably, the self-healing coating appeared no peeling, bubbling and other defects in both conditions. There were corrosion products identified in the pre-destructed area of the coating, which attributed to the localized inhibition of the self-healing coating. This paper investigated the corrosion resistance coating technology, including coating selection evaluation, typical tubing thread area protection technique, coating chemical and physical property analysis and evaluation. The study also recommended coating applicability for the target reservoir. The results suggested that phenolic epoxy based self-healing coating show robust anti-corrosion performance and can be used in the downhole containing CO2 and H2S.