Abstract
The chemical components of a crude oil significantly influence steel corrosion in crude oil production, and this influence comes in two flavors, occurring in tandem, specifically the effect on brine corrosivity and wettability. Brine corrosivity determines the corrosion that occurs when the surface is water wet, i.e., certain organic chemicals in the crude oil are soluble in the water phase directly affecting the corrosion rate and morphology. Wettability determines when water wetting occurs, i.e., the crude oil can influence the predisposition for either water or crude oil to wet the surface. The two together provide the total picture of the crude oil effect on steel corrosion.
The effect of crude oil on corrosion in a production environment can range from low corrosion rates with as much as 70% produced water in sweet crude oil production to severe localized corrosion with 1% produced water in sour crude oil production. This demonstrates the wide variability in the way the crude oil alters the corrosion of carbon and low alloy steel in both sweet and sour crude oil/water production, including the corrosion rate, corrosion morphology and protectiveness of the corrosion product layer formed on the steel. The water chemistry effect is observed when a separated water phase is in contact with the steel, such as stratified pipe flow and the bottom of production separators. When the crude oil and produced water are in an intimate mixture, such as in turbulent pipe flow and production tubulars, wettability is primary.
While there has been progress in understanding the effect of crude oil on steel corrosion, the details of the influence of crude oil chemistry are still under investigation. Due to the number and variation of polar organic compounds contained in a crude oil, possible synergistic effects, and the variability in these compounds from crude oil to crude oil, relating specific compounds to their effect on corrosion is extremely difficult at best. Utilizing more general data routinely obtained for crude oils such as total organic nitrogen and sulfur, acid number (TAN), and SARA analysis (organic saturates, aromatics, resins, and asphaltenes) is a much more practical alternative.
