Published December 2008
Acidic crude oils are grades of crude oil that contain substantial amounts of naphthenic acids (NAs) or other acids. They are also called high-TAN crudes after the most common measure of acidity, the Total Acid Number (TAN). Crude oils with as little as 0.5 mg KOH/g acid or petroleum fractions greater than about 1.0 mg KOH/g oil usually qualify as a high acid crude or oil. At the 1.0 mg/g level, crude oils begin to be heavily discounted in value. Other than acidity, there appear to be no other distinguishing properties that characterize these oils, although most high-TAN crudes’ gravities are often less than 29 API and often are low in sulfur (except for Venezuelan grades) and frequently produce high yields of gas oil. Acidic oils can vary widely in most other properties.
Refining acidic crude oils is of increasing interest due to their increased production and usually discounted value. According to one study, incremental high-TAN crude production will rise by 1.8 million B/D from 2005 to 2010. Output will continue to rise at least through 2015. Acidic crudes are produced in every oil producing region. China will dominate production, which is forecast to more than double from 2006 to 2015. Other locations historically noted for high NA crudes include Venezuela, India, Russia and some fields in California. Newer regions include the North Sea, West Africa, Mexico and offshore Brazil.
NAs are known to cause severe corrosion problems, especially when NAs are in low sulfur crude oils. Potential corrosion occurs at temperature between about 450 to 750°F (232 to 399°C). NAs and their salts stabilize the oil-water emulsion, creating problems during desalting and in downstream separators. Decomposition of NAs produces CO2 that increases the level of heat stable salts formation in the dry gas amine absorbers and cause foaming. CO2 in the aqueous phase increases the tendency for carbonate stress corrosion cracking.
Acidity in petroleum oils is due to two main sources: organic sulfur and naphthenic acids. This review emphasizes naphthenic acids since corrosion by sulfur compounds is much more well known and understood.