This paper describes the effect of chemical interactions on the efficiency of removing iron sulfide scale. Most types of iron sulfide can be removed by inorganic acids. A corrosion inhibitor (amine-type chemical) is added to the acid to protect the well tubulars. In addition, a hydrogen sulfide scavenger (aldehyde type) is used to prevent precipitation of elemental sulfur and iron sulfide. Other chemicals are also added to the acid to enhance acid-contact with the scale and prevent precipitation of iron once the acid is spent. Laboratory tests were conducted to evaluate the effect of chemical interaction on the efficiency of removing iron sulfide scale. The influences of various acid additives (anionic, cationic and non-ionic) on the dissolution power of the acid and the corrosion of oilfield steels were examined over a wide range of parameters.
Laboratory data indicated for the first time that the ability of the acid to remove iron sulfide scale depends, among other factors, on the ionic character of the acid additives. Corrosion inhibitors, which are used to protect the casing and well tubulars from the acid, were found to adsorb on the iron sulfide scale and inhibit acid reaction with the scale. Hydrogen sulfide scavengers, aldehyde type chemicals, were found to inhibit acid reaction, especially at high concentrations. A similar trend was noted with methanol and citric acid. On the other hand, TX-100 (a nonionic surfactant) and SDS (an anionic surfactant) was found to enhance the dissolution of iron sulfide by the acid.
Acid additives, especially anionic surfactants and mutual solvents were found to affect the ability of the corrosion inhibitor to protect the base metal.