Iron Sulfide Removal: A Nonacidic Alternative to Hydrochloric Acid Treatment

Abstract

Abstract Iron sulfide scale deposition is a common problem in sour oil and gas wells. Accumulation of iron sulfide in the production tubing causes reduction of well deliverability. Conventional chemical means to remove Iron sulfide scale usually involve hydrochloric acid and other mineral acids. The composition and crystalline structure of iron sulfide scales varies significantly. Although iron disulfide (FeS2), which is found in the minerals such as marcasite and pyrite, is not soluble in acid, pyrrhotite (Fe7S8) has a high solubility in hydrochloric acid. Therefore, although hydrochloric acid and other acids can be applied to remove iron sulfide scales, their effectiveness in different treatments could be quite different depending on the scale composition. Even in cases where hydrochloric acid could be applied for iron sulfide removal, the applied hydrochloric acid is very corrosive to downhole tubulars especially at high temperatures and high acid concentrations. In the meantime, the generated H2S poses a great threat because of its high toxicity and enhanced corrosion tendency due to the combination of hydrochloride acid and H2S. To overcome the drawbacks of hydrochloric acid treatment, i.e., the risk of H2S evolution and the corrosion tendency in standard carbon steel, an alternative chemical treatment to hydrochloric acid treatment is preferred. This paper illustrates the development of a new chemical treatment solution for iron sulfide removal. This polymer-based system demonstrated high dissolving capacity for iron sulfide scales in neutral or nearly neutral pH environments. Testing related to its performance also has revealed advantages such as low corrosion rate, and low H2S evolution. Additionally, laboratory testing demonstrated this polymer-based system is an effective general purpose stimulation fluid to stimulate carbonate and dolomite formations.