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.
Iron sulfide scale is present in sour wells,1 seawater injectors,2 water injection,3–5 and supply wells6 that are contaminated with sulfate reducing bacteria (SRB). It is also present in produced water disposal wells7 where hydrogen sulfide present in crude oil partitions into the produced water. Hydrogen sulfide reacts with iron surfaces of uncoated vessels, pipelines and well tubulars and produces iron sulfide.
Iron sulfide is present in various forms in the wellbore area. Analysis of the scale present in sour gas wells indicated that there are several types of iron sulfide species.1 Mackinawite, troilite, pyrrhotite, pyrite, and marcasite are found on tubular surfaces. Smith and Miller8 examined the crystalline structure and physical properties of various iron sulfide minerals. Hydrochloric acid can be used to dissolve iron sulfide deposits with varying degrees of success. In general, iron sulfide deposits with low sulfur content have higher solubility in acids.9 Acid reaction with iron sulfide produces hydrogen sulfide, as shown in Equation 1.
Hydrochloric acid is very reactive at temperatures encountered in most gas and oil reservoirs. Acidizing fluids contain a variety of special additives; the type and amount depend on the nature of the treatment. The fluids always contain corrosion inhibitors to suppress the attack of the acid on the tubulars and surface equipment. Other additives include wetting agents (surfactants), chelants or reducing agents for control of iron precipitation, anti-sludge agents and various solvents (methanol or glycol ethers). Sulfide suppressors will also be required in sour wells. 10–14 While required for a successful treatment, acid additives can affect scale dissolution characteristics15 and can also adversely affect corrosion inhibition.16