Iron sulfide (FeS)
|Jmol-3D images||Image 1|
|Molar mass||87.910 g/mol|
|Appearance||black solid, sometimes in lumps or powder|
|Solubility in water||negligible (insoluble)|
|Solubility||reacts in acid|
|EU Index||Not listed|
|Main hazards||can be pyrophoric|
|Related compounds|| Iron(II) oxide|
| (what is: / ?) |
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Iron(II) sulfide or ferrous sulfide (Br.E. sulphide) is a chemical compound with the formula FeS. In the oil/gas industry, iron sulfides are often present in a sour system due to reaction between corrosion products and hydrogen sulfide. They can deposit in the production tubing or on the pipe wall and accumulate in separators. Dry powdered iron sulfide is pyrophoric (ignites spontaneously when exposed air), which poses a significant HSSE risk.
Forms of iron sulfide
- Pyrrhotite, Fe1-xS, a mineral, which displays ferrimagnetism and crystallizes in monoclinic system. Iron metal shows ferromagnetism; iron sulfides do not.
- Troilite, FeS, a stoichiometric compound that adopts hexagonal symmetry.
- Mackinawite, Fe1+xS the least stable form of iron sulfide; mackinawite has a layered structure.
- Pyrite and marcasite, which are diamagnetic minerals, have the formula FeS2.
- Greigite (Fe3S4) a ferromagnetic species akin to magnetite (Fe3O4).
During acid jobs, iron sulfide reacts with acid, releasing the pungent and very toxic hydrogen sulfide
- FeS + 2 HCl → FeCl2 + H2S
FeS can be form when the dissolved iron as a result of corrosion (or naturally present in the reservoir) reacts with H2S in a sour system:
- Fe2+ + H2S → FeS
Implications in oilfield
In small quantities it can enhance the protective film formed with corrosion inhibiting compounds. In combination with asphaltenes it may form a protective layer. In gas lines, under the appropriate conditions, it can form protective layers by itself. Its effect can be compared with that of calcium carbonate scale that protects water distribution lines in most cities.
When too much iron sulfide is formed, it no longer provides its protective service. It may crack and portions of it may flake off. The remaining deposits may provide the perfect environment for crevice or under-deposit corrosion.
The adherent deposits may interfere with the operation of pumps, valves and other equipment. It may decrease the efficiency of heat exchangers and, in extreme cases, inhibit the flow of fluids through lines, creating flow assurance issues.
In gas systems, over an extended period of time, enough pyrophoric iron sulfide may form to pose a safety hazard when the material is exposed to oxygen.
Iron sulfide is much more easily oil-wet than other mineral scales, so free-floating precipitates of iron sulfide is often found at the oil-water interface, where it stabilizes emulsions and interferes with the separation process.
Prevention and remediation
Since the iron in FeS can come from the production system due to corrosion, a robust corrosion problem helps mitigate the root cause of this problem. H2S scavenger helps take out the S2- part of the equation.
Scale inhibitor can also potentially help prevent FeS deposition.
FeS is one of the most soluble sulfide minerals.
- Acid stimulation can help remove FeS solids. However, there are several challenges associated with acid dissolution.
- High concentration of toxic H2S can be released during the process, creating a potentially lethal health hazard. Extreme caution must be taken.
- As iron sulfide ages and, especially, oxidizes, it may be converted to forms that are much less readily dissolved by acid.
- The tendency of iron sulfide to become oil-wet helps create a protective organic coating that inhibits attack by the water-soluble acid.
- The spent acid solution containing the dissolved iron sulfide may redeposit iron sulfide when physical or chemical conditions change.
- Tetrakis(hydroxymethyl) phosphonium sulfate (THPS) is typically utilized as a biocide, but it can be used as a solvent to dissolve FeS solids
- D., Vaughan; J., Craig (1978), Mineral chemistry of metal sulfides, Cambridge University Press, ISBN 0-521-21489-0
- John L. Przybylinski, Baker Petrolite Corporation, Iron Sulfide Scale Deposit Formation and Prevention under Anaerobic Conditions Typically Found in the Oil Field, SPE International Symposium on Oilfield Chemistry, 13-16 February 2001, Houston, Texas.
- Robert E. Talbot, Albright & Wilson Ltd.; Jan Larsen, Maersk Oil & Gas; Peter F. Sanders, Oil Plus Ltd., "Experience With the Use of Tetrakishydroxymethylphosphonium Sulfate (THPS) for the Control of Downhole Hydrogen Sulfide", CORROSION 2000, March 26 - 31, 2000 , Orlando, Fl