Development and Field Application of a New Hydrogen Sulfide Scavenger for Acidizing Sour-Water Injectors
- Hisham A. Nasr-El-Din (Saudi Aramco) | Shrihari Kishor Kelkar (Schlumberger) | Mathew Samuel
- Document ID
- Society of Petroleum Engineers
- International Symposium on Oilfield Chemistry, 28 February-2 March, Houston, Texas, U.S.A.
- Publication Date
- Document Type
- Conference Paper
- 2007. Society of Petroleum Engineers
- 4.3.4 Scale, 3.4.1 Inhibition and Remediation of Hydrates, Scale, Paraffin / Wax and Asphaltene, 2.5.2 Fracturing Materials (Fluids, Proppant), 3 Production and Well Operations, 4.2 Pipelines, Flowlines and Risers, 1.8 Formation Damage, 1.6 Drilling Operations, 4.2.3 Materials and Corrosion, 4.1.2 Separation and Treating, 3.2.4 Acidising
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In treating sour, water injectors in carbonate formations in Saudi Arabia, sulfide scavenging, prevention of sulfur and iron sulfide precipitation is a major requirement during acidizing treatments. As the acid spends on the carbonate surfaces, and in the presence of ferric iron in the injected acid, iron sulfide precipitates at pH values less than 1.9.
Iron sulfide and elemental sulfur have been implicated in injectivity decline following acid treatments on water injectors under sour conditions. Previous research work has led to the use of a new generation of hydrogen sulfide scavengers (mixture of aldehyes) for use in acidizing treatments.1 However, one of the main drawbacks of this scavenger in acidizing treatments is the formation of oily material, which can adversely affect the performance of water injectors.
This paper discusses the main reactions of hydrogen sulfide with various scavengers that can be used during well stimulation. A new scavenger that is based on hydroxyalkyl triaziane was thoroughly examined. Laboratory work included compatibility of the old and new hydrogen sulfide scavenger with live acids. The impact of the new scavenger on corrosion of low-carbon steel was also addressed.
Based on encouraging results from the laboratory, field trials were conducted on a number of wells. Post acidizing flowback samples were collected and analyzed to confirm laboratory results. Finally, injectivity results from the field trial were compared with offset well performance to validate the technical benefits of the new hydrogen sulfide scavenger.
This paper continues the discussion on the need and importance of using proper sulfide scavengers in well acidizing fluids. It will highlight the fact that there is more science than art involved in carbonate acidizing, especially for sour wells.
In mydrogen sulfide scavengers are typically aldehyde-based chemicals.1 In mmmmmmany wells, pipelines, or hydrocarbon processing units of refineries, iron-based surfaces may come into contact with sulfur-containing fluids. At the temperature present in the various sections or reactors, and during long periods of contact, iron sulfide deposits (generally FeS, but sometimes, FeS2) will form. Iron sulfide is present in various forms in the wellbore. Analysis of the scale present in sour gas wells indicated that there are several types of iron sulfide species.2 Mackinawite, troilite, pyrrhotite, pyrite, and marcasite are found on tubular surfaces.
While iron sulfide scale removal using mineral acids is a very effective procedure, it produces large amounts of hydrogen sulfide.
FeS + 2H+ = Fe2+ + H2S (1)
Hydrogen sulfide causes severe safety and operational problems once the acid leaves the system being treated, and H2S stimulates corrosion of the base metal. Hydrogen sulfide scavengers are added to HCl when stimulating sour wells to prevent re-precipitation of iron sulfide and elemental sulfur.3-5 In treating sour oil and gas wells, as compared with treating surface equipment, corrosion suppression (not elimination of sulfide gas) and dissolution of FeS are of major concern. The inhibitor package must protect several types of steel at high temperatures in the presence of concentrated acid containing numerous additives. Various additives are required since the purpose of the treatment may include removal of inorganic and organic damage from producing formations.
Tubular cleaning prior to acidizing the formation is commonly performed and is strongly recommended.6-7 During acidizing treatments, control of iron and sulfur precipitation is an important requirement. The control is necessary to prevent formation damage due to precipitation of iron compounds and elemental sulfur. As the acid spends on carbonate surfaces, and in the absence of H2S, ferric iron precipitates at pH > 1-2,8 and ferrous iron precipitates at pH values > 6.0. Elemental sulfur can precipitate in sour wells, if ferric iron is present.9
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