Root Cause Analysis for Iron Sulfide Deposition in Sour Gas Wells

Abstract

Iron sulfide (FeS) deposition is a ubiquitous phenomenon in sour oil and gas wells, especially for these producing from high temperature and high pressure reservoirs. Hydrogen sulfide (H2S) gas is highly soluble in water and readily reacts with carbon steel and dissolved iron once in contact, which leads to the formation of FeS scale. The surface deposition or bulk precipitation of FeS scale is detrimental to flow assurance, such as flow restriction, pitting corrosion and stabilized emulsion. Compared to the conventional carbonate and sulphate scales, the mitigation of iron sulfide deposition is notoriously difficult. It is essential to understand its root causes in order to develop a suitable strategy to manage the problem effectively. By combining laboratorial tests and model simulations, new progresses have been made on the FeS root cause analysis for high temperature high H2S gas wells. The iron sources were determined over different stages of well life from drilling, completion, acidizing to production. Results from this study demonstrate that the iron contributed by the sour reservoir connate water is limited and is not the major cause to FeS deposition on downhole tubular in sour gas wells. Carbon steel corrosion during production stage is one source of FeS deposition. However, the rate of iron sulfide deposition during production is minor and far less than the deposit observed in the field. Other sources of iron sulfide deposition should be further investigated. Another major source is the iron released from tubing due to acid corrosion during acidizing stimulation, which potentially leads to severe formation damage and associated deposition problems in the production tubing and equipment. In addition, the iron contamination in the drilling fluid could contribute to FeS scaling problem. This paper presents a fundamental study to understand the sources of iron for FeS deposition in high H2S sour wells producing from carbonate reservoirs. Appropriate mitigation strategies are recommended accordingly.