Iron Sulfide Scale Dissolvers: How Effective Are They?

Abstract

Abstract Many Khuff reservoir gas wells in Saudi Aramco fields are classified as sour producers with H2S levels as much as 8 mol% and CO2 contents as much as 5 mol%. Despite being completed with carbon steel tubular, corrosion failure has not been a major issue even after more than 20 years of service. On the other hand, most wells are experiencing scale buildup in production strings, causing an estimated production loss of up to 7 MMscfd for some wells. Besides, scale deposits are limiting well access for downhole surveillance and intervention. Significant efforts have been taken to understand the scale formation mechanisms. Extensive scale samples have been collected and characterized over the years. Although the exact chemical compositions change from well to well and also vary with depth in a given well, the scale deposits are usually mixtures of many compounds and often dominated with iron sulfide minerals. These iron sulfides include pyrrhotite, troilite, mackinawite, greigite, pyrite and marcasite. Other iron containing compounds, such as iron oxide and iron carbonate, are also found in significant amounts in most cases. Additionally, mineral scales, such as calcium carbonate and barium, strontium and calcium sulfate, are often present. Both mechanical and chemical methods have been applied for scale removal. Chemical dissolvers based on concentrated hydrochloric acid have been the most effective solvents. This types of dissolvers are very corrosive to well completion metallurgy at elevated temperatures, and the spent acids can cause severe formation damage due to re-precipitation of iron sulfides, if entering the near wellbore area. The rapid generation of large amounts of H2S gas creates a potentially lethal health hazard. This paper presents an effort to identify alternative dissolvers with high dissolving power and low corrosivity to carbon steel.