Iron Sulfide Scale Dissolver for Downhole Application: Where Are We Now?

Abstract

Downhole scale deposition in the Khuff sour gas wells in Saudi Arabia has been a persistent problem, which negatively affects operation and production. Scale deposits are composed of predominantly iron sulfides with other types of minerals also present. Mechanical descaling treatment, although expensive and time-consuming, is often required. Effective scale dissolver is highly desirable to enhance descaling efficiency and to reduce treatment cost. An ideal dissolver is required to have high scale dissolving power, no damage to downhole completion and well productivity, and minimal H2S liberation. This paper presents the laboratory studies on the new scale dissolvers developed by service companies. These products have pH values ranging from strong acidic (pH < 2) to high alkaline (pH > 12). Dissolvers were evaluated for thermal stability, corrosivity to mild steel, and compatibility with formation water at downhole temperatures. The potentials of iron sulfide re-precipitation in spent solutions and free H2S generation were also examined. The qualified chemicals were then evaluated for their dissolving capacity using authigenic pyrrhotite and field scales at elevated temperatures. The obtained results show that most effective acidic dissolvers evaluated in this study were very aggressive to low alloy carbon steel at downhole temperatures. For these with acceptable corrosivity, formation of iron sulfide reprecipitation in spent dissolvers and the generation of a large quantity of free H2S gas also prevented them from field application. Some dissolver products were disqualified due to incompatibility with formation water. Dissolvers with near neutral and alkaline pH values, in general, were inefficient to dissolve the heterogeneous iron sulfide scales. The performance of tested dissolvers varied with scales from different wells, attributed by differences in composition, microstructure, and the presence of hydrocarbon. Results also suggested that pyrite and marcasite were possibly formed during the dissolution process. This paper presents an objective assessment on the currently available iron sulfide scale dissolvers, highlights the challenges on downhole scale dissolution in high temperature sour wells, and provides new insights on the scale dissolution process. The results suggest that further R&D efforts are required to develop more effective chemical solutions to mitigate the iron sulfide scale problem.