Iron sulfide scales create well deliverability and integrity problems such as reduced production rates and damage to well tubulars. Chelating agents have scarcely been studied for iron-sulfide dissolution. This paper evaluates EDTA, DTPA, and HEDTA for its iron sulfide (FeS) dissolution capacity and kinetics at 300°F (149°C). 

A comparative study of the chelating agents with a low pH (3-5), moderate pH (5-9), and high pH (9-14) determined the optimum pH for the scale treatment. These tests were conducted in glass bottles and a high-temperature autoclave apparatus under anoxic conditions. Results show that the iron-sulfide dissolution increases tremendously at 300°F (149°C) when using chelating agents with a pH>5. At 300°F (149°C), the bond strength is reduced, allowing the chelating agents to remove the metal ion from the solid surface. The ranking of the chelating agents in terms of iron-sulfide scale dissolution capacity and effectiveness was DTPA>EDTA>HEDTA at all pH conditions. The role of chelating agents in iron sulfide dissolution at 300°F (149°C) has not been thoroughly investigated. There is no study that reports the optimum treatment time at that temperature. This paper investigates chelating agents and provides the optimum dissolver composition and treatment time for field operations at high temperature conditions.