Evaluation of Chelating Agents for Iron Sulfide FeS Scale Removal
Authors: Ramanathan Raja , Nasr-El-Din Hisham 
Published: November 2019

Abstract
Scaling of well tubulars can become a very expensive problem because of well deliverability and integrity issues such as reduced production rates and damage to well tubulars. Alternative chemicals such as chelating agents are necessary to dissolve iron sulfide scales because of the the conventional HCl treatment is corrosive and toxic. However, chelating agents have been scarcely studied for iron sulfide dissolution. This paper investigates Ethylenediaminetetraacetic acid (EDTA), Diethylenetriaminepentaacteic acid (DTPA), and N-(2-Hydroxyethyl)ethylenediamine-N,N’,N’-triacetic acid (HEDTA) for its iron sulfide (FeS) dissolution capacity and kinetics at 150°F.
Chelating agents are expensive chemicals and must be investigated to determine the optimum concentration, pH, and treatment time at 150°F. 0.1g of iron sulfide composed of mainly troilite was used as the scale. 10 cm3 of EDTA, DTPA, and HEDTA solutions were prepared at different concentrations ranging from 0.05 to 0.4 mol/L using deionized water with a resistivity of 18.2 MΩ-cm. The pH of the dissolvers were dependent on the concentration and the degree of neutralization. The ligands were deprotonated at higher pH using sodium hydroxide or potassium hydroxide. A comparative study of the chelating agents with a low pH (3-5), moderate pH (5-9), and high pH (10-14) determined the optimum pH for the scale treatment. The sampling time of the dissolution process set at 1, 2, 4, 8, 20, 30, and 72 hours determined the kinetics of the scale dissolution process and helped optimize the treatment time. The iron concentration in the dissolver was quantified using an Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES).
Two calculated parameters, dissolution capacity and dissolver consumption, determined the effectiveness of the chelating agent in dissolving iron sulfide. From the bottle tests at 150°F, lower pH solutions were more effective. 100% of the iron from iron sulfide was complexed by 0.3 mol/L K2-DTPA after 20 hours of soaking. 0.2 mol/L Na2-EDTA and 0.3 mol/L K-HEDTA was able to remove 69 and 96% of the initial iron present in 0.1 g of iron sulfide. The mechanism of dissolution at pH &lt; 5 was determined to be H+ attack with surface complexation. At alkaline conditions (pH &gt; 10), the dissolution of the scale was negligible and was a result of solution complexation after surface hydrolysis. The order of the chelating agents in terms of dissolution capacity was DTPA &gt; HEDTA &gt; EDTA at all pH conditions. The kinetics study showed that the optimum treatment depended on the pH of the chelating agent. For pH &lt; 5 dissolvers, 16-20 hours was sufficient to obtain the maximum dissolution capacity. For dissolvers with a pH greater than 10, the dissolution continued for more than 72 hours and was minimal. Increasing the concentration of the chelating agent aided the solubility of the scale at pH &lt; 5 only. A SEM study showed the changes in the morphology of the iron sulfide particles after dissolution with low and high pH solutions of the chelating agent.
The role of chelating agents in iron sulfide dissolution has not been thoroughly investigated. There is no study that reports the optimum treatment time. The role of pH of the dissolver also needs more attention. This paper fills these gaps in literature and provides the optimum dissolver composition and treatment time for field operations.