New Insights into the Dissolution of Iron Sulfide Using Chelating Agents
Authors: Ramanathan Raja Subramanian , Nasr-El-Din Hisham A. , Zakaria Ahmed S. 
Published: July 2020

Iron sulfide (FeS) scales create well deliverability and integrity problems such as decreased production rates and damage to well tubulars. The application of chelating agents for production enhancement has been successful because of its high-temperature stability and its clean characteristic nature without the need for expensive additives. However, chelating agents have not been studied adequately for FeS dissolution. This paper investigates ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacteic acid (DTPA), and N-(2-hydroxyethyl) ethylenediamine-N,N′,N′-triacetic acid (HEDTA) for their FeS dissolution capacities and kinetics at 150°F and 300°F.

Chelating agents are expensive chemicals and must be investigated carefully to determine the optimum concentration, pH, treatment time, and dissolver/scale ratio. FeS (0.1 g) composed of mainly troilite was used as the scale. EDTA, DTPA, and HEDTA solutions (10 cm3) 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 was 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 to 5), moderate pH (5 to 9), and high pH (10 to 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. A dissolver/scale ratio of 100:1, 50:1, and 20:1 cm3/g were tested. The iron concentration in the dissolver was quantified using 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 FeS sulfide. From the bottle tests at 150°F, lower pH solutions were more effective. One hundred percent of the iron from FeS was complexed by 0.3 mol/L dipotassium DTPA (K2-DTPA) after 20 hours of soaking; 0.2 mol/L disodium EDTA (Na2-EDTA) and 0.3 mol/L potassium HEDTA (K-HEDTA) were able to remove, respectively, 69 and 96% of the initial iron present in 0.1 g FeS. The mechanism of dissolution at pH &lt; 5 was determined to be hydrogen ion (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 FeS dissociation. 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 to 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 only at pH &lt; 5. At 300°F and pH &gt; 5, there was an improvement in the effectiveness of the ligands because of the increase in the system energy and increased activity of the chelating agent. A scanning electron microscopy (SEM) study showed the changes in the morphology of the FeS particles after dissolution with low- and high-pH solutions of the chelating agent.

The role of chelating agents in FeS dissolution has not been thoroughly investigated. No study reports the optimum treatment time and dissolver/scale ratio. The role of the pH of the dissolver also needs more attention. This paper fills these gaps in the literature and provides the optimum dissolver composition and treatment time for field operations.