Molecular and electronic structure elucidation of Fe²⁺/Fe³⁺ complexed chelators used in iron sulphide scale removal in oil and gas wells

Quantum chemical calculations based on DFT are employed to study the electronic structure and binding affinity of chelators used in the removal of iron sulphide scales. Three chelating agents, EDTA, HEDTA, and DTPA, are considered in this work. The complexes showed a coordination number of 5, 6, and 7 for Fe²⁺ and Fe³⁺ ions with HEDTA, EDTA, and DTPA, respectively. However, regarding EDTA, Fe³⁺ could coordinate with an additional water molecule and form a seven-coordinate complex. The calculated binding energies agreed with the experimental stability constants of the chelators in the order DTPA > EDTA > HEDTA for both Fe²⁺/Fe³⁺ complexes. The binding free energies showed a spontaneous reaction with Fe³⁺ having a stronger binding affinity than Fe²⁺ due to electrostatic forces. This investigation provides insights regarding how chelators that are applied in iron sulphide scale removal may be designed by increasing the number of nitrogen atoms to above the number of carboxylate groups.