Molecular Modeling Study toward Development of H(2)S-Free Removal of Iron Sulfide Scale from Oil and Gas Wells

[Image: see text] A common problem that faces the oil and gas industry is the formation of iron sulfide scale in various stages of production. Recently an effective chemical formulation was proposed to remove all types of iron sulfide scales (including pyrite), consisting of a chelating agent diethylenetriaminepentaacetic acid (DTPA) at high pH using potassium carbonate (K(2)CO(3)). The aim of this molecular modeling study is to develop insight into the thermodynamics and kinetics of the chemical reactions during scale removal. A cluster approach was chosen to mimic the overall system. Standard density functional theory (B3LYP/6-31G*) was used for all calculations. Low spin K(4)Fe(II)(4)(S(2)H)(12) and K(3)Fe(II)(S(2)H)(5) clusters were derived from the crystal structure of pyrite and used as mimics for surface scale FeS(2). In addition, K(5)DTPA was used as a starting material too. High spin K(3)Fe(II)DTPA, and K(2)S(2) were considered as products. A series of K(m)Fe(II)(S(2)H)(n) complexes (m = n–2, n = 5–0) with various carboxylate and glycinate ligands was used to establish the most plausible reaction pathway. Some ligand exchange reactions were investigated on even simpler Fe(II) complexes in various spin states. It was found that the dissolution of iron sulfide scale with DTPA under basic conditions is thermodynamically favored and not limited by ligand exchange kinetics as the activation barriers for these reactions are very low. Singlet–quintet spin crossover and aqueous solvation of the products almost equally contribute to the overall reaction energy. Furthermore, seven-coordination to Fe(II) was observed in both high spin K(3)Fe(II)DTPA and K(2)Fe(II)(EDTA)(H(2)O) albeit in a slightly different manner.