Molecular Modeling Study toward Development of H2S-Free Removal of Iron Sulfide Scale from Oil and Gas Wells

Wim Buijs; Ibnelwaleed A Hussein; Mohamed Mahmoud; Abdulmujeeb T Onawole; Mohammed A Saad; Golibjon R Berdiyorov

doi:10.1021/acs.iecr.8b01928

Molecular Modeling Study toward Development of H₂S-Free Removal of Iron Sulfide Scale from Oil and Gas Wells

Ind Eng Chem Res. 2018 Aug 8;57(31):10095-10104. doi: 10.1021/acs.iecr.8b01928. Epub 2018 Jul 5.

Authors

Wim Buijs¹, Ibnelwaleed A Hussein², Mohamed Mahmoud³, Abdulmujeeb T Onawole², Mohammed A Saad⁴, Golibjon R Berdiyorov⁵

Affiliations

¹ Engineering Thermodynamics, Process & Energy Department, Delft University of Technology, Leeghwaterstraat 39, 2628 CB Delft, The Netherlands.
² Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar.
³ Department of Petroleum Engineering, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia.
⁴ Chemical Engineering Department, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar.
⁵ Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, P.O. Box 5825, Doha, Qatar.

Abstract

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 (K₂CO₃). 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 K₄Fe(II)₄(S₂H)₁₂ and K₃Fe(II)(S₂H)₅ clusters were derived from the crystal structure of pyrite and used as mimics for surface scale FeS₂. In addition, K₅DTPA was used as a starting material too. High spin K₃Fe(II)DTPA, and K₂S₂ were considered as products. A series of K _m Fe(II)(S₂H) _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 K₃Fe(II)DTPA and K₂Fe(II)(EDTA)(H₂O) albeit in a slightly different manner.