Effect of pH on acidic and basic chelating agents used in the removal of iron sulfide scales: A computational study

Iron sulfide scale is a major challenge in the upstream section of the oil and gas industry. It is responsible for precipitate formation in tubulars, and it damages equipment. This leads to a loss in production. Mechanical and chemical treatment are methods used in scale removal. However, the former causes a Cobra effect by making things worse due to corrosion. Hence, chemical treatments are more widely used. THPS (Tetrakis hydroxymethyl phosphonium sulfate) and DTPA (diethylenetriaminepentaacetic acid) which are acidic and basic respectively are chelating agents which are part of the chemical methods used in the removal of iron sulfide scales. However, they are used at extreme pH conditions. Herein, the pH effects on the chelating ability are studied via binding affinity to the central metal ion using DFT (Density Functional Theory) calculations. The robust B3LYP (Becke's 3 Lee Yang and Parr) parameter exchange-correlation functional was employed in this study. The binding affinity of THPS to the central metal ion decreased as the deprotonation increased despite forming six coordinate bonds with the central metal ion whereas in DTPA the binding affinity increased as the protonation increased and formed seven coordinated bonds to the central metal ion. Therefore, DTPA can be used more effectively if the pH is reduced to a neutral pH. The results herein provide molecular insights in understanding the possibility of using DTPA effectively in mild condition while still increasing its chelating power in removing iron sulfide scales.