Pyrite-scale removal using glutamic diacetic acid: A theoretical and experimental investigation

Abstract

Iron sulfide scale causes major losses in both upstream and downstream sectors of the hydrocarbon industry. Pyrite is one of the most-difficult forms of iron sulfide scale from a removal point of view. Inorganic acids such as hydrochloric acid (HCl) are not recommended for removing pyrite scales because they have many drawbacks, including low pyrite solubility, high corrosivity to the tubular system, and generation of toxic hydrogen sulfide (H2S). In this work, pyrite-scale dissolution is studied using an ecofriendly formulation of glutamic diacetic acid [L-glutamic acid, N, N-diacetic acid (GLDA)] as an alternative to HCl. Although GLDA has shown potential for removing iron sulfide in general and pyrite scale in particular, still GLDA/pyrite kinetics have not been well-understood. Both experimental and theoretical techniques have been used. The reaction kinetics has been investigated in a rotating-disk apparatus (RDA) at typical reservoir conditions of 150°C and 1,000 psi (Conway et al. 1999). Characterization techniques, including X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM), have been used to study the surface chemistry before and after treatment with GLDA, and the results support pyrite removal. Furthermore, density-functional-theory (DFT) calculations have been performed to understand the ability of GLDA to dissolve iron sulfide scale at the atomistic level. From the laboratory results, the reaction rate using 20-wt% GLDA (pH of 3.8) was 5.378×10−8 mol/cm2·s. The measured rate outperformed other proposed formulations according to the tetrakis(hydroxymethyl)phosphonium sulfate (THPS) formulation by 15 times. In addition, GLDA surpassed the most recent results on diethylenetriamine penta-acetic acid (DTPA) by nearly an order of magnitude. Moreover, pyrite dissolution in GLDA increases as the disk rotational speed increased, which indicates mass-transfer control with a diffusion coefficient of 1.338×10−7 cm2/s. Furthermore, from molecular modeling using DFT, the binding energy between GLDA and Fe2+ is calculated as –105.97 kcal/mol. The negative value observed correlates with the stability constant and indicates the strong binding affinity to Fe2+. Finally, GLDA could be recommended for pyrite-scale removal because it is biodegradable, less corrosive, free of H2S, and achieved solubility that outperformed THPS- and DTPA-based formulations.