Calcium Sulfate Scale Inhibitor: Does the Carbon Backbone Length Matter?

Abstract

Abstract Calcium sulfate (CaSO4) scale precipitation is a challenge faced in the oil and gas industry where it can cause a variety of issues that lead to productivity decline. Amino-phosphonate-base scale inhibitors are used to prevent this type of inorganic scale and have shown great success in the industry. A common misconception is to believe that high molecular weight amino-phosphonates perform better than their low molecular weight counterparts. This study systematically compares performance of scale inhibitors which have the same number of functional groups but different molecular size and backbone chain length. The study shows the influence of changing the molecular size and backbone chain length on the inhibition efficiency. Two amino-phosphonate-based scale inhibitors are tested in this work: Ethylenediamine tetramethylenephosphonic acid (EDTMP) and Hexamethylenediaminetetramethylene phosphonic acid (HDTMP). These inhibitors have the same number of functional groups, but they differ in the length of the carbon backbone chain. The inhibition efficiency of each inhibitor is measured through its ability to prevent formation of CaSO4 in a brine containing up to 6,000 ppm calcium ions (Ca2+) and an equivalent amount of sulphate ions (SO42-). Moreover, the remaining concentration of Ca2+ in the solution is measured using Inductive Coupled Plasma (ICP). The results of the study shows that the inhibitor molecules with shorter chain length, EDTMP, have higher inhibition efficiency than the ones with longer chain length, HDTMP. That is because a small dose of EDTMP can efficiently inhibit CaSO4 precipitation while a big dose of HDTMP to do the same job efficiently. Environmental scanning electron microscope (ESEM) is employed to study the effect of the inhibitors on the morphology of the formed scale. The morphology of the formed scale after using the low doses of the inhibitors are investigated by Environmental Scanning Electron Microscope (ESEM), and the resulted images show that the CaSO4 precipitates exhibit deformed shapes compared to the CaSO4 precipitates where no inhibitor is used.