Structure and stoichiometry of mixed divalent metal (Ca²⁺/Mg²⁺/Sr²⁺/Ba²⁺) phosphonate scale inhibitor complexes for application in precipitation squeeze processes

In oilfield applications, various divalent cations are present in field produced waters, e.g. Ca²⁺, Mg²⁺, Sr²⁺ and Ba²⁺. The interaction of these cations with phosphonate scale inhibitors (SI) is important both for scale inhibition and inhibitor retention mechanisms. In this paper, the properties of precipitated "mixed" Ca²⁺/M²⁺ phosphonate SI complexes formed by 9 common phosphonate species are investigated, where M²⁺ may be Mg²⁺, Sr²⁺, or Ba²⁺. These 2 and 3 metal ion complexes with SI are of the form SI-Ca<inf>N1</inf>-Sr<inf>N2</inf> and SI-Ca<inf>N1</inf>-Mg<inf>N2</inf>-Ba<inf>N3</inf>, where the stoichiometry N<inf>1</inf>, N<inf>2</inf> and N<inf>3</inf>, are the respective (M²⁺/SI) molar ratios. Static precipitation tests were performed varying the amounts of Ca²⁺ and M²⁺ present in the system (at a constant ionic strength), at 95°C, pH 8.5, and fixed [SI]= 2,000ppm. The stoichiometries of the solid precipitates were determined by re-dissolving the precipitated deposits in acidified distilled water (DW/HCl), and then assaying for Ca²⁺, M²⁺, and P directly, under each test condition, by Inductively Coupled Plasma (ICP) spectroscopy. It is shown experimentally that, for all 9 phosphonates tested, these stoichiometries depend on the number of M²⁺ binding sites per molecule, solution pH, the relative SI binding constants to Ca²⁺ and M²⁺ at the test pH, and the solution molar ratio of M²⁺/Ca²⁺.