Structure, stoichiometry, and modelling of mixed calcium magnesium phosphonate scale inhibitor complexes for application in precipitation squeeze processes

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    Abstract

    In this paper, the properties of precipitated "mixed" calcium and magnesium phosphonate scale inhibitor (SI) complexes formed by 9 common phosphonate species are investigated. These complexes are of the form SI- CaNl -MgN2 where the stoichiometry (Ca2+/SI and Mg2+/SI molar ratios, i.e. N1, and N2) in various precipitates are established experimentally, and the effect of solution pH on the stoichiometry is determined. Static precipitation tests were performed varying the amounts of Ca2+ and Mg2+ present in the system (at a constant ionic strength), at test temperatures ranging from 20°C to 95°C, at a fixed [SI] = 2, OOOppm. The stoichiometries of the solid precipitates were determined by assaying for Ca2+, Mg 2+, and P in the supernatant liquid, under each test condition, by ICP spectroscopy. It is shown experimentally that, for all 9 phosphonates tested, these stoichiometries (i.e. N1, and N2 in SI- CaNl-MgN2) depend on (i) the nature of the SI (i.e. M 2+ binding sites per molecule); (ii) solution pH, which affects the speciation of the SI; (iii) the relative magnitude of the SI binding constants to Ca2+ and Mg2+at the test pH (Kb1 and K b2, respectively); and (iv) the solution molar ratio of Mg 2+/Ca2+. It is found that, as pH increases, the combined molar ratio of Ca2+ and Mg2+ to SI, i.e. Nt, = N1, + N2 in the complex, increases up to a theoretical maximum, Nt,max, depending on the chemical stmcture of the phosphonate (corroborating earlier work, SPE 155114, SPE 164051). In addition, the precipitation behaviour of the various compounds is modelled theoretically by developing and solving a set of simplified equilibrium equations. Very good agreement is seen between the modelling and experimental results. Such models can be used directly in the simulation of field phosphonate precipitation squeeze treatments in order to design and optimize squeeze lifetimes.
    Original languageEnglish
    Title of host publicationSPE International Conference and Exhibition on Oilfield Scale 2014
    Place of PublicationRichardon, Texas
    PublisherSociety of Petroleum Engineers
    Pages46-63
    Number of pages18
    ISBN (Electronic)9781613993255
    ISBN (Print)9781632665911
    DOIs
    Publication statusPublished - 2014
    EventSPE International Oilfield Scale Conference and Exhibition - Aberdeen, United Kingdom
    Duration: 14 May 201415 May 2014

    Conference

    ConferenceSPE International Oilfield Scale Conference and Exhibition
    CountryUnited Kingdom
    CityAberdeen
    Period14/05/1415/05/14

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