Thomas Welton - The Relation Between Barite Inhibition by Phosphonate Scale Inhibitors and the Structures of Phosphonate-Metal Complexes

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      Shaw, Scott, Welton, Thomas Donovan, Sorbie, Kenneth Stuart -

      Abstract: Abstract In produced waters from   fields undergoing seawater (SW) flooding, inhibiting mineral   scaling can be problematic because the SW/formation water (FW)   ratio is constantly changing. For barium-sulfate scale, for   example, the barite saturation ratio (SR), the yield of barite   precipitate, and the molar ratio Ca2+/Mg2+ in the produced waters   all evolve over time. This paper describes the effects of SR and   molar ratio Ca2+/Mg2+ on the barium-sulfate inhibition efficiency   (IE) of nine phosphonate scale inhibitors (SIs): OMTHP   (hexa-phosphonate), DETPMP and HMTPMP (penta-phosphonates), HMDP   and EDTMPA (tetra-phosphonates), NTP (tri-phosphonate), EABMPA   and HEDP (di-phosphonates), and HPAA (mono-phosphonate and   mono-carboxylate). IE experiments were carried out testing a   range of SW/FW compositions (i.e., SR and molar ratio Ca2+/Mg2+   varying). The minimum inhibitor concentration (MIC) level of   these phosphonate SIs might correlate with either the level of SR   for the SW/FW mixing ratio in question (Type 1) or the Ca2+ and   Mg2+ levels in solution (Type 2). When experiments were repeated,   but the produced brine molar ratio Ca2+/Mg2+ was fixed, the MIC   for both Type 1 and Type 2 species always correlated with the SR.   The performance of these phosphonate SIs in consumption   experiments, where supernatant [SI] and [Ba2+] are both assayed   by inductively coupled plasma (ICP) spectroscopy at multiple   residence times, is also briefly discussed. In this paper, the   reasons behind Type 1 and Type 2 IE behavior in phosphonate SIs   are discussed, in terms of SI molecular structure, pH, SI   speciation, SI binding constants to Ca2+ and Mg2+ cations, and   the possible mononuclear or polynuclear chelate structures with   M2+ cations that can form under the test conditions. Possible   SI-M2+ complex structures are proposed, and through molecular   modeling, explanations are provided for why Type 1 and Type 2   behavior is exhibited by phosphonate SIs. 

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