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.