The Effects of Barium Sulfate Saturation Ratio, Calcium, and Magnesium on the Inhibition Efficiency—Part II: Polymeric Scale Inhibitors

Abstract

Summary In oilfield-produced waters, the barite mineral-scaling problem is a "moving target" because the seawater/formation-water (SW/ FW) mixing ratio is constantly changing. Therefore, the barite saturation ratio (SR), the yield of barite precipitate, and molar ratio Ca2+/Mg2+ in the produced waters are all evolving over time. This paper describes the effects of SR and molar ratio Ca2+/ Mg2+ on the barium sulfate inhibition efficiency (IE) of nine polymeric scale inhibitors (SIs): phosphino poly carboxylic acid (PPCA); maleic acid ter-polymer (MAT)—a green SI; sulfonated PPCA (SPPCA); phosphino methylated polyamine (PMPA)—a poly-phosphonate; a generic P-functionalized copolymer (PFC); polyvinyl sulfonate (PVS); vinyl sulfonate acrylic acid copolymer (VS-Co); and cationic ter-polymers A and B (CTP-A and CTP-B). The behavior for polymers is compared with similar results for phosphonate SIs (Shaw et al. 2010). IE experiments were carried out for a wide range of SW/FW compositions (i.e., SR and molar ratio Ca2+/Mg2+ varying). The minimum-inhibitor-concentration (MIC) levels of these polymeric SIs sometimes correlate with the level of SR, but not always, which is because of Ca2+ and Mg2+ effects. When experiments were repeated but the produced-brine molar ratio Ca2+/Mg2+ was fixed, MICs always correlate with the level of SR for all nine polymers studied. However, it was observed that for SIs PPCA, MAT, and PFC, the base-case MICs (i.e., molar ratio Ca2+/Mg2+ varying) were less than the fixed-case MICs (molar ratio Ca2+/Mg2+ fixed), whereas in testing SPPCA, PMPA, PVS, VS-Co, and both the cationic terpolymers, the converse is true (i.e., the fixed-case MIC is less than the base-case MIC). It has been demonstrated conclusively that the high [Ca2+] in the fixed-case tests causes some SI precipitation with brine Ca2+ when PPCA is being evaluated (as PPCA-Ca). Sulfonate functional groups present on polymeric SI molecules may help to prevent such incompatibility problems encountered with brine Ca2+ (e.g., SPPCA does not precipitate as SPPCA-Ca). Low levels of brine calcium (e.g., approximately 500 to 700 ppm) can be very beneficial to the IE performance of PPCA. However, when the [Ca2+] reaches a certain level (approximately 1,000 ppm or higher), this causes some precipitation of a Ca–PPCA compound. It has been illustrated that low-to-moderate Ca2+ levels are often best, and it is concluded that this is why SIs PPCA, MAT, and PFC perform better under base-case experimental conditions (lower Ca2+ mix). It has been illustrated that PMPA behaves remarkably similarly to conventional phosphonate SIs with regard to Ca2+ and Mg2+. In fact, the behavior of PMPA suggests that it is not polymeric in nature. PVS, VS-Co, and both the cationic ter-polymers are mildly affected by divalent ions Ca2+ and Mg2+, with the latter being affected more than PVS because of the presence of carboxylate functional groups.