SPE Production & Operations
Volume 27, Number 4, November 2012, pp. 390-403

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 ter-polymers, 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.

© 2012. Society of Petroleum Engineers

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History

• Original manuscript received: 24 June 2011
• Meeting paper published: 27 May 2010
• Revised manuscript received: 19 August 2011
• Manuscript approved: 6 December 2011
• Published online: 24 May 2012
• Version of record: 13 November 2012