The effects of barium sulphate saturation ratio, calcium and magnesium on the inhibition efficiency: II polymeric scale inhibitors

In oilfield produced waters, the mineral scaling problem is a “moving target” since the seawater/formation water (SW/FW) mixing ratio is constantly changing. Therefore, for barium sulphate for example, the barite saturation ratio (SR), the yield of barite precipitate and molar ratio of Ca2+/Mg2+ in the produced waters are all evolving over time.

This paper describes the effects of saturation ratio (SR) and Ca2+/Mg2+ ratio on barium sulphate inhibition efficiency (IE) of the polymeric scale inhibitors (SI), PPCA (phosphino poly carboxylic acid), MAT (maleic acid terpolymer), and SPPCA (sulphonated PPCA). The behaviour for polymers is compared with similar results for phosphonates reported in Paper I (SPE 130373). IE experiments were carried out for a wide range of SW/FW compositions (i.e. SR and Ca2+/Mg2+ varying). The minimum inhibitor concentration (MIC) levels of these polymeric SIs broadly correlate with the level of barite saturation ratio (SR). When experiments were repeated but the produced brine Ca2+/Mg2+ ratio was fixed, a similar IE trend is maintained. However, using the non-sulphonated polycarboxylate SIs (PPCA and MAT), the base case MICs are lower than the fixed case MICs whereas using SPPCA, the converse is true i.e. the fixed Ca2+/Mg2+ case MIC < base case MIC. We have demonstrated that the high [Ca] in the fixed case tests causes some SI precipitation with brine Ca2+ when PPCA is being evaluated. Sulphonate functional groups on polymeric SI molecules help to prevent such problems encountered with brine Ca2+. Low levels of brine calcium (~500-700ppm) can be very beneficial to the IE performance of PPCA. However, when the [Ca2+] reaches a certain level (~1000ppm+), this causes some precipitation of a Ca_PPCA complex. We show that lower [Ca2+] levels are optimum (~500-700ppm) and we deduce this is why non-sulphonated polycarboxylate SIs PPCA and MAT perform better under base case mixing conditions (lower [Ca2+] mix).