Removing scale-forming cations from produced waters†
The formation of precipitates (scales) during reinjection limits the reuse of oil and gas production water (produced water) for additional oil recovery. Selective removal strategies that target Ba and Sr, the primary scale-forming cations, would limit produced water treatment costs, reduce waste generation, and increase produced water reuse. A novel treatment technique for targeted Ba and Sr removal, complexation with polyelectrolyte polymers, is compared with chemical precipitation (sulfate addition and precipitative softening) for the removal of Ba and Sr from Kansas oil field brines. Four polymers were examined for cation removal, both with and without ultrafiltration: poly-vinyl sulfonate (PVS), poly(4-styrenesulfonate) (PSS), polyacrylic acid (PAA), and poly(4-styrenesulfonic acid-co-maleic acid) (PSSM). PSSM and PSS were effective for Ba and Sr removal from the lower salinity brine (TDS of 31 000 mg L−1), but exhibited limited Sr removal in the absence of Ba in the high salinity brine (TDS of 92 000 mg l−1). Similar results were achieved in both brines using sulfate addition. PSSM used in conjunction with ultrafiltration removed >99% of initial Sr and Ba from the lower salinity brine, while removing only 65% and 78% of Mg and Ca, respectively. These results compare favorably to precipitative softening, which removed >90% of all divalent cations from the same brine but was less selective for Ba and Sr. PAA plus ultrafiltration removed 58% of Sr (and 68% of Ca) from the high-salinity brine at pH 9. While increased Sr removal can be achieved by polymer-assisted ultrafiltration, further development of this process, including methods for polymer recovery and regeneration, will be needed to improve its performance compared to precipitative softening.