Complexation and precipitation of scale-forming cations in oilfield produced water with polyelectrolytes

Divalent cations such as Ba2+, Sr2+, Ca2+, and Mg2+, commonly found in oil/gas produced water at high concentrations, adversely affect oil and gas production by forming scales in pipes, tanks, process equipment such as oil-water separators, and treatment equipment such as filters and reverse osmosis membranes. If the water is reinjected, e.g., for water flooding, scales may also form on the well screens and in the producing formation. Water soluble polyelectrolytes have been used to inhibit scale formation in oil and gas production and to eliminate cationic metals from water via membrane separation. However, the underlying interactions between scale-forming cations and various polyelectrolytes are not well understood. To this end, two commercially available anionic polyelectrolytes with different repeating functional groups, poly(acrylic acid) (PAA) and poly(sodium 4-styrenesulfonate) (PSS), were used to evaluate complexation and precipitation of Ba2+, Sr2+, Ca2+, and Mg2+ in binary systems, multi-ion mixtures, and produced water. The apparent dissociation constant (pKa) of PAA was determined by titration to be 6.7, indicating a high affinity of PAA for H+. The strength of the complexes formed between PAA and scale-forming cations (binding strength) decreased in the order Ba2+ > Ca2+ > Sr2+ > Mg2+ at pH 5. Higher pH values led to a higher affinity of PAA for divalent cations, which is attributable to the increase in binding sites for cations as the PAA more completely ionizes. By contrast, the affinity of PSS decreases in the order Ba2+ > Sr2+ > Ca2+ > Mg2+, with no significant pH effect. The formation and precipitation behavior of polyelectrolyte complexes (PECs) was investigated by reacting cations with the sodium salt of PAA (sodium polyacrylate) or with PSS. Only Ba2+ formed precipitates with PSS in single-cation solutions, although small amounts of Sr2+ were also removed by PSS from a mixed cation solution. However, all of the scale-forming cations formed PEC precipitates with PAA in both binary and mixed-ion solutions, depending on the cation concentration and molar ratio of PAA to divalent cations. The maximum amount of precipitation occurred at a molar ratio of PAA to M2+ of 1.51.7, while overdosing of PAA caused dispersion of the precipitates. High concentrations of monovalent cations inhibited precipitation of divalent cations with PAA. Up to 73% of scale-forming cations were removed from field-collected produced water samples through repeated (4 times) addition of PAA at a molar concentration ratio of 1. The release of scale-forming cations from PAA-M2+ precipitates, thereby regenerating PAA, was achieved by adding HCl.