Experimental and theoretical investigation of a copolymer combined with surfactant for preventing scale formation in oil wells

Abstract

The present work aimed to evaluate the water-soluble copolymer poly(4-styrene sulfonic acid-co-maleic acid) (P(SS-MA)) and its supramolecular complex with the anionic surfactant sodium dodecyl sulfate (SDS) as calcium carbonate (CaCO3) scale inhibitors. The performance of the products was evaluated through an automated laboratory device consisting of a tube blocking system to examine the precipitation and deposition of scale and salt at simulated pipeline and water conditions. In addition, the chemical compatibility between the scale inhibitor and the scaling cations was also performed. The minimum effective concentration (MEC) of the inhibitor was determined under temperature, pressure and flow conditions of oil wells. Besides, the influence of temperature, salinity, concentration of iron(III), and headspace volume on the precipitation process was also evaluated. The results showed that the P(SS-MA) was efficient and chemically compatible with the selected brines, presenting MEC of 10 mg L−1, at pH 6.5 and 100 °C. XRD spectra demonstrated that the presence of P(SS-MA) during CaCO3 precipitation modified the polymorphisms of aragonite to a mixture of calcite/aragonite. Nevertheless, the efficiency of P(SS-MA) was reduced at higher temperatures (>100 °C) and in lower salinity waters containing iron(II) ions. On the other hand, the association P(SS-MA)–SDS at the molar ratio 1/46 (critical aggregation concentration) resulted in an improvement of inhibitor performance, increasing its chemical compatibility with the scaling brine at temperatures above 100 °C. As the headspace volume increased, the rate of scale formation increased too, indicating the conversion of the HCO3− to CO32− by the evolution of CO2. Quantum chemistry calculations were performed to evaluate structural parameters and thermodynamic quantities related to the interaction between the P(SS-MA) copolymer and the Ca2+ ion. Theoretical calculations support that the P(SS-MA) copolymer can efficiently capture calcium ions through its carboxylate groups. In addition, the cooperative coordination between the sulfate (SDS) and carboxylate (copolymer) groups can improve the efficiency of the sequestering effect, improving the inhibitor performance.