Coupled effects of sulfonated PAM concentration and degradation on separation kinetics of oil/water emulsions: interplay of flocculation, viscosity, and homogenization

• Inverted N-shaped phase separation profile in O/W emulsions with PAM-based polymer. • Bridging flocculation occurs up to ≈10 ppm of sulfonated PAM-based polymer. • Between 10 ppm and a critical concentration c app ∗ , viscosity hinders oil creaming. • Above c app ∗ , viscosity impairs O/W homogenization and the large droplets cream fast. • Degraded polymers yield slower separation and higher oil content in produced water. In polymer Enhanced Oil Recovery (EOR), back-produced polymers can increase oil–water (O/W) emulsion stability, hampering phase separation and raising costs. In this EOR technique, high-molecular-mass polyacrylamides (PAMs) are commonly used, but under fixed shear production conditions, emulsions undergo the same homogenization and polymer degradation is unavoidable. This study systematically investigates the influence of sulfonated PAM (S-PAM) concentration and degradation on the phase separation kinetics of heavy O/W emulsions under reservoir-relevant conditions (high temperature, high salinity). The experimental approach quantified separation kinetics, bulk viscosity, oil–water interfacial properties, oil content, and droplet size distribution. A consistent, previously unreported inverted N-shaped relationship was observed between water release and polymer concentration for both intact and degraded S-PAM. Our findings reveal a dual mechanism governing this profile: at very low concentrations (≤ 10 ppm), polymer-bridging flocculation dominates, accelerating phase separation; at moderate concentrations, increased bulk viscosity and polymer steric stabilization hinder oil droplets creaming, decelerating phase separation by up to 1800 % compared to the polymer-free system. Above a critical concentration, the higher viscosity impairs homogenization, forming larger initial oil droplets that cream faster despite the viscous continuous phase, thereby re-accelerating separation. Degraded polymers exhibited slower phase separation and higher residual oil in the aqueous phase compared to intact counterparts. S-PAM showed no interfacial activity, and the proposed mechanisms were validated with a medium oil. These findings highlight the complex interplay between flocculation and viscosity effects in controlling O/W emulsion stability during polymer flooding, offering critical insights for optimizing produced water management strategies.