Preparation and demulsification performance study of polyethylenimine polyether demulsifiers for surfactant-polymer flooding produced water

Traditional cationic reverse-phase demulsifiers for oilfield produced water treatment often lead to oil sludge formation and aged oil issues. To address this, we synthesized a series of nonionic polyethyleneimine (PEI) polyether demulsifiers with tunable branching degrees and hydrophilic-lipophilic balances by sequential polymerization of ethylene oxide (EO), propylene oxide (PO) and EO onto PEI initiators of varying molecular weights. The demulsification performance was systematically evaluated through interfacial film rupture kinetics, droplet coalescence dynamics, and interfacial dilational rheology. Results demonstrate that the demulsifier QPE8, synthesized from high-molecular-weight PEI (10 kDa), significantly weakens the interfacial film strength , reducing the rupture time from 36.3 ± 1.5 s (control) to 5.6 ± 0.6 s. At concentration higher than 10 mg/L, QPE8 achieves coalescence half-lives below 5 s and reduce oil content in surfactant-polymer (SP) flooding produced water to < 50 mg/L, corresponding to oil removal rates exceeding 95 %. The superior performance correlates with their optimized branching structure and moderate hydrophilicity, which enhance competitive adsorption at the oil-water interface. This study provides a practical strategy for designing nonionic demulsifiers to enable efficient and environmentally friendly treatment of complex oilfield wastewater.