Anionic–Nonionic Surfactant POES for Enhanced Emulsification and Recovery of Extra-Heavy Oil at High Temperatures

Traditional surfactants used in high-temperature extra-heavy oil reservoirs often exhibit poor thermal stability and weak emulsification capability. These limitations hinder enhanced oil recovery (EOR). To address these challenges, a novel anionic–nonionic surfactant, sodium polyoxyethylene (10) hexadecyl ether sulfonate (POES), was synthesized. The synthesis involved extending the carbon chain length of alcohol ethoxylate sodium hydroxypropyl sulfonate (AEOSHS) and eliminating the hydroxyl group. FTIR and 1 H NMR analyses verified the molecular structure. Comparative tests demonstrated that POES offers superior emulsification performance over AEOSHS and unsulfonated polyoxyethylene (10) hexadecyl ether (POE). A 0.3 wt % POES concentration yielded a surface tension of 34.19 mN/m and an interfacial tension (IFT) of 0.052 mN/m. Thermogravimetric analysis indicated thermal stability up to 220 °C. Furthermore, the surfactant retained a 95.45% viscosity reduction rate (VRR) after aging at 200 °C. Core flooding experiments revealed that POES enhanced steam flooding recovery by 13.8%. This recovery rate significantly exceeded that of POE (4.1%) and AEOSHS (5.8%). Mechanism analysis attributed this enhancement to effective wettability alteration (contact angle: 23.2°) and reduced emulsion particle size (10–25 μm). Additionally, POES effectively emulsified heavy components (>C80). Consequently, POES serves as a highly efficient emulsifier for EOR applications in high-temperature extra-heavy oil reservoirs.