Anionic-nonionic and nonionic mixed surfactant systems for oil displacement: Impact of ethoxylate chain lengths on the synergistic effect

Compound surfactant system has been extensively studied in enhanced oil recovery (EOR) as the synergistic effect of mixed surfactants can increase the density of surfactants distributed at the oil/water interface, enhancing the interfacial activity and decreasing the interfacial tension. The system can further increase the capillary number and enhance oil recovery for oil-wet reservoirs. Studying the interactions between surfactant molecules is of great significance to the enhancement of practical application performances. In this study, N, N-dihydroxyethyl alkyl amide (CDEA), and Alkylphenol polyoxyethylene carboxylate (APEC) with different EO numbers were mixed to prepare a binary compound oil displacement system. The influence of the EO number on the synergistic effect of the CDEA/APEC system and the EOR potential was measured by interfacial tension, nanomechanical tests and oil-displacement experiments. The results indicate a synergistic effect between CDEA and APEC due to the formation of hydrogen bonds between the hydroxyl groups of CDEA and the EO chains of APEC. An optimal synergistic effect in the system was obtained when the EO number was four. The CDEA/APEC-4 system exhibited an oil/water interfacial tension decrease to 10−2 mN/m at 75 °C and the oil recovery was 57.14 %, which was increased by 19.2 % compared to primary water flooding. However, a longer EO chain length weakened the interaction forces between CDEA and APEC, which diminished the synergistic effect of the system, leading to an increase of the interfacial tension. Due to the flexibility of the EO chain, the increase in the EO number leads to the bending of APEC molecules. The longer EO chain length weakened the interaction forces between CDEA and APEC, which diminished the synergistic effect of the system, leading to an increase of the interfacial tension. The oil-displacement experiment revealed that the EO chains changed the oil recovery and injection pressure by affecting the interfacial tension of the mixed system. This study has significant implications for the formulation design and optimization of agents for oilfield development.