Synergistic Effect of Gemini Cationic/Anionic Surfactant Mixtures for Enhanced Oil Recovery

The surface and interfacial properties of gemini multiquaternary ammonium salt (CD3N-12), sodium dodecyl polyoxyethylene ether sulfate (AES), and their mixtures were investigated. The nonionic polyoxyethylene hydrophilic groups in AES significantly improve the mixtures’ solubility, having good potential for practical applications. In addition, the Amott method and oil displacement experiment were used to evaluate the imbibition properties and oil displacement performance in low-permeability reservoirs. Finally, the microvisualization experiment was employed to reveal the displacement mechanism. The results show that when the mass fraction of AES is 0.7 (corresponding to an AES:CD3N-12 molar ratio of 7:3), the interaction parameter (βm) for the mixtures attains a value of −15.06, indicating a stronger synergistic interaction, higher than conventional anionic–cationic surfactant mixtures (βm ≈ −11). Moreover, the αAES = 0.7 mixtures reduce the oil–water interfacial tension to 10–3 mN/m. Besides, they demonstrate tolerance to NaCl concentrations of 1 × 105 mg/L and CaCl2 concentrations of 8 × 103 mg/L while still maintaining the oil–water interfacial tension to an ultralow level. Furthermore, they also exhibit an excellent oil film removal performance, achieving an oil film removal efficacy of 89%. These properties enable the alteration of rock surface wettability from hydrophobic to hydrophilic, as evidenced by a reduction in the water contact angle from 107 to 33.6°. The αAES = 0.7 mixtures enhance oil recovery by 20.37%. Eventually, the microvisualization proves that the surfactant mixtures enhance oil recovery through multiple mechanisms. These findings provide valuable guidance in designing compound surfactant mixtures for the efficient development of low-permeability reservoirs.