Molecular insight into the enhanced oil recovery potential of a seawater-based zwitterionic/anionic surfactant compound for heavy oil reservoirs

Zwitterionic/anionic surfactant mixtures can produce many synergistic effects that benefit heavy oil production. In this work, a seawater-based zwitterionic/anionic surfactant compound was prepared using cocoamidopropyl betaine (CAB) and alkyl polyoxyethylene ether sodium sulfate (AES), emphasizing their synergistic effects in enhancing conventional heavy oil recovery. First, the emulsification capability and the oil displacement efficiency of the surfactant compound were tested through laboratory experiments. Then the impacts of salinity, cation types, surfactant formulation and solution pH on surfactant-surfactant or surfactant-heavy oil interactions were systematically explored through molecular dynamics simulations to unveil the underlying mechanisms of enhanced oil recovery. The results showed that, (1) in a seawater-based system, when the AES/CAB concentration ratio was 1:1, a large wormlike micelle was formed due to the cation bridging of Ca2+ and Mg2+ ions, which not only strengthened the attractive interactions between the AES-SO4- and the CAB-N+ functional groups, but also screened the repulsive interactions between the AES-SO4- and the CAB-COO- functional groups. (2) Through mixed adsorption, the arrangement of AES/CAB molecules at the aqueous/heavy oil interface was more compact. (3) By incorporating the AES/CAB mixture into the associative structures of asphaltene and resins, it could effectively weaken the attraction interactions between the asphaltenes and the resins. Through these synergies, an additional oil recovery of 24.93 % was yielded by combining AES/CAB flooding and subsequent seawater flooding, compared to 19.38 % in the pure AES case. This study provides important insights into the microscopic mechanisms of zwitterionic/anionic surfactant mixtures, which can facilitate the effective development of heavy oil reservoirs and guide the complex formulation of cost-effective surfactant compounds.