Nanoemulsions stabilized by anionic and non-ionic surfactants for enhanced oil recovery in ultra-low permeability reservoirs: Performance evaluation and mechanism study

Nanoemulsions stabilized by surfactant have a high application potential in enhancing oil recovery in ultra-low permeability reservoirs. In this work, a new nanoemulsion flooding system were prepared by microemulsion dilution method using D-limonene, non-ionic surfactant fatty alcohol polyoxyethylene ether (AEO), anionic surfactant Sodium dodecyl sulfate (SDS), n-amyl alcohol and NaCl solution. The displacement performances of nanoemulsions were evaluated by spontaneous imbibition and displacement experiments, and the enhanced oil recovery (EOR) mechanisms were studied by Zetasizer Nano Z instrument, interfacial tensiometer, and contact angle meter. The results showed that the nanoemulsion system with optimal mass fraction (0.30 wt%) could get an additional oil recovery of 20.32% and reduce the injection pressure by 10.62%. Besides, the spontaneous imbibition recovery could reach 51.39%, which was 10.04% higher than that of SDS+AEO. The greater EOR effects of the nanoemulsion system can attribute to three perspectives. Firstly, nanoemulsions with particle sizes ranging from 80 nm to 100 nm make it possible to enter small pore throats of ultra-low permeability reservoirs to start-up crude oil without increasing insert pressure. Secondly, the nanoemulsion system exhibits ultra-low IFT (less than 1 x10(-2) mN/m) and can change the core surface from hydrophobic to hydrophilic, which could roll up the crude oil and separate them from the rock surface. Finally, the dissolving effect of the oil phase can disperse crude oil to enhance the solubilization effect of the micelles and extract more crude oil. The findings of this study can help for better understanding of the application of nanoemulsions to enhance oil recovery in ultra-low permeability reservoirs.