The exploration and development of super-low permeability reservoirs have become a global focus in recent years. However, conventional flooding systems commonly face problems of high injection pressure and poor displacement efficiency in super-low permeability reservoirs. Thus, it is imperative to find new flooding agents that tackle such problems.

In this study, a novel ultra-low interfacial tension (IFT) nanofluid was formulated, consisting of surfactants to achieve ultra-low IFT and silica nanoparticles to reduce injection pressure. The compatibility test between the surfactants and silica nanoparticles in 10,000 mg/L NaCl solution at 90 °C was conducted to ensure their adaption to harsh reservoir conditions. Also, the effects of silica nanoparticles on the IFT and emulsion stability of the surfactant solution as well as wettability of reservoir rock were evaluated to determine the optimum concentration of nanoparticles. Finally, oil displacement efficiency of the nanofluid was assessed and compared from respective nanofluid flooding, surfactant flooding and surfactant-free nanofluid flooding.

The compatibility results showed that the ultra-low IFT surfactant solution with silica nanoparticles remained clear and stable at 90 °C for one month. The surfactant solution can effectively emulsify oil, and the stability of the oil emulsion could be further improved in the presence of silica nanoparticles. In addition, the solution could achieve lower IFT at both low and high temperature with the addition of 0.01% silica nanoparticles. The silica nanoparticles could effectively alter the wettability of the rock, making it become more water-wet with increasing silica nanoparticle concentration. The displacement experiments through 0.2–0.3 mD tight cores indicated that the enhanced oil recovery could reach 21.12%OOIP by the nanofluid flooding after water flooding, higher than that of surfactant flooding (18.84% OOIP), and much higher than that of surfactant-free nanofluid flooding (3.48% OOIP). Moreover, the injection pressure difference was able to decrease nearly 50% after nanofluid injection in comparison with the occurrence of an increase in pressure along the surfactant solution injection. Thus, the combined surfactant and nanoparticles behaved excellent synergistic effect.

The newly formulated surfactant based silica nanofluids can efficiently enhance oil recovery in comparison with water flooding, and significantly lower the injection pressure compared with the surfactant flooding. This work lays the foundation for the application of ultralow IFT nanofluid flooding technology in super-low permeability reservoirs.

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