Exploring the microscopic synergism of hydrophobic nanoparticles and surfactants in surfactants-assisted nanofluids (SAN) for enhanced oil recovery

Enhancing oil recovery (EOR) from sandstone reservoirs with high formation salinity and high temperature has gained prominence as most of the easy-to-produce oil has already been produced.It has been approved that the surfactants-assisted nanofluids (SAN) perform well as a viable EOR technique. Most of the studies have macroscopically presented that surfactants-assisted nanofluids (SAN) can enhance oil recovery by reducing interfacial tension and wettability alteration. The effect of nanoparticles on the phase behaviour of microemulsions and the mechanism of surfactants-assisted nanofluids (SAN) to enhance oil recovery at the micro-scale are not clear. In this study, the performance in EOR of alcohol ethoxylate (AEO-15)/sodium alpha-olefin sulfonate (AOS) and hydrophobic nanoparticles (S16) was analysed. 2D visualization experiments systematically elucidated the mechanism of surfactants-assisted nanofluids (SAN) mobilisation of residual oil in the pores. Dynamic light scattering and zeta potential measurements showed that when the S16 concentration was less than 0.1 wt%, the SAN remained stable at 70 °C with a particle size of 64.27 nm and a zeta potential of −30.88 mV.Contact angle and adsorption experiments demonstrated that the addition of 0.1 wt% S16 decreased the contact angle by 21.72°, making the rock surface more water wet and the surfactants adsorption on the sample surface was decreased by 16.9%. The microemulsion induced by the SAN had lower interfacial tension (IFT) and exhibited wider “salinity window” compared to the surfactants-induced microemulsions, which might make the proposed SAN more applicable in reservoirs with high salinity and residual oil saturation. Furthermore, spontaneous swelling of trapped oil in the dead end and elongation of oil droplets were clearly observed in 2D visual microscopic model during the SAN flooding, which somehow explained the exceptional EOR performance of the developed SAN in the core flooding experiment at 70 °C and 80 g/L of salt. This study is expected to provide useful insights into the development of SAN involving hydrophobic nanoparticles and deepen the EOR mechanism of SAN at the microscopic scale.