Study on oil-in-water emulsions stabilized by SiO2 nanoparticles for enhancing oil recovery in harsh reservoirs

In high-temperature and high-salt environments, emulsions stabilized by surfactants are susceptible to instability phenomena, such as droplet coalescence, thereby limiting their utility in tertiary oil recovery. Addition of nanoparticles to the emulsion systems is able to improve the stability of emulsions by several mechanisms. In this paper, two kinds of SiO₂ nanoparticle stabilized emulsions, i.e., the electrostatic repulsion stabilized emulsions (ERS) and the Pickering emulsions, are investigated to clear their potential for enhancing oil recovery. The ERS emulsions are prepared by adding SiO₂ nanoparticle to a SDS stabilized emulsion. It is found that the critical surfactant concentration for forming emulsions is reduced from 0.06 to 0.006%, and the ERS emulsions are stable at salinity lower than 1% NaCl with no oil phase releasing. The cryo-SEM experiments show that the nanoparticles mainly disperse in the aqueous phase and prevent the droplets from coalescence by electrostatic repulsion. On the other hand, Pickering emulsions are prepared using nonionic surfactant modified SiO₂ nanoparticles. By adjusting a surfactant-to-nanoparticles ratio (such as 0.1%:1.0%), the hydrophilic-lipophilic equilibrium is obtained. Laser confocal and cryo-scanning electron microscopy results indicated that SiO₂ nanoparticles in Pickering emulsions are dispersed at the oil–water interface, forming a network structure between the emulsion droplets. Further experiments indicates that the ERS emulsions are effective at salinity lower than 1% NaCl, and the Pickering emulsions adapt to salinity lower than 4% NaCl. In the visual 2-D oil displacement experiments, the ERS emulsion and the Pickering emulsion contribute to 8% and 15% oil recovery, respectively, since the droplets of the Pickering emulsions may aggregate and plug large pores.

Graphical abstract