Synergism between lipophilic surfactant and lipophilic SiO2 nanoparticles enhances self-emulsification performance

Water control technology for high-temperature and -salinity gas wells is immature, and there is a lack of effective water control methods for condensate gas reservoirs and horizontal wells. Water control can be achieved by conducting subsurface self-emulsification to generate nanoparticle emulsions (Pickering emulsions). While this has been tested in oilfields, the overall success rate is low. Elucidating the mechanism of nanoparticle and surfactant synergy would help to improve the success rate of using self-emulsifying water-in-oil (W/O) Pickering emulsions for water control in gas wells. In this study, a self-emulsification process was simulated based on the formation conditions of the Tahe gas well using an active oil (diesel containing 1 wt% non-ionic surfactant) containing various concentrations of nanoparticles. By measuring the droplet size, viscosity, viscoelasticity and high-temperature stability of the emulsion, and in combination with previous research results, the synergistic effects of nanoparticles and surfactant on the generation and stability of emulsions were clarified. Finally, we explored the conditions under which synergy at the oil-water interface occurs and whether the nanoparticle surfaces were secondarily modified under formation conditions. The strength of the synergistic effect in the self-emulsifying system was found to be strongest at a 1:1 nanoparticle-to-surfactant concentration ratio. The strength of the synergistic effect was proportional to the emulsion's volume, droplet size, viscosity, viscoelasticity and other properties. The most synergistic self-emulsifying emulsion had up to five times the volume of emulsions made at other oil-to-water ratios. The emulsion droplet size showed a bimodal distribution, with mean particle sizes of 3 μm and 100 μm, respectively. The viscosity of the emulsion generated at 80 °C reached 2000 mPa s−1 at room temperature (26 °C) and increased to 2500 mPa s−1 as the temperature increased to 80 °C. Viscoelastic tests at 80 °C show that the emulsion generated at a concentration ratio of 1:1 exhibited the greatest and most stable elastic properties and that agglomerations of emulsion plugging pore throats were not easily penetrated by subsurface fluids. At this point, the nanoparticles were uniformly distributed at the droplet interface and the particle surfaces remained unchanged. The mechanism by which the synergistic effect stabilizes the emulsion changed at high temperatures (140 °C). The nanoparticles at the interface were wrapped in a thick oil film of high viscosity, like the inner phase of an emulsion. Then, concave high-viscosity oil-phase channels were formed between the ‘emulsified-like nanoparticles’. This change at the oil-water interface ensured the stability of the W/O emulsion at high temperatures. The synergy between the surfactant and nanoparticles was primarily based on the formation of an emulsion at the oil-water interface by the surfactant, followed by the gradual adsorption of nanoparticles at the interface, which enhanced the emulsion properties. The nanoparticle interface did not undergo secondary modification under formation conditions, and the synergistic effect of the surfactant and nanoparticles remained dominated by electrostatic forces.