Study on the Influence Mechanism of Modified SiO₂ Nanoparticles on the Stability of SDS Foam

Foam stability is significantly important for enhancing oil recovery. SiO₂ nanoparticles have been widely used to improve foam stability, but hydrophilic SiO₂ nanoparticles are limited in improving the stability of foam because they cannot stably adhere to the gas-liquid interface; therefore, surface modification is needed to improve their hydrophobicity. In this study, silane coupling agent (KH570) was used to modify the surface of SiO₂ nanoparticles, and the effects of concentration and wettability of SiO₂ nanoparticles on the property of SDS foam were explored. The results show that the concentration and wettability of SiO₂ nanoparticles have little effect on the foam ability but have a great impact on the foam stability. The stability of the foam increases as the concentration of the SiO₂ nanoparticles increases. As the contact angle increases, the stability of the foam first increases and then decreases. The optimal contact angle for stable foam is 70.4°, at which point the foam's half-life time with 2 wt % SiO₂ nanoparticles is 1450 s longer than that of unmodified SiO₂ nanoparticles (950 s); the salt and oil resistance are also the best at this contact angle. A two-phase molecular dynamics model at the foam interface was constructed to study the effects of different SiO₂ nanoparticle methylations on the stability of the SDS foam. The results indicate that as the degree of methylation of SiO₂ nanoparticles increases, foam stability first rises and then decreases, reaching an optimum at 40% methylation. When the methylation degree is 40%, SiO₂ nanoparticles not only adsorb near the gas-liquid interface but also jam in the plateau borders, resulting in the minimum water diffusion coefficient, and weaken the repulsive forces between DS^- groups and Si-OH groups, thus slowing down the liquid drainage and coarsening process, thereby enhancing foam stability.