Interfacial Synergistic Stabilization of CO₂ Foams by Surface-Modified SiO₂ Nanoparticles and Surfactants

The CO₂ channeling issue has long plagued many oilfields implementing CO₂ flooding. Foam flooding serves as an important method for controlling gas channeling, and developing an efficient foam-stabilizing system for CO₂ is thus of great significance. In this study, silica nanoparticles (NPs) with a diameter of 20 nm were surface-modified to produce NP3-P4 by grafting a hydrophilic group, (γ-glycidoxypropyl)trimethoxysilane, at a density of 3.0 μmol/m² and a hydrophobic long-chain alkyl group, propyltrimethoxysilane, at a density of 0.4 μmol/m². The resulting NP3-P4 exhibited a contact angle of 60.2° and retained excellent stability for more than 30 days in 0.25 API brine at 80 °C. When combined with AOS31, the hybrid system produced CO₂ foam with a half-life extended to 24 h under harsh conditions (0.25 API brine, 80 °C, and 8 MPa), demonstrating outstanding foam stabilization. In sand-packed tube flow experiments, the composite of AOS31 and NP3-P4 achieved a resistance factor exceeding 70, indicating strong plugging performance. Furthermore, measurements of surface tension and surface dilational modulus revealed that NP3-P4 imparted high expansion elasticity to the interfacial film. Observations via confocal laser scanning microscopy (CLSM) and cryogenic scanning electron microscopy (Cryo-SEM) confirmed a pronounced synergy between NP3-P4 and AOS31. Specifically, in a CO₂-saturated environment, the adsorption of betaine surfactants from AOS31 promoted the migration of NP3-P4 to the gas-liquid interface, inducing synergistic interactions and forming a thicker mixed interfacial film in the composite system. This study not only provides deep insights into the underlying stabilization mechanism but also holds considerable potential for broad foam-based applications, particularly demonstrating significant value in carbon dioxide sequestration and enhanced oil recovery (EOR).