Surfactant-stabilized CO₂ foams for storage in saline aquifers: Interfacial performance and foam stability

Foam-assisted CO₂ storage has the potential to enhance CO₂ mobility control and improve storage security in saline aquifers. However, its effectiveness depends on the ability of surfactant formulations to stabilize CO₂-brine interfaces under high-pressure conditions with varying salinities typical of saline formations. In this study, the interfacial and bulk foam performances of three field-relevant surfactant formulations, namely anionic (sulfonate-based, SUR-402), cationic switchable (amine-based, D -TTM) and viscoelastic-forming (quaternary ammonium-based, FTS-20) were systematically evaluated at low surfactant concentrations under aquifer-relevant conditions. Experiments were conducted at 60 °C and 8.6 – 13.8 MPa using low-salinity (38.44 g/L) and high-salinity (241.67 g/L) brines. The apparent interfacial critical micelle concentration (CMC_IFT) was identified from the CO₂-brine interfacial tension (IFT)-concentration responses and subsequently employed to define surfactant dosages (0.10 – 0.25 wt%) for foam stability measurements. The results revealed a pronounced salinity dependence on surfactant performances. At low salinity, the anionic formulation achieved the lowest IFT (4.5 mN/m) and longest foam half-life at 0.10 wt% (132.8 min). Under high-salinity conditions, however, its performance deteriorated markedly. In contrast, the switchable amine formulation maintained low IFT values (≈5.3 mN/m) and exhibited long foam half-lives (>300 min at 0.25 wt%), reflecting strong salinity tolerance. The viscoelastic system produced robust foam persistent across both salinity regimes, with maximum half-life exceeding 400 min at 0.25 wt%. Importantly, a clear decoupling was observed between equilibrium IFT reduction and long-term foam stability. These findings indicate that dynamic interfacial film properties, dictated by surfactant chemistry and brine composition, control foam persistence under saline environments. This study provides practical guidance for surfactant selection by showing that optimal foam stabilization for CO₂ storage is strongly formation-specific and governed by salinity-dependent stabilization pathways. These findings establish a comparative screening framework for surfactant selection under reservoir-relevant conditions and provide a basis for subsequent porous media validation.