Characterizing Foam Generated by CO2-Switchable Surfactants for Underground CO2 Storage Application

CO2-switchable surfactants, applicable for mitigating CO2 geological storage efficiency challenges, offer promising control over foam stability under reservoir conditions, but their performance under extreme pressure, temperature, and salinity still needs thorough investigation. This study experimentally characterizes the performance of CO2-switchable surfactants by evaluating their interfacial tension (IFT) reduction, foamability, and foam stability under reservoir-relevant conditions. Six surfactants, including cationic (cetyltrimethylammonium bromide (CTAB) and benzalkonium chloride (BZK)) and nonionic amine-based surfactants (N,N-Dimethyltetradecylamine, N,N-Dimethyldecylamine, and N,N-Dimethylhexylamine), were assessed using synthetic brine mimicking a depleted North Sea oil reservoir. A fractional factorial design was employed to minimize experimental runs while capturing key interactions between surfactant type, temperature, salinity, and divalent ion concentrations. Foam switchability was analyzed by alternating CO2 and N2 injections, and interfacial properties were measured to establish correlations between foam generation and IFT. Experimental findings demonstrate that cationic surfactants (BZK and CTAB) exhibit CO2-switchability and moderate foam stability. Nonionic surfactants show tail length-dependent responsiveness, where D14 demonstrated the highest foamability due to its optimal hydrophilic–hydrophobic balance. IFT measurements revealed that BZK consistently maintained lower IFT values, facilitating stronger foam generation, while CTAB exhibited higher variability. The inverse correlation between IFT and foamability was observed. These insights contribute to the development of tailored surfactants for subsurface CO2 storage applications, improving foam-based mobility control in CCS projects.