CO 2 Foam Stability in the Presence of Oil, Salinity, Pressure, and Porous Media: Experimental Insights for Enhanced Oil Recovery and Subsurface CO 2 Storage

Global climate change, driven by rising CO2 emissions, is a critical environmental concern. As a mitigation measure and to counteract its effects, CO2 foam injection has emerged as a promising method for enhanced oil recovery (EOR) and storage in depleted oil reservoirs or saline aquifers. The present study focuses on the foamability and characterization of CO2 foam, as well as the effects of salinity, oil components, pressure, and porous media on its stability. An amphoteric surfactant, cocamidopropyl hydroxysultaine (CAHS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB), were used as foam stabilizers. The 25:75 v/v CTAB (1500 ppm):CAHS (500 ppm) blend at a total concentration of 750 ppm exhibits strong synergy with CO2, markedly reducing interfacial tension while significantly enhancing both foamability and stability compared to individual surfactants at equivalent total loading. Further, the half-life time (t1/2) of the CO2 foam first increases with salinity due to the salt-out effect of CO2 solubility in water, but reaches a maximum at 1 wt % (2-fold increase in t1/2 vs salt-free) salinity because of optimum IFT (22.1 mN/m), and then further decreases as salt ions reduce electrostatic repulsion between surfactant head groups. It is observed that the presence of oil destabilizes the foam as it disrupts the thin liquid films and interfaces that support foam, leading to bubble coalescence. An increase in pressure compresses CO2 gas, causing lamella thinning, reduced coalescence, and Ostwald ripening, while strengthening the surfactant films, resulting in higher stability of the CO2 foam. The rheological studies of CO2 foam stabilized by the designed surfactant blend show enhanced apparent viscosity with shear-thinning behavior, demonstrating its potential to improve the sweep efficiency and storage performance. Considering the injection of the CO2 foam into the porous media for its application either in EOR or CO2 storage, visualizations in glass bead packs revealed prolonged foam lifetime via capillary lamella pinning, reduced drainage, and gas diffusion, while separate coreflood experiments confirmed superior in situ stability and mobility control for the CAHS+CTAB blend, ideal for EOR/CO2 storage.