Foam is injected in the subsurface to improve mobility control through the increase in the effective gas viscosity, e.g., in CO2\documentclass[12pt]{minimal}
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\begin{document}$$_2$$\end{document}-based enhanced oil recovery processes. As fine-textured foam has higher viscosity, it is envisaged to achieve an optimal foam texture and to maintain it for the entire period of an application. However, mechanisms of foam formation and destruction, which affect texture, are difficult to regulate. In this study, we investigate the synergic effect of nanoparticles and surfactant on the foam texture and the effective gas viscosity (μgf\documentclass[12pt]{minimal}
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\begin{document}$$\mu _g^f$$\end{document}) during transport in a porous medium. Experiments using glass-bead packs were performed injecting CO2\documentclass[12pt]{minimal}
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\begin{document}$$_2$$\end{document} and a solution containing either only surfactant or surfactant and nanoparticles. During each experiment, the pressure drop (Δp\documentclass[12pt]{minimal}
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\begin{document}$$\Delta p$$\end{document}) through the porous medium was measured to follow the generation of the foam. A two-phase flow mechanistic model combining the mass conservation law for water and CO2\documentclass[12pt]{minimal}
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\begin{document}$$_2$$\end{document} and the population balance equation of the lamellae was implemented to analyze the experiments and predict foam transport under the investigated conditions. The constitutive equations for foam generation and destruction were based on the dominant role of pressure gradient on lamella division and of capillary pressure on bubble coalescence, and their parameters were estimated using pressure drop measurements. Both equations were formulated for a surfactant-stabilized foam, and it was the aim of this work to understand their validity also for the case of a nanoparticle-stabilized foam. The experiments compare well with the theory showing that a foam stabilized with nanoparticles and surfactant can be modeled as a surfactant-stabilized foam. Overall, Δp\documentclass[12pt]{minimal}
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\begin{document}$$\Delta p$$\end{document} increases smoothly while the foam forms and, upon breakthrough, it stabilizes around a constant value while approaching steady state. During this phase, oscillations occur, particularly when high-quality foam is generated as the system is close to its critical conditions of capillary pressure and water saturation. When steady state is reached, the effective gas viscosity varies with fg\documentclass[12pt]{minimal}
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\begin{document}$$f_g$$\end{document} and solution composition and significantly increases when surfactant and nanoparticles are added. The maximum value of μgf\documentclass[12pt]{minimal}
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\begin{document}$$\mu _g^f$$\end{document} is 0.110 Pa s for fg\documentclass[12pt]{minimal}
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\begin{document}$$f_g$$\end{document} = 0.75, which is almost twofold of the maximum value attained when only a surfactant is used, corresponding to 0.067 Pa s at fg\documentclass[12pt]{minimal}
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\begin{document}$$f_g$$\end{document} = 0.4. This suggests that when nanoparticles and surfactant are employed, they can favor the formation of a strong high-quality CO2\documentclass[12pt]{minimal}
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\begin{document}$$_2$$\end{document}-foam.
