Hybrid surfactant-nanoparticles assisted CO₂ foam flooding for improved foam stability: A review of principles and applications

Miscible carbon dioxide (CO₂) flooding is a well-established and promising enhanced oil recovery (EOR) technique whereby residual oil is recovered by mixing with injected CO₂ gas. However, CO₂, being very light and less viscous than reservoir crude oil, results in inefficient sweep efficiency. Extensive research is ongoing to improve CO₂ mobility control such as the development and generation of CO₂/water foams. The long-term stability of foam during the period of flooding is a known issue and must be considered during the design stage of any CO₂ foam flooding project. The foam stability can be improved by adding surfactants as stabilizers, but surfactants generated foams have generally a shorter life because of an unstable interface. Furthermore, surfactants are prone to higher retention and chemical degradation in the porous media, particularly under harsh reservoir conditions. Research has shown that nanoparticles (NPs) can act as an excellent stabilizing agent for CO₂/water foams owing to their surface chemistry and high adsorption energy. The foams generated using NPs are more stable and provide better mobility control compared to surfactant-stabilized foams. One limitation of using NPs as foam stabilizers is their colloidal stability which limits the use of low-cost NPs. Combining surfactants and NPs for CO₂ foam stabilization is a novel approach and has gained interest among researchers in recent years. Surfactants improve the dispersion of NPs in the aqueous phase and minimize particle aggregation. NPs on the other hand create a stable barrier at the CO₂/water interface with the help of surfactants, thus generating highly stable and viscous foams. This paper presents a comprehensive review of the basic principles and applications of stabilized CO₂ foams. A brief overview of CO₂ foam flooding is discussed first, followed by a review of standalone surfactant-stabilized and NPs-stabilized CO₂/water foams. The application of hybrid surfactant-NPs stabilized CO₂ foams is then presented and areas requiring further investigation are highlighted. This review provides an insight into a novel approach to stabilize CO₂/water foams and the effectiveness of the method as proved by various studies.