Nano-technologically boosted CO2-EOR techniques: A review of theoretical, experimental, and field aspects

As global oil demand and carbon dioxide (CO2) emissions continue to increase, CO2-enhanced oil recovery (CO2-EOR) becomes an effective way of CO2 utilization inside the oil industry. However, problems such as gravity segregation, viscous fingering, and gas channeling have greatly limited its development. The addition of nanoparticles in CO2-EOR, known as NPs-CO2-EOR technology, can not only enhance the properties of CO2 nanofluids but also improve the flow characteristics of CO2 displacement fluids in porous media of underground rock reservoirs, thereby increasing oil recovery. In this review, several CO2-EOR techniques are introduced, namely immiscible and miscible flooding, polymer-assisted CO2 flooding, surfactant-assisted CO2 flooding, and NPs-CO2-EOR. The theoretical, experimental, and field test research of NPs-CO2-EOR is systematically reviewed. It is concluded that nanoparticles strengthen NPs-CO2-EOR through four main mechanisms: wettability alteration, interfacial tension reduction, rheology modification, and foam stability improvement. Additionally, factors that affect NPs-CO2-EOR, including NP characteristics and reservoir conditions, are analyzed. Furthermore, the mechanism of nanoparticles boosting the CO2-EOR process is addressed. Finally, challenges and future prospects in NPs-CO2-EOR are proposed, highlighting that further research is needed on the nanoparticles enhanced displacement mechanism under harsh conditions to realize the transition of this technology from indoor research to field implementation.