CO2 Nanobubbles as an Emerging EOR–CCUS Technology: Comparative Review of Laboratory Studies, Underlying Mechanisms, and Preliminary Assessment of CO2 Storage Potential

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Abstract
Nanobubbles (NBs) are emerging as a promising area of research across multiple scientific and industrial domains due to their unique physicochemical characteristics. NBs exhibit distinctive properties compared to normal bubbles, including high internal pressure, a large specific surface area, high interfacial activity, and long-term stability in liquids. Therefore, NBs have gained increasing attention as a novel enhanced oil recovery (EOR) technique, offering potential advantages over traditional gas flooding and chemical flooding. CO2-NB specifically represents a particularly promising approach as an intersection of EOR and carbon capture, utilization, and storage (CCUS), as CO2-NB enables hydrocarbon recovery and in situ CO2 utilization and storage at reservoir conditions. This paper presents a structured comparative discussion of currently identified experimental EOR studies that employ CO2-NBs. Based on the observations of these experiments, this paper discusses the proposed mechanisms in those experiments or other studies that could scientifically play a role in achieving incremental recovery. The main mechanisms discussed include interfacial tension reduction, wettability alteration, CO2 transfer from NBs into the oil liquid phase, and suppression of gravity segregation. Other possible contributors discussed in the literature include buoyancy-assisted mobilization, induced shock waves, and drag force reduction. These mechanisms are examined in relation to the distinctive properties of CO2-NBs, showing how these properties contribute to the occurrence of the proposed mechanisms, showcasing the potential of CO2-NBs as an emergent EOR–CCUS technology. A preliminary probabilistic assessment was performed to estimate CO2 storage potential during CO2-NBs EOR injection. The results suggest that the majority of the injected CO2 is dissolved in the saturated liquid phase, while the amount of free NBs is negligible, indicating that CO2-NB injection may provide secure storage through solubility trapping, but with lower storage capacity compared to conventional geological sequestration in saline aquifers.

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