Carbon Capture, Utilization, and Storage (CCUS) stands as one of the effective means to reduce carbon emissions and serves as a crucial technical pillar for achieving experimental carbon neutrality. CO₂-enhanced oil recovery (CO₂-EOR) represents the foremost method for CO₂ utilization. CO₂-EOR represents a favorable technical means of efficiently developing extra-low-permeability reservoirs. Nevertheless, the process known as the direct injection of CO₂ is highly susceptible to gas scrambling, which reduces the exposure time and contact area between CO₂ and the extra-low-permeability oil matrix, making it challenging to utilize CO₂ molecular diffusion effectively. In this paper, a comprehensive study involving the application of a CO₂ nanobubble system in extra-low-permeability reservoirs is presented. A modified nano-SiO₂ particle with pro-CO₂ properties was designed using the Pickering emulsion template method and employed as a CO₂ nanobubble stabilizer. The suitability of the CO₂ nanobubbles for use in extra-low-permeability reservoirs was evaluated in terms of their temperature resistance, oil resistance, dimensional stability, interfacial properties, and wetting-reversal properties. The enhanced oil recovery (EOR) effect of the CO₂ nanobubble system was evaluated through core experiments. The results indicate that the CO₂ nanobubble system can suppress the phenomena of channeling and gravity overlap in the formation. Additionally, the system can alter the wettability, thereby improving interfacial activity. Furthermore, the system can reduce the interfacial tension, thus expanding the wave efficiency of the repellent phase fluids. The system can also improve the ability of CO₂ to displace the crude oil or water in the pore space. The CO₂ nanobubble system can take advantage of its size and high mass transfer efficiency, among other advantages. Injection of the gas into the extra-low-permeability reservoir can be used to block high-gas-capacity channels. The injected gas is forced to enter the low-permeability layer or matrix, with the results of core simulation experiments indicating a recovery rate of 66.28%. Nanobubble technology, the subject of this paper, has significant practical implications for enhancing the efficiency of CO₂-EOR and geologic sequestration, as well as providing an environmentally friendly method as part of larger CCUS-EOR.

Miaoxin Zhang

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