Investigation of CO₂ Micro/Nano Bubble-Aqueous Multiphase Flow Patterns during Microfluidic Experiments

CO₂ micro- and nanobubbles have been identified as an effective technique for enhanced oil recovery and carbon sequestration, and have great potential in the development of diverse reservoirs. However, most reservoir environments are characterized by high temperature, high salinity, and low permeability, which negatively influences the stability and oil displacement efficiency of micro- and nanobubbles. A heterogeneous porous medium model was constructed by using CT scanning combined with CAD, and an etched chip was fabricated to perform visualized microfluidic experiments. The mechanism during the displacement process, such as the oil/water interface, oil droplet detachment, and migration, was observed, and the effects of CO₂ micro- and nanobubble mobility regulation on oil recovery efficiency and residual oil distribution were analyzed. The results indicate that at a low flow rate (0.1 μL/min), bubbles blocked high-permeability channels through the Jamin effect, resulting in a 53.8% improvement compared to water flooding; at a high flow rate (2 μL/min), shear-induced bubble coalescence formed preferential flow paths, enhancing the driving efficiency by 34.8%; Compared with N₂, H₂, and O₂ systems, CO₂ bubbles exhibited the best plugging effect in dead-end pores, reducing residual oil by 38%~45%. However, the reduced displacement pressure at low flow rates led to a 7.3% reduction in recovery efficiency compared to N₂; The CO₂ recovery efficiency increased monotonically with increasing flow rate, while the efficiency of the N₂ system decreased by 6.5% after exceeding 0.5 μL/min. CO₂ micro- and nanobubbles achieved a breakthrough in traditional displacement limits through a "plugging-shearing" dynamic equilibrium. Their self-adaptive flow regulation mechanism (reconstructing the flow field at low flow rates and connecting channels at high flow rates) increased the recovery rate enhancement by 2.8-fold as the flow rate increased. This study provides theoretical foundations and technical guidance for the application of CO₂ micro- and nanobubble flooding technology.