Microfluidic investigation on microscopic flow characteristics and displacement dynamics during CO2-enhanced water recovery in porous media

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Two-phase fluid displacement in porous media is a key physical process in subsurface energy and environmental engineering, particularly for CO2-enhanced water recovery (EWR) and geological CO2 storage. However, the effects of pore-space heterogeneity and brine salinity on pore-scale displacement dynamics and storage performance remain poorly understood. In this study, high-pressure and high-temperature microfluidic experiments were conducted using three representative porous-media chips at 50 °C and 10 MPa. High-resolution microscopic imaging combined with quantitative image analysis was used to systematically investigate the effects of pore structure and brine salinity on pore-scale CO2-brine displacement behavior and mechanisms. The results show that increasing pore-structure heterogeneity reduces both brine displacement efficiency and CO2 storage performance. The homogeneous structure exhibits a nearly piston-like displacement pattern and achieves the highest displacement efficiency of 81.98%, whereas the heterogeneous and rock structures promote capillary fingering, preferential channeling, and flow bypassing, resulting in lower displacement efficiencies of 63.86%–73.38%. As pore-structure complexity increases from the idealized homogeneous model to the realistic rock structure model, the CO2 storage capacity decreases significantly by 62.59%. Increasing the injection rate from 0.002 to 0.01 ml/min improves the displacement efficiency by approximately 10% across all three pore structures. In addition, increasing brine salinity modifies the interfacial tension and viscosity ratio while decreasing the capillary number, thereby enhancing capillary-dominated fingering and flow bypassing and reducing the displacement efficiency from 72.43% to 63.86%. These findings quantitatively demonstrate the critical roles of pore-space heterogeneity and brine salinity in pore-scale CO2-brine displacement and provide mechanistic insights into CO2 migration and trapping in saline aquifers, with implications for the optimization of CO2-EWR and geological CO2 storage.