The accumulation of immobile residual water during CO₂ injection for brine displacement significantly impairs storage efficiency, injectivity, and fluid migration—key factors for scaling up CO₂-based energy technologies. This study investigates the factors governing residual water saturation under different CO₂ phases and effective stress conditions in simulated subsurface environments. The results indicate that under constant effective stress, gaseous CO₂ yields the highest residual water saturation, followed by its supercritical and liquid states. As such, an inverse relationship is observed between residual water saturation and storage efficiency/capacity, underscoring the potential for jointly optimizing energy recovery and CO₂ sequestration. The analysis of the CO₂-brine-rock system confirms that capillary forces control residual water saturation. Increased interfacial tension or contact angle cosine value raises capillary entry pressure, hindering displacement and elevating irreducible water saturation. Moreover, higher effective confining pressure reduces capillary radius and creates "dead pores", thereby increasing capillary pressure and enhancing water trapping in the core. The findings give critical insights into how CO₂ phase behavior and confining pressure govern residual water saturation, displacement efficiency and migration in the reservoir, directly informing strategies for optimal CO₂ storage reservoir selection and enhanced oil recovery operations.

The significance of pore size as a determinant in two-phase flow dynamics is widely acknowledged. However, the micro-scale behavior of flow including pore channel capillary and air-water interface development are not systematically interpreted due to the limitation of test and computation methodologies. In the present study, an investigation was conducted into the impact of varying pore throat widths on the flow of a two-phase fluid. For the investigation, numerical simulations were integrated with microfluidic experimentation to provide a comprehensive analysis. The results indicate that large pore diameters in porous media are associated with accelerated infiltration rates, leading to quicker stabilization. However, an inverse correlation exists between pore throat diameter and seepage area, with larger diameters yielding larger residual air areas. In this investigation, the “queuing effect” was observed across all tests, irrespective of pore throat diameter. Water initially permeated the central region of the pore network, sequentially inducing a breakthrough in adjacent pores. It was found that smaller pore throat diameters necessitated higher breakthrough pressures. Consistently, under unchanged inlet flow rates, narrower channels exhibited greater capillary resistance, impeding water flow. Specifically, for the four models with increasing pore throat widths, the critical capillary resistances are decreasing continuously from 87.6 to 38.2 Pa ultimately.