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.

Gas and water migration through the hydrate-bearing sediment are characteristic features in marine gas hydrate reservoirs worldwide. However, there are few experimental investigations on the effect of water-gas flow on the gas hydrate reservoir. In this study, gas-water migration in gas hydrate stability zone (GHSZ) was investigated visually employing a high-resolution magnetic resonance imaging (MRI) apparatus, and the formation of hydrate seal was experimentally investigated. Results revealed that normal flow of gas-water at the low flow rate of 1–0.25 mL/min will induce the hydrate reformation. Conversely, higher gas-water flow rates (at 2–0.5 and 4–1 mL/min) need higher reservoir pressure to induce the hydrate reformation. In addition, the hydrate reformation during the gas-water flow process produced the hydrate seal, which can withstand an over 9.0 MPa overpressure. This high overpressure provides the development condition for the underlying gas and/or water reservoir. A composite MRI image of the whole hydrate seal was obtained through the MRI. The pore difference between hydrate zone and coexistence zone produces a capillary sealing effect for hydrate seal. The hydrate saturation of hydrate seal was more than 51.6%, and the water saturation was more than 19.3%. However, the hydrate seal can be broken through when the overpressure exceeded the capillary pressure of the hydrate seal, which induced the sudden drop of reservoir pressure. This study provides a scientific explanation for the existence of high-pressure underlying gas below the hydrate layer and is significant for the safe exploitation of these common typical marine hydrate reservoirs.