Objective CO₂ storage in saline aquifers serves as a critical technology used to dramatically reduce greenhouse gas emissions. Owing to the low-temperature marine environment and the pressure from overlying seawater, shallow offshore saline aquifers exhibit significantly different temperature and pressure conditions compared to onshore saline aquifers at equivalent burial depths, allowing CO₂ to occur in a liquid state. Compared to supercritical CO₂, liquid CO₂ features higher density, viscosity, and solubility in formation water, which affect the CO₂ migration and storage processes. Previous studies focus primarily on supercritical CO₂, lacking a deep understanding of the migration and storage patterns of liquid CO₂ in saline aquifers.

Methods Considering the distinct characteristics of liquid and supercritical CO₂, this study constructed a mathematical model for CO₂ migration and storage under the action of buoyancy and capillary pressure. Using the high-precision numerical simulations of two-phase seepage, this study compared the laws of changes in the migration characteristics and storage forms of liquid and supercritical CO₂ in saline aquifers after gas injection. Results and Conclusions The results indicate that compared to supercritical CO₂, liquid CO₂ manifested reduced vertical migration rates and swept volumes under buoyancy-dominated conditions. After 25 a, the storage amounts of liquid CO₂ in different storage forms were significantly lower than those of supercritical CO₂, making it more difficult to fully leverage the storage capacity of saline aquifers. Among the different CO₂ storage forms, local capillary trapping, residual gas trapping, and solubility trapping represent 55%, 40%, and 5%, respectively, with the CO₂ phase states posing minor impacts on the storage forms. An increase in geothermal gradient enhanced the vertical migration and swept volume of liquid CO₂, the CO₂ storage amounts of different storage forms, and the utilization efficiency of the storage capacity of saline aquifers. At the same burial depths, supercritical CO₂ displayed significantly different migration characteristics and storage amounts in onshore and offshore saline aquifers. The inhibited vertical migration of supercritical CO₂ in offshore saline aquifers reduced the CO₂ storage amounts of local capillary trapping and residual gas trapping, hampering the effective utilization of the storage capacity of saline aquifers. The results of this study can serve as a guide for efficient CO₂ storage in onshore and offshore saline aquifers.