ObjectiveCO2 storage in saline aquifers serves as a critical technology used to dramatically reduce greenhouse gas emissions. Owing to the l
ObjectiveCO2 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. MethodsConsidering 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.
