Transport and trapping characteristics of liquid and supercritical CO₂ in saline aquifers

Objective CO₂ sequestration in saline aquifers is a critical technology for achieving large-scale reductions in greenhouse gas emissions. In offshore shallow saline aquifers, the low-temperature marine environment and the pressure from overlying seawater create temperature and pressure conditions that differ significantly from those in onshore aquifers at equivalent burial depths. These conditions can allow CO₂ to exist in a liquid state. Compared to supercritical CO₂, liquid CO₂ has a higher density, viscosity, and solubility in formation brine, which influences its migration and sequestration behavior. While existing studies have primarily focused on supercritical CO₂, there remains a gap in understanding the migration and sequestration processes of liquid CO₂ in saline aquifers. Methods Considering the distinct characteristics of liquid and supercritical CO₂, a mathematical model is developed to analyze CO₂ migration and sequestration under the influence of buoyancy and capillary forces. Using high-resolution two-phase flow numerical simulations, this study compares the migration behavior and changes in sequestration forms of liquid and supercritical CO₂ in saline aquifers following the completion of gas injection. Results and Conclusions The results indicate that, compared to supercritical CO₂, the vertical migration rate of liquid CO₂ under buoyancy-dominated conditions is lower, resulting in a smaller swept volume. After 25 a, the total amount of liquid CO₂ sequestered across various trapping forms is significantly less than that of supercritical CO₂, making it more challenging to fully utilize the storage capacity of the saline aquifer. Local capillary trapping accounts for 55% of the total, residual trapping for about 40%, and dissolution trapping for 5%, with the impact of phase state on the contribution of different trapping forms being relatively minor. An increase in geothermal gradient enhances the vertical migration of liquid CO₂, increases its swept volume, and raises the sequestration quantity across different trapping forms, thereby improving the utilization of the saline aquifer’s storage capacity. Under the same burial depth, the migration characteristics and sequestration quantity of supercritical CO₂ differ significantly between onshore and offshore saline aquifers. In offshore saline aquifers, the vertical migration of supercritical CO₂ is inhibited, reducing sequestration quantities under local capillary and residual trapping, which hampers the effective utilization of the aquifer’s storage capacity. These findings provide valuable guidance for efficient CO₂ sequestration in both onshore and offshore saline aquifers.