Numerical analysis of migration and sequestration dynamics of dense liquid CO2 in offshore shallow saline aquifers

Geological sequestration of CO₂ in saline aquifers presents a promising strategy for large-scale greenhouse gas mitigation. While most existing studies have focused on the storage of supercritical CO₂, in the high-pressure and low-temperature conditions typical of offshore shallow saline aquifers, CO₂ may exist in a dense liquid phase. This phase exhibits distinct properties such as higher density and viscosity, which significantly influence its migration behavior and trapping forms. In this study, we develop numerical models to simulate CO₂ injection into offshore shallow saline aquifers, incorporating key trapping processes, including local capillary trapping, residual trapping, and dissolution trapping. High-resolution two-phase flow simulations are employed to investigate the spatiotemporal evolution of CO₂ plume and the dynamic transition of sequestration forms. We systematically evaluate the impacts of the CO₂ phase state, formation heterogeneity (characterized by dimensionless horizontal/vertical autocorrelation lengths and heterogeneity degree), and injection rate on CO₂ migration and storage performance. The results highlight distinct differences in migration patterns and trapping mechanisms between liquid-phase and supercritical CO₂ in offshore saline aquifers. Formation heterogeneity, particularly the autocorrelation length and global heterogeneity of the permeability field, plays a critical role in controlling plume evolution and sequestration efficiency. Saline aquifers with larger horizontal autocorrelation lengths or higher heterogeneity exhibit underutilized storage capacity despite reduced leakage risk through the caprock. In contrast, saline aquifers with larger vertical autocorrelation lengths tend to achieve higher storage efficiency but are associated with increased leakage potential. Under high injection rates, CO₂ is more likely to invade pores with higher capillary entry pressures, resulting in elevated saturations near the injection zone. These findings offer valuable insights into the migration dynamics and long-term stability of liquid-phase CO₂ in offshore shallow saline aquifers and guide the safe and efficient implementation of CO₂ sequestration strategies in such settings.