In-situ parameterization and scaling analysis of CO₂ convective dissolution for offshore saline aquifer sequestration in the South China Sea

Offshore saline aquifers in the South China Sea are promising targets for large-scale CO₂ sequestration because of their substantial capacity and proximity to major coastal emission sources. However, reliable prediction of CO₂ plume migration and dissolution trapping remains challenging because key fluid properties are often estimated from empirical correlations rather than measured under in-situ conditions. This study develops an integrated experimental–numerical framework to investigate the mechanisms and scaling laws of CO₂ convective dissolution. The diffusion coefficient, solubility, and density of the CO₂–brine system were determined from formation brine experiments under reservoir conditions representative of the South China Sea. CO₂ diffusion coefficients measured by Raman spectroscopy range from 2.89 × 10⁻⁹–4.78 × 10⁻⁹ m²·s⁻¹ at 313.15–363.15 K and 7.5–17 MPa. Diffusivity increases significantly with temperature, decreases systematically with salinity, and is only weakly affected by pressure. Solubility and density measurements further confirm that CO₂ dissolution generates sufficient density contrast to trigger density-driven convection. These experimentally constrained fluid properties were incorporated into Darcy-scale numerical simulations to quantify dissolution dynamics. The results identify the Rayleigh number (Ra) as the key parameter controlling the transition from diffusion-dominated to convection-dominated dissolution. The onset time of convection and the maximum mass transfer rate are described by t_onset=1.45 × 10⁷Ra^− 1.98 and Sh_max=0.045Ra^1.12, respectively. Simulations for South China Sea aquifers indicate that reservoir properties have limited influence on final dissolved CO₂ mass, but affect dissolution kinetics. These findings provide a calibrated basis for reservoir screening, injection design, and long-term assessment of offshore CO₂ sequestration.