Convective Mixing Characteristics under High Pressure and Temperature Condition during CO₂ Geological Storage

Carbon capture, utilization, and storage (CCUS) is an important pathway that can effectively reduce CO2 emissions, and the convective mixing phenomenon driven by a density difference in CO2 brine reservoir storage is very important for long-term storage. In this study, the convective mixing process of CO2 and brine under a series of reservoir conditions was investigated by high-pressure Hele-Shaw cell experiments and numerical simulations. The effects of pressure and salinity on the convective mixing characteristics were explored. The number of fingers increased to the peak and then decreased with small fluctuations. Then, the patterns of variation in dissolution concentration, dissolution rate, and density parameters over time were obtained. The density of the mixed solution and the concentration of dissolved CO2 both increased with time. Higher pressure promotes convective mixing, and the increase in salinity rather inhibits the convective mixing to proceed. At a pressure of 8.5 MPa and a salinity of 10 g/L, the density difference as a driving force of convective mixing is the largest; therefore, the process is the fastest. Additionally, the numerical simulation modeled the convective mixing process and clearly showed the flow state of each fluid, with most of the fingers moving downward and the brine moving upward, with boundary effects affecting the fluid at the boundary. The numerical simulations also produced quantitative analysis results, which agreed with the experimental data. This study provides a more detailed understanding of the CO2 convection mixing process under reservoir conditions, which has positive implications for the development of CCUS technology.