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Density-driven convection, which can accelerate the dissolution rate of carbon dioxide (CO 2 ) in resident brine, is critical for the long-term fate of the injected CO 2 permanence and security of CO 2 geological storage. Visualization experiments and pressure–volume–temperature (PVT) testing were conducted to investigate the influence from gravitational convection. For investigate gravitational instabilities and convective diffusion, we designed a Hele-Shaw cell rated to 70 MPa and Rayleigh number of 346 to conduct visualization experiments with the micro-schlieren technique. The average diffusion coefficient and time-dependent values were measured in the PVT experiments. We also calculated the convection parameters, including Rayleigh number and critical onset time, with a series of PVT testing at the temperature ranging 293.15–423.15 K and pressure ranging 14–24 MPa by using the constant-pressure method. Through visualization experiments, we observed convective currents triggered by the density gradient in the gas–liquid interface, which noticeably enhanced the CO 2 dissolution rate. The PVT testing confirmed that the diffusion coefficient increased sharply under the influence of the gravitational convection at the early stage and then decreased toward the average diffusion coefficient with time. The PVT testing also demonstrated the Rayleigh number increasing with temperature or pressure increasing under the reservoir conditions. The gravitational convection will be more likely to occur and more rapid with a greater pressure or higher temperature.

In this paper, an experimental technique has been developed to study the interfacial interactions of the reservoir brine and CO2 system at high pressures and elevated temperatures. Using the axisymmetric drop shape analysis (ADSA) for the pendant drop case, this new technique makes it possible to determine the interfacial tension (IFT) and visualize the interfacial interactions phenomenon between the reservoir brine and CO2 under practical reservoir conditions. The laboratory experiments were conducted for the measurement of the IFT between CO2 and pure water or brine covering three interesting salinities (0 mg/L, 14224.2 mg/L and 21460.6 mg/L). Comparing the data of the CO2–pure water interfacial tension with other papers at 45 °C, at low pressures (<7.38 MPa), all data accord well; however, at high pressures (>7.38 MPa), all data have slight deviations. The dynamic and equilibrium IFT are measured as a function of pressure at the stratum temperature (97.53 °C). It is found that the dynamic IFT gradually approaches to a constant value, which is the equilibrium IFT. The equilibrium IFT decreases as the pressure increases but increases as the salinity increases. In addition, the wettability of the reservoir brine and CO2 system changes from the hydrophilic case to the hydrophobic case as the pressure increases, and the same phenomenon decreases as the salinity of brine increases.

Solubility trapping is one important storage method in CO 2 geological sequestration, which is affected by many factors such as temperature, pressure, and salinity. At present, the solubility of single mineral (such as pure water, NaCl, MgCl 2 ) solution is mostly studied, and the dynamic dissolution process under actual reservoir conditions is less studied. In this study, based on the improved experimental method, we carried out static and dynamic solubility experiments under reservoir conditions. With pore permeability measurements, scanning electron microscopy, and other experimental means, the dynamic change rule and reason for CO 2 solubility were clarified. The results show that the static solubility experiment has more advantages in identifying the law of influencing factors of CO 2 solubility in the short term and quickly, and the dynamic solubility experiment is more accurate in simulating the actual CO 2 geological storage process change rule. Temperature is the most sensitive to solubility, followed by pressure, and salinity is almost unaffected by temperature and pressure for solubility. Under reservoir conditions, the solubility of CO 2 is closer to the solubility value (3.2/100 g) after dynamic equilibrium, and the solubility of CO 2 shows a dynamic process of "first decreasing, then increasing, and then balancing", reaching the lowest at about 15 days, about 1.76/100 g. But with a longer reaction time, improving solubility values, eventually, it gradually becomes saturated after 30 days, and eventually reaches equilibrium. The equilibrium solubility is about 5% lower than the static solubility value, due to the dynamic competition between mineral dissolution and precipitation under reservoir conditions, and at the same time, "chemical retention" and "residual capture" play a key role.