Effects of CO2 Solubility on Density and Mineral Trapping in Saline Aquifers

The Redlich-Kwong equation of state is used to develop a PVTx model that simulates appropriate reservoir conditions for the sequestration of CO₂ injected into saline aquifers. The PVTx model is based on the fundamental properties of CO₂ and predicts how temperature, pressure and salinity affect the solubility of supercritical CO₂ in brine, the resulting density of the CO₂-brine system, and the subsequent mineral trapping of CO₂ in the form of precipitates. The model is validated with experimental data available in the literature. It is found that at low temperatures, the density of the ternary H₂O-CO₂-NaCl solution does not vary monotonically with pressure but displays a minimum which is proportional to salinity. At high temperatures, however, density increases monotonically with pressure. Increasing salinity tends to lower the solubility of CO₂ in brine and limits the density increase. Similarly, the density of the H₂O-CO₂ solution has been found to behave like the density of the ternary H₂O-CO₂-NaCl solution in response to the change in pressure and temperature. The range of the respective values for the density of the ternary H₂O-CO₂-NaCl solution, however, is greater than the range of the respective values for the H₂O-CO₂ solution. We also present a model to find the dissolution and precipitation rates of the minerals by taking into account the pressure, temperature, salinity and pH of the system. This study also finds that both dissolution and precipitation rates of minerals increase with pressure and temperature. These results show good agreement with those obtained from experimental work reported in previously published studies. This study also validates earlier findings based on relatively less precise models, with respect to the increase in CO₂ solubility at higher pressures and a decrease in solubility associated with increasing values of temperature and salinity.