Evolution characteristics of gravitational instability and CO2 transport area in porous media under density-driven reactive solute transport

The dissolution of CO2 in brine leads to gravitational instability, as well as the interaction between CO2, brine, and reservoir minerals. This complex reactive solute transport can further alter the porosity and permeability, while changing the characteristics of natural convection process. In this study, a density-driven reactive solute transport model considering the evolution of porosity and permeability is developed, which is coupled with the evolution of reaction rate, specific area and porosity and permeability. A method for quantifying gravitational instability and CO2 transport area is proposed. On this basis, the impact of neglecting porosity and permeability evolution on density-driven reactive solute transport is discussed, and the influence of Damköhler (Da) number and pore structure heterogeneity on gravitational instability and CO2 transport area is further analyzed. It is found that density-driven reactive solute transport is highly sensitive to the evolution of porosity and permeability. Neglecting the evolution of porosity and permeability leads to an underestimation of gravitational instability and transport area. Gravitational instability and CO2 transport area generally decrease with the increase in Da number, while the increase in heterogeneity leads to the increase in both. In the process of density-driven reactive solute transport, the dominant mechanisms can be divided into three types: reaction consumption, concentration-driven, and density-driven mechanisms. When the reaction consumption mechanism is dominant, CO2 is significantly consumed by the reaction with CaCO3, leading to local changes in porosity and permeability. When concentration-driven and density-driven mechanisms are dominant, higher concentration and density gradient promote the development of CO2 fingering.