To support effective geological CO2 sequestration design and operation, dissolution and reaction in CO2-brine-clay mineral particle systems (sepiolite and montmorillonite) were studied under subcritical to supercritical CO2 conditions (10 bar to 150 bar at 45 degrees C and 65 degrees C). The order of ion dissolution from the framework of sepiolite in the brine was slightly different under each experimental condition, whereas the order of dissolved ion concentration from the montmorillonite was not varied. The solubility of CO2 was lower in the CO2-brine-clay mineral particle system than in a CO2-brine system. Precipitation of amorphous silica as a secondary mineral formation was observed after the reaction of both sepiolite and montmorillonite. The CO2 solubility model, considering ion concentration and aqueous silica, reasonably predicted the CO2 solubility from subcritical to supercritical conditions. The kinetic rate constant of the dissolution reaction of sepiolite was correlated with the initial pH of the brine. After reaction with high-pressure CO2-saturated brine, the micro-crystallinity of sepiolite did not change, whereas the basal (001) plane of montmorillonite showed deformation in micro-crystallinity after the dissolution reaction. By contrast, reaction with the CO2-saturated brine led to a decrease in the surface area of sepiolite and an increase in the surface area of montmorillonite.