Influence of supercritical CO₂ on bentonite properties

Wet bentonite was used to simulate a caprock for geologic CO₂ sequestration. Its properties were characterized after reacting with supercritical (SC) CO₂ alone and in the presence of salt solutions. High-pressure reaction tests were conducted to investigate the interaction of SC CO₂, bentonite, and water. Wet bentonite mixed with 1 N NaCl solution and then reacted with SC CO₂ resulted in noticeable changes in volume and bentonite mineral components compared with unreacted bentonite, due to SC CO₂ induced dehydration. Plagioclase, K-feldspar, montmorillonite, and pyrite were dissolved but Ca-bearing minerals, i.e., calcite, dolomite, and gypsum, and halite were precipitated. In contrast, wet bentonite mixed with the salt solution reacted with SC CO₂ in the presence of the salt solution resulted in less distinct changes in volume and mineral components, likely due to the dissolution of SC CO₂ in the salt solution. The fewer changes in mineral components in the bentonite reacted with SC CO₂ in the presence of the salt solution were attributed to the greater quantity of water, causing CO₂ dissolution. These results suggest implicitly that caprock in the immediate vicinity of an injection well may significantly deteriorate due to the interaction between caprock and dry SC CO₂, and caprock far from the injection well may deteriorate less because the SC CO₂ may react in the presence of large quantities of water.