Element mobilization and immobilization from carbonate rocks between CO₂ storage reservoirs and the overlying...

Despite the numerous studies on changes within the reservoir following CO₂ injection and the effects of CO₂ release into overlying aquifers, little or no literature is available on the effect of CO₂ release on rock between the storage reservoirs and subsurface. This is important, because the interactions that occur in this zone between the CO₂ storage reservoir and the subsurface may have a significant impact on risk analysis for CO₂ storage projects. To address this knowledge gap, relevant rock materials, temperatures and pressures were used to study mineralogical and elemental changes in this intermediate zone. After rocks reacted with CO₂-acidified 0.01 M NaCl, liquid analysis showed an increase of major elements (e.g., Ca and Mg) and variable concentrations of potential contaminants (e.g., Sr and Ba); lower aqueous concentrations of these elements were observed in N₂ control experiments, likely due to differences in pH between the CO₂ and N₂ experiments. In experiments with As/Cd and/or organic spikes, representing potential contaminants in the CO₂ plume originating in the storage reservoir, most or all of these contaminants were removed from the aqueous phase. SEM and Mössbauer spectroscopy results showed the formation of new minerals and Fe oxides in some CO₂-reacted samples, indicating potential for contaminant removal through mineral incorporation or adsorption onto Fe oxides. These experiments show the interactions between the CO₂-laden plume and the rock between storage reservoirs and overlying aquifers have the potential to affect the level of risk to overlying groundwater, and should be considered during site selection and risk evaluation.