DEPTH DEPENDENT CO2-INDUCED ELEMENTAL MOBILIZATION FROM CARBONATE-RICH ROCKS USING FIELD-RELEVANT PRESSURE AND TEMPERATURE

In the past decade, many studies have been conducted to determine changes within the reservoir following CO₂ injection as well as effects of CO₂ release into overlying groundwater aquifers. There is little or no literature available, however, on the effect of CO₂ release on rock between the storage reservoir and the subsurface. To fill this current knowledge gap, this study used relevant rock materials, temperatures and pressures to study solid phase mineralogical changes and measure elements released into or removed from the aqueous phase through mineral dissolution or mineral precipitation. Using a NaCl background solution to represent reservoir brine, and depth dependent pressures ranging from 3,800-14,000 kPa and temperatures from 50-61°C, rocks samples (either <212 µm or larger rock chips) were reacted for 7 days. The liquid samples were then analyzed with IC, ICP-OES and ICP-MS, and the solids were analyzed with SEM/EDS, QXRD, and Mössbauer spectroscopy. A significant increase of major elements (e.g., Ca, Mg, Si, Mn, K) and variable releases of potential contaminants (e.g., Sr, S, Ba, Cs, B, Cu, and Cr) were observed in the liquid phase from CO₂ experiments; lower concentrations of these elements were observed in the N₂ experiments. SEM and Mössbauer spectroscopy results showed the formation of new minerals and Fe oxides in some CO₂-reacted samples. This has important implications for contaminant removal from the aqueous phase due to potential incorporation into precipitated minerals or the strong adsorption properties of Fe oxides. The rock chemistry (determined with extractions), and the mineralogical and physical (particle size) properties were the major controlling factors for elemental release; each of these properties, as well as the calculated pressure and temperature for each experiment are, in some extent, related to the depth of the samples. These experiments show the interactions between the CO₂-rich brine and the rock between deep storage reservoirs and overlying groundwater aquifers have the potential to affect the level of risk to overlying groundwater resources, and should be considered during site selection and risk evaluation.