Evaluating impacts of CO₂ gas intrusion into a confined sandstone aquifer: Experimental results

Deep subsurface storage and sequestration of CO₂ has been identified as a potential mitigation technique for rising atmospheric CO₂ concentrations. Sequestered CO₂ represents a potential risk to overlying aquifers if the CO₂ leaks from the deep storage reservoir. Experimental and modeling work is required to evaluate potential risks to groundwater quality and develop a systematic understanding of how CO₂ leakage may cause important changes in aquifer chemistry and mineralogy by promoting dissolution/precipitation, adsorption/desorption, and redox reactions. Sediments from the High Plains aquifer in Kansas, United States, were used in this investigation. This aquifer was selected to be representative of consolidated sand and gravel/sandstone aquifers overlying potential CO₂ sequestration repositories within the continental US. In this paper, we present results from batch experiments conducted at room temperature and atmospheric pressure with four High Plains aquifer sediments. Batch experiments simulate sudden, fast, and short-lived releases of the CO₂ gas as would occur in the case of well failure during injection. Time-dependent release of major, minor, and trace elements were determined by analyzing the contacting solutions. The strong acid extraction tests confirmed that in addition to the usual elements present in most soils and sediments, the High Plains aquifer sediments had appreciable amounts of As, Cd, Pb, Cu, and occasionally Zn, which potentially may be mobilized from the solid to the aqueous phase during or after exposure to CO₂. However, the results from the batch experiments showed that these sediments mobilized only low concentrations of trace elements (potential contaminants), which were detected occasionally in the aqueous phase during these experiments. Importantly, these occurrences were more frequent in the calcitefree sediment. Results from these investigations provide useful information to support site selection, risk assessment, and public education efforts associated with geological CO₂ storage and sequestration.