Expected CO2-Water-Rock Interactions and Changes in Formation Porosity in a Deep Saline Aquifer in Florida, United States

It has been proposed that deep saline aquifers in the state of Florida might be suitable repositories for geologic storage of carbon dioxide (CO2) during carbon capture and sequestration (CCS). In particular, the Cedar Keys Formation and the underlying Cretaceous Lawson Formation of south-central and south Florida have been identified as a potentially suitable repository. These formations are believed to consist predominantly of porous dolomite, with smaller amounts of calcite and gypsum also present. A key question related to the suitability of these formations as a CO2 repository is how the porosity of the formations will change in response to injection of supercritical CO2, e.g., due to dissolution or precipitation of minerals caused by the CO2 injection. In response to this question, we performed modeling studies to estimate the magnitude of likely changes in formation geochemistry, with particular attention to changes in porosity. Our simulations suggest that, as CO2 moves laterally outward from an injection well, dissolution of CO2 into native brine will cause a decrease in pH. This will, in turn, cause dissolution of dolomite and calcite and accompanying precipitation of gypsum. However, these dissolution/precipitation reactions are predicted to cause only slight changes in formation porosity over an injection period of 100 years, even at high injection rates (several million tons of CO2 per year). Furthermore, this result is relatively insensitive to model input parameters such as the background pH of the brine. Therefore, based on results to date, we have no reason to rule out the proposed formations as suitable repositories for long-term CO2 storage.