Limiting the risk inherent to geological CO2 storage: The importance of predicting inorganic and organic chemical species behavior under supercritical CO2 fluid conditions

Field tests have clearly demonstrated that injecting CO2 in geological storage sites results in the release of heavy metals and organic species to groundwater, implying that CO2 injection may have potentially dramatic consequences for the environment. Numerous laboratory experiments using rock and cement samples from different geological formations typical of injection sites show that rocks reacting with synthetic or natural fluids and supercritical CO2 at their respective temperature and pressure conditions generate fluids with As, Cr, Cu, Cd, Pb, Fe, and Mn concentrations above Environmental Protection Agency drinking water standards. The solubility of a compound in supercritical-CO2 (sc-CO2), expressed in terms of the compound's activity or fugacity, also depends on the composition of the phases present at the pressure and temperature of the storage site. In a brine sc-CO2 system, estimating the activity of an inorganic compound or the fugacity of an organic compound is a prerequisite to predicting the solubility of a compound in sc-CO2 phases. Available models (e.g. Pitzer equations) require the use of binary salt concentrations and are best applicable to polar ionic compounds; but the effect of brines on larger hydrocarbons has not yet been explored. New experimental data will be needed to determine the magnitude of pH effects on the partitioning behavior of organic acids and trace metal complexes from brine to sc-CO2.