GEOCHEMICAL CONSIDERATIONS OF CO2 LEAKAGE INTO USDWs FROM CCS SITES

Carbon Capture and Storage (CCS) has been proposed as one of several options to mitigate atmospheric carbon dioxide emissions. The prospect of injecting CO₂ under high pressures into deep geologic formations is accompanied by concerns of possible leakage of CO₂ into overlying underground sources of drinking water (USDWs). Possible leakage pathways include abandoned leaky wells, fractures, faults and discontinuities in caprock strata. Dissolution of high amounts of CO₂ into water may lead to relatively acidic conditions, which may cause release of major and trace metals into USDWs with the potential for human health risk.

Important release mechanisms are primarily desorption and dissolution; yet for many trace metals of concern, such as lead, arsenic and barium and nickel, literature values for desorption and dissolution rates, as well as equilibrium constants, vary over orders of magnitude. For example, equilibrium sorption coefficient (K_D) values for arsenic span five orders of magnitude. The dissolution and desorption kinetics, and amount of metal release, depend on variables such as pH, concentration, competition with other cations, presence of organic matter, aquifer mineralogy, redox state and groundwater velocities. Additionally, buffering capacity of the aquifer, determined by aquifer mineralogy, can mitigate pH fluctuations and potentially metal release.

In this work, we focus on quantifying the risk to USDWs from CO₂ leakage, and reducing uncertainties through literature review and geochemical modeling. For example, no generally accepted methodology exists to measure leakage rates, which poses a boundary-condition problem when trying to assess leakage human health risks. Therefore, we present a range of leakage rates as reported in the literature, from numerical modeling efforts, analytical solutions, laboratory core experiments and “worst-case scenario” regulatory concerns. In addition, we show an analysis of reported dissolution and desorption rates for a select group of metals and minerals. Future laboratory experiments will focus on conditions identified as risky through this literature review, supplemented by geochemical modeling.