Predicting δ13C DIC dynamics in CCS: A scheme based on a review of inorganic carbon chemistry under elevated pressures and temperatures

Abstract

Stable carbon isotopes are important tools to assess potential storage sites for CO 2, as they allow the quantification of ionic trapping via isotope mass balances. In deep geological formations high p/ T conditions need to be considered, because CO 2 dissolution, equilibrium constants and isotope fractionation of dissolved inorganic carbon (DIC) depend on temperature, pressure and solute composition. After reviewing different approaches to account for these dependencies, an expanded scheme is presented for speciation and carbon isotope fractionation of DIC and dissolution of CaCO 3 for pCO 2 up to 100 bar, pH down to 3 and temperatures of up to 200 °C. The scheme evaluates the influence of respective parameters on isotope ratios during CO 2 sequestration. The pCO 2 and pH are the dominant controlling factors in the DIC/ δ 13C/pH system. The fugacity of CO 2 has major impact on DIC concentrations at temperatures below 100 °C at high pCO 2. Temperature dependency of activities and equilibrium dominates at temperatures above 100 °C. Isotope ratios of DIC are expected to be about 1–2‰ more depleted in 13C compared to the free CO 2 at pCO 2 values above 10 bar. This depletion is controlled by carbon isotope fractionation between CO 2 and H 2CO 3* which is the dominant species of DIC at the resulting pH below 5.