Predicting δ13CDIC dynamics in CCS: A scheme based on a review of inorganic carbon chemistry under elevated pressures and temperatures

Stable carbon isotopes are important tools to assess potential storage sites for CO2, 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 CO2 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 CaCO3 for pCO2 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 CO2 sequestration. The pCO2 and pH are the dominant controlling factors in the DIC/δ13C/pH system. The fugacity of CO2 has major impact on DIC concentrations at temperatures below 100 °C at high pCO2. 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 CO2 at pCO2 values above 10 bar. This depletion is controlled by carbon isotope fractionation between CO2 and H2CO3* which is the dominant species of DIC at the resulting pH below 5.

► DIC speciation and carbon isotope fractionation in deep formations were investigated
► Relations to describe p, T and TDS dependence of isotope methods have been reviewed
► Schemes determine ambient conditions of fluids depending on pH, DIC and δ13C
► For pCO2 > 10 bar, d13CDIC is dominated by CO2, providing a stable end member
► δ13C is an accurate tracer in CCS, if ambient conditions are considered properly