The Extent of Reactive Solubility Trapping During CO2 Storage in Geological Carbonate Formations

Abstract Carbon dioxide (CO2) is trapped in geological formations during subsurface storage through structural, capillary, solubility, and mineral trapping mechanisms. However, the contribution of geochemical reactions in increasing the CO2 solubility in brines is often overlooked. The objective of this study is to evaluate the extent of additional CO2 uptake in CO2/brine/rock systems due to carbonate mineral dissolution driven by carbonic acid. Chemical thermodynamic modeling was performed to calculate CO2 solubility in brines in the presence and absence of calcium carbonate. Pitzer equations modeled excess free energy to account for deviation from ideality at high ionic strengths, while the Peng-Robinson equation of state was used to express the single component gas phase fugacity. The study investigated a wide range of temperatures (25–90 °C), pressures (1–300 atm), and salinities (0-5 mol sodium chloride (NaCl)/kgw and 0-5 mol calcium chloride (CaCl2)/kgw). The difference in solubility due to carbonate dissolution reaction is only significant at very low pressures; at 1 atm and 25 °C, the solubility trapping of CO2 is enhanced by 23.5% while at pressures exceeding 50 atm this increment is reduced to less than 1.6%. Increasing the temperature lowers the solubility of CO2 but can lead to a non-monotonic variation between physical and reactive solubility trapping. The effect of salinity, on the other hand, depends whether the cations in solution are common with those in the carbonate crystal lattice. For example, increasing the salinity with a CaCl2(aq) solution will suppress the dissolution of calcium carbonate due to the increased calcium chemical potential in the aqueous phase. The impact of non-common ions, such as in the case of NaCl(aq) solution, is concentration dependent with salting-in effect taking place at low molalities that maximize the solubility at a concentration between 1-2 mol NaCl/kgw followed by a salting-out regime. Consequently, the additional trapping due to geochemical reactions can be more than that of pure water, but is still insignificant at high pressures above 50 atm. This study provides new insights by quantifying the maximum additional CO2 solubility attributable to carbonate dissolution as a function of pressure, temperature, and salinity. The presented findings will be useful to determine the importance of incorporating reactive models in reservoir simulations and also conclude on whether computationally expensive reactive transport modeling is warranted for practical CO2 sequestration assessments.