Mixed-layer illite–smectite reactivity in acidified solutions: Implications for clayey caprock stability in CO2 geological storage

The reactivity of mixed-layer illite–smectite was investigated in batch and mixed-flow reactors in the presence of K-bearing mineral impurities (K-feldspar) in order to determine the reaction pathways and the kinetic rate of the reactions in a range of pH conditions pertinent to the context of CO2 geological storage.The mixed-layered illite–smectite (I–S) mineral was extracted from a natural clayey caprock and was studied at 80 °C, with a solution salinity of 0.1 M NaCl and over a pH range from 3 to 8.5. The experimental results show that the most important process taking place is illitization of the initial I–S. In the most acidic conditions, an intense illitization is observed, which was interpreted as a new proton-promoted illitization process. At neutral pH, illitization is less intense and is attributed to a classical thermal effect.In terms of the overall reaction rate (I/S and K-feldspar dissolution in a mixed flow experiment), the values calculated from the evolution of SiO2 concentrations fall in the range of the experimental values reported in the literature for smectites and illites.Implications of illitization processes on CO2 storage and clayey caprock integrity are also discussed.

Research Highlights
► We investigate the reactivity of mixed-layer illite–smectite in conditions pertinent to the context of CO2 geological storage.
► We show that the most important process taking place is a proton-promoted illitization in acidic conditions.
► The illitization may proceed both from the alteration of the initial I–S phase and the dissolution of K-feldspar present as impurities.
► We did not observe any illitization in basic conditions.