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Sequestration of CO2 in deep saline aquifers has attracted positive attention from the scientific community as a mitigatory measure to reduce anthropogenic CO2 in the atmosphere. Caprock layers/zones, which are saturated with brine and CO2 at different degrees of saturation (DOSs), behave differently. Since the risk of CO2 back-migration through different zones in the caprock varies, the focus of this study is on the effects of injected CO2 on the mineralogical and mechanical behavior of caprock with different DOSs due to chemical alteration. A series of reactivity tests were conducted on siltstone samples under different saturation conditions. The experimental results showed that fast dissolution of minerals such as kaolinite and muscovite due to carbonic acid increases the pore volume of caprock. Therefore, flow paths and porosity increase in the fully CO2–brine-saturated zone compared to other zones in the caprock that are not fully saturated with CO2 and brine. In addition, the caprock layer, which is fully saturated with CO2 and brine, has the highest stress corrosion and shear weakening due to mineral dissolution and wetting softening effects, which occur due to surface energy consumption. The next weakest zone in the caprock is the fully brine-saturated siltstone layer due to quartz hydrolysis and wetting softening effects. Therefore, serious attention should be paid to a caprock layer fully saturated with a CO2–brine mixture during sequestration because the risk of CO2 migration via this caprock zone is high, and this affects the permanence and security of injected CO2, especially during periodic CO2 injections and minor seismic events.

During CO2 injection into deep saline aquifers, the overlying caprock may be subjected to geochemical reactions which can alter the leakage pathways for injected CO2. Thus, it is crucial to identify the supercritical CO2 (scCO2) flow behaviour via fractures in caprock and its permeability to estimate the permanence of injected CO2. The objective of this study is to find the effect of scCO2 flow on fractured caprock permeability. A fractured siltstone sample was saturated in deionized water and conducted scCO2 permeability tests using a high-precision advanced core flooding apparatus under different injection pressures and confinements. Next, the siltstone sample was saturated in 10% w/w NaCl brine and conduced scCO2 permeability tests as described earlier. The results show that the brine-saturated sample has low permeability compared to water-saturated siltstone sample. The reason would be the deposition of evaporites during scCO2 flow through the fractured sample. This is known as CO2 dry-out phenomenon or absorbing moisture into the scCO2, making the remaining brine saturated with salts. Thus, the CO2 back-migration through the caprock discontinuities becomes minimized due to CO2 dry-out phenomenon, which is an advantage for the caprock integrity in deep saline aquifers. In addition, aquifers with high salinity contents show significant dry-out phenomenon because pore fluid easily becomes supersaturated with salts due to evaporation of moisture into the scCO2.