Reactive transport modeling of water-CO₂-rock interactions in clay-coated sandstones and implications for CO₂ storage

In this work, the potential influences of grain-coating clays on water-CO₂-rock interactions in sandstones and subsequent ramifications for CO₂ storage were investigated using reactive transport simulations. The results indicated that, compared to pore-filling smectite, grain-coating smectite leads to significant pH decrease, increases in the CO₂-species concentrations, and decreases in smectite dissolution and the precipitation of secondary minerals. Moreover, it was revealed that smectite and chlorite coats dissolve preferentially over detrital K-feldspar being covered, while K-feldspar is dissolved preferentially over illite and kaolinite coats. While the mineral trapping mechanism is only important for smectite and chlorite coats, sandstone porosity is significantly reduced for chlorite coat but increased for the other three clay coats. The main causes of the differences between pore-filling and grain-coating scenarios for smectite and chlorite coats are ascribed to the inhibitory effect of clay coats on the growth of secondary quartz and the dissolution of clay. In addition to the above two factors, the decelerating effect of clay coats on the dissolution of K-feldspar is also important for illite coat; meanwhile, for the kaolinite coat, the dissolution of clay is less important and the other two factors are more critical. Furthermore, the coverage and thickness of clay coats, fluid flow rate, detrital grain size, detrital lithology, partial pressure of CO₂, and temperature may all impact the role of clay coats.