Reactive transport modeling of mineral carbonation in unaltered and altered basalts during CO₂ sequestration

Basalt formation is a new promising CO₂ sink for providing secure long-term carbon storage via trapped CO₂ in mineral phases. When water enters into the crystal structures of the divalent cation-bearing unaltered basalt, the rock is altered and serpentinized. The serpentinization of the unaltered basalt occurs widely around the world. To evaluate the mineral carbonation efficiency of unaltered and altered basalt over a longer time frame, a predictive modeling frame is established in TOUGHREACT based on data from core-scale static batch experiments and the field-scale Carbfix project. Results indicate that unaltered basalt has higher carbonation efficiency than altered basalt both in the carbonation rate and extent due to serpentine kinetic limitations for aqueous phase CO₂ injection, and the carbonates mineralization extent and rate increase with the content of olivine minerals in basalt. Furthermore, under these conditions, the longer CO₂ migration distance from the injection site enhances the chance for mineral carbonation in the serpentinized basalt. In addition, rapid aqueous phase CO₂ injection may limit mineral carbonation and cause porosity reduction near wellbores in the basaltic reservoir, and this effect is more pronounced in case of unaltered basalt than in the altered basalt. Compared to the injection rate, the impact of reservoir conductivity is found to be significant on mineral carbonation in both the unaltered basalt and altered basalt; higher conductivity is more advantageous for CO₂ mineralization under certain circumstances. In combination, the injectivity rate should be carefully assessed at sites based on the reservoir conductivity and whether the basalt is altered or unaltered and the extent of alteration.