Spatiotemporal progression of CO2 mineralization: A micro-CT study of fracture-matrix interaction

Carbon trapping by mineralization in mafic rocks provides long-term stability and large storage capacity. The injection of CO2-dissolved water into fractured rock formations gives rise to advective-reactive transport along fractures and diffusive-reactive transport within matrix blocks. This study explores the intricate interaction between mineralogy and fluid chemistry, as well as the impacts of dissolution and precipitation on the evolving pore structure and flow pathways. Sequential tomographic images and complementary reactive-transport modeling allowed us to elucidate the evolution of fracture-matrix interaction in the near and far-field of the injection well. Within the matrix: the diffusing low pH front selectively dissolves minerals according to their reactivities; the products diffuse both out into the fracture space and into the matrix towards the center of rock blocks; eventually, the migrating reactants exceed solubility and precipitation follows. Within the fracture network: the injected low pH fluids start accumulating reaction products and losing protons until precipitation conditions prevail at some intermediate distance from the injection well; while dissolution enhances fracture transmissivity and may lead to flow channeling near the injection point, precipitation is self-stabilizing resulting in an annular zone of precipitation. The injection of seawater with dissolved CO2 results in the additional precipitation of clay minerals and gypsum. Most dissolution-precipitations are volume-positive reactions, therefore, there will be a marked decrease in matrix permeability away from the fracture wall, and in fracture transmissivity away from the injection well. These results support the improved assessment of mineral trapping efficiency and reservoir storage capacity, highlight the impact of the injection fluid composition, and affect site selection for carbon geological storage.