Hydrogeological challenges for carbon mineralization in terrestrial mafic/ultramafic rock bodies

The widespread and substantive occurrence of basalt in ocean basins has long been recognised as a potential reservoir for CO₂ removal via mineralization having stability over extended periods of geological time. Due to the significant cost of fully exploring this potential, more recent attention has focused on geochemically equivalent rocks in more accessible terrestrial terranes. Examples occur in ophiolite sequences, terrestrial volcanic environments, and even in stable cratonic settings that have undergone considerable metamorphism. There have been abundant studies of the hydrogeology of crystalline rock in general by the nuclear waste and mining industries, and for water supply, which clearly illustrate that flow and transport are governed in these rock types by a sparse network of discrete fractures having relatively small aperture (10s to a few 100 μm with a total void volume of <0.01%), embedded in a rock matrix of virtually zero permeability (<10^-18 m²)  and very low effective porosity (<0.05% to ∼0.8%). Thus, although the geochemical suitability of the rock is required, the injection permeability, CO₂ transport mechanisms, and pore volumes available for mineralization may be the more limiting factors to commercial viability. Examples will be presented illustrating the need for hydrogeological characterization of appropriate rock bodies, and more complete analysis of the process of matrix diffusion under complex geochemical conditions.