The importance of fractures and matrix diffusion to the success of carbon mineralization in terrestrial mafic/ultramafic rock bodies

Ocean basin basalt is well recognised as a potentially massive reservoir for CO₂ removal via carbon mineralization due to the appropriate mineralogy and the presence of moderate porosity and permeability in these rocks. Similar mafic and ultramafic mineralogy reside terrestrially in exhumed oceanic crust, tectonically active continental margins, and even in stable cratonic settings. As disposal of the necessary volumes of carbon via mineralization requires injecting large volumes of CO₂ in either supercritical or dissolved form, having sufficient permeability and porosity in the rock is critical to success. In the few studies that have been conducted on the hydrogeology of terrestrial ultramafics, fluid flow is largely governed by sparsely distributed fractures and faults, with little advection into the surrounding intact rock matrix. The process of carbon mineralization is therefore dependent on advective CO₂ transport in the fractures but primarily relies on diffusion-driven transfer into the intact rock matrix. The process of matrix diffusion is well understood from detailed studies of contaminant transport in fractured rock, and robust analytical and numerical models can be used to demonstrate the process, evaluate the timing, and resolve the efficacy for commercial-scale carbon disposal in these settings. To illustrate the impact of fracture and rock properties on the success of carbon mineralization, an analysis is conducted with a radial transport model which can simulate CO₂ injection in discrete fractures accounting for matrix diffusion. The analysis is based on a range of bulk permeabilities (1×10^-17 m² to 1×10^-12 m²) and matrix porosities (1-4%) obtained from site investigations, and a range of fracture apertures, fracture spacings, and injection pressures.  The cubic law is used to convert permeability to fracture aperture under given fracture spacings. Notwithstanding the geochemical reactions that will be involved, just the process of matrix diffusion illustrates that the matrix pore space can be largely filled with dissolved CO₂ given the presence of sufficient fractures and enough time. Considering that the CO₂ is also stripped via diffusion from the fractures over time/distance during injection and there is no significant form of CO₂ egress from the matrix, there is no need for overlying caprock protection. As has been previously recognised, the largest potential limitation is the limited permeability of these rock types which constrains the volume of fluid that can be injected under acceptable pressure gradients. This is a very similar problem to that experienced in the geothermal industry, whereby hydraulic stimulation (without proppant) of healed fractures is successfully employed.