Fracture Topology in Mafic Formations: Implications for Geological Carbon Storage

Carbon mineralization pilot projects have demonstrated effective CO₂ sequestration, yet uncertainties persist for large-scale deployment, particularly regarding the role of fracture networks and evolving fracture-matrix interactions. In this study, we integrate field data, numerical simulations and gravimetric-volumetric analyses to investigate the fracture characteristics of basalt formations and their implications for CO₂ storage. Fracture aperture is shown to be proportional to block size, governed by thermal contraction during lava cooling, with the aperture-to-block size ratio β ranging from ∼0.7·10^-2 to 6·10^-2 depending on mineralogy. Network modeling reveals that initial aperture variability is amplified by dissolution near the injection zone (high Peclet and low Damköhler numbers); however, the hexagonal fracture topology enhances mixing and delays hydrochemical feedback and flow localization. Chemo-gravimetric analysis indicates that mineralization can sequester 0.2-0.3 g CO₂ per gram of rock, significantly exceeding pore-space storage via supercritical or dissolved CO₂. However, volume-positive mineralization eventually reduces fracture transmissivity. Aperture shut-off depends on the aperture-to-block size ratio β and the mineralization expansion factor ε. The reacted volume fraction at shutoff can range from 7 to 24%. Mineralogy emerges as a primary control on fracture topology, chemical reactivity and storage capacity. Results underscore the need for careful reassessment of CO₂ storage capacity in mafic rocks.