Pore-scale salt precipitation and transport in fractures during carbon dioxide storage: roles of fracture geometry, brine chemistry, and phase state.

Ensuring caprock integrity is essential for maintaining long-term containment security in geological carbon dioxide (CO2) storage. Fracture networks of caprocks act as leakage pathways for stored CO2. Interactions between brine and CO2 trigger salt precipitation within fractures, potentially sealing fractures to restrict further leakage. The mechanisms governing salt precipitation in structurally diverse fractures remain poorly understood at the pore-scale. We employed microfluidics to examine the effects of the fracture geometry, CO2 phase, and brine composition on salt precipitation, aggregation, and migration. The fracture geometry influences salt dynamics, with salt coverage 1.6- and 3.3-fold that of the unfractured model in discrete and interconnected models, respectively. The brine composition alters salt aggregation behavior: CaCl2 brine yields larger, more stable precipitated salt, resulting in up to ∼51% greater salt coverage than NaCl. The CO2 phase exerts dominant control-supercritical carbon dioxide (scCO2) displacement enhances NaCl precipitation by ∼683% compared with gas-phase CO2, due to improved brine film retention and evaporation. The brine film reaccumulation mechanism under scCO2 displacement further suppresses salt migration, sustaining salt aggregation in interconnected fractures. Our findings offer fundamental insights into salt sealing and migration in multiscale porous media, with vital influence on leakage risk assessment and injectivity control in geological CO2 storage.