Local Mass-Transfer Boundary Conditions Control Basalt Carbonation Pathways and Pore-Micromechanical Restructuring during CO2 storage

Basalt can permanently store carbon dioxide by converting it into stable carbonate minerals, but these reactions may vary greatly between different pores and rock surfaces. We reacted a basalt sample with carbon dioxide-rich brine under high-temperature and high-pressure conditions and compared two surfaces exposed to different local fluid environments. The surface in contact with a larger volume of fluid developed scattered carbonate deposits. In contrast, the confined surface, separated from the reactor base by a thin water layer, promoted denser carbonate growth directly along mineral interfaces. These contrasting reaction conditions also changed the pore structure and local mechanical response of the basalt. Small pores were more likely to be filled by newly formed minerals, whereas some larger pores and fracture-like voids became enlarged through mineral dissolution. Our results show that the amount of nearby fluid and the distance over which dissolved elements can move strongly influence where carbonates form and how basalt is restructured. This helps explain why carbon mineralization can be highly uneven within basalt reservoirs, even under the same overall temperature, pressure, and fluid composition.