Mineralization and permeability evolution of naturally-fractured basalt: Insights from CO2-water-rock reactive-percolation experiments

In-situ carbon mineralization in basalts facilitates CO 2 conversion into stable carbonates for permanent geological storage. Fracture networks are primary spaces for fluid flow and chemical reactions, and their permeability evolution under hydro-chemical processes is essential for assessing the long-term injectivity and carbon storage potential of basalt reservoirs. This study conducted reactive-percolation experiments on a naturally-fractured basalt sample to investigate the mineralization mechanisms and the evolution of pore structure and permeability under varying flow rates. Mineralization reactions caused less than 0.2% porosity reduction, but weakened permeability by one order of magnitude, underscoring that permeability was more susceptible to the heterogenous geochemical alterations. At the higher flow rate (0.1 mL/min), rapid dissolution of fine particles on the fracture surface led to initially high ion concentrations, but subsequent slow release of Si restricted the dissolution rate and resulted in the formation of Si-rich layers. Although net porosity increased, particle migration and redistribution at the downstream of the fracture intersection led to a reduction in permeability. Furthermore, dissolution of asperities along the fracture interfaces potentially narrowed the fracture aperture, thereby exacerbating the decline in permeability. In contrast, non-uniform mineral precipitation at the lower flow rate (0.01 mL/min) caused a more significant reduction in permeability, predominantly as Ca-carbonate at the outlet of the longitudinal fracture and within the transverse fracture. These results implied that while open-end fractures acted as primary flow pathways, mineralization mainly occurred in dead-end fracture spaces. Importantly, particle accumulation downstream of fracture intersections emerged as a critical factor governing permeability evolution.