Controls of\nthe Nucleation Rate and Advection Rate\non Barite Precipitation in Fractured Porous Media

Mineral precipitation is ubiquitous in natural and engineered\nenvironments,\nsuch as carbon mineralization, contaminant remediation, and oil recovery\nin unconventional reservoirs. The precipitation process continuously\nalters the medium permeability, thereby influencing fluid transport\nand subsequent reaction kinetics. The diversity of preferential precipitation\nzones controls flow and transport efficiency as well as the capacity\nof mineral sequestration and immobilization. Taking barite precipitation\nas an example, previous studies have examined this process in porous\nand/or fractured media, but pore-scale mechanisms under varying flowing\nand geochemical conditions remain unexplored. In this study, we conducted\nreal-rock microfluidic experiments to investigate the precipitation\ndynamics within a fractured porous system. Direct observations of\nthe evolution of the porous structure and flow channel and quantifications\nof barite precipitation dynamics using X-ray diffraction (XRD) and\nscanning electron microscopy with energy-dispersive X-ray spectroscopy\n(SEM-EDS), revealed two distinct precipitation regimes: precipitation\non the fracture surface (regime I) and precipitation in the alteration\nzone (regime II). Through theoretical analysis of the rate of advection\nand nucleation, we defined a dimensionless number <i>Da</i> above which regime I occurs and regime II prevails otherwise. At\nthe large <i>Da</i> number, when the precipitation rate\nis large compared with the flow rate, precipitation on the fracture\nsurface is favored. As the precipitation regimes are expected to impact\ndifferently the permeability of the fractured porous media, the mass\ntransfer across matrix and fractures, and the spatial distributions\nof coprecipitated contaminants, our work sheds light on accurately\nmodeling reactive transport in fractured porous media across diverse\napplications.