Reactive Transport Modeling of Mineral Precipitation and Carbon Trapping in Discrete Fracture Networks.

In this study we use numerical experiments to analyze reactive flow and transport behavior in discrete intersecting fracture networks, focusing on (a) how reaction‐induced changes in physical and chemical properties affect flow connectivity and (b) how fracture networks developed in the Earth's critical zone contribute to carbon sequestration via mineral weathering reactions. In the first part of the study, we used two‐dimensional reactive flow and transport simulations to analyze the impacts of mixing in a natural discrete fracture network. We concluded that reaction‐induced changes can substantially alter the flow connectivity, especially at fracture intersections. The second set of simulations considered the problem of natural weathering of fractured mafic and ultramafic rocks in the partially saturated Earth's critical zone as a function of infiltration rates, fracture permeability, and partially saturated flow parameters. As a model system, we considered an incongruent reaction network with dissolution of forsterite and precipitation of magnesite. The behavior is complex in terms of the rate‐controlling processes because of the multicomponent nature of the system as shown by the grid Peclet number: the CO2 behavior is gas diffusion‐controlled in the partially saturated zone, while the rate of water flow via the Damkӧhler number controls Mg2+ transport through the fracture network. The amounts of carbon that can be trapped are modest, but the naturally fractured domain considered here provides a useful "base case" against which various engineered solutions can be compared. Plain Language Summary: In this study we use numerical experiments to analyze reactive flow and transport behavior in discrete intersecting fracture networks, focusing on (a) how the interplay between intersections of rough fractures and advection, diffusion, and chemical reaction affects flow connectivity via changes in geometry and physical and chemical properties and (b) how fracture networks developed in the Earth's critical zone contribute to carbon sequestration via mineral weathering reactions. In the first part of the study, we used reactive flow and transport simulations to analyze the impacts of mixing in a natural discrete fracture network. We concluded that reaction‐induced changes can substantially alter the flow connectivity, especially at fracture intersections. The second set of simulations considered the problem of natural weathering of fractured mafic and ultramafic rocks in the partially saturated Earth's critical zone as a function of infiltration rates, fracture permeability, and partially saturated flow parameters. The behavior is complex in terms of the rate‐controlling processes because of the multicomponent nature of the system. The amounts of carbon that can be trapped are modest, but the naturally fractured domain considered here provides a useful "base case" for the future design of efficient strategies for carbon trapping and mitigation. Key Points: Modeling of the interplay between intersecting rough fracture networks and advection, gas diffusion, and mineral dissolution/precipitationMineral precipitation due to mixing at fracture intersections can alter the flow connectivity of a fracture networkModeling demonstrates CO2 trapping in naturally fractured rocks within the critical zone, a base case for engineered technologies [ABSTRACT FROM AUTHOR]