Reactive Flow and Homogenization in Anisotropic Media

• Published in: Water resources research 2020-12, Vol.56 (12), p.n/a
• Main Authors: Roded, R., Aharonov, E., Holtzman, R., Szymczak, P.
• Format: Article
• Language: English
• Subjects: Acidic water; Analogs; Anisotropic media; Anisotropy; Aquifers; Carbon dioxide; Computational fluid dynamics; Corrosion; Diagenesis; Diffusion rate; Dissolution; Dissolving; Evolution; Flow velocity; Fluid flow; Fluids; Geological processes; Geology; Groundwater; Groundwater management; Homogenization; Karst; Limestone; Mathematical models; Network model; Numerical models; Permeability; Permeability evolution; Porous media; Reactive transport; Rock; Rocks; Subsurface flow; Surface reactions; Tortuosity; Transport processes; Transport properties; Uniform dissolution; Void space
• ISSN: 0043-1397; 1944-7973
• DOI: 10.1029/2020WR027518

The evolution of heterogeneous and anisotropic media in the uniform dissolution regime (low Damköhler number) is studied here using a numerical network model. The uniform dissolution extensively homogenizes the medium and therefore the flow field. The homogenization is further enhanced when the surface reaction is transport controlled—when slow diffusion of dissolved ions away from the mineral surface leads to the reduction of the global dissolution rate. Under those conditions, diffusive transport is more effective in narrow channels, which selectively enlarge, leading to an initial steep rise of the permeability. However, as dissolution proceeds, the void space widens and the overall dissolution rate drops, and permeability enhancement slows down. Finally, we review the relevance of these results to various processes in geological systems ranging from diagenesis and karst evolution, to carbon geosequestration. These findings provide fundamental insights into reactive transport processes in fractured and porous media and evolution of permeability, tortuosity, anisotropy, and bulk reaction rates in geological systems. Plain Language Summary When corrosive fluids flow in an aquifer, they dissolve the rocks and change the void‐space structure (e.g., acidic water invading limestone). In many geological processes and applications, the flow velocity is high compared to the chemical dissolution rate, and the corrosive fluid penetrates large distances before it loses its reactivity, resulting in relatively uniform dissolution everywhere. Here, using analog numerical model, we study how the void‐space structure of porous rocks and aquifers change during uniform dissolution. In particular, we study how different initial structures and reaction conditions change the flow field and transport properties. Finally, we review the relevance of these results to various geological systems. The study improves our understanding of rock and aquifer evolution, with implications to groundwater management and other subsurface flow‐related processes, such as CO2 geosequestration. Key Points Uniform dissolution homogenizes heterogeneous and anisotropic medium and its flow field Homogenization is enhanced when the reaction is transport controlled at the pore scale, due to selective dissolution of constrictions Under transport‐controlled reaction, permeability increase is initially steep but later slows down due to a decrease in reaction rate