Wormholing in Anisotropic Media: Pore‐Scale Effect on Large‐Scale Patterns

• Published in: Geophysical research letters 2021-06, Vol.48 (11), p.n/a
• Main Authors: Roded, R., Szymczak, P., Holtzman, R.
• Format: Article
• Language: English
• Subjects: Optimum injection rate; Permeability evolution; Pore network model; Unstable growth processes; Wormhole competition
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL093659

The formation of dissolution conduits by focused reactive flow (i.e., wormholing) in anisotropic media is studied using a pore network model. Simulations reveal a significant effect of anisotropy on wormholing dynamics and medium permeability evolution. Particularly, anisotropy controls wormhole competition and their characteristic spacing. It also affects the flow through the individual wormholes and their shapes, and consequently, shifts the optimum injection rate at which breakthrough is achieved at a minimal expense of reactant. For anisotropic media with low transverse pore conductivities, wormhole distribution ceases to be scale‐invariant and pronounced side‐branches develop. Wormholing is further compared to viscous fingering in an anisotropic network, and other unstable growth processes of similar underlying dynamics. Despite several similarities, few important differences are identified. Our findings contribute to the understanding of wormholing in geological media and demonstrate how pore‐scale features can fundamentally affect the emergence of large‐scale morphologies. Plain Language Summary The flow of corrosive fluids in an aquifer (e.g., acidic water in limestone) can become focused in conductive pathways leading to the formation of pronounced dissolution conduits—wormholes. Wormholes can form across a large range of scales, from microns to the extended systems of karst conduits. Wormholing patterns evolve by competitive dynamics: Longer wormholes drain more flow and hence grow faster, increasing their conductivity, in turn focusing even more flow. In the meantime, shorter wormholes become devoid of reactant and stop growing. This results in a hierarchical distribution of wormhole lengths, with many small and only a few long ones. Here, using a numerical model, we study wormholing in anisotropic media characterized by different permeabilities along different directions—a common feature of geological media. We find that anisotropy markedly affects wormhole dynamics and the evolution of overall medium permeability. Particularly, anisotropy affects wormhole competition and thus their number, shapes, and branching. Wormholing is further compared to other pattern‐forming processes in nature, and similarities and differences are analyzed. These findings contribute to the understanding of wormholing, with implications to subsurface flow‐related processes such as karst and contaminant migration. The results demonstrate how micro‐scale features contro