Visualizing and evaluating wormholes formation dynamics under flow competition in an intermediate-scale dissolution experiment.

Wormholes are highly conductive channels that develop in high solubility rocks. They are especially important for environmental and industrial sustainability in saline karst aquifers (e.g. Salar de Uyuni, Salar de Atacama). Wormholes dynamics (i.e., the space and time evolution of these preferential flow paths) depends on the hydrodynamic and geochemical conditions during formation, as well as on wormholes competition for flow. Despite the importance of wormholes interaction for their development, experimental efforts have focused on the evolution of a single flow-path. Direct observation and quantification of wormholes dynamics is still lacking. We propose an experimental set-up to visualize and characterize the dynamics of multiple wormholes, which may help to understand the changes in flow and transport behaviour of aquifers. We performed a dissolution experiment in a 2D synthetic evaporitic aquifer, and simultaneous fluorescent tracer tests before and during wormhole growth. We visualized the growth by sequential photographs, with the fluorescent tracer highlighting the evolving structures. We quantified wormholes dynamics by measuring pressure and mass changes of the aquifer, and by image analysis. On the one hand, results show that wormholes tend to form along areas where flux was fastest prior to dissolution. They also show clear evidence of competition for water between wormholes and represent the first quantitative evidence of the amplifying factor that drives the self-organization in wormhole growth. On the other hand, we found that the competition is slower than predicted by current analytical and numerical theories, but consistent with other laboratory results. We conjecture that the discrepancy between theory and experiments can be attributed to the combined effect of non-linear kinetics and particle dragging during dissolution. We then compared experimental tracer breakthrough curves before and during the formation of preferential flow paths. They reflect wormhole growth by an accentuated non-Fickian behaviour, with reduced first arrival and increased tailing.