Application of neutron radiography to image flow phenomena in porous media

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Abstract
Neutron transmission imaging is a technique ideally suited for imaging various flow phenomena in consolidated porous media because the porous matrix is virtually transparent to thermal neutrons. Hydrogen‐containing fluids, however, provide a contrast in the image due to their large thermal neutron cross‐sections. This technique has uncovered new phenomena in the study of dissolution and precipitation in porous media. Neutron images of acid‐etched porous patterns show that the degree of branching, and the tortuosity of the etched patterns depend on competing effects of acid flow rate, dissolution rate and precipitation rate of the reaction products. The fine structure of etched pathways show that they can propagate against the direction of flow, a phenomenon not previously observed. In a second study, realtime neutron transmission imaging was used to visualize miscible tracer dispersion. It was shown how quantitative information such as the in‐situ spatial distributions of the tracer concentrations can be obtained with this technique.