Fluorescence-visualization experimental system for flow-dissolution dynamics in salt rock fractures and the impact of roughness

Flow-dissolution in rock fractures is prevalent in engineering practices such as salt cavern hydrogen storage, long-term anti-seepage safety of dam foundations, and CO2 geological sequestration. Dissolution patterns and rates directly govern the permeability evolution of fractures. To elucidate the interplay between flow and dissolution in real three-dimensional (3D) fractures, this study develops a flow-dissolution visualization method based on fluorescence reflection. Specifically, a fluorescent tracer is introduced into the reactive fluid, and resulting fluorescence within the fracture is excited by an external excitation light source and captured through single-sided optical path. The relationship between fluorescence intensity and local fracture aperture is calibrated, enabling high-precision characterization of the 3D salt rock fracture aperture field and dissolution rate. On this basis, rough and smooth salt rock fracture models with identical average apertures are fabricated, and flow-dissolution visualization experiments are conducted under varying injection flow rates and solution concentrations. A phase diagram encompassing three dissolution patterns including compact dissolution, channel dissolution and uniform dissolution in 3D salt rock fractures is established. The results demonstrate that with increasing injection flow rate, the dissolution morphology gradually transitions from compact dissolution dominated by concentrated dissolution at the inlet to channel dissolution characterized by the development of preferential channels, and ultimately to uniform dissolution with uniformly increased fracture apertures. Roughness does not alter the formation and transition of dissolution patterns; however, it enhances the spatial heterogeneity of the flow field and mass transfer through geometric undulations, exerting a dual regulatory effect on dissolution rates. In the channel dissolution pattern, roughness provides initial perturbations for unstable dissolution, accelerating the penetration of preferential channels, and thus the dissolution rate is higher than that in smooth fractures. In contrast, in the compact and uniform dissolution patterns, low-velocity stagnant zones exist in rough fractures, which reduces the effective utilization of reactants, leading to a lower dissolution rate compared to smooth fractures. The fluorescence reflection visualization method developed in this study provides a core-scale experimental approach for deciphering the quantitative relationship between 3D fracture roughness, dissolution patterns, and dissolution rates, and offers important experimental basis and theoretical support for the long-term anti-seepage safety assessment of salt rock repositories and karst dam foundations.