Density contrast effects on tracer dispersion in variable aperture fractures

We investigate the development of preferential flow paths and anomalous dispersion resulting from weak density contrasts in the course of tracer experiments in a tortuous natural fracture. The processes are first documented by the non-invasive measurement of the fracture aperture and of the time-resolved distribution of the tracer using Positron Emission Projection Imaging. Then, numerical simulations of the three-dimensional tracer transport in the fracture are performed to explore the parameters that control the development and the persistence of the tracer localization, as a function of the density contrast between the tracer and the resident solution. Results reveal that density contrasts representative of what could be expected in borehole and laboratory tracer tests can induce irreversible localization along preferential channels. As density contrast increases, the correlation between velocity and aperture distributions vanishes whereas (i) velocity field increasingly correlates with the fracture median plan elevation and (ii) the longitudinal dispersion coefficient increases. The anomalous velocity distribution may persist well after the injection stops due to the occurrence of tracer trapped zone.