Direct observation of preferential transport pathways in salt rocks by means of GeoPET

Migration of radionuclides out of a salt-based repository requires transport pathways, where substances propagate by molecular diffusion or even pressure-driven advection. Diffusion coefficients and permeability, being very low in most intact saline rocks, are likely to increase considerably under geomechanical stress, inevitably occurring for instance in the excavation damage zone. Observation and evaluation of transport processes in such heterogeneously structured materials are most demanding, because they likely occur in either extremely localized, macroscopic but rare fractures or in dispersed, mostly indiscernible but still connected microscopic voids.
Applying salt solutions labelled with radionuclides we are able to observe such processes directly and - presumably for the first time with due resolution and sensitivity – with our GeoPET method. The same processes are studied with Lattice-Boltzmann flow simulations on µCT-based images of the same samples. This strategy contrasts the common use of computer simulations based on a priori material parameters, and allows for the matching of flow simulation results in “real structures” with real time flow monitoring in the same samples. Here we report on results obtained from a number of drill cores from the ancient Stassfurt salt mine and its overburden, including aged backfill material, which has been analysed in the framework of our joint research project, funded by the Federal Ministry of Education and Research (BMBF) (Wolf et al., 2010).