Transport in tight material enlightened by process tomography

The analysis of fluid transport through tight barrier materials poses two major challenges: (i) Long equilibration periods require long minimum experiment durations, and, (ii) the fluid transport frequently results in complex pattern formation. Too short measuring times may feign too small transport rates; intact homogeneous samples are often missing problematic features, e.g. fractures. Both issues are detected and analyzed by using process tomography techniques, hence providing an improved understanding of transport processes in complex materials.
We thus continuously develop and apply the positron emission tomography (PET) method for geomaterials (Kulenkampff et al., 2016). It is able to trace very low concentrations of β+-emitting radionuclides during their passage through drill cores of barrier material with reasonable resolution (1 mm) and over variable periods (hours to years). The method yields time-resolved quantitative tomographic images of the tracer concentration (e.g. https://doi.org/10.5281/zenodo.166509), in contrast to input-output experiments like common permeability measurements, diffusion cells, or break-through curves.
Our current research includes the analysis of diffusive transport in heterogeneous shales (sandy facies of the Opalinuston) (BMBF and HGF iCross project), the reactive flow in fracture-filling materials of crystalline rocks (Eurad FUTURE project), and transport in engineered barriers and the contact zone (Euratom Cebama, Eurad Magic, as well as MgO and Stroefun BMWi projects). The efforts combine flow field tomography, structural imaging and reactive transport modelling for improving process understanding and to provide a bridge from the molecular to the macroscopic scale.
The benefits include:

• Insight into temporal stability and spatial heterogeneity of the observed transport process
• Parameterization of local velocity distribution and effective volume, comparability with pore-scale model simulations
• Ability to quantify multiple internal transport rates without necessity to register the delayed output signal
• Transparent and palpable visualization of processes hidden in the opaque material

The method requires specific constraints of the experimental setup (size, fluid pressure, temperature). Nevertheless, it provides unique insight into reactive transport processes observed in potential materials for nuclear waste management.
References
Kulenkampff, J.; Gründig, M.; Zakhnini, A.; Lippmann-Pipke, J., Solid Earth 7, 1217-1231, DOI: 10.5194/se-7-1217-2016
Kulenkampff, J.: Geophysical Research Abstracts, Vol. 20, EGU2018-8813-1, 2018.