Spatiotemporal observation of transport in fractured rocks

We apply positron emission tomography (PET) with a high-resolution "small-animal" PET-scanner (ClearPET by Raytest, Straubenhardt) for process observation in rocks. Without affecting its physico-chemical properties, the fluid is labelled with the PET-tracer, a positron-emitting isotope. The annihilation radiation from individual decaying tracer atoms is detected with high sensitivity, and tomographic reconstruction of the recorded events yields a quantitative 3D-image of the tracer concentration. Sequential tomograms during tracer injection are used for the spatiotemporal observation of transport.
The raw data have to be corrected, prevalently with respect to background radiation (randoms) and Compton scattering, which is more significant than in common biomedical applications. Although these effects can be considered exactly in principle, we had to develop and apply simplified correction methods for performance reasons. Deficiencies of these correction algorithms generate some artefacts, that cause a lower limit of the tracer concentration in the order of 1 kBq/µl or about 107 atoms/µl, outranging other methods (e.g. nmr or resistivity tomography) by many orders.
A number of injection experiments in different rocks have been conducted with PET-process-tomography. New 3D-visualizations of the process-tomograms in fractured rocks showed strongly localized and complex flow paths and some unexpected deviations from the fractures that are deducible from µCT-images.
At least, the results demonstrate the large discrepancy between the µCT-derived volume and specific surface area and the hydraulic effective parameters, which also can be analyzed quantitatively with this method. Possibly, these discrepancies and the complexity of the process show the limits of parameter determination methods with model simulations based on structural pore-space models - as long as the simulations are not verified by experimental data.