High-resolution Positron-Emission-Tomography for ultrasensitive spatio-temporal monitoring of tracer transport in porous media

Positron-Emission-Tomography (PET) has favourable characteristics for monitoring the propagation of tracers in opaque media on the drill core scale. PET directly yields quantitative tracer concentrations, without retroaction on the process, with ultimate sensitivity (less than 109 atoms per voxel) and selectivity, with negligible matrix effect, and with expedient spatial resolution. It only requires radiolabelling of the observed substance with a positron-emitting isotope and therefore allows investigating processes in fluids without physical or compositional contrasts.
PET is a widely applied nuclear medical technique for 3D functional imaging. Since 1988 [1], some exemplary applications for the visualization of flow processes in various rock formations and other non-medical applications were conducted with clinical PET scanners with low spatial resolution and with short living radionuclides (e.g. ¹⁸F, T_1/2=109.77 min).
In contrast, we take advantage of a high-resolution PET-scanner, originally dedicated for biomedical research on small animals, with a resolution at the physical limit of 1 mm [2]. Also, we are able to conduct long-time experiments in our laboratory, applying radionuclides with longer lifetimes (e.g. ²²Na, T_1/2=2.602 a).
Our applications cover conservative and reactive flow of solutes and particles in soils and rocks, as well as diffusion processes in clays [3]. We strive to characterize heterogeneous transport effects on the basis of our visualisations of the propagation of the tracer. Frequently, we observe the evolution of strongly localized pathways, which depict the reduced effective volume, coming along with a reduced efficiency of the internal surface area for sorption processes. Our data can be used also for experimental calibration and verification of computer simulations of transport processes.
REFERENCES
[1] van den Bergen, E.A., Jonkers, G., Strijckmans, K., Goethals, P., 1989. Industrial Applications of Positron Emission Computed Tomography. Nuclear Geophysics 3, pp. 407-418.
[2] Zakhnini, A., Kulenkampff, J., Sauerzapf, S., Pietrzyk, U. and Lippmann-Pipke, J., 2013. Monte Carlo simulations of GeoPET experiments: 3D images of tracer distributions (¹⁸F, ¹²⁴I and ⁵⁸Co) in Opalinus Clay, anhydrite and quartz. Computers & Geosciences, 57: 183-196.
[3] Kulenkampff, J.; Gründig, M.; Korn, N.; Zakhnini, A.; Barth, T.; Lippmann-Pipke, J., 2013. Application of high-resolution positron-emission-tomography for quantitative spatiotemporal process monitoring in dense material. 7. World Congress on Industrial Process Tomography, Krakow, Poland, free available on http://www.isipt.org/world-congress/7/902.html