Benchmarking of reactive transport visualisation (PET) by numerical modelling with COMSOL Multiphyiscs and PhreeqC

For about a decade we apply positron emission tomography (PET) as molecular imaging modality for non-destructive process visualisation in geological material on laboratory scale [1]. Sequential PETimages directly yield the spatiotemporal concentration distribution of a PET radiotracer (3D+t) in the course of the process, with a resolution of ~1 mm and tracer sensitivity to some ten thousand atoms per 1 mm³ voxel. Our PET data sets are typically complemented by μCT-images. We demonstrated its advantages for elucidating heterogeneous processes on different time scales, conservative and reactive flow experiments, including the transport of nano-particles. We considered our 3D+t data sets as highly valuable for benchmarking of reactive transport modelling results. In the recent past we have consequently gained some experiences in aligning differently complex data sets (up to 3D, (non-)reactive) with available modelling tools. As suitable for aligning our data with well-established transport codes we identified Comsol Multiphysics for the transport process parameter estimation, PhreeqC for the geochemical reaction simulation, as well as couplings of both for reactive transport processes in 2D at the mm to cm scale. Benchmarks in 3D+t have their strength in emphasising on the role of heterogeneity on macroscopic reaction rates. Here, retroactions from geochemical reactions on the effective transport pathways in geological media (dissolution, precipitation, filtration, etc.) cause spatial and temporal variations of their transport properties - processes verifiable by our non-destructive PET method.
We aim at developing into this highest complexity level of reactive transport benchmarking; the backcoupling of reaction on structural transport parameter values (porosity, permeability).
REFERENCES
[1] Kulenkampff, J. et al., 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, http://www.isipt.org/worldcongress/7/902.html (free access after registration)