Heterogeneities at the pore scale influence flow and reactive transport processes in porous media such as those relevant in natural (clay) and engineered (cement-based materials) barriers in a radioactive waste disposal system. Flow properties of porous media are controlled by the geometry of their pore space (two-phase system, i.e. solid or pore). Reactive transport properties additionally depend on the spatial distribution of different minerals within the solid matrix (multiphase system) and their specific characteristics. Therefore, 3D reconstruction of the pore and solid structure is crucial as input for pore-scale modelling of flow and reactive transport processes and to upscaling towards effective properties allowing for multiscale modelling over a large continuum of scales (i.e. ranging from the micrometre scale to the scale of a repository).
Reconstruction of porous structures requires accurate imaging of the pore space and mineral phases at different scales. Recent advances in 2D imaging techniques such as broad-ion beam scanning electron microscopy (BIB-SEM), focused ion-beam serial cross-sectioning (FIB-SEM) in combination with 3D multiple x-ray computed tomography (CT) scans with different resolutions (from nano- to medical CT) have made this possible. The use of 2D images for computer-based 3D reconstruction of porous media together with the integration of 2D and 3D images with different scales and resolutions, are therefore the subject of intensive research, aiming at reconstructions at a scale representative to determine effective properties. A first step in an upscaling workflow is to integrate 2D (training image for small-scale heterogeneity) and 3D (training images for larger scale heterogeneity) images and is recently introduced by Claes (2015) in the µCT to medical CT range for a two-phase system (solid – pore). Gerke et al. (2015) present a workflow for merging µCT and FIB-SEM/SEM images, for a four-phase system, assuming independency of the structures at different scales. These workflows are scale-independent and can in theory address any kind of scale range. Therefore in the proposed research, the scale of information will be enhanced spatially (e.g. by going from FIB-SEM to medical CT imaging), with the inclusion of different phases in the solid matrix.
In addition, the choice of an optimal algorithm for performing porous media modelling depends strongly on the characteristics of the porous media. Although multiple point statistics (MPS) or different kinds of two-point correlation functions were used in the workflows mentioned above, other complex porous media as clay and concrete may require other techniques as solid texture synthesis approaches, plurigaussian simulations, process-based simulations, … Here also, extension to multiphase solid matrices is a huge challenge. Hence, accurate 3D multiscale and multiphase porous media reconstruction based on 2D and/or 3D data involves many scientific and computational challenges.
Claes, S. (2015). Pore classification system and upscaling strategy in travertine reservoir rocks. PhD thesis, KU Leuven.
Gerke, K., Karsanina, M., & Mallants, D. (2015). Universal Stochastic Multiscale Image Fusion: An Example Application for Shale Rock. Scientific Reports, Accepted.