Ph.D. projects

Heterogeneity of the diffusive flux in the sandy facies of the Opalinus clay

Eu atoms (green) at an etch pit surface on the
base of muscovite (kMC simulation)

Till Bollermann

PD Dr. Cornelius Fischer (HZDR)

Reactive Transport

02/2018–01/2021

Clay rock formations are considered as host rocks for underground radioactive waste repositories. Reliable predictions of diffusive transport heterogeneity are critical for assessing the sealing capacity of argillaceous rocks. The predictive power of numerical approaches to flow field analysis and radionuclide migration depends on the quality of the underlying pore network geometry. Both sedimentary and diagenetic complexity are controlling factors.

In this study, we demonstrate a cross-scale approach to reconstruct the pore network geometries of the s andy facies of the Opalinus Clay rock. We identified diagenetic and sedimentary subfacies components based on the concentration of diagenetic carbonates and sulfides and grain size variability, and quantified their pore size distributions and pore network geometries. A viable approach for use in transport modeling is to combine μ-CT data segmentation followed by filling the resulting volumes with representative pore network geometries based on FIB-SEM data. The resulting generalized pore network geometries are applied in digital rock models to calculate effective diffusivities, using a combined upscaling workflow for transport simulations from nanometer to micrometer scales.

Positron emission tomography (PET) diffusion experiments validated the transport simulation results. We introduced a statistical treatment of the PET and μ-CT tomographic datasets based on the spatial variability of both PET tracer concentrations and rock density. The analyzed effective diffusivities confirmed the numerical results.

This study illustrates three important steps in migration analysis: (i) a workflow of general applicability for cross-scale identification of pore network data in argillaceous rocks, (ii) application of the pore network data for the numerical analysis of diffusive transport, and (iii) validation of numerical results via combined PET - μ-CT diffusion experiments. Although the conceptual approach is not feasible for large numbers of samples, it opens up a strong potential for generalization: the validated results of effective diffusivities can now be easily used in a variety of segmented geometries. This allows to efficiently test upscaling concepts for the continuum scale on this basis.

For details see: https://doi.org/10.1016/j.chemgeo.2022.12099⁽¹⁾

The variability in crystal surface reactivity under identical chemical conditions is responsible for a significant intrinsic reaction rate variation. Such variability is quantified using rate maps of reacting surfaces and rate spectra as a statistical concept. In this study, we exemplify the existence and the temporal variability of multiple dissolution rate contributors that combine to an overall rate of dissolving polycrystalline calcite. At least three different high rate contributors control the overall dissolution rate and show no steady state behavior over a reaction period of 10 hours. We conclude that the data about spatial and temporal rate evolution combined with information about crystal size, orientation, and grain boundaries provide constraints for the prediction of porosity pattern in polycrystalline materials. We discuss the density and distribution of surface kink sites as the critical parameter controlling the observed rate variability. Rate spectra measurements of crystal corner, edge, and face surface portions substantiate this conclusion.

For details see: https://ajsonline.org/article/60933.pdf⁽²⁾

URL of this article
https://www.hzdr.de/db/Cms?pOid=60489

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