Influence of microstructural heterogeneity on water diffusion in porous clay media: A case study of illite

Understanding the diffusion of water and solutes in clay-rich materials is critical for several applications, notably radioactive waste disposal. Most of the available experimental data on tracer diffusion in clay porous media are interpreted using only bulk porosity and, to a lesser extent, on particle orientation, implicitly assuming their homogeneity in the sample. Nevertheless, these parameters can be heterogeneously distributed in both natural and laboratory-made clay samples, and a quantitative assessment of their impact on water diffusion is still lacking. To address this gap, synchrotron two-dimensional micro-X-ray diffraction was used to map the microstructural heterogeneity in porosity and clay particle orientation in two laboratory-made illite porous media obtained by compaction in dry conditions or centrifugation in wet conditions. The resulting maps were coupled with the CrunchODiTi reactive transport code to derive the macroscopic water effective diffusion coefficient by accounting for porosity and the orientation tensor of clay particles at each map pixel. The results indicate that heterogeneity in clay particle orientation has a negligible effect on the water effective diffusion coefficients, such that the use of an average value is sufficient. Conversely, pronounced variations in water diffusion values among different porosity heterogeneity patterns (i.e., homogeneous, bedding, and random) demonstrate that the porosity microstructure can either amplify or attenuate diffusion anisotropy associated with the preferred orientation of clay particles. The methodology developed in this work, which integrates microstructural heterogeneity mapping with diffusion calculations, provides a new framework for incorporating the textural properties of clay-rich rocks into the prediction of water and solute transport.