Influence of texture on anion-accessible porosity fraction explored by µCT, SEM & N2 adsorption data

The anion-accessible porosity fraction (f_a) is an important parameter controlling solute transport in claystones. A safe disposal of nuclear waste in such rocks relies on a comprehensive understanding of transport in clays. In stratigraphic sequences, established Cl or Br profiles provide insights into the paleo-hydrogeological evolution. Anion concentrations in the accessible porewater can be calculated from measured bulk porewater concentrations and the anion-accessible porosity fraction f_a. Since experimental data for f_a are scarce, and data extrapolation within a heterogeneous stratigraphy is challenging due to the f_a dependency on multiple parameters (e.g., pore/grain shapes and size distributions), detailed understanding of texture and its influence on macroscopic transport parameters is paramount.

In this study, imaging techniques (µCT and SEM) and other methods (e.g., N₂ adsorption) were combined to characterise texture and the pore network of rock samples from Opalinus Clay and confining units. The techniques unravel pore characteristics at different scales: N₂ adsorption from nanometers to a micrometer, SEM larger than 50 nm, and µCT larger than a few µm. Samples with different mineralogical compositions, lithologies, and experimentally determined f_a for Cl (f_Cl) were analysed.

Two sand/siltstone samples with different f_Cl but similar clay content show identical ratios of grains to porous clay regions, but different pore sizes in high-resolution SEM images. This can qualitatively explain the different f_Cl for these samples. However, SEM cannot resolve small pores (<50nm), and a structural model is additionally required to derive quantitative results.

The gained textural insights make clear that f_Cl does not necessarily correlate with the clay fraction. Moreover, extended correlations of f_Cl with quantified textural information allow a better prediction of f_Cl for formations where this parameter was not measured. The outcome of this study encourages further investigations for verifications such as transmission electron microscopy (TEM) imagery to explore the nanometric pore space within and around clay minerals.

 

[1] C. Zwahlen, T. Gimmi, A. Jenni, M. Kiczka, M. Mazurek, L. R. Van Loon, et al., "Chloride accessible porosity fractions across the Jurassic sedimentary rocks of northern Switzerland," Appl. Geochem., vol. 162, p. 105841, 2024. DOI: 10.1016/j.apgeochem.2023.105841