Tracer transport in claystones at the core scale: Experiments and modelling

Within the Swiss deep drilling campaign 2019–2022, a series of Jurassic (Dogger, Lias) claystone samples were mineralogically and geochemically characterised and used in small-scale transport experiments. The advective-diffusive transport experiments clearly showed a faster breakthrough of anions relative to neutral tracers, which agrees with the accumulation of anions in a relatively faster moving porewater fraction due to anion repulsion by negatively charged clay surfaces. Electroneutral tracers were not affected by the clay charge and thus evenly distributed in all porewater domains, including stationary ones, and were transported more slowly than anions. The experiments showed consistently a faster breakthrough of Br relative to Cl, which was intuitively not expected based on the equal charge and chemical behaviour of these ions. Simulations with a continuum finite volume model approach that considers electrostatic effects in clay accurately described the different breakthrough curves of charged and neutral tracers. Matching of breakthrough behaviour required a model explicitly considering heterogeneous flow, because the simple Fickian advection-dispersion equation failed to predict the experimental data of all tracers in a consistent way. The model indicated that although the exclusion behaviour of Cl and Br as a function of concentration and surface potential is the same, the effective retardation factor (and thus breakthrough) of each ion depends on the difference in concentration of initial in-situ porewater and that of the infiltrating solution. Heterogeneous flow, electrostatic interactions, and multicomponent transport are key processes governing solute transport in claystone cores under large hydraulic gradients. These mechanisms were therefore incorporated into the presented model, enabling successful simulation of the experimental data. • An extensive dataset of claystone transport experiments was interpreted and modelled. • Heterogeneous flow and electrostatic effects dominate advective transport in clays. • Difference in advective breakthrough behaviour of equally valent anions was measured. • This often observed phenomenon has been interpreted and simulated for the first time. • Core-scale experiments provide a fundamental understanding of transport in clays.