The influence of sedimentary and diagenetic heterogeneity on the radionuclide diffusion in the sandy facies of the Opalinus Clay at the core scale

Abstract

Molecular diffusion is an important transport mechanism for radionuclide migration in low-permeable argillaceous host rock such as Opalinus Clay (OPA). In this study, the influence of sedimentary and diagenetic heterogeneity on heterogeneous diffusion in sandy facies of OPA (SF-OPA) from lamina scale to drill core scale is investigated using an upscaling workflow to model diffusive transport from the pore scale to the core scale. Our numerical results based on the simplified structural model show fast diffusion fronts in clay laminae (7 mm displacement after 6 days of diffusion) and slow diffusion fronts in carbonate lenses and sand laminae (4 mm displacement after 6 days of diffusion), demonstrating the endmembers of heterogeneous diffusion patterns in SF-OPA. Moreover, our results show that the diffusion fronts begin to homogenize after 22 days of diffusion with the specific influence of carbonate lenses (here: length = 1 cm, thickness = 3 mm). This example illustrates how material heterogeneities affect heterogeneous diffusion on a small temporal and spatial scale. The sensitivity studies show that the diffusion length and homogenization time increase by up to 190% when the length and thickness of the carbonate lenses are doubled. Using four compositional endmembers, we show the generalized diffusion behavior to demonstrate the influence of thin laminae and thick layers as well as dispersed small and large diagenetic concretions on the homogeneity of diffusion. These results demonstrate that the geometry of sedimentary and diagenetic material and the subfacies composition are the controlling factors for quantifying diffusion length and homogenization time. This study provides quantitative constraints on the temporal and spatial evolution of heterogeneous diffusion at the core scale. This quantitatively improves the predictability of radionuclide migration in host rocks as a function of compositional and pore network-specific parameters.