Impact of sand content on solute diffusion in Opalinus Clay

•Exploring mesostructural effects on solute diffusion in shales•Determination of geometric tortuosity and constrictivity on the base of tomographic data•Evaluating the respective effects of tortuosity and constrictivity on diffusion

The mesostructure (micro to millimeter scale) of clay rocks was considered as a two-component mixture consisting of impermeable non-clayey sand grains embedded in a permeable clay matrix. Provided that representative diffusion properties can be defined, the clay matrix can be treated as a continuum. Under these conditions diffusion at larger scales will depend on geometric properties of the mesostructure. The objective of this study, then, is to analyze geometric parameters, which control diffusion at larger scales. In a first step a set of different clay matrix mesostructures were reconstructed on the base of synchotron X-ray computed microtomography applied to clay rock samples from northern Switzerland (e.g. Opalinus Clay). In a second step mesostructural effects on diffusion were quantified by applying diffusion simulations to reconstructed mesostructures. Further analysis revealed that constrictivity is the most dominant parameter, which controls diffusion on the millimeter scale in Opalinus Clay. Regarding diffusion, the mesostructure of the clay matrix is near isotropic. Hence, the reason for anisotropic diffusion in Opalinus Clay must be searched on the nanometer to micrometer scale and it is caused by anisotropic pore path tortuosity related to shape preferred orientation of clay platelets.