Analysis of Pore Structure Effects on Diffusive Reactive Transport in Opalinus clay

Opalinus clay has a high sealing capacity and is therefore considered as viable candidates for hosting radioactive waste in future deep geological repositories. In order to validate and to predict the sealing capacity of the clay and the long-term containment performance of radioactive waste repositories, a comprehensive understanding of transport processes in the clay is necessary. Pore network models are useful for linking material structure to longer length-scale diffusion, but in principle require accurate description of the morphology of real porous media to support model parameterisation, i.e. pore and throat size distribution, connectivity information. Opalinus clay has tight pore spaces and current experimental techniques do not allow for extracting sufficient connectivity data and throat sizes (if at all). This work extends previous works on pore network modelling of porous media with limited structural information. Measured solid phase characteristics balance the insufficient geometrical and topological information for the clay pore systems to construct pore network. The constructed networks are described as mathematical graphs with diffusion modelled as a continuous process in water-saturated bonds. The effects of length scale parameters and shaper factors on effective diffusivities are analyzed in this work. Calculated results are in the range of experimental data in different clay directions. The analyses suggest that the micro-pores, those smaller than 5nm, have major effect on diffusivity. Diffusion of larger complexes can be used for experimental validation of the model. The proposed methodology can be used for any micro and meso-porous material with limited information and be useful for further development of multi-phase and reactive transport.