The Effect of Mineral Dissolution-Precipitation Reactions on the Diffusivity of Porous Media: Innovative Experiments & Reactive Transport Modelling

A reliable assessment of the evolution of geological repositories for radioactive wastes requires a profound understanding of coupled hydrogeochemical processes across various temporal and spatial scales. At barrier interfaces, chemical and thermal gradients promote mineral precipitation reactions, reducing the porosity and potentially leading to clogging of diffusion-controlled porous media [1]. Recent experiments and pore-scale modelling investigations conducted to benchmark continuum-scale reactive transport models revealed the deficiency of conventional and revised porosity-diffusivity relationships based on Archie’s law to account for changes in effective diffusivity of evolving porous media [1-2]. To identify key parameters at the pore-scale that might explain the discrepancies between experimental and modelling studies, we designed a “lab-on-a-chip” approach for counter-diffusion precipitation experiments, combining time-lapse optical microscopy and operando Raman spectroscopy. As the 2D microfluidic pore network became clogged, the transport of deuterium through the evolving microporosity of the precipitated celestine was visualized by Raman imaging, demonstrating the dynamic nature of porosity clogging. Pore scale simulations were conducted on the 2D images of the evolving pore network to determine its effective diffusivity. The application of a revised porosity-diffusivity relationship improved the agreement between modelling results and experimental observations, but also strongly emphasised the need for further pore-scale investigations. Our innovative combination of microfluidic experiments and pore-scale modelling opens new possibilities to validate and identify relevant pore-scale processes and provides data for upscaling approaches to derive key relationships for continuum-scale reactive transport simulations.[1] Chagneau et al. (2015), Geochem. Trans. 16, 13.[2] Deng et al. (2021), Water Resour. Res. 57(5), 1-16.