Understanding mineral precipitation induced porosity clogging and being able to quantify its non‐linear feedback on transport properties is fundamental for predicting the long‐term evolution of energy‐related subsurface systems. Commonly applied porosity‐diffusivity relations used in numerical simulations on the continuum‐scale predict the case of clogging as a final state. However, recent experiments and pore‐scale modeling investigations suggest dissolution‐recrystallization processes causing a non‐negligible inherent diffusivity of newly formed precipitates. To verify these processes, we present a novel microfluidic reactor design that combines time‐lapse optical microscopy and confocal Raman spectroscopy, providing real‐time insights of mineral precipitation induced porosity clogging under purely diffusive transport conditions. Based on 2D optical images, the effective diffusivity was determined as a function of the evolving porous media, using pore‐scale modeling. At the clogged state, Raman isotopic tracer experiments were conducted to visualize the transport of deuterium through the evolving microporosity of the precipitates, demonstrating the non‐final state of clogging. The evolution of the porosity‐diffusivity relationship in response to precipitation reactions shows a behavior deviating from Archie's law. The application of an extended power law improved the description of the evolving porosity‐diffusivity, but still neglected post‐clogging features. Our innovative combination of microfluidic experiments and pore‐scale modeling opens new possibilities to validate and identify relevant pore‐scale processes, providing data for upscaling approaches to derive key relationships for continuum‐scale reactive transport simulations.

Reactive transport modelling is a powerful tool to assess subsurface evolution in various energy-related applications. Upscaling, i.e., accounting for pore scale heterogeneities into larger scale analyses, remains one of the biggest challenges of reactive transport modelling. Pore scale simulations capturing the evolutions of the porous media over a wide range of Peclet and Damköhler number in combination with machine learning are foreseen as an efficient methodology for upscaling. However, the accuracy of these pore scale models needs to be tested against experiments. In this work, we developed a lab on a chip experiment with a novel micromodel design combined with operando confocal Raman spectroscopy, to monitor the evolution of porous media undergoing coupled mineral dissolution and precipitation processes due to diffusive reactive fluxes. The 3D-imaging of the porous media combined with pore scale modelling enabled the derivation of upscaled transport parameters. The chemical reaction tested involved the replacement of celestine by strontianite, whereby a net porosity increase is expected because of the smaller molar volume of strontianite. However, under our experimental conditions, the accessible porosity and consequently diffusivity decreased. We propose a transferability of the concepts behind the Verma and Pruess relationship to be applied to also describe changes of diffusivity for evolving porous media. Our results highlight the importance of calibrating pore scale models with quantitative experiments prior to simulations over a wide range of Peclet and Damköhler numbers of which results can be further used for the derivation of upscaled parameters.

An accurate mechanistic understanding of solute diffusion in partially saturated clays is critical for assessing the safety of deep geological repositories for radioactive waste. In this study, a pore‐scale numerical framework is developed to simulate water and ion diffusion in partially saturated clays. First, the two‐phase Shan‐Chen Lattice Boltzmann method is employed to establish the liquid‐gas distribution in a reconstructed three‐dimensional pore geometry of a clay. An equivalent solute method is also developed and validated to improve the numerical stability of the solution at the liquid/gas interface corresponding to steep variations of the concentration and diffusion coefficient of the water tracer. By using a mobility‐distance relationship from molecular simulations, Fick's law is numerically solved to simulate water diffusion in nanopores, while the coupled Poisson‐Boltzmann‐Nernst‐Planck equations are solved to simulate ion diffusion under the influence of the electrical double layer (EDL). Our model reveals that the decrease of relative effective diffusion coefficients during the desaturation is more pronounced for ions than for water, due to the additional transport pathway of water tracers in the gas phase. The obtained effective diffusion coefficients of tritiated water and ions agree well with reported data from compacted sedimentary rocks. By comparing the local electric potential and the distribution of ion concentrations in single pores, the simulation results suggest that the EDL in unsaturated clays has a more complex influence on ion distribution than under fully water‐saturated conditions. This study provides critical insights into the coupled transport processes of solutes in partially saturated clays.

Accurate understanding of diffusion of anionic radionuclides in different clays is significant to predict long‐term performance of high‐level radioactive waste (HLW) repositories. The importance of electrical double layer (EDL) on anionic tracer diffusion in different clays has been studied by both through‐diffusion experiments and pore‐scale simulations in this work. The through‐diffusion experiments measured the effective diffusion coefficient and the accessible porosity of Re (VII) in compacted montmorillonite, Na‐bentonite, and illite/smectite mixed layer (I/S) at different salinities. The results showed that the accessible porosity and the effective diffusion coefficient of Re (VII) in montmorillonite and Na‐bentonite were similar but lower than those in I/S under the same conditions. For mechanism analysis and predictions, a pore‐scale modeling was implemented to simulate the diffusion of anions in compacted clays. In the simulations, the characteristics (porosity, density, and total surface area) of microstructures of montmorillonite and I/S were used to regenerate three‐dimensional pore structures numerically by a Quartet Structure Generation Set method. The Re (VII) diffusion was then simulated by directly solving coupled Poisson‐Nernst‐Planck equations via the lattice Boltzmann method. The diminished effect of EDL was therefore calculated and compared with Donnan and multiporosity models. As EDLs overlap in compacted clays, the Donnan model overestimates the influence of EDL on Re (VII) diffusion, while the multiporosity model underestimates it. The pore‐scale modeling, which captures the structure of overlapping EDLs automatically, can simulate the diffusion of anionic radionuclides in compacted clays without any fitting parameters.