Investigation of partial water saturation effects on diffusion in shale

Nunn, J. A.; Xiang, Yan; Al, Tom A.

doi:10.1016/j.apgeochem.2018.08.004

Cited by 7 publications

(3 citation statements)

References 68 publications

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“…The observed reduction in D e was larger for cations and anions as compared to water tracers, with factors up to 7 for water tracers and up to 60 for anions upon a reduction of the water saturation from 1 to 0.8 in the claystone. A new technique to estimate iodide diffusion coefficients in unsaturated claystones based on X-ray radiography was recently developed by Nunn et al (2018). A reduction of D e values by about 20% for an average desaturation of 4 to 6-7% of samples from a Canadian shale was reported.…”

Section: Influence Of Partial Saturationmentioning

confidence: 99%

Water retention and diffusion in unsaturated clays: Connecting atomistic and pore scale simulations

Gimmi

Churakov

2019

Applied Clay Science

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Molecular diffusion is the dominant solute transport process in clays and claystones that are considered as sealing materials in the deep geological disposal of radioactive waste. These materials are typically water saturated, but during construction and later, at elevated temperatures or when gas may be produced, unsaturated conditions prevail. Investigating the clay's water retention properties as well as solute transport under unsaturated conditions is therefore mandatory. Here, functional dependencies of these properties were derived from atomistic and pore-scale simulations. In the absence of tomographic maps that resolve all pores in clays, model clay structure maps with different pore size distributions were generated using a previously developed algorithm. Upscaled water retention functions and upscaled diffusion coefficients of unsaturated samples were derived from these maps based on the shifted Young-Laplace equation that considers film adsorption and capillary condensation. Pore-scale parameters (diffusion coefficients, water film thickness) used for the upscaling were taken from molecular dynamics (MD) simulations, thus connecting MD and pore-scale simulations. We focused on effects of the pore size distribution and of the adsorbed water film on upscaled parameters. Samplescale diffusion coefficients were clearly reduced in unsaturated samples compared to the saturated state, with less reduction when including adsorbed water films. The reduction was stronger in samples with a narrow size distribution of the interparticle pores as compared to those with a wide distribution (but equal mean size). The results follow the trends of the experimental data, even though the scale of the simulations is still clearly smaller than that of typical experiments.

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Section: Influence Of Partial Saturationmentioning

confidence: 99%

Water retention and diffusion in unsaturated clays: Connecting atomistic and pore scale simulations

Gimmi

Churakov

2019

Applied Clay Science

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“…Before reaching equilibrium, the pressure changes closer to the boundaries will first cause the pore water to vaporize. Hence, the water saturation profiles exhibit a rough U-shape, as observed in shale (Nunn et al, 2018), with a lower water saturation at the inlet and outlet boundaries and a higher saturation in the center. Additionally, the pressure gradient could also induce a deformation of the clay material (Guo & Fall 2021;Ma et al, 2020).…”

Section: Comparison With Reported Experimental Datamentioning

confidence: 81%

Pore‐Scale Modeling of Water and Ion Diffusion in Partially Saturated Clays

Yang,

Churakov,

Patel

et al. 2024

Water Resources Research

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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.

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“…This is a challenging task, as illustrated by the fact that only a few studies have been reported in the literature. For instance, Nunn et al (2018) presented a new method using X-ray radiography and iodide tracers for quantifying the degree of partial saturation of shale samples and measuring De. They showed that the De value decreased by 22% when saturation decreased from 100% to 93.3%.…”

Section: Introductionmentioning

confidence: 99%