Abstract

The effective diffusivity is a key parameter in the diffusive transport calculations, thus decisive for predicting the radionuclide migration in low-permeable clay-rich formations. Potential host rocks such as the Opalinus clay exhibit pore network heterogeneities, critically modified due to compositional variability in the sandy facies and owing to diagenetic minerals. Meaningful estimation of the effective diffusivity requires an understanding of transport mechanisms at the nanometer-scale as a starting point and a combination with upscaling strategies for considering compositional heterogeneities at the micrometer-scale. In this study, we propose an upscaling workflow that integrates transport simulations at both the nanometer-scale and the micrometer-scale to predict the effective diffusivities of radionuclides in the sandy facies of the Opalinus clay. The respective synthetic digital rocks provide conceptually two types of materials at the pore scale, in which the pore space and pore network in the clay matrix at the nanometer scale and mineral complexity in shales at the micrometer scale are considered. The numerical approach using the introduced digital rocks is validated with published experimental data that confirm the general applicability of the models. Sensitivity studies reveal the increase of effective diffusivity of shales as a function of increased pore space, reduced tortuosity, and an increased sheet silicate concentration compared to other rock components. Thus, such spatial variabilities at the pore scale of more complex sedimentary rocks are now addressed in the proposed approach and available for studying heterogeneous diffusion patterns compared to commonly assumed homogeneous behavior. Finally, and as a starting point for further upscaling strategies, we investigate anisotropic diffusion by studying the effect of lamination of the shales toward enhanced predictability of radionuclide migration.

Article Highlights

• Upscaling workflow to predict effective diffusivity of radionuclides (RN) diffusion in shales showing compositional heterogeneity (sandy facies of Opalinus clay)

• Sensitivity studies demonstrate and quantify the effect of the pore network geometry in the clay matrix as well as the effect of clay mineral concentration variability in shales on RN diffusion

• Prediction of the heterogeneity of diffusivity based on multiple types of imaging data including compositional rock data

中文翻译：

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使用集成的放大工作流对粘土地层中放射性核素的有效扩散率预测

摘要

有效扩散率是扩散输运计算中的关键参数，因此对于预测低渗透性富粘土地层中的放射性核素迁移具有决定性作用。潜在的宿主岩石（例如Opalinus粘土）表现出孔隙网络的异质性，由于砂岩相中的成分变化和成岩矿物的原因，这些岩石被严格地改性。有效扩散率的有意义的估算需要以纳米尺度为起点来理解传输机制，并需要结合放大策略来考虑微米尺度上的组成异质性。在这项研究中，我们提出了一个升级工作流程，该工作流程集成了纳米级和微米级传输模拟，以预测Opalinus粘土砂相中放射性核素的有效扩散率。各个合成数字岩石在孔隙尺度上概念上提供了两种类型的材料，其中考虑了纳米尺度上的粘土基质中的孔隙空间和孔隙网络以及微米尺度下的页岩中的矿物复杂性。使用引入的数字岩石的数值方法已通过已发布的实验数据进行了验证，这些数据证实了模型的普遍适用性。敏感性研究表明，与其他岩石组分相比，页岩的有效扩散率随孔隙空间的增加，曲折度的降低和层状硅酸盐浓度的增加而增加。因此，现在，在提出的方法中解决了更复杂沉积岩孔隙尺度上的这种空间变异性，并且与通常假定的均质行为相比，可用于研究非均质扩散模式。最后，作为进一步升级策略的起点，我们通过研究页岩层合对放射性核素迁移的增强可预测性的影响来研究各向异性扩散。

文章重点

• 升级工作流程，以预测页岩中放射性核素（RN）扩散的有效扩散率，从而显示出组成异质性（Opalinus粘土的砂岩相）

• 敏感性研究证明并量化了粘土基质中孔隙网络几何形状的影响以及页岩中粘土矿物浓度变化对RN扩散的影响

• 基于多种类型的成像数据（包括成分岩石数据）预测扩散率的非均质性