BACKGROUND In geochemically perturbed systems where porewater and mineral assemblages are unequilibrated the processes of mineral precipitation and dissolution may change important transport properties such as porosity and pore diffusion coefficients. These reactions might alter the sealing capabilities of the rock by complete pore-scale precipitation (cementation) of the system or by opening new migration pathways through mineral dissolution. In actual 1D continuum reactive transport codes the coupling of transport and porosity is generally accomplished through the empirical Archie's law. There is very little reported data on systems with changing porosity under well controlled conditions to constrain model input parameters. In this study celestite (SrSO4) was precipitated in the pore space of a compacted sand column under diffusion controlled conditions and the effect on the fluid migration properties was investigated by means of three complementary experimental approaches: (1) tritiated water (HTO) tracer through diffusion, (2) computed micro-tomography (µ-CT) imaging and (3) post-mortem analysis of the precipitate (selective dissolution, SEM/EDX). RESULTS The through-diffusion experiments reached steady state after 15 days, at which point celestite precipitation ceased and the non-reactive HTO flux became constant. The pore space in the precipitation zone remained fully connected using a 6 µm µ-CT spatial resolution with 25 % porosity reduction in the approx. 0.35 mm thick dense precipitation zone. The porosity and transport parameters prior to pore-scale precipitation were in good agreement with a porosity of 0.42 ± 0.09 (HTO) and 0.40 ± 0.03 (µ-CT), as was the mass of SrSO4 precipitate estimated by µ-CT at 25 ± 5 mg and selective dissolution 21.7 ± 0.4 mg, respectively. However, using this data as input parameters the 1D single continuum reactive transport model was not able to accurately reproduce both the celestite precipitation front and the remaining connected porosity. The model assumed there was a direct linkage of porosity to the effective diffusivity using only one cementation value over the whole porosity range of the system investigated. CONCLUSIONS The 1D single continuous model either underestimated the remaining connected porosity in the precipitation zone, or overestimated the amount of precipitate. These findings support the need to implement a modified, extended Archie's law to the reactive transport model and show that pore-scale precipitation transforms a system (following Archie's simple power law with only micropores present) towards a system similar to clays with micro- and nanoporosity. Graphical abstract.

中文翻译：

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矿物沉淀引起的孔隙度降低及其对扩散控制的多孔介质中输运参数的影响。

背景技术在孔隙水和矿物组合不平衡的地球化学扰动系统中，矿物的沉淀和溶解过程可能会改变重要的输运性质，例如孔隙度和孔隙扩散系数。这些反应可能会通过系统的完整孔隙尺度沉淀（胶结）或通过矿物溶解打开新的迁移途径而改变岩石的封闭能力。在实际的一维连续介质反应性输运编码中，通常通过经验阿奇定律实现输运与孔隙度的耦合。在控制良好的条件下，为限制模型输入参数而改变孔隙度的系统上，几乎没有报道过的数据。在这项研究中，在扩散控制的条件下，天青石（SrSO4）沉淀在压实砂柱的孔隙中，并通过三种互补的实验方法研究了对流体迁移特性的影响：（1）tri水（HTO）示踪剂通过扩散，（2）计算机断层扫描（μ-CT）成像和（3）沉淀物的事后分析（选择性溶解，SEM / EDX）。结果扩散实验在15天后达到稳定状态，此后天青石沉淀停止并且非反应性HTO通量变得恒定。使用6 µm µ-CT空间分辨率，在沉淀区的孔隙空间保持完全连通，孔隙率降低了约25％。0.35毫米厚的密集降水区。孔尺度沉淀之前的孔隙度和输运参数与孔隙度分别为0.42±0.09（HTO）和0.40±0.03（µ-CT）高度吻合，µ-CT在25±25°C时估算出的SrSO4沉淀质量也是如此。 5 mg和选择性溶出分别为21.7±0.4 mg。然而，使用该数据作为输入参数，一维单一连续反应传输模型不能准确地再现天青石的沉淀锋面和剩余的连通孔隙度。该模型假设在所研究系统的整个孔隙度范围内，仅使用一个胶结值就可以将孔隙度与有效扩散率直接联系起来。结论一维单一连续模型要么低估了降水带中剩余的连通孔隙度，要么高估了降水量。这些发现支持将修正的，扩展的阿奇定律应用于反应性输运模型的必要性，并表明，孔隙规模的降水将系统（遵循仅具有微孔的阿奇简单幂定律）转化为类似于具有微孔和纳米孔隙度的粘土的系统。图形概要。