Abstract
Many popular models have been proposed to study the fractal properties of the pores of porous materials based on mercury intrusion porosimetry (MIP). However, most of these models do not directly apply to the small-micro pores of loess, which have a significant impact on the throat pores and tunnels for fluid flow. Therefore, in this study we used a combination of techniques, including routine physical examination, MIP analysis, and scanning electron microscope (SEM) image analysis, to study these small-micro pores and their saturated water permeability properties. The techniques were used to determine whether the fractal dimensions of six MIP fractal models could be used to evaluate the microstructure types and permeability properties of loess. The results showed that the Neimark model is suitable for analysis of small-micro pores. When applied to saturated water permeability, the results from this model satisfied the correlation significance test and were consistent with those from SEM analysis. A high clay content and density cause an increase in the number of small-micro pores, leading to more roughness and heterogeneity of the pore structure, and an increase in the fractal dimensions. This process further leads to a decrease in the content of macro-meso pores and saturated water permeability. Furthermore, we propose new parameters: the *Ellipse and its area ratios (*EAR). These parameters, coupled with 2D-SEM and 3D-MIP fractal dimensions, can effectively and quantitatively be used to evaluate the types of loess microstructures (from type I to type III) and the saturated water permeability (magnitude from 1×10−4 cm/s to 1×10−5 cm/s).
目的
基于压汞法(MIP)的6种分形模型常被用来研究多孔介质的分形特征,但是这些模型中的大多数并不能直接适用于黄土的小微孔,且小微孔隙对水的渗流吼道有显著影响。本文旨在结合常规变水头渗透试验、MIP分析和扫描电镜(SEM)图像等,研究小微孔及其饱和渗透特性,并验证6种MIP分形模型是否可用于评估原状黄土的微结构类型和饱和渗透特征。
创新点
1. 结合饱和渗透系数和SEM图像计盒维数等参数进行相关性分析和显著性检验,以确定最适合黄土小微孔的MIP分形模型;2. 建立SEM分形维数和MIP分形维数的耦合,并结合黏粒含量,对原状黄土微结构类型进行分类和对饱和渗透系数从1×10−4 cm/s变到1×10−5 cm/s进行评价。
方法
1. 通过压汞试验和6种MIP分形模型公式,得出原状黄土小微孔隙在不同分形模型下的分形维数。2. 通过SEM图像和计盒维数计算得出原状黄土小微孔隙的二维分形维数。
结论
1. 模型2(Neimark模型)对小微孔隙显示出非常好的线性拟合(拟合系数大于0.94);其分形维数值与黏土含量、密度和饱和渗透系数呈强正相关,并与SEM图像的计盒维数相匹配。2. 黏土颗粒含量高会导致小微孔数量增加;这些小微孔会导致孔隙结构的表面粗糙度和不均匀性增加,从而呈现出较大的分形维数;这一过程最终导致大中孔隙和主导流线减少,从而降低饱和渗透性。3. 本研究提出了新的分维椭圆及其面积比(*EAR)参数,其中两个主要的半轴由Neimark模型的结果(减2)和计盒维数(减1)组成;*EAR和黏土含量可作为定量评价黄土微结构类型和饱和渗透性的有效参数;当*EAR在35%至50%之间、黏土含量在9%至15%之间时,原状黄土微观结构从I型转变为II型;当*EAR大于50%、黏土含量大于15%时,微结构进一步从II型转变为III型;相应地,饱和渗透系数从1×10−4 cm/s转变为1×10−5 cm/s。
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Acknowledgments
This work is supported by the China Geological Survey Project (No. DD20190642) and the Shaanxi Provincial Key Research Program of China (No. 2019ZDLSF07-07-02).
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Yaming TANG designed the research. Tuo LU, Bo HONG, and Wei FENG processed the corresponding data. Tuo LU wrote the first draft of the manuscript. Yongbo TIE helped to organize the manuscript. Yaming TANG revised and edited the final version.
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Tuo LU, Yaming TANG, Yongbo TIE, Bo HONG, and Wei FENG declare that they have no conflict of interest.
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Lu, T., Tang, Y., Tie, Y. et al. Fractal analysis of small-micro pores and estimation of permeability of loess using mercury intrusion porosimetry. J. Zhejiang Univ. Sci. A 24, 584–595 (2023). https://doi.org/10.1631/jzus.A2200528
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DOI: https://doi.org/10.1631/jzus.A2200528
Key words
- Malan loess
- Fractal models
- Small-micro pores
- Mercury intrusion porosimetry (MIP)
- Microstructure
- Saturated water permeability