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<p>Delving into the elastic anisotropy mechanisms of argillaceous rocks is pivotal for understanding the deformation and failure of subterranean argillaceous formations. Nevertheless, in contrast to layered shale, the anisotropy inherent in non-layered mudstone remains inadequately understood. Here, depth-sensing nanoindentation (DSNI), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and mercury intrusion porosimetry (MIP) tests were conducted on a polymineralic mudstone to analyze the elastic anisotropy mechanism of mudstones. XRD method was used to quantify the mineralogical compositions in the mudstone. SEM–EDS was adopted to visualize the compositions at the mesoscale resolution for targeting the tested regions and to observe the orientation of clay particles. The pores in the mudstone were quantified by the MIP method. The clay-rich mudstone consists of kaolinite and quartz minerals; quartz particles embedded in the kaolinite matrix that is formed with kaolinite particles aligning along normal plane and sub-micrometer sized pores. At the mesoscale, the indenter was impressed into the kaolinite matrix and quartz particle on the mudstone surfaces normal and parallel to the sedimentary direction. At the macroscale, uniaxial compressive tests were also conducted on the mudstone along both the normal and parallel directions. The elastic modulus of the kaolinite matrix on the parallel plane (<i>E</i><sub>1</sub> = 23.70 GPa) is much larger than that on the normal planes (<i>E</i><sub>3</sub> = 13.16 GPa), with the elastic anisotropy ratio (<i>R</i><sub><i>a</i></sub> = <i>E</i><sub>1</sub>/<i>E</i><sub>3</sub>) of 1.74. Mudstone anisotropy originates from intrinsic anisotropy of kaolinite matrix, but its anisotropic ratio (<i>R</i><sub><i>a</i></sub> = 1.51), mitigating by the random-distributed quartz particles, is slightly less. Three cross-scale models, i.e., Mori–Tanaka (MT), Kuster–Toksöz (KT), and Voigt–Reuss (VR), were used to upscale the elastic properties from the mesoscale kaolinite matrix and quartz particle to the macroscale mudstone. MT model describes the macroscale experimental data best.</p>

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Polymineralic Mudstone Exhibits Anisotropy due to the Preferential Orientation of Kaolinite Particles

  • 期刊论文

  • 发表日期：

    2025年4月26日

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  • Changlun Sun,
  • Guichen Li,
  • Zhenqian Ma,
  • Jiahui Xu,
  • Yuantian Sun

摘要

Delving into the elastic anisotropy mechanisms of argillaceous rocks is pivotal for understanding the deformation and failure of subterranean argillaceous formations. Nevertheless, in contrast to layered shale, the anisotropy inherent in non-layered mudstone remains inadequately understood. Here, depth-sensing nanoindentation (DSNI), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and mercury intrusion porosimetry (MIP) tests were conducted on a polymineralic mudstone to analyze the elastic anisotropy mechanism of mudstones. XRD method was used to quantify the mineralogical compositions in the mudstone. SEM–EDS was adopted to visualize the compositions at the mesoscale resolution for targeting the tested regions and to observe the orientation of clay particles. The pores in the mudstone were quantified by the MIP method. The clay-rich mudstone consists of kaolinite and quartz minerals; quartz particles embedded in the kaolinite matrix that is formed with kaolinite particles aligning along normal plane and sub-micrometer sized pores. At the mesoscale, the indenter was impressed into the kaolinite matrix and quartz particle on the mudstone surfaces normal and parallel to the sedimentary direction. At the macroscale, uniaxial compressive tests were also conducted on the mudstone along both the normal and parallel directions. The elastic modulus of the kaolinite matrix on the parallel plane (E1 = 23.70 GPa) is much larger than that on the normal planes (E3 = 13.16 GPa), with the elastic anisotropy ratio (Ra = E1/E3) of 1.74. Mudstone anisotropy originates from intrinsic anisotropy of kaolinite matrix, but its anisotropic ratio (Ra = 1.51), mitigating by the random-distributed quartz particles, is slightly less. Three cross-scale models, i.e., Mori–Tanaka (MT), Kuster–Toksöz (KT), and Voigt–Reuss (VR), were used to upscale the elastic properties from the mesoscale kaolinite matrix and quartz particle to the macroscale mudstone. MT model describes the macroscale experimental data best.