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<p>The borehole geometry and cuttings particle characteristics of using self-rotating multi-jet (SRMJ) drilling in coal are significantly affected by the geo-stress and the intrinsic properties. Therefore, coal drilling experiments using SRMJ under true triaxial stress conditions were carried out. The geometric features of the boreholes were obtained by computed tomography (CT), and the distribution of coal cuttings area was obtained by image processing. The research results indicate that the borehole morphology is influenced not only by in situ stresses, but also by the direction between bedding and drilling, exhibiting strength-oriented drilling characteristics. The increase in triaxial stress reduces the morphological differences between drilling perpendicular and parallel to bedding. As the confining pressure increases, the borehole diameter, depth, and volume decrease (with the maximum reduction of 30.78%, 48.45%, 53.17%, respectively, in perpendicular direction; with the maximum reduction of 34.27%, 55.99%, 78.63%, respectively, in parallel direction). As the confining pressure increases, the particle size of coal cuttings changes to a smaller range. When drilling perpendicular to bedding, the thickness of coal cuttings is controlled by butt cleats. When drilling parallel to bedding, the length of coal cuttings is controlled by face cleats. This work establishes an experimental foundation for future research into borehole morphology.</p>

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The Effect of Triaxial Stress on Borehole Geometry and Cuttings Particle Characteristics in Coal Using Self-Rotating Multi-jet

  • 期刊论文

  • 发表日期：

    2026年2月20日

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  • Xiangjie Liu,
  • Zhe Zhou,
  • Zhaolong Ge,
  • Yilong Tang,
  • Hongfei Cao,
  • Jianming Shangguan

摘要

The borehole geometry and cuttings particle characteristics of using self-rotating multi-jet (SRMJ) drilling in coal are significantly affected by the geo-stress and the intrinsic properties. Therefore, coal drilling experiments using SRMJ under true triaxial stress conditions were carried out. The geometric features of the boreholes were obtained by computed tomography (CT), and the distribution of coal cuttings area was obtained by image processing. The research results indicate that the borehole morphology is influenced not only by in situ stresses, but also by the direction between bedding and drilling, exhibiting strength-oriented drilling characteristics. The increase in triaxial stress reduces the morphological differences between drilling perpendicular and parallel to bedding. As the confining pressure increases, the borehole diameter, depth, and volume decrease (with the maximum reduction of 30.78%, 48.45%, 53.17%, respectively, in perpendicular direction; with the maximum reduction of 34.27%, 55.99%, 78.63%, respectively, in parallel direction). As the confining pressure increases, the particle size of coal cuttings changes to a smaller range. When drilling perpendicular to bedding, the thickness of coal cuttings is controlled by butt cleats. When drilling parallel to bedding, the length of coal cuttings is controlled by face cleats. This work establishes an experimental foundation for future research into borehole morphology.