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<p>Rocks undergo interactions such as dissolution and erosion upon exposure to water, which contribute to the degradation of their mechanical properties. Water–rock interactions primarily occur through the fractures within the rock, while the mineralogical makeup of the rock substantially affects these interactions. The inherent anisotropy of rocks results in non-uniform mineral distribution and fracture orientation. This non-uniformity may trigger hazardous issues such as surrounding rock instability and collapse during engineering operations involving water-rich rock formations under confining pressure. This study investigates rock softening and anisotropy of softening under confining pressure to prevent such occurrences. Drilling experiments were performed on four distinct rock types under a matrix of four confining pressure levels and three saturation states, each tested along three mutually orthogonal directions. The methods for calculating drilling strength (<i>S</i>) and specific energy (<i>E</i>) were established based on a rock–bit interaction model. A drilling softening coefficient (<i>C</i>) and a drilling anisotropy index were then defined. By integrating the anisotropy index, the directional, depth-dependent, and saturation-induced variations of the softening coefficient under confining stress were systematically analyzed. The proposed indicators were further validated through comparisons with other mechanical and anisotropy parameters that reflect rock softening. Results indicated that <i>C</i> decreased as saturation increased, but rose with higher levels of confining pressure. Shale and red sandstone exhibited the most pronounced softening, with minimum coefficient values of 0.37 and 0.38. The drilling anisotropy index declined with both rising saturation and confining pressure with shale showing the greatest reduction at 24%. Furthermore, this study revealed the coupling between drilling-induced rock damage and water-induced softening, as well as the phenomenon of dominant force reversal in the <i>S–E</i> curve. The proposed method offers a reliable approach to quantifying rock softening and anisotropy using drilling parameters compared to previous studies.</p>

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Effect of confining pressure and saturation states on softening and anisotropy of rock during drilling process

  • 期刊论文

  • 发表日期：

    2026年7月11日

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  • Mingchen Ding,
  • Mingming He

摘要

Rocks undergo interactions such as dissolution and erosion upon exposure to water, which contribute to the degradation of their mechanical properties. Water–rock interactions primarily occur through the fractures within the rock, while the mineralogical makeup of the rock substantially affects these interactions. The inherent anisotropy of rocks results in non-uniform mineral distribution and fracture orientation. This non-uniformity may trigger hazardous issues such as surrounding rock instability and collapse during engineering operations involving water-rich rock formations under confining pressure. This study investigates rock softening and anisotropy of softening under confining pressure to prevent such occurrences. Drilling experiments were performed on four distinct rock types under a matrix of four confining pressure levels and three saturation states, each tested along three mutually orthogonal directions. The methods for calculating drilling strength (S) and specific energy (E) were established based on a rock–bit interaction model. A drilling softening coefficient (C) and a drilling anisotropy index were then defined. By integrating the anisotropy index, the directional, depth-dependent, and saturation-induced variations of the softening coefficient under confining stress were systematically analyzed. The proposed indicators were further validated through comparisons with other mechanical and anisotropy parameters that reflect rock softening. Results indicated that C decreased as saturation increased, but rose with higher levels of confining pressure. Shale and red sandstone exhibited the most pronounced softening, with minimum coefficient values of 0.37 and 0.38. The drilling anisotropy index declined with both rising saturation and confining pressure with shale showing the greatest reduction at 24%. Furthermore, this study revealed the coupling between drilling-induced rock damage and water-induced softening, as well as the phenomenon of dominant force reversal in the S–E curve. The proposed method offers a reliable approach to quantifying rock softening and anisotropy using drilling parameters compared to previous studies.