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<p>To investigate the macroscopic and microscopic damage mechanisms of rocks under the coupled effects of prestress and cyclic thermal shock, this study systematically examines the evolution of microscopic pore structure, damage behavior, and permeability in red sandstone subjected to 0.3 times its uniaxial compressive strength as prestress and repeated thermal cycles—up to 30 cycles of water and air cooling from 200&#xa0;°C—using low-field nuclear magnetic resonance (NMR) technology. The research results demonstrate that: First, the cooling rate is a critical factor in controlling the extent of damage. Water cooling generates a larger temperature gradient than air cooling, resulting in significantly greater attenuation of the wave velocity, higher pore growth rates, and increased damage variables. Second, the impact of thermal shock cycles on the pore structure shows a phased characteristic. During the initial 10 cycles, the number of mesopores increases rapidly; afterward, macropores develop prominently, with the average pore diameter growing exponentially. Pore tortuosity and spatial complexity initially decrease but then rise progressively. Third, based on the linear correlation between the NMR signals and P-wave velocity, a unified macro–microdamage variable correction model was developed. This model effectively resolves the over-limit issue inherent in the traditional NMR weighting method and substantially enhances measurement accuracy. Finally, the permeability increases exponentially with the number of thermal cycles and shows a positive correlation with the MRI entropy values, indicating that the increasing complexity of the pore space is the primary driver of enhanced permeability. This research is anticipated to offer effective guidance for assessing the operational lifespan of geothermal systems and the microstructural evolution of reservoir rocks.</p>

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Investigation of Microdamage and Seepage Evolution in Sandstone Under Coupled Thermomechanical Loading and Cooling Shocks via NMR

  • 期刊论文

  • 发表日期：

    2026年3月17日

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  • Lin Ning,
  • Jing Bi,
  • Yu Zhao,
  • Chaolin Wang,
  • Yongfa Zhang,
  • Lin Zhang,
  • Yuhang Zhao,
  • Xiaojiang Deng

摘要

To investigate the macroscopic and microscopic damage mechanisms of rocks under the coupled effects of prestress and cyclic thermal shock, this study systematically examines the evolution of microscopic pore structure, damage behavior, and permeability in red sandstone subjected to 0.3 times its uniaxial compressive strength as prestress and repeated thermal cycles—up to 30 cycles of water and air cooling from 200 °C—using low-field nuclear magnetic resonance (NMR) technology. The research results demonstrate that: First, the cooling rate is a critical factor in controlling the extent of damage. Water cooling generates a larger temperature gradient than air cooling, resulting in significantly greater attenuation of the wave velocity, higher pore growth rates, and increased damage variables. Second, the impact of thermal shock cycles on the pore structure shows a phased characteristic. During the initial 10 cycles, the number of mesopores increases rapidly; afterward, macropores develop prominently, with the average pore diameter growing exponentially. Pore tortuosity and spatial complexity initially decrease but then rise progressively. Third, based on the linear correlation between the NMR signals and P-wave velocity, a unified macro–microdamage variable correction model was developed. This model effectively resolves the over-limit issue inherent in the traditional NMR weighting method and substantially enhances measurement accuracy. Finally, the permeability increases exponentially with the number of thermal cycles and shows a positive correlation with the MRI entropy values, indicating that the increasing complexity of the pore space is the primary driver of enhanced permeability. This research is anticipated to offer effective guidance for assessing the operational lifespan of geothermal systems and the microstructural evolution of reservoir rocks.