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<p>Liquid nitrogen (LN<sub>2</sub>) fracturing has emerged as a promising technique for enhancing the permeability of hot dry rock (HDR) reservoirs. Since most geological formations exhibit varying degrees of anisotropy, this study investigates the fracture behavior of anisotropic rocks subjected to LN<sub>2</sub> treatment under thermal shock conditions. Gneiss, a foliated metamorphic rock, was selected as the representative anisotropic material. A series of mode I fracture toughness tests were conducted using semi-circular bend (SCB) specimens to evaluate the combined effects of temperature and anisotropy. Specimens were heated to five temperature levels, 25&#xa0;°C (room temperature), 100&#xa0;°C, 300&#xa0;°C, 500&#xa0;°C, and 700&#xa0;°C, followed by either air cooling or LN<sub>2</sub> cooling. To assess the influence of foliation, four anisotropy angles were considered: 0°, 30°, 60°, and 90°. The results show that fracture toughness generally decreases with increasing temperature. At a given temperature, toughness also increases with increasing anisotropy angle from 0° to 90°. LN<sub>2</sub>-cooled specimens exhibited a more pronounced reduction in fracture toughness compared to air-cooled ones under equivalent thermal and anisotropic conditions. Scanning electron microscopy (SEM) was used to examine the fracture process zone (FPZ) near the crack tip, revealing extensive microcrack development at elevated temperatures. Post-failure analysis of SCB specimens showed increasingly tortuous fracture paths at higher temperatures, particularly in samples with anisotropy angles of 60° and 90°.</p>

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Thermal Shock-Induced Microstructural and Fracture Evolution of Anisotropic Gneiss Under Air and Liquid Nitrogen Cooling

  • 期刊论文

  • 发表日期：

    2026年7月27日

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  • Amirreza Kavandi,
  • Kamran Goshtasbi,
  • Mir Raouf Hadei,
  • Hamid Reza Nejati

摘要

Liquid nitrogen (LN2) fracturing has emerged as a promising technique for enhancing the permeability of hot dry rock (HDR) reservoirs. Since most geological formations exhibit varying degrees of anisotropy, this study investigates the fracture behavior of anisotropic rocks subjected to LN2 treatment under thermal shock conditions. Gneiss, a foliated metamorphic rock, was selected as the representative anisotropic material. A series of mode I fracture toughness tests were conducted using semi-circular bend (SCB) specimens to evaluate the combined effects of temperature and anisotropy. Specimens were heated to five temperature levels, 25 °C (room temperature), 100 °C, 300 °C, 500 °C, and 700 °C, followed by either air cooling or LN2 cooling. To assess the influence of foliation, four anisotropy angles were considered: 0°, 30°, 60°, and 90°. The results show that fracture toughness generally decreases with increasing temperature. At a given temperature, toughness also increases with increasing anisotropy angle from 0° to 90°. LN2-cooled specimens exhibited a more pronounced reduction in fracture toughness compared to air-cooled ones under equivalent thermal and anisotropic conditions. Scanning electron microscopy (SEM) was used to examine the fracture process zone (FPZ) near the crack tip, revealing extensive microcrack development at elevated temperatures. Post-failure analysis of SCB specimens showed increasingly tortuous fracture paths at higher temperatures, particularly in samples with anisotropy angles of 60° and 90°.