Laboratory experiments are performed on Laizhou granite samples after heating and rapid water-cooling treatment to investigate the thermal effects on the physical and mechanical properties and fracture initiation of granite during the utilization of hot dry rock. Experimental results show that P-wave velocity and tensile strength monotonously decrease because of the generation of thermally induced microcracks when the thermal treatment level increases from 25 to 500 °C. Scanning electron microscopy analysis indicates that few and isolated microcracks are observed before 300 °C, whereas microcrack networks are generated at 400 °C and 500 °C. Porosity and gas permeability sharply increase with the enhancement in the density and connectivity of microcracks. The variation trend of compressive strength tends to show a transition from increasing to decreasing at the threshold temperature. Uniaxial compressive strength and cohesion increase and the Young’s modulus slightly changes before 300 °C. Afterward, the three parameters decrease, indicating that the dominant mechanism transforms from thermal hardening to thermal cracking. Failure process of uniaxial compression shows that the proportion of crack closure region in the pre-peak stage gradually increases, whereas those of the elastic and stable crack growth regions decrease after 300 °C with the increase of the thermal treatment level. Experimental results of laboratory fracturing tests show that the decreasing tendency of breakdown pressure is similar to that of tensile strength. The pressure build-up rate substantially decreases because of the enhancement in fluid leak-off, and hydraulic fracture initiation occurs before the breakdown of granite samples when the thermal treatment levels are higher than 300 °C. Shear failure becomes dominant based on the hypocenter mechanism analysis of acoustic emission events.

中文翻译：

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水力压裂过程中热效应对莱州花岗岩物理力学性能及断裂起裂的影响

对莱州花岗岩样品进行加热和快速水冷处理后的室内试验，研究了干热岩利用过程中热对花岗岩物理力学性能和断裂起始的影响。实验结果表明，当热处理水平从 25°C 增加到 500°C 时，由于热致微裂纹的产生，P 波速度和抗拉强度单调下降。扫描电子显微镜分析表明，在 300°C 之前观察到很少且孤立的微裂纹，而在 400°C 和 500°C 时会产生微裂纹网络。随着微裂纹密度和连通性的增强，孔隙度和气体渗透率急剧增加。抗压强度的变化趋势在阈值温度处趋于呈现由增加到减少的转变。在 300 °C 之前，单轴抗压强度和内聚力增加，杨氏模量略有变化。之后，三个参数下降，表明主导机制从热硬化转变为热开裂。单轴压缩破坏过程表明，随着热处理水平的提高，在峰值前阶段裂纹闭合区的比例逐渐增加，而弹性和稳定裂纹扩展区的比例在300 ℃后逐渐减少。实验室压裂试验结果表明，破坏压力的下降趋势与抗拉强度的下降趋势相似。由于流体泄漏的增强，压力建立率显着降低，当热处理水平高于 300°C 时，水力压裂发生在花岗岩样品破裂之前。基于声发射事件的震源机制分析，剪切破坏成为主导。