Abstract Thermal stimulation for shale gas reservoirs may cause water to evaporate, accelerate desorption of adsorbed gas and induce micro-scale cracking in shale matrix, which will increase shale gas productivity. However, few researches have been focused on the thermal micro-cracking behavior of shale matrix. In this paper, a novel energy functional considering the effect of thermal contribution and initial stress field is established, which then is used to formulate coupled thermo-mechanical phase-field method by the variational principle. The validations with analytical, numerical and experimental results show reliability and robustness of presented model. The quartet structure generation set is used to reconstruct the heterogeneous shale matrix, including quartz, feldspar and dolomite. The image function recognizes pixel points to assign fixed parameter values for different minerals in COMSOL. The three reconstructed shale matrixes subjected quasi-static thermal shock are simulated based presented model. The simulation results indicate that a higher initial stress ratio can selectively suppress or accelerate crack propagation. The thermal expansion coefficient of different minerals can significantly affect the crack initiation temperature, crack pattern and thermal cracking efficiency. Furthermore, the presented phase-field can be extended easily and applied to coupled thermo-hydro-mechanical-chemo fracture problem in underground rock engineering.

中文翻译：

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热采过程中页岩基质的微裂纹行为：相场建模的见解

摘要 页岩气藏热增产可引起水分蒸发，加速吸附气解吸，引起页岩基质微裂化，提高页岩气产能。然而，关于页岩基质的热微裂化行为的研究很少。本文建立了一种新的考虑热贡献和初始应力场影响的能量泛函，然后利用变分原理将其用于制定耦合热-机械相场方法。分析、数值和实验结果的验证显示了所提出模型的可靠性和稳健性。四方结构生成装置用于重建非均质页岩基质，包括石英、长石和白云石。图像函数识别像素点，为 COMSOL 中的不同矿物分配固定参数值。基于所提出的模型，对受准静态热冲击的三个重建页岩基质进行了模拟。模拟结果表明，较高的初始应力比可以选择性地抑制或加速裂纹扩展。不同矿物的热膨胀系数对裂纹萌生温度、裂纹模式和热裂效率有显着影响。此外，所提出的相场可以很容易地扩展并应用于地下岩石工程中的热-水-机械-化学耦合断裂问题。模拟结果表明，较高的初始应力比可以选择性地抑制或加速裂纹扩展。不同矿物的热膨胀系数对裂纹萌生温度、裂纹模式和热裂效率有显着影响。此外，所提出的相场可以很容易地扩展并应用于地下岩石工程中的热-水-机械-化学耦合断裂问题。模拟结果表明，较高的初始应力比可以选择性地抑制或加速裂纹扩展。不同矿物的热膨胀系数对裂纹萌生温度、裂纹模式和热裂效率有显着影响。此外，所提出的相场可以很容易地扩展并应用于地下岩石工程中的热-水-机械-化学耦合断裂问题。