Coalbed methane (CBM) plays an indispensable role in total natural gas production and consumption, and for the bottlenecks encountered in stimulating CBM reservoirs by conventional fracturing methods, studying more environment-friendly and efficient stimulation methods is imperative. Cryogenic fracturing using liquid nitrogen has been trialed for the development of coalbed methane, demonstrating great significance as more and more research corroborated. This study extended our previous coupled thermo-mechanical-damage (TMD) numerical procedure to investigate the cryogenic fracturing or damage process in 3D coal cubes. The cryogenic fractures on concrete cubes along with analytical solutions of temperature and first principle stress distributions for the borehole cooling process were first employed to verify the correctness and feasibility of this method, which incorporates the boundary conditions affected by the Leidenfrost effect at the initial stage. Then sensitivity analyses of cryogenic fracture generation including the maximum fracture cross-sectional area and the fractured volume were carried out. Results show that the coal rock with a high modulus of elasticity and coefficient of thermal expansion is more prone to be damaged in terms of the cross-sectional area and volume than other parameters within the parameter range analyzed. Damage cross-sectional area and damage volume expand with both the increasing coefficient of thermal expansion and the increasing modulus of elasticity, while they shrink with increasing heat capacity and coal density. Cryogenic fractures are relatively less sensitive to the thermal conductivity, density, and heat capacity of coal. This 3D modeling study provides a technical route and a panorama of the cryogenic fracturing process under varied conditions in opaque coal rocks.

中文翻译：

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真三轴应力下煤中液氮低温开裂的 3D 建模

煤层气在天然气生产和消费总量中发挥着不可或缺的作用，针对传统压裂方法改造煤层气藏遇到的瓶颈，研究更加环保、高效的改造方法势在必行。液氮低温压裂技术已在煤层气开发中进行了尝试，越来越多的研究证实其意义重大。这项研究扩展了我们之前的耦合热机械损伤 (TMD) 数值程序，以研究 3D 煤立方体中的低温破裂或损伤过程。首先利用混凝土立方体的低温断裂以及钻孔冷却过程的温度和第一主应力分布的解析解来验证该方法的正确性和可行性，该方法考虑了初始阶段受莱顿弗罗斯特效应影响的边界条件。然后进行了低温裂缝产生的敏感性分析，包括最大裂缝横截面积和裂缝体积。结果表明，在分析的参数范围内，具有高弹性模量和热膨胀系数的煤岩在横截面积和体积方面比其他参数更容易受到破坏。损伤截面积和损伤体积随热膨胀系数和弹性模量的增大而增大，随热容量和煤密度的增大而减小。低温裂缝对煤的导热系数、密度和热容的敏感度相对较低。这项3D建模研究提供了不透明煤岩中不同条件下的低温压裂过程的技术路线和全景。