Abstract Conventional hydraulic fracturing often causes problems such as reservoir damage, water consumption, and pollution. Recently, liquid nitrogen (LN2) fracturing, an environmentally friendly fracturing technology, has attracted more attention. In this paper, the mechanism of surface crack propagation in coal and sandstone induced by LN2 is explored through laboratory tests and numerical simulations. An ultrasound detection analyser is employed to investigate the longitudinal wave velocity ( V p ) changes in specimens before and after LN2 cooling. The micromorphology of specimens is observed through scanning electron microscopy (SEM). Furthermore, the heat transfer characteristics and damage zone of specimens induced by LN2 are determined. The experimental results indicate that LN2 cooling is more effective for improving coal permeability than it is for improving sandstone permeability. After LN2 cooling, a complex fracture network is observed from the coal surface. However, limited damage to the sandstone surface is observed. V p decreases by 24.7%–38.1% for coal specimens after cooling but by less than 0.06% for sandstone. The simulation results indicate that the low-temperature region and the tensile area gradually expand into the interior of the specimens with time. However, the temperature of sandstone drops faster than coal when specimens contact LN2. The maximum tensile stress induced by the instantaneous contact between the specimens and LN2 can be generated at the outer surface of the specimens. Moreover, the maximum damage area of coal extends 3.3 mm from the outer surface, while the damage zone of the sandstone is almost zero.

中文翻译：

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液氮低温冷却煤和砂岩地表裂纹扩展机理研究

摘要 常规水力压裂常造成储层破坏、耗水、污染等问题。近年来，液氮（LN2）压裂作为一种环保型压裂技术受到越来越多的关注。本文通过室内试验和数值模拟，探讨了LN2诱导煤和砂岩表面裂纹扩展的机理。采用超声检测分析仪研究 LN2 冷却前后试样的纵波速度 (Vp) 变化。通过扫描电子显微镜（SEM）观察样品的显微形态。此外，还确定了由 LN2 引起的试样的传热特性和损伤区。实验结果表明，LN2 冷却对提高煤的渗透率比提高砂岩渗透率更有效。LN2 冷却后，从煤表面观察到复杂的裂缝网络。然而，观察到对砂岩表面的损坏有限。煤样冷却后V p 下降24.7%~38.1%，砂岩下降不到0.06%。模拟结果表明，随着时间的推移，低温区和拉伸区逐渐扩展到试样内部。然而，当试样接触 LN2 时，砂岩的温度下降速度比煤快。试样与 LN2 瞬时接触引起的最大拉应力可在试样的外表面产生。而且，煤炭的最大破坏范围扩大了3。