Damage precursors during hydraulic fracturing in shale gas reservoirs may be better understood if the deformation, failure, and acoustic emission (AE) characteristics under cyclic loading are known. Therefore, the purpose of this paper is to investigate the quantitative damage based on the Felicity effect under constant stress lower limit uniaxial cyclic loading-unloading rates (0.5, 1.0, 1.5, 2.0, and 2.5 kN/s). Variations in the b-value and the spatiotemporal evolution of cumulative AE were also used to observe how shale fractures formed. The findings reveal that during the unloading stage, there are many cumulative AE events when the stress level is low (≤1.50 kN/s) but that this number drops significantly when the stress level increases above (>2.0 kN/s). The AE amplitude, AE counts, and cumulative AE energy of each cycle in a loading-unloading test show an increasing trend, but the rate increases in the last cycle. During the whole process of loading and unloading, the Kaiser effects were present in the 3rd cycle at stress levels (≤1.5 kN/s). Still, the Felicity effect appeared in the 2nd and 1st cycles during 2.0 and 2.5 kN/s cyclic loading. The Kaiser effect occurs in the linear elastic stage, while the Felicity effect occurs in the crack initiation and crack damage stage. Furthermore, the Felicity ratio (FR) variations during shale deformation and failure can be divided into four phases: (Phase I = 1.01 ≥ FR > 0.89), (Phase II = 0.89 ≥ FR > 0.48), (Phase III = 0.48 ≥ FR > 0.23), and (Phase IV = FR ≤ 0.23). The b-value is relatively higher under the loading rate below (≤1.50 kN/s), indicating an increase in the number of small AE events. In contrast, the fact that the b-value is relatively smaller under the loading rate above (>2.0 kN/s) indicates that, the number of large AE events increases the number of cracks and fractures. These findings provide important design references for damaged precursors during hydraulic fracturing in shale gas reservoirs.

中文翻译：

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基于福祉效应的页岩损伤前兆演化


如果已知循环载荷下的变形、破坏和声发射（AE）特征，则可以更好地理解页岩气储层水力压裂过程中的损伤前兆。因此，本文的目的是研究恒应力下限单轴循环加卸载速率（0.5、1.0、1.5、2.0和2.5 kN/s）下基于Felicity效应的定量损伤。 b 值的变化和累积 AE 的时空演化也被用来观察页岩裂缝是如何形成的。研究结果表明，在卸载阶段，当应力水平较低（≤1.50 kN/s）时，会出现许多累积AE事件，但当应力水平增加至高于（>2.0 kN/s）时，该数字显着下降。加卸载试验中每个周期的AE幅值、AE计数和累积AE能量均呈现增加趋势，但最后一个周期的速率有所增加。在整个加载和卸载过程中，在第3个循环的应力水平（≤1.5 kN/s）下存在凯撒效应。尽管如此，在 2.0 和 2.5 kN/s 循环加载期间，Felicity 效应出现在第二个和第一个循环中。 Kaiser效应发生在线弹性阶段，而Felicity效应发生在裂纹萌生和裂纹破坏阶段。此外，页岩变形破坏过程中的幸福比（FR）变化可分为四个阶段：（阶段I=1.01≥FR>0.89），（阶段II=0.89≥FR>0.48），（阶段III=0.48≥FR） > 0.23），（第四阶段 = FR ≤ 0.23）。在以下加载速率（≤1.50 kN/s）下，b值相对较高，表明小AE事件的数量有所增加。相比之下，在上述加载速率下（>2.0 kN/s）表明，大AE事件的数量增加了裂纹和断裂的数量。这些发现为页岩气藏水力压裂过程中受损前驱体的设计提供了重要参考。