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Effect of frequency during cyclic hydraulic fracturing and the process of fracture development in laboratory experiments
International Journal of Rock Mechanics and Mining Sciences ( IF 7.2 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.ijrmms.2020.104474
Melvin B. Diaz , Kwang Yeom Kim , Sung Gyu Jung

Abstract Cyclic injection in hydraulic fracturing has been reported to reduce the level of induced seismicity compared to the conventional continuous injection. We investigate the evolution of acoustic emission (AE) activity, hydraulic energy, fracture area, and fracture development for cyclic injection schemes with different cycle time durations (td) at a repeated maximum cyclic pressure through laboratory experiments. These test were compared with a case under conventional continuous injection at an injection rate of 50 mm3/s. Pocheon granite samples with an outer diameter of 50 mm, height of 100 mm, and an inner borehole of 8 mm were used. The maximum pressure of 5.5 MPa for each cycle represents approximately 77% of the average breakdown pressure from continuous injection cases. Four td values of 30 s, 60 s, 120 s, and 240 s were selected. The results show an exponential decrease in the number of cycles until failure with increasing td. Similarly, the total number of AEs, the peak AE rate, and the cumulative AE energy also decreased with increasing td. These values contrasted with those for continuous injection which reported the lowest number of AEs, and the highest cumulative AE energy. The td influence was also evident on the AE occurrence, with fewer high amplitude AEs at a high td, and all lower than continuous injection. In general, the hydraulic energy and cumulative AE energy decreased with increased with increasing td, although a slight deviation was related to the total testing time. The fracture areas during cyclic injection reported a positive correlation with the cumulative AE energy. Moreover, detailed analysis of the instantaneous injection rate, and the cumulative AEs suggested a three-stage damage process, which was supported with X-ray computed tomography observations. The second stage was characterized by a steady linear injection rate increase, and its slope correlated with the fatigue life or number of cycles to failure. Finally, based on the injection rate evolution, an interpretation of the damage evolution and fracture onset during cyclic hydraulic fracturing could be made.

中文翻译:

室内试验中循环水力压裂频率的影响及裂缝发育过程

摘要 据报道,与传统的连续注入相比,水力压裂中的循环注入可以降低诱发地震活动的水平。我们通过实验室实验研究了在重复最大循环压力下具有不同循环持续时间 (td) 的循环注入方案的声发射 (AE) 活动、水力能、裂缝面积和裂缝发育的演变。将这些测试与在 50 mm3/s 注入速率下的常规连续注入情况下进行比较。使用外径为 50 mm、高度为 100 mm、内孔为 8 mm 的抱川花岗岩样品。每个循环 5.5 MPa 的最大压力代表了连续注入情况下平均击穿压力的大约 77%。选择了 30 s、60 s、120 s 和 240 s 四个 td 值。结果表明,随着 td 的增加,直至失效的循环数呈指数减少。同样,AE 总数、峰值 AE 率和累积 AE 能量也随着 td 的增加而减少。这些值与连续注射的值形成对比,后者报告了最低数量的 AE,以及最高的累积 AE 能量。td 对 AE 的影响也很明显,在高 td 时高振幅 AE 较少,并且都低于连续注入。一般来说,水力能量和累积 AE 能量随着 td 的增加而减少,尽管与总测试时间略有偏差。循环注入期间的裂缝面积与累积 AE 能量呈正相关。此外,对瞬时喷射率的详细分析,累积的 AE 表明了一个三阶段的损伤过程,这得到了 X 射线计算机断层扫描观察的支持。第二阶段的特点是稳定的线性注射率增加,其斜率与疲劳寿命或失效循环数相关。最后,基于注入速率演化,可以对循环水力压裂过程中的损伤演化和裂缝开始进行解释。
更新日期:2020-10-01
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