Abstract
Shale formations suffer dynamic loading and unloading stresses during hydraulic fracturing processes that cause cracks to open and shear-slip to develop, while simultaneously producing microseismicity. Incremental cyclic loading tests were conducted on shale specimens using various stress levels and measured via real-time acoustic emission (AE) monitoring at the laboratory scale because AE parameters are useful in understanding heterogeneous material damage behaviour. Parallel (θ = 0°) and vertically (θ = 90°) cored shale specimens were cyclically loaded from lower stress levels of 0.3 and 0.47 and higher stress levels of 0.8 and 0.84, respectively, to analyze the effects of the bedding plane and loading level on the fracture process. Based on temporal variation of the AE amplitude in response to the stress paths, the longer AE activity quiet period noted during the loading stage in each cycle for 90° shale specimens shows that the Kaiser effect is more pronounced for vertically cored specimens than for parallel cored specimens. This phenomenon is also evidenced by the smaller load ratios observed with θ = 0° (as compared to θ = 90°) specimens. This indicates easier crack generation at lower stresses and is independent of the loading path. Meanwhile, high calm ratios indicate the initiation of numerous cracks during the unloading stage. Therefore, the load ratio can be combined with the calm ratio to evaluate the difficulty of crack generation in shale. In addition, the AE amplitude distribution shows that the proportion of large cracks increases when shale specimens are loaded from higher stress levels. Moreover, the average frequency and RA value (obtained from the rise time and amplitude) were used to characterize damage mechanisms. A decreasing average frequency and increasing RA value for parallel cored specimens from lower stress levels or small average frequency and large RA value from higher stress levels both indicate the dominant shear type that contributes to the failure process and causes splitting along the bedding plane. The higher average frequency and lower RA values noted for vertically cored specimens imply that tensile modes control the failure process and cause finally side-step main fracture plane across the bedding plane.
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Acknowledgements
This work was supported by the Natural Science Foundation of Shaanxi Province (2020JQ-356) and the Fundamental Research Funds for the Central Universities, CHD (No.300102219113).
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Wang, M., Shao, X., Zhu, L. et al. Use of acoustic emission to determine the effects of bedding and stress paths on micro-cracking evolution of anisotropic shale under cyclic loading tests. Environ Earth Sci 80, 476 (2021). https://doi.org/10.1007/s12665-021-09761-w
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DOI: https://doi.org/10.1007/s12665-021-09761-w