Evaluation of fracture morphology of shale in Brazilian tests and analysis of power spectral characteristics

The quantitative evaluation of the fracture morphology of shale is an important prerequisite for assessing the complexity of hydraulic fracturing fracture networks. The Brazilian tests coupled with digital image correlation and acoustic emission technique are conducted on black shale, the corresponding relationships between the characteristics of the frequency band of acoustic emission power spectra and the micro-damage mechanism of rock specimens are established, and the fracture morphology is quantitatively evaluated. The results show that the bedding layer leads to the differences of power spectral characteristics, micro-damage mechanism and fracture morphology of shale. The tension and shear failures of shale matrix induce high-frequency acoustic emission signals, and the tension and shear failures of shale bedding induce low-frequency acoustic emission signals. With the increase of the angle between the bedding layer and the loading direction, the dominant frequencies and the secondary dominant frequencies gradually diffuse from the low-frequency band to the high-frequency one, and the quantitative ratio of high frequency to low frequency H：L gradually increases. The H：L of 0^° shale specimen is 4.28%∶95.72%, and the fracture is a straight line in shape; the H：L of 30^° and 60^° shale specimens are 15.89%∶84.11% and 36.93%∶63.07%, respectively, and their fractures are arched in shape; the H：L of 90^° specimen is 93.85%∶6.15%, and the fracture is a composited arc-straight line in shape. The results can provide references for analyzing in situ micro-seismic data and a theoretical basis for controlling fracture trajectory in hydraulic fracturing in shale reservoirs.