页岩巴西劈裂裂缝形态评价及功率谱特征分析

定量评价页岩破坏的裂缝形态是评估水力压裂裂缝网络复杂程度的重要前提。通过对黑色页岩试件进行巴西劈裂试验,同时结合数字图像相关技术和声发射技术,建立声发射功率谱频带特征与页岩试件微损伤机制的对应关系,并对裂缝形态进行定量评价。结果表明：层理是页岩功率谱特征、微损伤机理、裂缝形态差异的根本原因。页岩基质张拉、剪切破坏引起高频声发射信号,页岩层理张拉、剪切破坏引起低频声发射信号。随层理与加载方向之间的夹角角度增加,主频、次主频逐渐从低频带向高频带扩散,且高低频数量比H：L也逐渐增加。0°页岩试件的H：L为4.28%∶95.72%,裂缝为直线形;30°和60°页岩试件的H：L分别为15.89%∶84.11%和36.93%∶63.07%,裂缝为圆弧形;90°页岩试件的H：L为93.85%：6.15%,形成圆弧-直线复合型裂缝。研究结果将为解析现场微震数据提供借鉴,为控制页岩储层水力压裂裂缝轨迹提供理论基础。

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.