Study on the Microscale Tensile Properties of Lower Cambrian Niutitang Formation Shale Based on Digital Images

Abstract

Tensile strength is an important parameter that affects the initiation and propagation of shale reservoir fractures during hydraulic fracturing. Shale is often filled with minerals such as calcite. To explore the effect of calcite minerals on the tensile strength and failure mode of shale, in this paper, lower Cambrian shale cores were observed by microslice observations and core X-ray whole-rock mineral diffraction analysis, and 7 groups of numerical direct tensile tests were performed on simulated shale samples with different azimuth angles. The test results show that as the azimuth angle α increases, the tensile strength of the samples gradually decreases, and the fracture rate also shows a decreasing trend. The failure modes can be summarized as root-shaped (0° and 15°), step-shaped (30 and 45°), fishbone-shaped (60°), and river-shaped (75° and 90°) fracturing. The smaller the azimuth angle α, the easier it is for hydraulic fractures to propagate along the direction of the calcite veins and inhibit the formation of fracture networks in the shale matrix. Considering the correlation between the acoustic emission characteristics and failure mode, the fractal dimension is used to reflect the microscopic failure mode of shale. The larger the fractal dimension, the higher the fracture rate is, the more microcracks exist at the edge of the main crack, the more severe the internal damage is, and the more complex the failure mode of the sample is. As the azimuth angle α increases, the fractal dimension shows a decreasing trend, and the crack becomes smoother. This research has important reference value for the study of hydraulic fracture initiation mechanisms and natural fracture propagation.

Direct tensile damage process of calcite vein shale based on 3D CT reconstruction

“…The Yangzi platform has been influenced by multiple tectonic movements of the Xuefeng, Early-Middle Garidon, Late Garidon, Haixi, Yinzhi, Yanshan, and Xishan movements, resulting in a complex tectonic pattern. 13 Figure 1 shows the tectonic map of the northern part of Guizhou. As shown in Figure 1, 21,22 the folds and fracture structures are very developed in the northern part of Guizhou Province, and the folds are mainly north-east oriented on the whole, while north-south, east-west, and north-west folds and fractures are also developed.…”

“…The northern part of Guizhou is geotectonically located in the Upper Yangzi Platform area, which is consistent with the regional tectonic evolution of the Yangzi Platform. The Yangzi platform has been influenced by multiple tectonic movements of the Xuefeng, Early‐Middle Garidon, Late Garidon, Haixi, Yinzhi, Yanshan, and Xishan movements, resulting in a complex tectonic pattern 13 . Figure 1 shows the tectonic map of the northern part of Guizhou.…”

“…Song et al used twodimensional rock damage process analysis (RFPA 2D ) to numerically simulate uniaxial tension in shales containing calcite veins and found that calcite veins inhibit fracture extension in hydraulic fracturing. 13 However, the actual rock fracture process occurs in three dimensions, and the two-dimensional images lose much of the three-dimensional (3D) spatial information, making it difficult for the researcher to reproduce the complete structure and changes within the specimen, which greatly limits the validity of the simulation results and their conformity to real conditions. 14,15 Computed tomography (CT) microscopic imaging is a nondestructive testing method that uses the penetrating ability of X-rays to analyze the interior of an object to obtain information such as internal images of rocks, minerals, and pore distribution.…”

“…The variation of the damage pattern of shale with the azimuth of calcite veins under two‐dimensional loading conditions was investigated, and the damage characteristics of shale were effectively characterized by the fractal dimension. Song et al used two‐dimensional rock damage process analysis (RFPA 2D ) to numerically simulate uniaxial tension in shales containing calcite veins and found that calcite veins inhibit fracture extension in hydraulic fracturing 13 . However, the actual rock fracture process occurs in three dimensions, and the two‐dimensional images lose much of the three‐dimensional (3D) spatial information, making it difficult for the researcher to reproduce the complete structure and changes within the specimen, which greatly limits the validity of the simulation results and their conformity to real conditions 14,15 …”