Combined effects of bedding anisotropy and matrix heterogeneity on hydraulic fracturing of shales from Changning and Lushan, South China: An experimental investigation

Abstract

Shales are intrinsically anisotropic and heterogeneous due to the existence of bedding planes and natural microfractures. Although the roles of shale bedding anisotropy and inherent heterogeneity have been well studied, their combined effects remain unclear. To fill this knowledge gap, we performed six hydraulic fracturing tests on shales with varying bedding inclinations (0°, 45° and 90°) collected from Changing (Sichuan) and Lushan (Jiangxi), South China. The mineral components and microstructure of shales collected from these two places were compared. The results indicate that the shale from Lushan is more heterogeneous due to its higher content of microfractures and pores than the shale from Changning. Through hydraulic fracturing analysis, we show that the fracture initiation pressure and breakdown pressure first increase and then decrease with increasing bedding inclination, and a strong linear correlation between the two pressures is found to be independent of shale heterogeneity. Second, we find inconsistency between the high breakdown pressure and large hoop deformation, which can be attributed to the anisotropy effect of shale’s bedding strength relative to the maximum principal stress. In addition, the analysis of the amplitudes and frequency of acoustic emission signals suggests that tensile cracks preferentially occur in a short-transverse mode, while deviating the bedding plane from the axial direction suppresses the initiation of tensile cracks. Shale heterogeneity is mainly reflected in reducing the magnitude of critical pressures and deflecting local fracture branch behaviors, which are less significant than the impacts of bedding anisotropy during the hydraulic fracturing process.