Propagation and characterization of the micro cracks induced by hydraulic fracturing in shale

Hydraulic fracturing is commonly used for the exploitation of shale gas. However, the mechanism of permeability increase after the fracturing has still remained unclear so far. In this study, a hydraulic fracturing experiment was performed on Longmaxi shale specimens in order to create the hydraulic fractures. In order to investigate the cracks at micro-scale, several small slices were collected from the fractures surface and scanned with the scanning electron microscope (SEM). The regions of interest were captured through SEM for analyzation and investigation of the micro cracks. The captured images show that the densely distributed micro cracks can increase the permeability by connecting pores with macro fractures. Micro cracks generally propagate along the boundaries of hard mineral grains and soft organic matters. We selected three parameters: length, tortuosity and branching to quantitatively characterize the cracks at micro-scale, which generally obey to the logarithmic normal distribution. (C) 2019 Elsevier Ltd. All rights reserved.