An Experimental Analysis of Microcrack Generation during Hydraulic Fracturing of Shale

Hydraulic fracturing is commonly applied in the shale gas exploitation industry. However, the mechanical mechanism of permeability under fracturing has so far been unclear. In this study, an analysis of laboratory experiments on hydraulic fracture propagation and bedding plane reactivation in shale is presented. To investigate microcrack occurrence under fracturing, several small slices were collected from the fracture surface and scanned with a scanning electron microscope (SEM). It was found that observed microscopic microcracks could not be produced by fluid pressure as the latter generated compressive stresses at the site of microcracks. Rather, the microcracks were produced by tensile stress concentration in front of the propagating fracture. This implies that bedding plane reactivation was caused by fracture propagation along the plane. An analysis of microcrack lengths showed that shale exhibited anisotropy in fracture toughness with resistance to fracture propagation parallel to bedding planes being twice as small compared to resistance to fracture propagation in the direction normal to bedding planes.