Experimental Investigation on the Morphology of Fracture Networks in Hydraulic Fracturing for Coal Mass Characterized by X-ray Micro-Computed Tomography

Hydraulic fracture (HF) network at the meso-macro scale is crucial in the development of coalbed methane contained in coal seams with low permeability. The characterization of a real meso-macro HF network in coal mass helps to further understand its formation mechanism. The meso-macro morphology of different HFs in the HF network of coal mass is characterized by the X-ray micro-computed tomography according to different factors such as principles stress, joints, and natural cracks. The basic law of HF network growth under the stress disturbance of main hydraulic fracture (MHF) is discussed. The results show that: the MHF has an approximate shape of an oblate spheroid. The shear stress concentration and the deflection of the maximum principal stress emerge in the near field under the stress disturbance of MHF. Along the deflected maximum principal stress, the branching hydraulic fracture (BHF) laterally initiates. Induced by the water pressure and the stress disturbance of adjacent deflecting HF or penetrating HF, the bedding and cleats near the tip would be prone to open under the effect of a water wedge, resulting in the propagation of the HF network. The natural cracks may cause the deflection of approaching HF and the natural cracks linking with the HF network are found to continue to propagate at the tip, contributing to the extension of the HF network. The BHF, the opened bedding and cleats, the new HFs which connect with natural cracks and continue to extend, and the MHF form the HF network of coal mass at the meso-macro scale under the stress disturbance of MHF.