ABSTRACT:

Laboratory-scale experiments were performed on cubic shale specimens containing multiple bedding planes (BPs) to investigate into dynamics of hydraulic fracturing. Acoustic emission (AE) monitoring and computerized tomography scanning techniques were used to analyze the failure behavior and obtain the ultimate geometry of internal hydraulic fracture (HF). The generation process of a complex fracture network was revealed by analyzing time dependency between AE activity and injection pressure. Experimental results showed that shear events were induced prior to tensile events and could be detected around weak BPs far away from the wellbore before HF initiation. When the fracturing fluid slowly leaked into the BPs, the injection pressure remained stable and AE activity became low. Only a few AE events with low amplitude were located along the BP near the wellbore. Numerous shear and tensile AE events and drastic pressure changes occurred during the generation of the fracture network because of the breakdown of rock matrix and the activation of BPs. Statistical results indicated that shear failure were dominant. In addition, the shear instability of weak BPs caused by the stress perturbation during pressurization and HF growth resulted in a shear event zone in the hydraulically disconnected region.

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