Acoustic Emission Response of Laboratory Hydraulic Fracturing in Layered Shale

Understanding the generation process of complex fracture network is essential for optimizing the hydraulic fracturing strategy in shale formations. In this study, laboratory fracturing was performed on shale specimens containing multiple bedding planes (BPs) combined with acoustic emission (AE) monitoring and computerized tomography scanning techniques. The injection pressure curve and the time dependency and hypocenter mechanisms of AE events in different stages were analyzed in detail. The relationship between AE spatial localization and hydraulically connected region were then further quantitatively discussed. Experimental results show that the characteristics of the pressure curve and AE response reflect well the hydraulic fracture (HF) growth behavior in layered shale. Shear events were detected around some weak BPs far away from the wellbore before the HF initiation. Stable injection pressure and a few AE events with low amplitude along the BP may indicate the stage of fluid leak-off. Numerous shear and tensile AE events and drastic pressure changes occur during the generation of a fracture network including breakdown in rock matrix and activation of multiple BPs. The shear instability of weak BPs caused by the stress perturbation during pressurization and HF growth tends to result in overestimation of the effective stimulated reservoir volume/hydraulically connected region.