The results from two laboratory hydraulic fracture experiments are presented in this paper. Experiments were conducted in a tri-axial geophysical imaging cell equipped with 18 broadband acoustic emission transducers. Samples were loaded to a differential stress of 115 MPa and then distilled water (1 cP) was injected at a constant flow rate through a central cased borehole until failure. Two flow rates, 0. 2 mL/min and 0.5 mL/min were investigated and their failure sequences were analyzed in detail. A flow rate of 0.2 mL/min caused gradual failure with AE source locations aligning on two conjugate planes and AE activity lasting for ~120 ms. Using an injection rate of 0.5 mL/min initiated a two stage fracture process. AE hypocenters initially located around the borehole and then rapidly moved to the edge of the sample along a vertically oriented plane. The final burst of AE activity resembled a tremor-like-signal and lasted for approximately 22 ms. Frequency analysis of continuous acoustic  emission waveforms show that the maximum energy is initially held at high frequency and gradually shift to lower frequencies as the fracturing nears completion. X-ray computed tomography scans revealed complex fracture geometry.

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