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Micro-seismic monitoring of scaled laboratory hydraulic fracturing experiments for different fracture propagation regimes
  • Awais Butt,
  • Ahmadreza Hedayat,
  • Omid Moradian
Awais Butt
Colorado School of Mines

Corresponding Author:ambutt@mines.edu

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Ahmadreza Hedayat
Colorado School of Mines
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Omid Moradian
ETH Z├╝rich
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Abstract

While hydraulic fracturing is a widely employed process, the underlying fracturing processes are not clearly understood. Scaled laboratory hydraulic fracturing experiments with seismic monitoring can help with better understanding of the relationship between the generated hydraulic fracture network and the induced micro-seismicity while taking into account the effect of different HF parameters (injection fluid type and rate, stress conditions). In this study, hydraulic fracturing experiments were performed on true-triaxially loaded Barre granite cubes, with real-time micro-seismic monitoring, to identify and characterize the stimulation processes associated with the viscosity and toughness dominated hydraulic fracturing propagation regimes. Water and gear oil were used as the fracturing fluids. Moment tensor inversion technique was employed to determine the fracture mechanisms (tensile, shear, or mixed-mode). Viscosity propagation regime experiments involved higher breakdown pressures and larger injection fluid volumes relative to toughness propagation regime experiments. The micro-seismicity from toughness propagation regime experiments resulted in relatively larger b-value (2.35 compared to 1.62), indicating dominance of small magnitude events. Overall, tensile fractures were dominant in both propagation regimes (ranging from 52% to 58%), which can be attributed to the very low permeability of the granite rock. These results indicate that even for a relatively impermeable rock, theoretical assumptions of mode-I tensile fracturing and the scaling analysis may only be applicable to the near borehole region and as the fracture propagates away from the borehole, the fracturing pattern varies depending on the locally encountered conditions.