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Microseismic monitoring of laboratory hydraulic fracturing experiments in granitic rocks 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 (HF) is a widely employed process, the underlying fracturing processes are not clearly understood. Laboratory HF experiments with seismic monitoring can help with better understanding of the relationship between the generated HF network and the induced microseismicity while taking into account the effect of different HF parameters (injection fluid type and rate, stress conditions). In this study, HF experiments were performed on true-triaxially loaded Barre granite cubes, with real-time microseismic monitoring, to identify and characterize the stimulation processes associated with the viscosity and toughness dominated HF 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 microseismicity from toughness propagation regime experiments resulted in slightly larger b-value (2.25 compared to 2), indicating higher percentage of small magnitude events. The spatio-temporal evolution of fracture mechanisms indicated very dominant tensile fracturing (82-85%) during fracture initiation phase surrounding the injection region. As the fracture propagated away from the injection borehole, the number of shear and mixed-mode fracturing events increased. Overall, tensile fractures were dominant in both propagation regimes (ranging from 52% to 58%), which can be attributed maily to the absence of significant pre-existing faults/discontinuities in the very low permeability granite rock.