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.

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.