53rd U.S. Rock Mechanics/Geomechanics Symposium,
New York City, New York
2019. American Rock Mechanics Association
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ABSTRACT: A laboratory hydraulic fracture experiment was conducted on a Montney shale specimen under the conventional triaxial compressional stress condition. Acoustic emissions (AEs) were recorded using 7 piezoelectric transducers throughout the duration of the test to investigate the hydraulic fracture propagation processes. Distilled water was injected at an average rate of 2.5 mL/min until the sample failed. As expected, a hydraulic fracture formed perpendicular to the externally applied minimum principal stress. Due to the nature of tensile fractures, the relative plasticity of the Montney shale lithology compared to more brittle granitic specimens used in many previous studies, and the reverberatory character of the recorded waveforms, AE seismicity was relatively sparse (600 events) and only a small fraction (about 6%) of AE events could be located. Nevertheless, the hydraulic fracture plane in the middle of the specimen was clearly evident through the hypocenter location plot. In general, locating the AE hypocenters is very challenging compared with granitic materials due to the low signal-to-noise ratio of the waveforms emitted from shale during the hydraulic fracturing process. Moreover, event amplitudes recorded in this experiment are significantly lower than in a previously reported AE experiment where failure occurred under compression.
Hydraulic fracturing has been widely recognized as an efficient method in the development of unconventional oil and gas reservoirs. Microseismic monitoring is useful for imaging the spatial and temporal changes of fracture network (Eaton, 2018). These two techniques have been used extensively in extracting shale gas from ultra-low permeable shale reservoirs, which usually exhibit transversely isotropic characteristics and shale/fluid interactions (Lal, 1999). Current simulation models are often limited by the inability to consider the complex shale behaviors in reality (Weng, 2015), which makes it important to elucidate shale responses to hydraulic fracturing based on experimental observations.
Acoustic emissions (AE) are considered to be a proxy of induced microseismicity in higher frequencies (Mogi, 1967), and can therefore provide a further insight on the failure process of hydraulic fractures. Previous studies have demonstrated that the hydraulic fracturing process in shale can be estimated from the sample mechanical behaviour and the AE spatial-temporal distributions (Stanchits et al., 2014; Liang et al., 2016). This study aims to document the hydraulic fracturing process in a Montney shale specimen during fluid injection while monitoring the AE activity to examine the processes of crack propagations. Laboratory setup and procedure are introduced in the first section, and then the results are presented including fracture geometry, AE hypocenters and temporal changes.
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