Hydraulic-fracturing induced seismicity in Alberta, Canada: Analysis and interpretation using dense local arrays
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AbstractUnconventional oil and gas development is associated with anthropogenic earthquake activity worldwide. In particular, hydraulic fracturing, which involves the injection of high-pressure fluids into the subsurface, has been observed to trigger induced seismicity in some areas. The Duvernay Formation in Alberta is a target of hydraulic fracturing and has experienced induced earthquakes up to magnitude 4.2. To study hydraulic fracturing-induced seismicity in greater detail, a local dense array of sensors was used to collect four weeks of passive seismic data over a 4-well pad. In this thesis, this dataset is processed and interpreted using a variety of techniques. Supplementary information, such as waveform data from broadband seismometers and an accelerometer that were co-located with the dense geophone array, is also used to aid the interpretation. In some cases, it is found that the magnitude-frequency distribution obtained from the monitored seismicity deviates from the expected power-law form associated with the Gutenberg-Richter relationship. In particular, the largest earthquakes, greater than Mw 3.0, are larger than statistically expected given the observed seismicity. Spectral analysis of a subset of events reveals complex source spectra. For example, some of the events show evidence of a double corner frequency and a source time function with multiple peaks. A detailed analysis of the timing of events relative to the hydraulic fracturing stages reveals that natural fracture networks within the Ireton Shale, overlying the target Duvernay Formation, provided permeable conduits for an expanding region of elevated pore pressure. Using simple diffusive modelling, it is shown that these fracture networks allowed pore pressure to perturb fault systems at distances of over 1 km, consistent with observations of fault activation during treatment. In two datasets that are used in this thesis, shear-wave splitting analysis of microseismic waveforms provides evidence for stress rotation near the largest faults. Integration of these investigations provides a basis for a new working model for induced seismicity in the Fox Creek, Alberta, area.
CitationIgonin, N. (2021). Hydraulic-fracturing induced seismicity in Alberta, Canada: Analysis and interpretation using dense local arrays (Unpublished doctoral thesis). University of Calgary, Calgary, AB.
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