Moment-Tensor Analysis of Seismicity Related to Hydraulic Fracturing in North America
Date
2019-01-30
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Abstract
Various types of seismic arrays are being used to monitor seismicity associated with oil and gas operations. These different types of recording systems exhibit significantly different characteristics in azimuthal coverage, magnitude detection threshold, signal-to-noise ratio and waveform frequency content. Taking these factors into account, this thesis focuses on implementing source-mechanism inversions for three typical monitoring geometries including the regional seismic network, sparse surface array and dense shallow borehole array. With the aim of investigating potential indicators for the discrimination between fluid-injection induced seismicity and natural earthquakes in the Western Canada Sedimentary Basin, the waveform-fitting-based moment-tensor inversion is performed for eight induced earthquakes with M > 3 and a nearby M 5.3 inferred natural earthquake, all of which were recorded by regional seismic networks. Based on the inverted parameters, the focal depth is found to be the most robust for parameter to distinguish the induced seismicity from natural earthquakes since the induced events considered here are significantly shallower than one observed nature event and most intraplate earthquakes in the Canadian Shield. Moreover, in addition to a dominant double-couple (DC) mechanism this is common to nearly all events, non-negligible non-DC components (typically > 25%) are observed within the moment-tensor solutions for most of the induced events. To overcome limitations of typically low signal-to-noise ratio and poor azimuthal coverage for the sparse surface array, a novel regularized approach is developed to estimate a composite focal mechanism from a set of microearthquakes recorded by this type of array. It operates by minimizing the weighted misfits of both SH/P amplitude ratios (in absolute sense and logarithmic scale) and P-wave polarities, using a regularization parameter determined from the trade-off curve for these values. The regularized approach reduces the multiplicity of solutions and avoids the use of signed amplitude ratios, which may be ambiguous for data with low signal-to-noise ratio. For the hydraulic-fracturing induced event sequences recorded by a dense shallow borehole array near Fox Creek area, a least-squares inversion based on 3C P-wave amplitudes has been implemented, which avoids fitting the relatively high-frequency waveforms and the picking of S-wave amplitudes that are contaminated by P-wave coda. The recovered source mechanisms are dominantly strike-slip with sub-vertical nodal planes, although a distinct cluster of events is characterized by more complex mechanisms with slip on a shallow-dipping plane accompanied by significant (> 30%) non-DC components. The non-DC components may be due to tensile crack opening and/or co-slipping on several fault strands that have been mapped using 3D seismic data. In addition, in the absence of direct stress measurements, moment-tensor solutions of the hydraulic-fracturing induced event sequences are used to estimate the local stress field near the Fox Creek area. The estimated orientation of SHmax differs from the median regional SHmax direction by ~ 15º, but it agrees with the nearest available borehole measurement from the World Stress Map. Mohr circle analysis indicates that N-S trending faults, which hosted the largest events (MW > 1.5), are mis-oriented for slip and required a relatively large increase in pore pressure (12 ± 4 MPa) in order to be brought to a state of incipient failure.
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Keywords
Moment-Tensor Inversion, Stress Inversion, Hydraulic Fracturing, Induced Seismicity, Microearthquake
Citation
Zhang, H. (2019). Moment-tensor analysis of seismicity related to hydraulic fracturing in North America (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.