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For tectonic earthquakes, slip rate spans a continuum from creep to supershear earthquakes, where slow slip events (SSEs) are important in releasing stress without radiating damaging seismic energy. Industrial-scale subsurface fluid injection has caused induced earthquakes, but the role of SSEs in fault activation is currently unclear. Ground-deformation observations, measured by satellite radar, show that SSEs up to magnitude 5.0 occurred during hydraulic fracturing (HF) operations in northwestern Canada, corroborated by reported deformation of the steel well casing. Although the magnitude 5.0 SSE exceeded the magnitude of the largest induced earthquake in this region (magnitude 4.55), it was undetected by seismograph networks. The observed SSEs occurred within a buried thrust belt and their magnitude and duration are consistent with scaling behavior of SSEs in unbounded natural systems, e.g. slab interfaces in subduction zones. [ABSTRACT FROM AUTHOR]

One of the largest induced seismic events attributed to hydraulic-fracturing operations, with a moment magnitude (M w) of 4.1, occurred on 12 January 2016 in Alberta, Canada. The hydraulic-fracturing treatment was monitored using a dense array of 93 shallow wellbores, each containing three geophones, producing a high-resolution data set that is presented here as a case study. Over 9700 smaller events were observed both before and after the event, with a magnitude of completeness of approximately M w −1.9. The use of perforation shots with known locations enabled refinement of the velocity model, such that event hypocentres could be located with high accuracy and precision. Most of the hypocentres of induced events were located within the sedimentary section, 100s of metres above the treatment zone. Event locations of the foreshocks and aftershocks reveal fault activation along both small and large structures. The mechanisms and locations suggest a complex north–south oriented strike-slip fault system that we interpret to be part of a regional flower structure. The b -value for the data set is close to unity, as expected for fault activation events; however, the three largest magnitude events deviate significantly from the fit, suggesting that it would be difficult to forecast these larger events based on the Gutenberg–Richter relation. This has significant implications for the mitigation of hydraulic-fracturing induced seismicity. The three largest events appear to be located on fault segments that are less optimally oriented for slip than those imaged by other event clusters, suggesting that larger events require a larger perturbation to the stress state in order to nucleate. Monitoring setups with the capability of the system presented here, if operated in real time, could be of great value to discern fault activation due to seismicity aligning on planar structures prior to a main shock. This approach may have considerable potential to improve short-term forecasts of induced seismicity. [ABSTRACT FROM AUTHOR]

Development of a resource play is subject to intrinsic risks. These may include the risk of incurring unexpected costs or shutdowns due to anomalous induced seismicity, or risks associated with reservoir stimulation that differs from prior empirical or numerical models. These two risks may be related causally, as completion effectiveness could be adversely affected by loss of fracture fluids into fault systems and/or damage zones, irrespective of the occurrence of induced seismicity. We outline a novel strategy for pre-operational risk assessment that is guided by the interpretation of 3-D seismic data, principles of structural geology and available induced seismicity data from surrounding operations. We integrate regional seismic structural mapping with results of previous experimental studies to develop a corridor-based risk-assessment approach. This approach is illustrated by a case study from the Duvernay play in Alberta, Canada, where subtle basement-rooted strike-slip faults and associated Riedel shears have been activated during previous hydraulic-fracturing completion programs. Unlike previous case studies in this area, we combine analysis of several adjoining 3-D seismic and induced seismicity datasets in order to map regional risk corridors. Given the general infeasibility of exhaustive detection and mapping of all potentially seismogenic faults, our approach relies on established structural principles, analog settings (flower structures) and analysis of fault-slip potential to quantify the probability of activation within a defined corridor that contains interpreted faults. • pre-existing faulting can be evaluated for fault slip potential for fluid injection, or hydraulic fracture operations • the fault reactivation risk assesment used here incorporated 3-D seismic reflection mapping and microseismic hypocenteres • the formation ananalised here was the Duvernay Formation, near Fox Creek, Alberta Canada • the fault types identified in this study were, NNW bounding faults, N-S shear, and NNE trending Riedel shears • larger magnitude seismic events were associated with N-S faults, smaller events were associated with NNE tfault trends • fault slip potential modeling of fault reacivation explained the correlation between event magnigude, fault type and b value [ABSTRACT FROM AUTHOR]