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The characterization of the failure process of induced fractures by hydraulic stimulation is fundamental to understanding the generation and evolution of the discrete fracture network within the reservoir. A more detailed analysis of the fracture mechanism can be a powerful tool for identifying fluid flow paths and proppant placement within the reservoir. For example, during the rupture process the energy release is partitioned into different physical processes for which the relative ratio is changed by the presence of fluids or rotations in the local stress field with relation to the frictional resistance in the fracture plane. Fractures occurring in the same host rock under the same stress conditions are expected to rupture similarly, independent of their size (self-similarity). Changes in the scaling relationships of fractures are indicative of a change in the failure process, host rock or in-situ stress. Correlation of failure process data with reservoir rock properties and the in-situ stress field will help identify regions within the reservoir with characteristic types of failures. Further correlation with hydrocarbon production data can be used to develop efficient treatment plans and production diagnostic tools.
In this study we investigate the failure process of ~ 27,000 microseismic (M < -1) fractures induced during a hydraulic fracturing shale completion program in NE British Columbia, Canada by estimating static and dynamic source parameters, such as dynamic and static stress drop, radiated energy, seismic efficiency, moment tensor, fracture plane orientation, slip direction and rupture velocity. On average, the microseismic events have low radiated energy, low dynamic stress and low seismic efficiency, consistent with the obtained slow rupture velocities. Events fail in overshoot mode (slip weakening failure model), with fluids lubricating faults and decreasing friction resistance. Events occurring in deeper formations tend to have faster rupture velocities and are more efficient in radiating energy. Variations in rupture velocity tend to correlate with variation in depth, fault azimuth and elapsed time, reflecting a dominance of the local stress field over other factors.
Further identification of spatial and temporal distribution of families of events with similar characteristic rupture behaviors, based on either rock formation, depth, source type, fracture plane orientation, stress drop, pad or proppant stage, may be used as a proxy for specific fracture network development and hydrocarbon production. This information may be used to determine reservoir properties, constrain reservoir geo-mechanical models with measured physical parameters, classify dynamic rupture processes for fracture models and improve fracture treatment designs. These will be the focus of future studies.
Abstract Induced seismicity resulting from fluid injection is a growing concern with a number of operations, including hydraulic fracturing. The vast majority of hydraulic stimulations results in no felt seismicity. However, three examples of larger, anomalous seismicity have been attributed to hydraulic fracturing, which seem to be associated with operations in unique geologic and geomechanical settings. In response, a number of operational protocols have been developed and include specific requirements for seismic monitoring. Seismological aspects are obviously central to these protocols, including characterizing the seismic source strength and associated seismic hazard. The typical microseismicity recorded during hydraulic fracturing represents a small portion of the hydraulic energy associated with the injection. However, the energy balance of the relative amount of seismic energy increases in the cases of anomalous seismicity, which may provide a monitoring tool to potentially help mitigate induced seismicity. Although the number of cases with anomalous seismicity is relatively small, other examples have been observed from geothermal stimulations. In these cases, the ratio of seismic energy is relatively larger but of potentially interest remains significantly less than the hydraulic energy. Furthermore, the ratio of seismic moment to injected volume also increases but typically remains less than a limit suggested by McGarr (1976). Potentially the energy and volume balances could be useful monitoring tools to assist in ongoing operation decision processes.
Besides the typical negative magnitude fractures induced in the treated formations by hydraulic fracture stimulation programs, small positive magnitude events associated to small faults located underneath or cross-cutting the reservoir also frequently occur. In this study we investigate discriminant source and rupture characteristics to distinguish between the two event types and also between reservoir fracture types. To achieve this goal we estimate source and failure properties of –M3 to M1 seismic events recorded during a hydraulic fracturing stimulation of a shale reservoir in Horn River Basin, Canada, for which the >M0 events are associated with slip on a pre-existing fault underneath the reservoir. Comparison between static and dynamic source parameters suggests distinct signatures of the two event types associated to two distinct failure processes. Positive magnitude events occurring beneath the reservoir have slightly higher static and dynamic stress drops, higher apparent stress and energy release, and rupture faster than shallower reservoir events. These differences reflect fracturing of harder rocks at higher confining stresses for the deep events, but also a possible release of a larger quantity of strain energy stored within the fault zone. The lower stress and energy release and slower rupturing fractures observed in the reservoir fractures, as well as overshoot type failure (slip weakening failure) indicates fluid lubrication by pore pressure increase and frictional resistance reduction. Some trends are also observed when looking only at reservoir fractures, where variations in average rupture velocities correlate with variations in formation depth and fault azimuth reflecting a dominance of the local stress field over other factors. Average rupture velocities also correlates with elapsed time showing an observable imprint of the changed local conditions during treatment over the regional conditions. Identification of more spatial and temporal families of events with similar rupture behaviors and source characteristics can be used as a proxy for specific fracture network development and hydrocarbon production and included in geo-mechanical models and fracture treatment designs. Reservoir and fault related induced events release less stress and radiates less energy than natural occurring tectonic earthquakes of comparable size at similar depths indicating a potential fluid influence in these failures. Considering the ongoing debate regarding the cause-effect relationship between fluid injection programs and nearby deeper earthquakes this study suggests that source parameters can be used as a discriminant factor between the two types of earthquakes.
Abstract Typically, seismic networks are deployed downhole to monitor injection-related activity and are designed to detect and analyze microseismic events at very short distances. During hydraulic fracture treatments, thousands of events can be generated with magnitudes in the range of -M4 to –M0. These networks record relatively high frequency signals (> 15Hz) and use time records with a short duration; however, they are limited in low-frequency response. This lack of bandwidth results in underestimated magnitudes for events with larger magnitudes (from about –M0 to +M4) that may occur during stimulations. To correct this issue, recording with high -sensitivity, near-surface sensors with low-frequency capabilities (>0.5Hz) extends the frequency range thereby allowing for the correct assessment of magnitude. In this paper, we investigate the spatial and temporal variations in seismicity associated with hydraulic fracture treatments of a shale play in North America using two different monitoring configurations, with multiple, fibre-optic based wireline arrays, each with twenty to forty-eight three-component levels of omni 15Hz geophones deployed vertically in close proximity to the treatment well and using a near-surface network of five stations consisting of three component Force Balance Accelerometers and 4.5Hz geophones. The near-surface monitoring network detected hundreds of events ranging in magnitude from M0 to M3, the largest of which were also recorded on a nearby national seismic network station. The downhole events did not see such large events, but recoreded thousands of events up to M0. Here, we compare parameters from the downhole data to the near-surface data to try to answer what role the larger events are playing in the reservoir and how they can be put into proper context with the smaller, downhole data. The failure mechanisms and scaling relationships for these large events suggest that they are the response of larger features in the reservoir, slipping in response to the stress perturbations induced by the treatment. Based on these observations, we suggest that a hybrid solution is appropriate for reservoirs with structural controls in order to appropriately assess the reservoir response to stimulations.
The regulations mandate suspension of operations generally use traffic light protocols (TLPs) with staged if an M 4 event is induced. Operations may continue after magnitude thresholds, which limit the maximum allowable the approval of the mitigation plan by the Commission. Due size of induced events to well below human perception level to high costs associated with interruption of well completion at surface. Such techniques primarily rely on the observation operations, implementation of an effective induced of preceding moderate-magnitude seismicity that would seismicity risk mitigation program is important.