Hydraulic fracturing induced hybrid earthquakes in the Montney Basin, British Columbia, Canada may mark the transition from aseismic slip near the wellbore to seismic slip at greater distances

2020 ◽  
Author(s):  
Rebecca M. Harrington ◽  
Hongyu Yu ◽  
Honn Kao ◽  
Yajing Liu ◽  
Bei Wang
Keyword(s):  
British Columbia ◽  
Hydraulic Fracturing ◽  
Stress Drop ◽  
High Frequency ◽  
Local Network ◽  
Maximum Magnitude ◽  
Strengthening Effect ◽  
Seismic Velocities ◽  
Well Bore ◽  
Seismicity Rates

<p>Seismicity related to fluid injection during unconventional oil and gas exploration has increased dramatically in North America in the last decade. The Western Canadian Sedimentary Basin experienced a significant increase in the number of M3+ earthquakes, including several M4+ associated with high-pressure stimulation during Hydraulic Fracturing (HF) activity. The vigorous seismic response to injection activity and low historical seismicity rates pose critical questions as to the triggering mechanism(s) and seismic hazard assessment in the affected areas. To monitor seismicity linked to injection, a dense local network of eight broadband seismic stations was installed in 2015 at distances of ~2 km around an active well pad with the purpose of monitoring seismicity prior to, and following, a HF well stimulation in the Montney Play in British Columbia, Canada. Here we present an earthquake source process study using observations from the local station network, and provide evidence for a slow-rupture seismic signal which may bridge the spectrum of fault slip rates from aseismic near the well bore, to typical seismic velocities at distances beyond ~1 km.</p><p>Initial detection and relocation of seismicity between May 28 – October 15, 2015 yielded 350 well-constrained hypocenters of high-frequency events with a maximum magnitude of M<sub>w</sub> 1.8 that resemble typical tectonically generated earthquakes. The detection procedure also yielded a total of 31 events with high-frequency (or broadband) onsets, that transition to protracted, low-frequency ringing relative to event magnitude, which we term hybrids. Both hybrid and high-frequency events occur at similar depths to the active well bore and at distances of ~1-2 km from injection stages, yet exhibit varying source characteristics in spite of their proximal source volumes. Hybrid waveforms are marked by broader P- and S-wave arrival pulse shapes, and spectral fitting suggests that the stress drop values are roughly an order of magnitude lower than high-frequency events, with average static stress drop values of 0.3 MPa and 4.9 MPa, respectively. We interpret wider phase arrival pulse widths and lower stress drop values as resulting from lower rupture velocities of hybrid events relative to high-frequency events. A dilatant strengthening effect would be expected in close proximity to the well bore, and near the hybrid sources, where material is weaker and pore pressures are elevated, which may result in slower rupture propagation when slip is initiated relative to further distances where material damage and pore pressure perturbation are both lower. Thus, hybrid earthquakes may mark regions where slip velocities transition from aseismic sliding directly next to the well bore, which has been observed in laboratory and meso-scale experiments, to typical seismic velocities at further distances. The size-duration scaling of the induced hybrids observed here also extends the scaling of slow earthquakes occurring in tectonic fault transition zones, and may provide the first observations to extend the scaling down to seismic moment values of ~10<sup>10</sup>.</p>

Insights into downhole array spectra of hydraulic-fracturing induced seismicity in the Horn River Basin, British Columbia

2020 ◽  
Author(s):  
Adam Klinger ◽  
Max Werner
Keyword(s):  
British Columbia ◽  
Hydraulic Fracturing ◽  
Stress Drop ◽  
High Frequency ◽  
Source Parameters ◽  
Low Frequency ◽  
Corner Frequency ◽  
Downhole Array ◽  
Horn River Basin ◽  
Downhole Arrays

<p>Hydraulic fracturing underpins tight shale gas exploration but can induce seismicity. During stimulations, operators carefully monitor the spatio-temporal distribution and source parameters of seismic events to be able to respond to any changes and potentially reduce the chances of fault reactivation. Downhole arrays of geophones offer unique access to (sub) microseismic source parameters and can provide new insights into the processes that induce seismicity. For example, variations in stress drop might indicate changes in the seismic response to injection (e.g. pore pressure variations). However, borehole arrays of geophones and the high frequencies of small events also present new challenges for source characterization. Stress drop depends on the corner frequency, a parameter with great uncertainty that is sensitive to attenuation, especially for (sub-) microseismicity. Here, we explore the behavior of microseismic spectra measured along borehole arrays and the effect of attenuation on estimates of corner frequency. We examine a dataset of over 90,000 microseismic events recorded during hydraulic fracturing in the Horn River Basin, British Columbia. We only see clear phase arrivals for events M<sub>w</sub> > -1 and restrict our initial analysis to a subsample of M<sub>w</sub>> 0 events that vary in space and time.</p><p>Our first observation is that some stations in the borehole array show an unexpected increase in the displacement energy from the low frequency to the corner frequency in the P and SH phases as well as high-frequency energy spikes inconsistent with a smooth Brune source model. A shorter time window that only captures the direct arrival results in a flatter low frequency plateau and reduces the amplitude of the pulses but compromises the resolution. The spikes may be caused by high frequency coda energy. We also find that corner frequency estimates decrease with decreasing station depth along the array in both the P and SH phases, a likely result of high frequency attenuation along the downhole array. The findings suggest Brune corner frequencies of moment magnitudes < 0.5 may not be resolvable even with downhole arrays at close proximity. Our results will eventually contribute to a better characterization of microseismic source parameters measured in borehole arrays.</p><p> </p>

Seismic b-value within the Montney Play of Northeast British Columbia, Canada

2021 ◽  
Author(s):  
Alireza Babaie Mahani
Keyword(s):  
British Columbia ◽  
Hydraulic Fracturing ◽  
Induced Seismicity ◽  
B Value ◽  
Systematic Difference ◽  
Magnitude Distribution ◽  
Seismicity Rate ◽  
Northern Area ◽  
Seismicity Rates ◽  
Frequency Magnitude Distribution

Critical analysis of induced earthquake occurrences requires comprehensive datasets obtained by dense seismographic networks. In this study, using such datasets, I take a detailed investigation into induced seismicity that occurred in the Montney play of northeast British Columbia, mostly caused by hydraulic fracturing. The frequency-magnitude distribution (FMD) of earthquakes in several temporal and spatial clusters, show fundamental discrepancies between seismicity in the southern Montney play (2014-2018) and the northern area (2014-2016). In both regions, FMDs follow the linear Gutenberg-Richter (G-R) relationship for magnitudes up to 2-3. While in the southern Montney, within the Fort St. John graben complex, the number of earthquakes at larger magnitudes falls off rapidly below the G-R line, within the northern area with a dominant compressional regime, the number of events increases above the G-R line. This systematic difference may have important implications with regard to seismic hazard assessments from induced seismicity in the two regions, although caution in the interpretation is warranted due to local variabilities. While for most clusters within the southern Montney area, the linear or truncated G-R relationship provide reliable seismicity rates for events below magnitude 4, the G-R relationship underestimates the seismicity rate for magnitudes above 3 in northern Montney. Using a well-located dataset of earthquakes in southern Montney, one can observe generally that 1) seismic productivity correlates well with the injected volume during hydraulic fracturing and 2) there is a clear depth dependence for the G-R b-value; clusters with deeper median depths show lower b-values than those with shallower depths.

A Study on the Largest Hydraulic-Fracturing-Induced Earthquake in Canada: Observations and Static Stress-Drop Estimation

2020 ◽  
Author(s):  
Bei Wang ◽  
Rebecca M. Harrington ◽  
Yajing Liu ◽  
Honn Kao ◽  
Hongyu Yu
Keyword(s):  
Hydraulic Fracturing ◽  
Stress Drop ◽  
Risk Mitigation ◽  
Source Parameters ◽  
B Value ◽  
Corner Frequency ◽  
Maximum Magnitude ◽  
Maximum Earthquake Magnitude ◽  
Tectonic Earthquakes ◽  
Maximum Earthquake

ABSTRACT On 17 August 2015, an Mw 4.6 earthquake occurred northwest of Fort St. John, British Columbia, possibly induced by hydraulic fracturing (HF). We use data from eight broadband seismometers located ∼50  km from the hypocenter to detect and estimate source parameters of more than 300 events proximal to the mainshock. Stress-drop values estimated using seismic moment and corner frequency from single-event spectra and spectral ratios range from ∼1 to 35 MPa, within the typical range of tectonic earthquakes. We observe an ∼5-day delay between the onset of fluid injection and the mainshock, a b-value of 0.78 for the sequence, and a maximum earthquake magnitude larger than the prediction based on the total injection volume, suggesting that the Mw 4.6 sequence occurred on a pre-existing fault and that the maximum magnitude is likely controlled by tectonic conditions. Results presented here show that pre-existing fault structures should be taken into consideration to better estimate seismic hazard associated with HF operations and to develop schemes for risk mitigation in close proximity to HF wells.

Investigation of regional seismicity before and after hydraulic fracturing in the Horn River Basin, northeast British Columbia

2015 ◽  
Vol 52 (2) ◽  
pp. 112-122 ◽  
Author(s):  
Amir Mansour Farahbod ◽  
Honn Kao ◽  
Dan M. Walker ◽  
John F. Cassidy
Keyword(s):  
British Columbia ◽  
Hydraulic Fracturing ◽  
River Basin ◽  
Seismic Station ◽  
Depth Resolution ◽  
Earthquake Catalog ◽  
Maximum Magnitude ◽  
Peak Period ◽  
Physical Relationship ◽  
Horn River Basin

We systematically re-analyzed historical seismograms to verify the existence of background seismicity in the Horn River Basin of northeast British Columbia before the start of regional shale gas development. We also carefully relocated local earthquakes that occurred between December 2006 and December 2011 to delineate their spatiotemporal relationship with hydraulic fracturing (HF) operations in the region. Scattered seismic events were detected in the Horn River Basin throughout the study periods. The located seismicity within 100 km of the Fort Nelson seismic station had a clearly increasing trend, specifically in the Etsho area where most local HF operations were performed. The number of events was increased from 24 in 2002–2003 (prior to HF operations) to 131 in 2011 (peak period of HF operations). In addition, maximum magnitude of the events was shifted from ML 2.9 to ML 3.6 as the scale of HF operation expanded from 2006–2007 to 2011. Based on our relocated earthquake catalog, the overall b value is estimated at 1.21, which is higher than the average of tectonic/natural earthquakes of ∼1.0. Our observations highly support the likelihood of a physical relationship between HF operation and induced seismicity in the Horn River Basin. Unfortunately, due to the sparse station density in the region, depth resolution is poor for the vast majority of events in our study area. As new seismograph stations are established in northeast British Columbia, both epicentral mislocation and depth uncertainty for future events are expected to improve significantly.

Latent seismicity driven by aseismic creep and enhanced pore-fluid pressure in NE British Columbia

2021 ◽  
Author(s):  
Rebecca O. Salvage ◽  
David W. Eaton
Keyword(s):  
British Columbia ◽  
Hydraulic Fracturing ◽  
Pore Pressure ◽  
Fluid Pressure ◽  
Fluid Injection ◽  
Maximum Magnitude ◽  
Dynamic Triggering ◽  
Global Pandemic ◽  
Pressure Diffusion ◽  
Montney Formation

<p>The global pandemic of COVID-19 furnished an opportunity to study seismicity in the Kiskatinaw area of British Columbia, noted for hydraulic-fracturing induced seismicity, during a period of anthropogenic quiescence. A total of 389 events were detected from April to August 2020, encompassing a period with no hydraulic-fracturing operations during a government-imposed lockdown. During this time period, observed seismicity had a maximum magnitude of M<sub>L</sub> 1.2 and lacked temporal clustering that is often characteristic of hydraulic-fracturing induced sequences. Instead, seismicity was persistent over the lockdown period, similar to swarm-like seismicity with no apparent foreshock-aftershock type sequences. Hypocenters occurred within a corridor orientated NW-SE, just as seismicity had done in previous years in the area, with focal depths near the target Montney formation or shallower (<2.5 km). Based on the Gutenberg-Richter relationship, we estimate that a maximum of 21% of the detected events during lockdown may be attributable to natural seismicity, with a further 8% possibly due to dynamic triggering of seismicity from teleseismic events. The remaining ~70% cannot be attributed to direct pore pressure increases induced by fluid injection, and therefore is inferred to represent latent seismicity i.e. seismicity that occurs after an unusually long delay following primary activation processes, with no obvious triggering mechanism. We can exclude pore-pressure diffusion from the most recent fluid injection, as is there is no clear pattern of temporal or spatial seismicity migration. If elevated pore pressure from previous injections became trapped in the subsurface, this could explain the localization of seismicity within an operational corridor, but it does not explain the latency of seismicity on a timescale of months. However, aseismic creep on weak surfaces such as faults, in response to tectonic stresses, in addition to trapped elevation pore-pressure could play a role in stress re-loading to sustain the observed pattern of seismicity.</p>

Testing hypotheses of stress drop variations with hydraulic fracturing induced seismicity in the Horn River basin

2021 ◽  
Author(s):  
Adam Klinger ◽  
Joanna Holmgren ◽  
Max Werner
Keyword(s):  
Hydraulic Fracturing ◽  
Stress Drop ◽  
High Frequency ◽  
Induced Seismicity ◽  
Fluid Injection ◽  
Magnitude Range ◽  
Source Models ◽  
Stress Drops ◽  
Horn River Basin

<div> <p>Source parameters can help constrain the causes and mechanics of induced earthquakes. In particular, systematic variations of stress drops of fluid-injection induced seismicity have been interpreted in terms of the role of fluids, differences between tectonic and induced events, and self-similarity. The empirical basis for the variations, however, remains controversial. Here, we test three hypotheses about stress drops with observations of seismicity induced by hydraulic fracturing in the Horn River basin (Canada). First, stress drop is self-similar and independent of magnitude. Second, stress drop increases with distance from the point of fluid injection, which might be expected if in-situ effective stresses increase away from the point of fluid injection. Third, stress drops estimated with empirical Green’s functions (EGFs) are systematically larger than those estimated from direct fits to source models, which is expected if seismic waves attenuate in a frequency-dependent manner or experience site effects.</p> </div><div> <p>We probe the hypotheses with a large microseismic dataset collected during hydraulic fracturing operations in the Horn River shale gas play in British Columbia. 90,000+ seismic events were recorded by three borehole geophone arrays with a moment magnitude range of -3 < M<sub>w </sub>< 0.5. To calculate corner frequencies, we assume small, co-located seismic events can be approximated as EGFs, which effectively remove propagation and site effects from a larger target event. We target 34 M<sub>w</sub> > 0 events and search for EGFs over a 100 m radius for each event, choosing only those EGFs that satisfy multiple quality criteria. This study builds on previous work that estimated stress drops from direct fitting of standard Brune source models and found systematic high frequency resonances recorded by the geophones.</p> </div><div> <p>Of the 34 target events, we retrieve corner frequency and stress drop estimates for 22 events to test the three hypotheses. We observe that stress drop appears relatively constant over M<sub>w </sub>, but the magnitude range (0 < M<sub>w </sub>< 0.5) is currently too limited to draw strong conclusions. Second, stress drop appears to decrease, rather than increase, with distance from the point of injection (with a moderate Pearson’s correlation co-efficient of -0.5 ± 0.2); this could be caused by a direct hydraulic connection causing a reduction of in-situ effective normal stresses distal to the point of injection. Third, we observe no systematic difference between stress drops from direct source fits and EGF-based estimates, although stress drop uncertainties are large compared to standard earthquake source studies because of limited azimuthal coverage and high-frequency instrument resonances. These initial results do not support the systematic variations of stress drop for fluid-injection induced seismicity that have been observed in other datasets.</p> </div>

scholarly journals Stress Drop, Seismogenic Index and Fault Cohesion of Fluid-Induced Earthquakes

2021 ◽  
Author(s):  
Serge A. Shapiro ◽  
Carsten Dinske
Keyword(s):  
Stress Drop ◽  
Induced Seismicity ◽  
Quantitative Characteristic ◽  
High Stress ◽  
Operation Time ◽  
Average Stress ◽  
Induced Earthquakes ◽  
Effective Stresses ◽  
Seismicity Rates

AbstractSometimes, a rather high stress drop characterizes earthquakes induced by underground fluid injections or productions. In addition, long-term fluid operations in the underground can influence a seismogenic reaction of the rock per unit volume of the fluid involved. The seismogenic index is a quantitative characteristic of such a reaction. We derive a relationship between the seismogenic index and stress drop. This relationship shows that the seismogenic index increases with the average stress drop of induced seismicity. Further, we formulate a simple and rather general phenomenological model of stress drop of induced earthquakes. This model shows that both a decrease of fault cohesion during the earthquake rupture process and an enhanced level of effective stresses could lead to high stress drop. Using these two formulations, we propose the following mechanism of increasing induced seismicity rates observed, e.g., by long-term gas production at Groningen. Pore pressure depletion can lead to a systematic increase of the average stress drop (and thus, of magnitudes) due to gradually destabilizing cohesive faults and due to a general increase of effective stresses. Consequently, elevated average stress drop increases seismogenic index. This can lead to seismic risk increasing with the operation time of an underground reservoir.

Combining Distributed Acoustic Sensing and Beamforming in a Volcanic Environment on Mount Meager, British Columbia.

2021 ◽  
Author(s):  
Sara Klaasen ◽  
Patrick Paitz ◽  
Jan Dettmer ◽  
Andreas Fichtner
Keyword(s):  
British Columbia ◽  
Power Distribution ◽  
High Frequency ◽  
Low Frequency ◽  
Volcanic Tremor ◽  
Extreme Environment ◽  
Volcanic Complex ◽  
Acoustic Sensing ◽  
Distributed Acoustic Sensing ◽  
Volcanic Environment

<p>We present one of the first applications of Distributed Acoustic Sensing (DAS) in a volcanic environment. The goals are twofold: First, we want to examine the feasibility of DAS in such a remote and extreme environment, and second, we search for active volcanic signals of Mount Meager in British Columbia (Canada). </p><p>The Mount Meager massif is an active volcanic complex that is estimated to have the largest geothermal potential in Canada and caused its largest recorded landslide in 2010. We installed a 3-km long fibre-optic cable at 2000 m elevation that crosses the ridge of Mount Meager and traverses the uppermost part of a glacier, yielding continuous measurements from 19 September to 17 October 2019.</p><p>We identify ~30 low-frequency (0.01-1 Hz) and 3000 high-frequency (5-45 Hz) events. The low-frequency events are not correlated with microseismic ocean or atmospheric noise sources and volcanic tremor remains a plausible origin. The frequency-power distribution of the high-frequency events indicates a natural origin, and beamforming on these events reveals distinct event clusters, predominantly in the direction of the main peaks of the volcanic complex. Numerical examples show that we can apply conventional beamforming to the data, and that the results are improved by taking the signal-to-noise ratio of individual channels into account.</p><p>The increased data quantity of DAS can outweigh the limitations due to the lower quality of individual channels in these hazardous and remote environments. We conclude that DAS is a promising tool in this setting that warrants further development.</p>

A dynamic model for far-field acceleration

1982 ◽  
Vol 72 (4) ◽  
pp. 1049-1068
Author(s):  
John Boatwright
Keyword(s):  
Stress Drop ◽  
High Frequency ◽  
Dynamic Stress ◽  
The Body ◽  
Body Waves ◽  
Far Field ◽  
Rupture Velocity ◽  
Frequency Level ◽  
Average Change ◽  
Dynamic Stress Drop

abstract A model for the far-field acceleration radiated by an incoherent rupture is constructed by combining Madariaga's (1977) theory for the high-frequency radiation from crack models of faulting with a simple statistical source model. By extending Madariaga's results to acceleration pulses with finite durations, the peak acceleration of a pulse radiated by a single stop or start of a crack tip is shown to depend on the dynamic stress drop of the subevent, the total change in rupture velocity, and the ratio of the subevent radius to the acceleration pulse width. An incoherent rupture is approximated by a sample from a self-similar distribution of coherent subevents. Assuming the subevents fit together without overlapping, the high-frequency level of the acceleration spectra depends linearly on the rms dynamic stress drop, the average change in rupture velocity, and the square root of the overall rupture area. The high-frequency level is independent, to first order, of the rupture complexity. Following Hanks (1979), simple approximations are derived for the relation between the rms dynamic stress drop and the rms acceleration, averaged over the pulse duration. This relation necessarily depends on the shape of the body-wave spectra. The body waves radiated by 10 small earthquakes near Monticello Dam, South Carolina, are analyzed to test these results. The average change of rupture velocity of Δv = 0.8β associated with the radiation of the acceleration pulses is estimated by comparing the rms acceleration contained in the P waves to that in the S waves. The rms dynamic stress drops of the 10 events, estimated from the rms accelerations, range from 0.4 to 1.9 bars and are strongly correlated with estimates of the apparent stress.

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