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>

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>

Fracture Initiation and Propagation in a Deep Shale Gas Reservoir Subject to an Alternating-Fluid-Injection Hydraulic-Fracturing Treatment

SPE Journal ◽  
2019 ◽  
Vol 24 (04) ◽  
pp. 1839-1855 ◽  
Author(s):  
Bing Hou ◽  
Zhi Chang ◽  
Weineng Fu ◽  
Yeerfulati Muhadasi ◽  
Mian Chen
Keyword(s):  
Hydraulic Fracturing ◽  
Shale Gas ◽  
Fracture Initiation ◽  
Fluid Injection ◽  
Hydraulic Fractures ◽  
Stress Difference ◽  
Gas Reservoirs ◽  
Complex Fracture ◽  
Initiation And Propagation

Summary Deep shale gas reservoirs are characterized by high in-situ stresses, a high horizontal-stress difference (12 MPa), development of bedding seams and natural fractures, and stronger plasticity than shallow shale. All of these factors hinder the extension of hydraulic fractures and the formation of complex fracture networks. Conventional hydraulic-fracturing techniques (that use a single fluid, such as guar fluid or slickwater) do not account for the initiation and propagation of primary fractures and the formation of secondary fractures induced by the primary fractures. For this reason, we proposed an alternating-fluid-injection hydraulic-fracturing treatment. True triaxial hydraulic-fracturing tests were conducted on shale outcrop specimens excavated from the Shallow Silurian Longmaxi Formation to study the initiation and propagation of hydraulic fractures while the specimens were subjected to an alternating fluid injection with guar fluid and slickwater. The initiation and propagation of fractures in the specimens were monitored using an acoustic-emission (AE) system connected to a visual display. The results revealed that the guar fluid and slickwater each played a different role in hydraulic fracturing. At a high in-situ stress difference, the guar fluid tended to open the transverse fractures, whereas the slickwater tended to activate the bedding planes as a result of the temporary blocking effect of the guar fluid. On the basis of the development of fractures around the initiation point, the initiation patterns were classified into three categories: (1) transverse-fracture initiation, (2) bedding-seam initiation, and (3) natural-fracture initiation. Each of these fracture-initiation patterns had a different propagation mode. The alternating-fluid-injection treatment exploited the advantages of the two fracturing fluids to form a large complex fracture network in deep shale gas reservoirs; therefore, we concluded that this method is an efficient way to enhance the stimulated reservoir volume compared with conventional hydraulic-fracturing technologies.

Simultaneous inversion for Q and source parameters of microearthquakes accompanying hydraulic fracturing in granitic rock

1991 ◽  
Vol 81 (2) ◽  
pp. 553-575 ◽  
Author(s):  
Michael Fehler ◽  
W. Scott Phillips
Keyword(s):  
Hydraulic Fracturing ◽  
Stress Drop ◽  
Source Parameters ◽  
Low Frequency ◽  
Seismic Energy ◽  
P Wave ◽  
Corner Frequency ◽  
Spectral Amplitude ◽  
Seismic Zone ◽  
Stress Drops

Abstract An inversion that fits spectra of earthquake waveforms and gives robust estimates of corner frequency and low-frequency spectral amplitude has been used to determine source parameters of 223 microearthquakes induced by hydraulic fracturing in granodiorite. Assuming a ω−2 source model, the inversion fits the P-wave spectra of microearthquake waveforms to determine individual values of corner frequency and low-frequency spectral amplitude for each event and one average frequency-independent Q for all source-receiver paths. We also implemented a constraint that stress drops of all microearthquakes be similar but not equal and found that this constraint did not significantly degrade the quality of the fits to the spectra. The waveforms analyzed were recorded by a borehole seismometer. The P-wave Q was found to be 1070. For Q values as low as 600 and as high as 3000, the misfit between model and spectra increased by less than 5 per cent and the average corner frequency changed by less than 15 per cent from those obtained with a Q of 1070. Average stress drop was 3.7 bars. Seismic moments obtained from spectra ranged from 1013 to 1018 dyne-cm. The low stress drops are interpreted to result from underestimation of the actual stress drops because of a nonuniform distribution of stress drop and slip along the fault planes. Spatially varying stress drops and slips result from the strong rock heterogeneity due to the injection of fluid into the rock. Stress drops were found to be larger near the edges of the seismic zone, in regions that had not been seismically active during previous injections. The seismic moments determined from spectra were used to obtain a coda length-to-moment relation. Then, moments were estimated for 1149 events from measurements of coda lengths from events whose moments could not be measured from spectra because of saturation or a low signal-to-noise ratio. The constant of proportionality between cumulative number of events and seismic moment is higher than that found for tectonic regions. The slope is so high that the seismic energy release is dominated by the large number of small events. In the absence of information about the number of events smaller than we studied, we cannot estimate the total seismic energy released by the hydraulic injection.

Seismic Magnitudes, Corner Frequencies, and Microseismicity: Using Ambient Noise to Correct for High-Frequency Attenuation

2020 ◽  
Vol 110 (3) ◽  
pp. 1260-1275 ◽  
Author(s):  
Antony Butcher ◽  
Richard Luckett ◽  
J.-Michael Kendall ◽  
Brian Baptie
Keyword(s):  
Stress Drop ◽  
High Frequency ◽  
Seismic Noise ◽  
Seismic Risk ◽  
Ambient Noise ◽  
Source Model ◽  
Lower Stress ◽  
Microseismic Activity ◽  
Stress Drops ◽  
Seismic Catalog

ABSTRACT Over recent years, a greater importance has been attached to low-magnitude events, with increasing use of the subsurface for industrial activities such as hydraulic fracturing and enhanced geothermal schemes. Magnitude distributions and earthquake source properties are critical inputs when managing the associated seismic risk of these activities, yet inconsistencies and discrepancies are commonly observed with microseismic activity (M<2). This, in part, is due to their impulse response being controlled by the medium, as opposed to the source. Here, an approach for estimating the high-frequency amplitude decay parameter from the spectral decay of ambient seismic noise (κ0_noise) is developed. The estimate does not require a pre-existing seismic catalog and is independent of the source properties, so avoids some of the main limitations of earthquake-based methods. We then incorporate κ0_noise into the Brune (1970) source model and calculate source properties and magnitude relationships for coal-mining-related microseismic events, recorded near New Ollerton, United Kingdom. This generates rupture radii ranging approximately between 10 and 100 m, which agrees with the findings of Verdon et al. (2018), and results in stress-drop values between 0.1 and 10 MPa. Calculating these properties without κ0_noise produces much higher rupture radii of between 100 and 500 m and significantly lower stress drops (∼1×10−2  MPa). Finally, we find that the combined κ0-Brune model parameterized with these source property estimates accurately capture the ML–Mw relationship at New Ollerton, and that stress drop heavily influences the gradient of this relationship.

scholarly journals Real‐Time Imaging, Forecasting, and Management of Human‐Induced Seismicity at Preston New Road, Lancashire, England

2019 ◽  
Author(s):  
Huw Clarke ◽  
James P. Verdon ◽  
Tom Kettlety ◽  
Alan F. Baird ◽  
J‐Michael Kendall
Keyword(s):  
Hydraulic Fracturing ◽  
Real Time ◽  
Shale Gas ◽  
Statistical Models ◽  
Induced Seismicity ◽  
Red Light ◽  
Fracture Network ◽  
Fluid Injection ◽  
Traffic Light ◽  
Subsurface Fluid

ABSTRACTEarthquakes induced by subsurface fluid injection pose a significant issue across a range of industries. Debate continues as to the most effective methods to mitigate the resulting seismic hazard. Observations of induced seismicity indicate that the rate of seismicity scales with the injection volume and that events follow the Gutenberg–Richter distribution. These two inferences permit us to populate statistical models of the seismicity and extrapolate them to make forecasts of the expected event magnitudes as injection continues. Here, we describe a shale gas site where this approach was used in real time to make operational decisions during hydraulic fracturing operations.Microseismic observations revealed the intersection between hydraulic fracturing and a pre‐existing fault or fracture network that became seismically active. Although “red light” events, requiring a pause to the injection program, occurred on several occasions, the observed event magnitudes fell within expected levels based on the extrapolated statistical models, and the levels of seismicity remained within acceptable limits as defined by the regulator. To date, induced seismicity has typically been regulated using retroactive traffic light schemes. This study shows that the use of high‐quality microseismic observations to populate statistical models that forecast expected event magnitudes can provide a more effective approach.

Fault Reactivation and Induced Seismicity During Multistage Hydraulic Fracturing: Microseismic Analysis and Geomechanical Modeling

SPE Journal ◽  
2019 ◽  
Vol 25 (02) ◽  
pp. 692-711 ◽  
Author(s):  
Fengshou Zhang ◽  
Zirui Yin ◽  
Zhaowei Chen ◽  
Shawn Maxwell ◽  
Lianyang Zhang ◽  
...  
Keyword(s):  
Hydraulic Fracturing ◽  
Seismic Activity ◽  
Induced Seismicity ◽  
Fault Reactivation ◽  
Fluid Injection ◽  
Fluid Viscosity ◽  
Multistage Hydraulic Fracturing ◽  
Budget Analysis ◽  
Casing Deformation ◽  
The Impact

Summary This paper presents a case study of fault reactivation and induced seismicity during multistage hydraulic fracturing in Sichuan Basin, China. The field microseismicity data delineate a fault activated near the toe of the horizontal well. The spatio-temporal characteristics of the microseismicity indicate that the seismic activity on the fault during the first three stages is directly related to the fluid injection, while after Stage 3, the seismic activity is possibly due to the relaxation of the fault. The fault-related events have larger magnitudes and different frequency-magnitude characteristics compared to the fracturing-related events. Three-dimensional (3D) fully coupled distinct element geomechanical modeling for the first two hydraulic fracturing stages and a shut-in stage between them is performed. The modeling result generates features of microseismicity similar to that of the field data. The energy budget analysis indicates that the aseismic deformation consumes a major part of the energy. The simulated fault shear displacement is also consistent with the casing deformation measured in the field. The model is also used to investigate the impact of possible operational changes on expected seismic responses. The results show that lower injection rate and lower fluid viscosity would be helpful in reducing casing deformation but not in mitigating seismicity. Decreasing the total fluid injection volume is an effective way to mitigate the seismicity, but it may hinder the stimulation of the reservoir formation and the production of the well.

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>

scholarly journals Parametric analysis of the elastohydrodynamic lubrication efficiency on induced seismicity

2020 ◽  
Vol 222 (1) ◽  
pp. 517-525 ◽  
Author(s):  
Chiara Cornelio ◽  
Marie Violay
Keyword(s):  
Stress Drop ◽  
Induced Seismicity ◽  
Parametric Analysis ◽  
Elastohydrodynamic Lubrication ◽  
In Situ Stress ◽  
Induced Earthquakes ◽  
Dynamic Stress Drop ◽  
Static Stress Drop ◽  
In Situ Stress Field

SUMMARY During reservoir stimulations, the injection of fluids with variable viscosities can trigger seismicity. Several fault lubrication mechanisms have been invoked to explain the dynamic stress drop occurring during those seismic events. Here, we perform a parametric analysis of the elastohydrodynamic fault lubrication mechanism to assess its efficiency during fluid-induced earthquakes. The efficiency of the mechanism is measured with the dimensionless Sommerfeld number S. Accordingly, we analysed eight well-documented cases of induced seismicity associated with the injection of fluids whose viscosities range from 1 mPa s (water) to 100 mPa s (proppant). We collected information related to the in situ stress field, fault orientation and geometry, moment of magnitude and static stress drop of the events. These parameters allow us to analyse the variation in the Sommerfeld number. Our results show that the estimated dynamic friction on the fault during the event is compatible with the fault weakening predicted by the elastohydrodynamic lubrication theory, particularly for highly viscous fluids.

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