Engineering, United States
Engineering, United States

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Timms N.E.,Curtin University Australia | Healy D.,Curtin University Australia | Healy D.,University of Aberdeen | Reyes-Montes J.M.,Applied Seismology Consultants | And 4 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2010

[1] Transgranular microcracking is fundamental for the initiation and propagation of all fractures in rocks. The geometry of these microcracks is primarily controlled by the interaction of the imposed stress field with the mineral elastic properties. However, the effects of anisotropic elastic properties of minerals on brittle fracture are not well understood. This study examines the effects of elastic anisotropy of quartz on the geometry of brittle fracture and related acoustic emissions (AE) developed during indentation experiments on single crystals at ambient pressure and temperature. A Hertzian cone crack developed during blunt indentation of a single crystal of flawless Brazilian quartz parallel to the c axis shows geometric deviation away from predictions based on the isotropic case, consistent with trigonal symmetry. The visible cone crack penetration depth varies from 3 to 5 mm and apical angle from 53° to 40°. Electron backscatter diffraction (EBSD) mapping of the crack tip shows that fracturing initiates along a ∼40 μm wide process zone, comprising damage along overlapping en echelon high-index crystallographic planes, shown by discrete bands of reduced electron backscatter pattern (EBSP) quality (band contrast). Coalescence of these surfaces results in a stepped fracture morphology. Monitoring of AE during indentation reveals that the elastic anisotropy of quartz has a significant effect on AE location and focal mechanisms. Ninety-four AE events were recorded during indentation and show an increasing frequency with increasing load. They correspond to the development of subsidiary concentric cracks peripheral to the main cone crack. The strong and complex anisotropy in seismic velocity (∼28% Vp, ∼43% V s with trigonal symmetry) resulted in inaccurate and high uncertainty in AE locations using Geiger location routine with an isotropic velocity model. This problem was overcome by using a relative (master event) location algorithm that only requires a priori knowledge of the velocity structure within the source volume. The AE location results correlate reasonably well to the extent of the observed cone crack. Decomposition of AE source mechanisms of the Geiger relocated events shows dominantly end-member behavior between tensile and compressive vector dipole events, with some double-couple-dominated events and no purely tensile or compressive events. The same events located by the master event algorithm yield greater percentage of vector dipole components and no double-couple events, indicating that AE source mechanism solutions can depend on AE location accuracy, and therefore, relocation routine that is utilized. Calculations show that the crystallographic anisotropy of quartz causes apparent deviation of the moment tensors away from double-couple and pure tensile/compressive sources consistent with the observations. Preliminary modeling of calcite anisotropy shows a response distinct from quartz, indicating that the effects of anisotropy on interpreting AE are complex and require detailed further study. Copyright © 2010 by the American Geophysical Union.

Chorney D.R.,Engineering Seismology Group | Grob M.,University of Alberta | Jam P.,University of Alberta | Van Der Baan M.,University of Alberta
48th US Rock Mechanics / Geomechanics Symposium 2014 | Year: 2014

A better understanding of the energy budget has important implications for enhancing the efficiency of hydraulic fracturing treatments. In particular, what percentage of the input treatment energy is released as radiated energy? What characterizes the deformation of the failure? We use a bonded-particle modeling approach to investigate both the radiated energy release and the amount of brittle failure. To test our model, we simulate triaxial compression tests on calibrated sandstone samples. Our results show that much of the failure is marked by a tensile component, despite the development of one or two large shear planes crosscutting the samples. Additionally, only 2.5% of the input energy is radiated as seismic waves. We propose an updated empirical energy-magnitude relation: log ER = 1.9Mw +8.5, where ER is the radiated energy and Mw is the event moment magnitude. This relation is an alternative to the commonly used Kanamori relationship and more applicable for the small-magnitude acoustic emissions in triaxial tests and likely microseismic events in hydraulic fracturing experiments, which are both marked by strong tensile deformation. Close examination of the source mechanisms of the induced acoustic emissions reinforce the complex nature of the micromechanics behind rock fracturing in general, due to strong deviations of the local stress field from the applied external field. Copyright © 2014 ARMA, American Rock Mechanics Association.

Baig A.,Engineering Seismology Group | Viegas G.,Engineering Seismology Group | Gallagher J.,Engineering Seismology Group | Urbancic T.,Engineering Seismology Group | Von Lunen E.,NexenCnooc Ltd.
Leading Edge | Year: 2015

Geomechanical modeling of hydraulic stimulations requires data to build and calibrate models to predict decline curves and other aspects of long-term reservoir performance. Initialization of these models requires knowledge of the preexisting fractures and geologic properties of the media. Orientations of different fracture sets, their intensities, and spacing, along with characterization of their size scales, critically impact geomechanical predictions of the stimulations in terms of proppant and fluid placement and the decline of reservoir productivity. With sufficient sampling of azimuths around the stimulation, the mechanisms and associated fracture planes and stress/strain conditions can be reconstructed through seismic-moment tensor inversion (SMTI) of recorded microseismic-event waveforms. At smaller scale lengths, signal-to-noise ratios can be low, and events might not be observed at high enough quality to permit SMTI. To extend the characterization of these fractures to smaller scales, a stochastic optimization algorithm is used, designed to search for optimally placed fractures in the reservoir that intersect with event locations while constraining their orientations from the same distribution observed at larger (SMTI-resolvable) length scales. Effectively, this technique allows for extension of the power law governing fracture distribution to smaller scales by invoking observed trends in self-similar behavior. In turn, characterization of the wider band of fractures in the reservoir provides necessary inputs into geomechanical models to predict fluid and proppant distributions from the full band of generated microseismicity and long-term behavior of the reservoir through decline-curve estimation. © 2015, Society of Exploration Geophysicists. All rights reserved.

Mace K.,Engineering Seismology Group | Urbancic T.,Engineering Seismology Group | Baig A.,Engineering Seismology Group
Society of Petroleum Engineers - Canadian Unconventional Resources Conference 2011, CURC 2011 | Year: 2011

We introduce the concept of Fracture Treatment Optimization (FTO) to find and analyze the Point of Diminishing Return (PDR) of stimulations. The approach is based upon locating the opening and closing points of fractures as identified through Seismic Moment Tensor Inversion (SMTI) analyses of microseismic data collected during treatments. By using SMTI data, it is postulated that the size of a treatment, the proppant concentrations and amounts, types of fluid pumped and amounts, the rates of treatment, and disposition of the fracture network may be analyzed to determine if they are optimal for the formation and the stress conditions of each individual fracture stage. As described, FTO is performed as a post-treatment approach to characterize the effectiveness of the treatment process. The PDR is identified by analyzing the SMTI data for each event in time and compared to the fracture treatment data. We show, through example, how the SMTI data can be used to establish when the transition between fractures with opening components of failure and those including significant closure components of failure. We suggest that these states are indicative of reaching a PDR thereby allowing for changes to the treatment to be considered. In the examples provided, we identify apparent PDRs due to leakoff dominated pad stages which are too large and when proppant is added too late in the stage. Additional PDRs are identified in the treatment when an energy balance has been reached, where leakoff and fluid injection is at equilibrium. At this point we suggest that introducing a change in energy or proppant concentrations can optimize the treatment. This empirical optimization allows for potentially more economic and efficient stimulations in subsequent hydraulic fracture treatments in the field. Copyright 2011, Society of Petroleum Engineers.

Urbancic T.I.,Engineering Seismology Group | Baig A.,Engineering Seismology Group | Goldstein S.,Engineering Seismology Group
74th European Association of Geoscientists and Engineers Conference and Exhibition 2012 Incorporating SPE EUROPEC 2012: Responsibly Securing Natural Resources | Year: 2012

Through seismic moment tensor inversion (SMTI) of microseismic events related to stimulation in a shale gas play utilizing well-conditioned geophone arrays, we have been able to define a complex three dimensional discrete fracture network consisting of sub-horizontal and sub-vertical fractures. Geologic data from the sites provided corroborative evidence that the moment tensor derived fracture orientations are in-line with the outcrop defined discrete fracture network, suggesting that in shale gas plays the preferred mode of failure during stimulations is the re-activation of pre-existing fractures and joints. It can be further suggested that the presence of sub-horizontal fractures are locally responsible for the transfer of stress resulting in failure along sub-vertical fractures and in the development of a well connect fracture network, required for effective proppant transport. The observed modes of failure can generally be categorized as being dominated by mixed-mode failures, representative of shear-dilatational failures. The opening-closure behavior of observed events, particularly for the sub-horizontal fractures can be related to surface roughness and the presence of asperities. The nature of fracturing appears to be fractal, following a power law distribution. It suggests that when stress levels are sufficiently high, fractures with similar orientations coalesce and develop longer fractures.

Cochrane A.,Engineering Seismology Group | Urbancic T.I.,Engineering Seismology Group | Baig A.M.,Engineering Seismology Group
48th US Rock Mechanics / Geomechanics Symposium 2014 | Year: 2014

During hydraulic fracture stimulations in shale, the activation of pre-existing fractures plays an important role in the development of the discrete fracture network. Understanding the behavior of the fracture network allows for the potential of incorporating the observed fracture network into reservoir models necessitating the identification of fracture behavior over a wide magnitude scale -3 < M < +3. Here, we discuss seismic events that were recorded by a multi-array multi-level network consisting of high frequency geophones located near the reservoir and arrays of accelerometers and low-frequency geophones deployed near the surface. The hybrid system captures a larger bandwidth, allowing for the integrated analysis of the source signal at various scales. Comparatively speaking, our observations suggest that the larger scale events identified on the near-surface network contribute upwards of 80% of the overall seismic budget or seismic energy release associated with the stimulation process. Additionally, these events accounted for a further 11,870 m2 of activated fracture surface area, approximately 10,295 m2 more than would have been estimated from the downhole array alone. Overall, the identification of the actual discrete fracture network over many size scales allows for a better understanding of the fracturing processes associated with stimulations.s. Copyright (2014) ARMA, American Rock Mechanics Association

Urbancic T.,Engineering Seismology Group | Baig A.,Engineering Seismology Group | Goldstein S.,Engineering Seismology Group
Society of Petroleum Engineers - SPE Hydraulic Fracturing Technology Conference 2012 | Year: 2012

The inclusion of fracture networks in reservoir models is generally based on the concept of failure associated with subvertical fractures. In general, it is surmized that fractures can grow irregularly in a stress field that is perturbed by a hydraulic fracture injection. It has also been considered that structural weaknesses in the rock such as pre-existing fractures and naturally occurring laminations commonly found in shale-gas reservoirs can be conduits for fracturing during stimulation and active pathways for fluid flow. We postulate that local stress perturbations through stress transfer allows for fractures to propagate and initiate failure along pre-existing fracture sets, which include sub-vertical and sub-horizontal fractures. Additionally, the degree of fracture interConnectivity and the type of fracturing will play a role in whether effective proppant transport is achieved. Through moment tensor inversion of microseismic events related to stimulation in the Horn River Basin utilizing well-conditioned geophone arrays, we have been able to define a three dimensional discrete fracture network consisting of sub-horizontal and sub-vertical fractures. Geologic data from the site provided corroborative evidence to the validity of the observed discrete fracture network, the presence of sub-horizontal fractures and fracture orientations in-line with current regional stress field. The fracture intensity and complexity appeared to be directly related to the degree of interaction between the sub-horizontal and sub-vertical fractures. Regions dominated by sub-horizontal fractures were also regions exhibiting poor fracture intensity and complexity. Based on these observations and moment tensor derived failure modes (opening component of failure), we were able to identify regions of enhanced fluid flow, further identifying regions of effective fluid transport. Regions with poor connectivity and dominance of sub-horizontal fractures also were identified as regions of poor fluid flow; these then become regions for potential re-stimulation. Based on these analyses, it can be suggested that sub-horizontal fractures can play an important role in the overall fracture development. Copyright 2012, Society of Petroleum Engineers.

Baig A.,Engineering Seismology Group | Urbancic T.,Engineering Seismology Group | Mace K.,Engineering Seismology Group | Prince M.,Engineering Seismology Group
Society of Petroleum Engineers - SPE Hydraulic Fracturing Technology Conference 2012 | Year: 2012

Hydraulic fracture completions seek to balance spacing of treatment wells and perforation clusters in order to minimize the costs of drilling wells and pumping fluids and proppant downhole while promoting the development of a discrete fracture network to connect even the most isolated pockets of hydrocarbon. To this end, numerous strategies for well completions have been proposed, such as avoiding the overlap of treatment volumes between adjacent wells and/or stages because of the risk that proppant and fluid will preferentially be diverted into earlier treated volumes. In counterpoint, it has also been suggested that the creation of new fractures in a previously treated volume promotes a complex fracture network enhancing drainage. When these stimulations are monitored from multiple arrays surrounding the treatment zone, seismic moment tensor inversion (SMTI) offers the ability to test these hypotheses by inferring if the events represent the opening of fractures or closure of pre-existing natural or newly created fractures. In this paper, we discuss two different completion programs. One common thread between the two data sets is that observed event clusters occur with a significant degree of overlap between neighbouring stages. Both completions were monitored with optimal multi-array configurations allowing for the calculation of SMTI with a high degree of robustness. The first stages in both examples showed significant opening components of failure. Neighbouring subsequent stages show closure events in the overlapping regions suggesting that the previously opened fractures were now closing due to local re-orientations of the stress-strain field stress induced by the later injection over-printing the region of overlap. Based on these analyses, it can be suggested that the moment tensor response can be used to identify the effective spacing for perforation clusters and establish optimal stimulation programs, which could include setting fracture ports farther apart. Copyright 2012, Society of Petroleum Engineers.

Urbancic T.I.,Engineering Seismology Group | Cochrane A.,Engineering Seismology Group | Bowman S.,Engineering Seismology Group | Baig A.,Engineering Seismology Group
Society of Petroleum Engineers - SPE/AAPG/SEG Unconventional Resources Technology Conference | Year: 2016

Understanding out of zone frac growth can lead to designing stimulation programs that can effectively enhance production from adjacent horizons. The effectiveness of the stimulation program can be assessed by incorporating monitoring programs that include instrumentation to detect seismic events over a broad range of magnitudes from the smallest detectable events with magnitudes below zero (microseismic) to larger events with magnitudes above zero (induced seismic), typically related to larger pre-existing fractures or faults. Stimulating these larger structures could lead to loss of fluid from the reservoir and affect estimates of stimulated volume. In this study, we examine data recorded using a typical downhole microseismic wireline supplemented with a near surface array designed to record induced seismic events. In this study, two horizons were stimulated. The intent of the program was to stimulate both zones by stimulating wells in the lower horizon by increasing pressure rates both early and late into the injection program in the upper horizon. About 4500 microseismic events and 28 induced seismic events were observed. These larger events represent approximately 83% of the total seismic energy released during the stimulation, which, if only using standard recording, would have been mis-interpreted as microseismic events and thereby would not have contributed to the overall energy dissipation levels. The larger events were associated with fractures with lengths varying from about 40 m to over 110 m, whereas the microseismic fracture lengths varied from ∼5 m up to ∼ 35m. The microseismic and induced seismic events could be used to identify growth from the upper to lower horizon at different pressure rates. The occurrences of larger magnitude events appeared to precede pressure increases in the program, suggesting that larger structures were activated as a result of the injection program even before pressures were increased. This observed process sets the foundation to better control stimulation programs. Copyright 2014, Unconventional Resources Technology Conference (URTeC).

Prince M.,Engineering Seismology Group | Baig A.,Engineering Seismology Group | Urbancic T.,Engineering Seismology Group
SPE Middle East Oil and Gas Show and Conference, MEOS, Proceedings | Year: 2011

Advanced seismic analysis approaches are used to examine possible correlations between various seismic characteristics and observed in-situ pressure readings as related to CSS heavy oil operations in Canada. In particular, we examined how seismic parameters such as cumulative strain, apparent volume, energy surplus, and cumulative occurrence rates can be used to identify the dynamic transfer of stress from the reservoir to generating depths, generating conditions for possible failure of the cap rock. These techniques are not only applicable to Canadian oil sands, but potentially to any environment where significant deformation results in seismic events giving the opportunity for a real-time analysis of failure conditions. Based on our analyses, changes in cumulative strain mimic observed pressure changes. Similarly, a resulting increase in energy surplus occurred at reservoir depth coincident with the occurrence of observed pressure spikes, followed by a subsequent increase in apparent volume and cumulative occurrence rates at shallower depth subsequent to the observed pressure spikes. Our observations suggest that advanced seismic analysis techniques can be used to potentially identify the transfer of strain and stress from the reservoir to shallower depths and provide the potential to provide real-time feedback mechanism on reservoir behaviour based on observed microseismicity. Copyright 2011, Society of Petroleum Engineers.

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