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4th EAGE Passive Seismic Workshop
- Conference date: 17 Mar 2013 - 20 Mar 2013
- Location: Amsterdam, Netherlands
- ISBN: 978-94-6282-040-1
- Published: 17 March 2013
41 results
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Microseismic Applications using DAS
Authors S. Grandi Karam, P. Webster, K. Hornman, P.G.E. Lumens, A. Franzen, F. Kindy, M. Chiali and S. BusaidiDistributed Acoustic Sensing (DAS) is a rapidly maturing fibre optic technology with many applications for wellbore monitoring and geophysical surveillance. DAS transforms a fibre optic cable into a distributed array of acoustic sensors. Shell is developing DAS technology in partnership with OptaSense, a subsidiary of QinetiQ U.K. DAS has been proven to work for VSP applications (Mateeva et al., 2012). The technology has been improved through numerous field trials and it has been tested in a variety of installations, where it has been compared to geophones and sonic logs as a check-shot tool, and as an imaging tool for walk away VSP data. Signal to noise ratio, directionality and repeatability among other aspects have also been studied through these field trials and laboratory experiments. The performance of DAS for micro-seismic monitoring applications is still under evaluation. Although DAS offers the advantage of recording along the whole well at once, significantly increasing the number of receivers, it has several challenges with respect to geophone arrays: the current DAS system records data with a higher noise floor and with a more constrained angular sensitivity since it behaves as a doublet of one-component geophones. In this paper we report the current status of this technology regarding micro-seismic monitoring. A field trial specifically designed to test this application in a micro-seismically active area is described. The trial was jointly carried out with Petroleum Development Oman (PDO) in a field in Oman.
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Bandwith Coupling Issues in Recording Passive Seismicity
Authors A. Wuestefeld, A.M. Baig, T.I. Urbancic and M. PrincePassive seismic recording systems are typically designed to monitor microseismicity (M < 0) and as such are frequency band-limited. For effective monitoring full field monitoring, a broadband recording approach needs to be considered in order to expand the magnitude range from -4 < M < +4. Based on data recorded using standard downhole approaches for both hydraulic fracture and long term reservoir monitoring we (1) outline frequency and bandwidth limitations, (2) examine resonance effects on interpretation, (3) evaluate coupling effects associated with temporary deployments, and (4) assess how these issues affect the interpretation. We (5) also analyze the role of magnitude saturation and define the need for a hybrid recording approach, utilizing both low frequency sensors situated near or at surface with downhole high frequency phones, in order to avoid some common pitfalls in recording passive seismicity.
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Long Beach 3D Seismic Survey: Data Mining Continuous Passive Seismic Data
Authors D. Hollis, C. Cox, R. Clayton, F. Lin, D. Li and B. SchmandtAutonomous-type seismic data recording systems for active-source seismic reflection exploration surveys provide a valuable by-product: a continuous seismic recording from a dense seismic network over the survey area. This dense exploration network serves as a scaled-down version of the global passive seismic network that provides natural seismicity and ambient noise data that can be utilized to study the earth’s interior. This presentation describes the results of applying various techniques to passive data acquired using an autonomous seismic recording system. A 57 square kilometer survey 3D seismic reflection survey acquired in early 2011 in Long Beach, California, that resulted in six-months of continuous passive seismic data from a static 5,400 receiver seismic network. The natural seismicity and ambient noise identified in this continuous passive data set were mined for signal used to extract p-wave and shear-wave velocities, anisotropy information, and seismic imaging products that add value to the active source data; as well as, other information that would benefit exploration and earthquake-related studies of the study area. Analyses on the mined dataset include: time-lapse ground motion studies, micro-earthquake detection and location, surface wave interferometry, ambient noise correlation for reflection imaging, receiver functions, and teleseismic event amplitude analysis.
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Validation of Microseismic Acquisition Geometry
Authors A. Rosca, G. Daniel, J. Vernier and T. BardainneThis paper presents an analysis of microseismic acquisition strategies for a range of monitoring applications from reservoir characterization to environmental warning systems
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Low-Frequency Tremor Signals from a Hydraulic Fracture Treatment in Northeast British Columbia, Canada
Authors D.W. Eaton, M. van der Baan, J.B. Tary, B. Birkelo and S. CuttenThe Rolla Microseismic Experiment (RME) was undertaken by the Microseismic Industry Consortium August 7-28, 2011 to record a multistage hydraulic fracture stimulation of a Triassic unconventional gas reservoir in northeastern British Columbia, Canada. The microseismic deployment included a 6-level downhole toolstring with low-frequency (4.5 Hz) geophones, a set of 21 portable broadband seismograph systems, and a 12-channel surface array comprised of 10-Hz geophones. Although we did not observe LPLD events based on how they have been previously described, our data exhibit high-amplitude signals in the 8-15 Hz band. These signals are monotonic and have been interpreted as resonance of fluid-filled cracks or successions of small repetitive events. We have also detected several instances of discrete microseismic events with unusually low frequency. An apparent tendency for low-frequency tremors to precede high-frequency microseismicity in our data provides a tantalizing suggestion that these processes may be genetically linked.
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Comparing Near-Surface and Deep-Well Microseismic Data and Methods for Hydraulic Fracture Monitoring
Authors T. Probert, D. Raymer and I. BradfordIn 2011, a comprehensive microseismic monitoring case study was acquired in the Fayetteville Shale using surface, shallow-well, and deep-borehole sensors to help evaluate and compare different hydraulic fracture monitoring techniques. We apply methods based on Coalesence Microseismic Mapping (Drew et al, SPE 2005) to the different arrays, introduce some new methods, and highlight some of the advantages and disadvantages of each data set.
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Velocity Uncertainties in Surface and Downhole Monitoring
By M. ThorntonPrevious work in modeling positional uncertainty for both surface and downhole passive monitoring approaches has largely ignored the uncertainty in velocity estimates. Incorporation of velocity uncertainty in the model produces more realistic error estimates, that tend to agree with case studies showing large positional differences in results processed by different analysts. Velocity uncertainty is analyzed within a maximum likelihood framework, where uncertainties for estimated parameters can be computed for a variety of imaging constraints and approaches.
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New Design and Advanced Processing for Frac Jobs Monitoring
Authors D. Pandolfi, E. Rebel-Schissele, M. Chambefort and T. BardainneThe main goal of frac monitoring is to obtain a clear definition of event characterization to ultimately compute SRV. Enhancement in signal-to-noise ratio is one of the key elements used to discriminate a successful technique from others. While in the buried sensors design the noise reduction is a wellaccepted advantage, the simple layout of surface sensors arrayis still under discussion. This abstract presents results obtained with a new surface design used in hydraulic fracturation monitoring showing how geometry and spatial distribution of the sensors can improve SNR and therefore sensitivity. This new design is named‘patch design’.The name is self-explanatory: OnePatch consists in a given number of sensors closely distributed; a patch design is a series of patches sparsely distributed above the region of interest. This design has several advantages; we will discuss here the economic and technical benefits of this method. Sensor deployment can be easily adapted to the environment optimizing permitting, line clearing, access and avoiding noise prone areas. The economic advantage does not weaken the technical benefit: at the patch scale, the short distance between sensors allowsadvanced summation to optimallypreserve the signal and reduce the noise. The patch design provides a much higher signal-tonoise ratio compared to the usual surface designs. Real cases examples will show how the patch design combines the advantages of both surface and shallow buried array designs.
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Imaging Moment Tensors from Surface Arrays
Authors K. Chambers, J. Clarke and S. WilsonWe employ a new method for imaging microseismic sources referred to as Moment Tensor Microseismic Imaging (MTMI). Unlike standard diffraction stack methods our method allows for polarity and amplitude changes produced by the anisotropic radiation pattern from the microseismic source, without a-priori knowledge of the source properties. This is achieved by projecting the data on to moment tensor components before stacking at each image point. In addition to the cancellation of radiation pattern effects the methods advantages include, the production of images for each moment tensor component (allowing determination of the source mechanism given a suitable geometry), as well as the option to include data from multiple component systems and multiple wave types in a single image.
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Optimization of Statistically Optimal (SO) Algorithms for Surface Location of Microseismic Sources with Complex Focal Mechanisms
Authors I. Dricker, A. Kushnir, M. Rozhkov, A. Varypaev, N. Rojkov, A. Epiphansky, P. Friberg and S. HellmanWe present several statistically optimal adaptive and robust algorithms for accurate location of microseismicity using small-aperture surface arrays which show superior results in comparison with semblance-based Seismic Emission Tomography algorithm for the events with double-couple focal mechanisms and better suppress coherent noise of hydraulic fracturing
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Source Mechanisms of Acoustic Emissions due to Failure Around a Borehole in a Triaxial Lab Experiment
Authors D. Kuehn, V. Vavrycuk, E. Aker, H.N. Gharti, V. Oye and K. HuynhAt the Norwegian Geotechnical Institute, a triaxial laboratory experiment was performed on a sample of Vosges sandstone. To study the development of borehole breakouts, a horizontal, borehole was drilled at mid-height through the sample. During the experiment, 12 piezoelectric receivers recorded waveforms of acoustic emissions. To locate events, the InSite software package (ASC) was used. Waveforms are complex and characterised by long coda waves produced by interaction of emitted waves with the surface of the borehole and the specimen’s walls. Often, no S-wave can be distinguished. In order to suppress effects of wave propagation, we perform a moment tensor inversion of P-wave first-onset amplitudes. For inversion, a subset of 162 high-quality events was chosen. Green’s functions were computed assuming a homogeneous velocity model. The source mechanisms basically coincide with the orientation of fractures developing in the sample. Since the waveforms are affected strongly by the presence of the horizontal borehole and probably also the specimen’s walls, a full waveform inversion and a 3-D velocity model taking into account the geometry of the sample would have to be applied. To this end, we will employ a spectral element method (SEM) to compute Green’s functions.
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Induced Seismicity Patterns in the Paralana Geothermal Reservoir, South Australia
Authors J. Albaric, V. Oye, M. Hasting, M. Messeiller and P. ReidWe are presenting the results of the analysis of seismicity induced by the first main stimulation of the geothermal reservoir at Paralana (south Australia) in July 2011. More than 7000 events were detected and processed automatically. A 3D velocity model was built from active seismic data and used to locate the events. Well-located microearthquakes were relocated using differential travel-times improved by waveform cross-correlation. Focal mechanisms of the four largest events (two Mw 2.4 and one 2.5) are all in thrust with a strike-slip component, consistent with the stress regime in the area.
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Microseismic Source Mechanisms: What the Waveform Can Tell Us
Authors T.B. O'Toole, J.H. Woodhouse, J.P. Verdon and J.M. KendallWe have developed a technique for inverting microseismic waveform data, recorded at the surface and/or downhole, to obtain simultaneously the centroid location and moment tensor of a microseismic event. Our inversion approach is conceptually similar to the “Centroid—Moment Tensor algorithm”, which has been very widely applied in the field of global seismology, except that we forward model the seismic wavefield using a method that is amenable to the efficient, accurate and stable computation of synthetic microseismic waveforms and their partial derivatives with respect to source parameters. We obtain an initial source model by performing a least-squares inversion of waveform data for the moment tensor, with the centroid position fixed at the hypocentral coordinates determined by conventional body wave travel-time methods. Assuming that non-linearity in the relationship between the source location and data is weak, we may use this source as the starting point for an iterative least-squares inversion for the centroid and moment tensor of a microseismic event. Here, we present centroid locations, moment tensors, and estimates of their uncertainty calculated using this method for microseismic events induced during hydraulic fracturing operations at Preese Hall, UK and Cotton Valley, USA.
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A Feasibility Study of Time-Lapse Seismic Noise Interferometry for CO2 Monitoring at Ketzin
Authors B. Boullenger, A. Verdel, J. Thorbecke and D. DraganovSince 2008, CO2 has been injected at the demonstration site for CO2 sequestration in Ketzin, Germany. Since 2009, a permanent array of seismic receivers installed by TNO at the injection site has recorded passive data continuously. It is the intention of TNO to use seismic interferometry (SI) by cross-correlation applied to the recorded ambient noise as a cost-effective technique for time-lapse monitoring. Under specific conditions, SI by cross-correlation can retrieve reflection data that may be interpreted in terms of subsurface layer properties. To assess the feasibility of this monitoring technique, we model base and monitor passive experiments for recording the response from noise sources and cross-correlate the recorded synthetic traces. The best results are obtained when the stationary-phase regions for the Green's functions are sampled densely enough by noise sources and, at the same time, the noise contributions from non-stationary regions cancel to a sufficient extent. Conversely, non-favourable noise conditions will cause wrong retrieval of the Green's functions and possibly deteriorate the monitoring potential of the technique. However, we show that being selective with the recorded noise in terms of illumination characteristics prior to correlation will improve the retrieved reflection data for further monitoring interpretation.
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On the Relationship Between Levels of Seismicity and Pump Parameters in a Hydraulic Fracturing Job
Authors S.J. Oates, A. Vogelaar, R. Herber and J. WinsorThe McGarr equation gives a means of estimating the amount of seismicity associated with a fluid injection/hydrofrac job. McGarr’s formula seems however to be little used: most examples in the literature contradict it. Here we analyse a number of hydrofrac datasets from gas shales to assess whether they satisfy the McGarr equation. In agreement with other authors we find that moment is proportional to injected volume but the equality is not satisfied. Combining McGarr’s formula with the Gutenberg-Richter Law allows estimates to be made of the number of events expected above a given magnitude. We show that the requirement of a finite moment budget implies that the Gutenberg-Richter b-value must be less than 1.5: b=1.5 corresponds to a fractal dimension for the underlying fault network of 3. Almost all of the datasets we analysed are characterized by b values greater than 1.5 implying that in these cases the Gutenberg-Richter Law is not consistent with the assumption of a finite moment budget. Based on the data analysed, we conclude that McGarr’s formula may well be valid for injection/fraccing but that most of the moment budget is lost in undetectably small events on length scales down to the grain-size.
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Microseismic Monitoring and Subseismic Fault Detection in an Underground Gas Storage
Authors D.A. Kraaijpoel, D.A. Nieuwland and B. DostA natural gas field with an induced seismic history is being converted to an underground gas storage. Microseismic monitoring with a downhole tool detects hundreds of microseismic events. The events delineate a subseismic fault that coincides with a previously interpreted flow baffle.
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High-Resolution Analysis of Microseismicity Related to Hydraulic Stimulation in the Berlin Geothermal Field, El Salvador
Authors G. Kwiatek, F. Bulut, M. Bohnhoff and G. DresenWe investigate microseismicity during a hydraulic stimulation at the Berlin Geothermal Field, El Salvador. The site was monitored using 13 3-component seismic stations deployed in shallow boreholes. Three stimulations were performed in the well TR8A with a maximum injection rate and well head pressure of 140l/s and 130bar, respectively. The catalog of 581 seismic events were relocated using the double-difference relocation algorithm based on cross-correlation derived differential arrival time data. We also recalculated source parameters using the the spectral ratio method. We investigated the source parameters and spatial and temporal changes of the seismic activity based on the refined dataset and studied the correlation between seismic activity and production. The achieved hypocentral precision allowed resolving the spatiotemporal changes in seismic activity down to a scale of a few meters. We observe clustering of the seismicity around the injection well as well as the migration of seismicity outside of injection point along the pre-existing faults. The migration of seismicity is determined by increasing injection rate (Kaiser effect). We observe larger magnitude events after the shut in of the injection well. We finally observe the decrease of the static stress drop with the distance from injection point.
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Seismogenic Index of a Reservoir Location Estimated from Tectonic Seismicity and Crustal Deformation
Authors C. Dinske, S.A. Shapiro and F. WenzelWe address the question whether tectonic seismic activity within an area where a fluid injection is planned can be used to evaluate its seismotectonic state. For this purpose, we expand and reformulate the theoretical framework which essentially applies to describe the occurrence of fluid-induced seismicity to the case of tectonic seismicity. Based on this model, we introduce the tectonic seismogenic index which can be determined prior to an injection if the tectonic seismicity rate and the crustal deformation rate are known in the reservoir region. We apply the derived formalism to reservoir locations where the seismogenic index for fluid-induced seismicity had already been obtained. Thus, we can examine whether the two differently defined seismogenic indices are comparable for an injection location. Our results show that the tectonic seismogenic index can be used as a proxy for the seismogenic index of fluid-induced seismicity. Thus, we conclude that our formalism can contribute to avoid the occurrence of large-magnitude fluid-induced earthquakes by properly selecting and developing reservoir locations.
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Cross-Correlation – An Objective Tool to Indicate Induced Seismicity
More LessThe qualitative correlation between earthquake rates and the injected volume has been an established tool for investigating possible induced or triggered seismicity. The method using direct values of earthquake rates and the injected volume in normalized cross-correlation (NCC) is a recent development. We show theoretically and by numerical example that the mathematical definition of NCC provided by a set of positive random functions, exhibits high cross-correlation values with a limit equal to 1 for large mean and low standard deviation time histories. Instead of positive-value time histories, their “Useful Functions” (the original functions with their weighted running mean subtracted) should be used for NCC. Then the NCC of such functions may be close to zero or oscillating between positive and negative values of +0.5 and -0.5 in cases where seismicity is probably not induced by injection. If the positive and negative peaks of NCC were statistically significant, it would mean that the increased seismicity is related to either lower or higher injection volumes. However, a NCC dominated by a positive maximum with a zero (or small) time-lag, indicates that seismicity is induced by the injection. We apply the method for case studies of known natural and triggered/induced seismicity.
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Evidence for Tensile Faulting from Full Waveform Moment Tensor Inversion of Induced Seismicity in Basel Geothermal Site
More LessIn summary, we have performed a full waveform inversion to retrieve both double couple (DC) and moment tensor (MT) source parameters for the 19 strongest induced microseismic events at the Basel geothermal site. The DC solutions agree well with the result from a previous study based on the fitting of projected P wave first-motion polarities at ~40 stations. In comparison, data from less than ten (mostly six) stations has been used in our study. The analysis of MT solutions indicates the presence of significant ISO components, including both fracture opening and closing, for events occurring during and immediately after the injection and pure DC solutions for later events. The spatio-temporal pattern of isotropic components of the moment tensor can be explained by the pore pressure perturbation.
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Fractal Dimension of Microseismic Events via the Two-Point Correlation Dimension, and its Correlation with b Values
By J.P. VerdonThe magnitude distribution of a microseismic dataset can be described by the b value. Similarly, the spatial distribution of events can be described by the two-point correlation dimension (Dc), which provides an objective and quantifiable assessment of the dimensionality of a datasets (i.e., do the events fall along a linear or planar feature, or fill up a 3D volume?). Typically, spatial distributions are identified by eye (the interpreter will indentify linear or planar features on a hypocentral map). We begin this paper by describing the calculation of Dc, before going on to evaluate the correlation between Dc and the seismic b value. We perform our analysis on a range of microseismic datasets, including hydraulic fracture stimulation, enhanced oil recovery and a mining dataset. It has been suggested that elevated b values indicate a more distributed deformation. We find strong, statistically significant correlation between b and Dc, implying that the inferences made regarding b values during reservoir stimulation stimulation - that elevated b values correspond to areas with greater stimulation complexity, while lower b values imply activity on simpler, planar features - are appropriate.
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Relation Between Elastic Heterogeneity, Stress and the Gutenberg Richter b-value: A b-value Estimation using Sonic Logs
Authors C. Langenbruch and S.A. ShapiroIn this paper we establish relations between the fractal distribution of elastic parameters, stress field variations and the Gutenberg-Richter b-value of earthquakes. We extract information about elastic parameter distribution from sonic well logs to create an elastically heterogeneous 3D random medium with a fractal spatial correlation function. Using the finite element program Abaqus we apply an externally homogeneous stress field and compute the resulting stress distribution inside the model medium. By applying geomechanical considerations we determine the distribution of Coulomb Failure Stress (CFS). We find that elastic heterogeneity causes strong spatial CFS variations. We assume that if the CFS is perturbed e.g. by a fluid injection, rupturing takes place inside clusters of interconnected critically stressed cells of the model. From the resulting size distribution of clusters we compute fault size and correspondingly magnitude scaling. We find that fault sizes exhibits power law scaling according to the Gutenberg Richter relation. The application of our method to well log data and stress profiles measured along the Continental Deep Drilling Site (KTB, Germany) main hole results in a b-value of b=0.95, which is in agreement with the b-value of b=1 computed from fluid injection induced seismicity at the KTB.
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The Importance of Accurate and Consistent Microseismic Event Magnitudes
Authors A.L. Stork, J.P. Verdon and J.M. KendallSeismicity induced by fluid injection during hydraulic fracturing; carbon capture and storage (CCS); and geothermal developments is of concern to industry and the public. However, the magnitudes of such events are often reported without a clear understanding of the meaning or implications. Using microseismic events recorded at injection sites we show how moment magnitude estimates depend on the monitoring set-up and the calculation, in the time or frequency domain. Magnitude estimates, and in particular moment magnitude estimates, vary by tenths of a magnitude unit depending on the geophone array configuration and how the calculation is made. This raises important questions for the monitoring of industrial activities, such as hydraulic fracturing, if the regulation of such activities is to be based on the magnitude of seismic events induced by industrial projects.
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Microseismic Permeability Characterisation from Hydraulic Fracturing of Shales
Authors N. Hummel and S.A. ShapiroWe present an approach to characterise the hydraulic transport during borehole fluid injections. We introduce a new type of scaling to transform clouds of seismicity from a hydraulically anisotropic medium into an effective isotropic medium. For this we assume that the pressure-dependence of permeability is independent of a specific direction although the permeability is anisotropic. We apply our approach to seismicity recorded during a hydraulic fracturing treatment in the Barnett Shale. We analyse spatio-temporal characteristics of transformed seismicity. Our results show a significant nonlinear fluid-rock interaction. To explain our findings we suppose a nonlinear diffusional relaxation of the pore-fluid pressure perturbation. In particular, we consider a hydraulic transport which is governed by a power-law pressure-dependence of permeability. For this type of nonlinear fluid-rock interaction we present a numerical modelling approach to generate synthetic seismicity distributions. Their spatio-temporal features are similar to the ones which we observe in Barnett Shale. Additionally, our approach allows us to compute the permeability evolution inside of the fracture stimulated rock. Our results indicate that the fluid transport during the hydraulic fracturing treatment in the Barnett Shale may be described by a nonlinear pressure diffusion based on a power-law dependent permeability.
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The Peak Frequency of Direct Waves for Microseismic Events
Authors L. Eisner, M. Hallo, I. Opršal, D. Gei and A. MohammedThe resolution of both the location and the source mechanism is a function of the peak frequency of the observed signal. The peak frequencies control the resolution of the microseismic event locations in surface monitoring. In downhole monitoring, the peak frequency determines the picking error which also controls the resolution of the microseismic event locations. Therefore, the peak frequency of the direct body-waves provides for better design of microseismic monitoring networks. In this study we show that the peak frequency can be determined for geophone recordings (i.e., particle velocity) as from global absorption factor only and this calculation very well explains observed data. Furthermore we show that, except for unusually large events, the peak in amplitude spectra is below the corner frequency for most of the typical microseismic monitoring techniques. We also show that the spectral peak is determined by attenuation of the media and is consistent with observations. We also show that the corner frequencies approach 10 Hz for earthquakes with a moment magnitude between 1 and 2, indicating that 10 Hz geophones (velocity transducers) may not be well suited for these and earthquakes of magnitude 2 and greater.
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Correlating the Distribution of Microseismic Events with the Stress State for Multi-Stage Stimulations
By I. BradfordThe aim of microseismic data interpretation is to enable reliable production predictions to be made using reservoir models that contain geological structure extracted from microseismic event clouds, and to enable real-time operational decisions that maximize the effectiveness of hydraulic fracturing. This paper moves towards the latter goal. Previously Bradford (2011) showed that the dynamic behaviour of a system comprising a single hydraulic fracture and neighbouring pre-existing faults can be modeled, using data from Cotton Valley. Specifically, the modeled spatio-temporal pattern of plastic shear deformation on the pre-existing faults matched the sequence and style of microseismic events. This paper extends this technique so that it is applicable to systems comprising tens of hydraulic fractures and multiple pre-existing faults. Data from a multi-well, multi-stage stimulation in the Fayetteville Shale, where there was a deep monitoring well, illustrate the technique. It is shown that the pattern of observed microseismicity is strongly influenced by the induced stress state. Specifically, deformation on pre-existing faults in the vicinity of the hydraulic fractures generates regions of varying compression. The zones of observed microseismicity and those regions of lower compression at, or near, failure, correlate well.
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Assessing Hydraulic Fracture Stimulation Effectiveness with Microseismicity, An Engineering Perspective
Authors T.I. Urbancic, A.M. Baig, K. Kocon and K. TremblayThe Kiel Process involves pumping a stage, stopping the pumps to let stresses redistribute, and then continuing to pump. Theoretically, the process should stimulate some fracture initially, and different fractures at the end of the stage. We show using seismic moment tensor inversion that this behaviour is indeed observed for a couple of stages where this technique was attempted.
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Tracking Fracks: Evolution of Fracture Normal/Tangential Compliance During Hydraulic Fracture Stimulation
Authors A.F. Baird, J.P. Verdon and J.M. KendallS-wave splitting (SWS) from microseismic events may be used to estimate anisotropy in the region around hydraulic fracture stimulations. The anisotropy can then be used to characterize the distribution of fractures in a reservoir. In addition to fracture orientation, SWS can be used to estimate the ratio of normal to tangential compliance (ZN/ZT). ZN/ZT is sensitive to (1) the stiffness of the infilling fluid, (2) fracture connectivity and permeability, and (3) the internal architecture of the fracture (e.g. fracture roughness, degree of cementation). Here we demonstrate the use of SWS to infer the evolution of ZN/ZT from two hydraulic stimulation datasets from tight gas reservoirs. In both examples we observe an apparent increase in ZN/ZT as the stimulation progresses. We suggest that this increase may be produced by the development of new, clean fractures that have a greater normal compliance than their natural counterparts, combined with increases in fracture network connectivity and permeability. The ability to monitor ZN/ZT during stimulations provides a means to gain insight into the evolving flow properties of the induced fracture network, and may be beneficial for assessing the success of drilling and stimulation strategies.
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Deploying Wireless Seismic Recording Systems for Real-time Monitoring and Analysis of Hydraulic Fracturing Projects
Authors D.B. Crice, M. Lambert, R. Evans and P. MortonUse of real-time wireless seismic system for microseismic recording with immediate transfer of data to the frac operators with case history
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Microseismic monitoring of a CO2 Injection: the case study at In Salah, Algeria
Authors B.P. Goertz-Allmann, V. Oye, K. Iranpour, D. Kühn, E. Aker and B. BohloliWe analyze microseismic data induced during CO2 injection at In Salah, Algeria. We detected over 5000 microseismic events using a master event waveform cross-correlation approach. Interpretation of the microseismic events is mainly based on S-P wave onset times and polarization angles of the P-waves but also includes source parameter determination. Events can be classified in various event clusters according to their S-P traveltimes and azimuth. The microseismic activity shows correlation between seismological parameters and injection data. Moment magnitudes have been determined in the frequency domain. Most magnitude estimates are between -1 and 0, with a few larger magnitude events up to Mw 1. B-values of events clusters have been determined and show significant variations between the clusters.
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Integrated P, S-phase Picking and Pulse Width Determination. Application on Surface Seismic Network during Acid Fracing
Authors N. Martakis, G.A. Tselentis, A. Lois, P. Paraskevopoulos and E. SokosMonitoring fracing induced seismicity is an important and developing technique of modern reservoir operation practice. The large numbers of small magnitude events, as well as the increasing number of instruments used, make manual detection, selection and picking of events a tedious, costly and time consuming task. In that scope, an algorithm is presented that automatically detects and picks arrival times for P-waves and S-waves, as well as pulse width picks. In particular, automatic processing of a seismic record consists of 3 steps: (i) detection of seismic events in a given record, (ii) estimation of the P-wave arrival time, as well as zero-crossing (pulse width) and finally, (iii) estimation of the S-wave arrival time. The algorithm is applied and validated on a dataset collected during the acid fracturing of a reservoir.
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Migration-based Deconvolution for High Resolution Locating of Microseismic Sources from Multi-Component Recordings
Authors J.B.U. Haldorsen, N.J. Brooks and M. MilenkovicWe describe and illustrate a locating method based on full-waveform 3D migration of three-component data to reconstruct components of the elastic wave field at any point in 3D space. To be a viable micro-earthquake source at a location, the compressional (P) and shear (S) wave fields reconstructed at this location should have similar wave shapes and be simultaneous. These two conditions are assured by an imaging condition based on inverse-energy-weighted correlation between the reconstructed P and S signals. This weighted correlation process is the least-squares solution to the problem of maximizing the zero-phase bandwidth of the common signal while minimizing the noise. The process measures, at optimal resolution, both the similarity and the synchronicity of the two signals, while the adaptation to the signal and noise conditions of the data ensures numerical stability. The process does not require an absolute timing of the data as the floating time reference common to the reconstructed P and S signals is eliminated by the correlation process. Without explicit phase picking and event association, the new method is well suited for automated data processing, and is equally applicable to the analysis of three-component data, acquired either in boreholes or on the surface.
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New 3D Wave Attribute Modelling Tools for Microseismic Monitoring Processing: Traveltimes, Amplitude and Polarization
Authors N. Belayouni, M. Noble, T. Bardainne and A. GesretMicroseismic monitoring has many applications that need very precise tools to model wave propagation through complex structures in order to obtain acceptable results. Here we propose a new set of accurate tools to compute numerically the wave attributes such as traveltimes, amplitudes and polarization. Our numerical scheme assumes spherical wave fronts that enables us to avoid source singularities due to plane wave assumption. We thus obtain very accurate gradients of the traveltimes, which are needed to compute correct amplitudes and polarization. Furthermore, in high contrast velocity models, traveltime gradients are generally discontinuous due to refracted arrivals. We compute amplitudes and polarization jointly to the traveltimes to have correct gradients and consequently better results of amplitudes and polarizations. These attributes are necessary for focal mechanism computation, detection sensibility estimation and microseisms location.
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Automatic Event-Detection and Time-Picking Algorithms for Downhole Microseismic Data Processing
Authors J. Akram, D. Eaton and A. St. OngeMicroseismic event detection and time picking are critical for accurate microseismic locations. Both processes are often done automatically, considering the large volume of recorded microseismic data. Many event detection algorithms require a static or dynamic threshold criterion. In this paper, we propose a dynamic threshold criterion and evaluate its performance in comparison with static thresholds using STA/LTA and peak eigenvalue (PEV) methods. The comparison is based on a subset of microseismic data acquired during two different hydraulic-fracturing treatments in western Canada. We also compare the performance of several time-picking algorithms, STA/LTA, modified energy ratio (MER), modified Coppens’s method (MCM) and Akaike information criterion (AIC). In addition, we propose a hybrid time picking algorithm, joint energy ratio (JER), which combines peak eigenvalue ratio (PER) with STA/LTA. We find that the dynamic threshold approach yields detection results between the intermediate and low static threshold parameters, reduces false noisy detections and is capable of identifying weak signals. We also find that JER, MER and STA/LTA all perform equally well with good S/N data. However, JER provides more stable results with mixed S/N data. We recommend that a manual QC step should follow the automatic time picking to verify time picking accuracy.
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Semi-Automatic Detection and Localization of Microseismicity Induced by a “Salt Dissolution Provoked” Cavity Collapse
Authors J.L. Kinscher, I. Contrucci, P. Bernard, A. Mangeney, J.P. Piguet and P. BigarreAn event detection and localization design was tested to study semi-automatically unusual seismic signals recorded during the growth, and the induced collapse of a salt cavity. In the proposed detection design, simple spectral signal characteristics are used to detect and characterize seismic events in complex swarming sequences. For event localization, an approach based on simple peak-to-peak amplitude estimation was tested which relied on the derivation of a site specific attenuation law. The results show the great potential of both methods to provide a reliable first order approximation of spatio-temporal attributes of the recorded seismicity.
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Realising the Benefits of VSP
By M. HumphriesVertical seismic profiles support micro-seismic in many practical ways but perhaps not to their full potential. A number of examples showing the link between VSP and micro-seismic are discussed and suggest that information from VSP and micro-seismic should be allowed to flow both ways, in order to maximise their full potential.
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Microtremor Analysis Over Carbonates with and without Oil
Authors A. Vesnaver, S.I. Kaka, S.R. Hussaini and A.K. PopoolaDuring the last few years, the spectra of microtremors have been proposed as a possible tool for detecting hydrocarbon reservoirs (Lambert et al., 2009), although criticisms were raised on both theoretical aspects (Broadhead, 2010) and experimental evidence (Hanssen and Bussat, 2008; Ali et al., 2010). In this paper, we analyse two different sets of passive seismic data acquired in areas of outcropping carbonate rocks, one in Italy and one in Saudi Arabia. The second area is located over a semi-depleted oil reservoir, which still could be produced by enhanced oil recovery techniques. In the first area, instead, there is no oil at all. We compare some signals features in the two cases to check the possibility of using microtremors as a diagnostic tool for detecting hydrocarbons.
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Clearing the Confusion: - Understanding Precision, Accuracy and Error for Both Inversion and Imaging Methods
Authors S.A. Wilson, B. Dando and K. Chambersadds further complexity to the semantics of separating bias from the random component of error. In this paper, • We attempt to clarify the meaning of uncertainty, precision, accuracy and error for different location methodologies using the scientific literature, and discuss the issues of precision bias and other logical fallacies • We describe a set of monitoring scenarios, then compare event accuracy obtained using (i) an imaging method based on a surface network geometry and (ii) an inversion method based on a down-hole wireline tool geometry . We show for each scenario which method offers the better solution. By doing so we provide some general guidelines on which monitoring scenarios are best suited to which monitoring objectives In conclusion we recommend that all parties are vigilant in using appropriate terminology and consider the objectives of any monitoring project, prior to deciding upon the network geometry. We advise that any decision to carry out a monitoring project is considered in the light of the nature of the sub-surface velocity field and the extent of our knowledge of this velocity field. With such consideration in mind one can decide whether this knowledge is likely to be sufficient to answer the project objectives in a constrained and meaningful fashion.
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Augmenting Downhole Seismic Networks with Surface Sensors to Monitor Hydraulic Fracture Stimulations for Accurate Hazard Assessment and Full Bandwidth Assessment of Scaling Relationships for Discrete
Authors A.M. Baig, G. Viegas, S. Karimi and T.I. UrbancicAn array of 4.5 Hz geophone and FBAs recorded a sequence of seismicity during a hydraulic fracture stimualtion that culminated in a couple ~M3 events. These sensors record accelerations and velocities that can be used to show the peak ground motions that are necessary for seismic hazard studies. We discuss that using the appropriate instrumentation during these monitoring efforts is essential to being able to quantigy hazard.
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Microtremors, Ocean Waves, and Other Items
Authors A.L. Vesnaver, L. Baradello, F. Da Col and D. NietoOcean waves are known to be a dominant component of microtremors worldwide in the frequency band from 1 to 10 Hz (Peterson, 1993). Recently, Lambert et al. (2009) claimed that spectral anomalies in microtremors can detect underlying hydrocarbon reservoirs. However, Hanssen and Bussat (2008) and Ali et al. (2010) got experimental results leading to opposite conclusions, because of the lack of signal repeatability and the correlation between spectral anomalies and near-surface topography. This paper expands the related work of Nieto et al. (2011) with new data from the OGS Seismological Network (OASIS Working Group, 2011) and the Marine Environmental Monitoring Network, showing that geological structures and active faults may be locally dominant factors too.
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