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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
1 - 20 of 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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