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75th EAGE Conference & Exhibition incorporating SPE EUROPEC 2013
- Conference date: 10 Jun 2013 - 13 Jun 2013
- Location: London, UK
- ISBN: 978-90-73834-48-4
- Published: 10 June 2013
1 - 20 of 1113 results
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Modal Elastic Inversion
By F. ErnstRecent years have seen the emergence of full-waveform inversion (FWI) approaches for generating detailed velocity models for imaging. For land seismic data, especially in areas of complex near-surface geology, a proper near-surface model is crucial for imaging. These two observations naturally lead to the question whether FWI approaches can also be used for near-surface model building purposes. As most wave propagation effects in the near surface are inherently elastic, this requires elastic FWI. The combination of small scales involved, and typical large sizes of modern land surveys, may make an approach based on finite differences or finite elements computationally very demanding. We propose a fast approximation to elastic FWI, modal elastic inversion, where wave propagation in the near surface is modeled by means of a (small) number of horizontally propagating waves: ground roll, its higher modes and guided waves. These waves comprise the vast majority of energy in land data and are sensitive to near-surface velocities. We apply modal elastic inversion to a 2D test line and show that both the shallow P-wave and S-wave velocity models can be recovered well.
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Q Estimation from Surface Waves
Authors D. Boiero, M. Werning and P. VermeerDespite the fact that surface waves in land seismic data are often regarded as noise, they can also be used to obtain valuable information about the subsurface. Because their propagation is directly related to the properties of the subsurface, the analysis and inversion of surface waves can provide a characterisation of the near surface. In this paper, surface waves were used to estimate the dimensionless quality factor Q, which is most commonly used to measure the attenuation of seismic waves. To invert for the quality factor Q, the surface-wave attenuation coefficient and the phase velocity, as well as the near-surface S- and P-wave velocities, are needed. A weighted damped least-squares algorithm was chosen for the inversion. The algorithm produced promising results both for synthetic data, used to test different subsurface conditions (not shown here), and for real data sets, providing information about the subsurface that is not yet obtainable with other techniques.
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Use of Simultaneous Joint Inversion as a Maximum Concordance Solver for Statics
Authors M. Mantovani, M. Clementi and F. CeciStatics are primarily derived from seismic refractions or first arrivals, and strongly rely on their quality of gathers. At the initial stage of seismic processing, refractions are the single data input. In common practise, inversions of first breaks are based on user’s interpretation of early arrivals and, therefore, they are subject to systematic errors, especially if picking is automated as for large 3D data sets. In such circumstances, it is desirable to avoid overfitting of observations in inversion. An active criteria counter to systematic mistakes is presented here, based on statistical benchmarking against independent non-seismic measures. The approach is based on Simultanous Joint Inversion for measure integration. While rock physics relations are normally unstable at the near surface, the qualitative concept of localized anomaly can be transported between various geophysical domains, as is normally done in prospect play evaluation. Anomaly distribution consistency between domains is here used as a discriminant of input data through a-posteriori inversion result benchmarking. A static solver can, therefore, weight more the first break data, which are experimentally confirmed by several independent measurements, rather than contradicting data. Concordance is based on concordant anomaly generation in the a-posteriori inverted model.
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Ice Scours as Trapping Mechanism for Shallow Gas
More LessShallow gas is commonly found on the Norwegian Continental Shelf and is a potential geohazard for drilling operations. Examples of shallow gas in Upper Pliocene sediments, in the Central North Sea is presented, and a model for how buried ice scours can act as a trapping mechanism for shallow gas is proposed. The examples are from conventional 3D seismic data.
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Mitigation of the 3D Cross-line Acquisition Footprint Using Separated Wavefield Imaging of Dual-sensor Streamer Seismic
Authors A.S. Long, S. Lu, D. Whitmore, H. LeGleut, R. Jones, N. Chemingui and M. FaroukiA modified one-way equation pre-stack depth migration of up-going and down-going pressure wavefields was applied to two datasets derived from 3D towed dual-sensor streamer data in offshore Australia and Malaysia. The primary objective was to mitigate the well-known cross-line acquisition footprint effects upon shallow data quality and interpretability. The new methodology introduced here exploits the illumination corresponding to surface multiple energy, and thus exploits what has historically been treated by the seismic industry as unwanted noise. Whereas a strong cross-line acquisition footprint affected the very shallow 3D data using conventional processing and imaging, the new results yield spectacular continuous high resolution seismic images, even up to, and including the water bottom. One implication of these results is that very wide-tow survey efficiency can be achieved without compromising shallow data quality if dual-sensor streamer acquisition and processing is used, even in very shallow water areas such as that discussed here. The imaging methodology can account for all degrees of lateral variability in the velocity model, full anisotropy, and angle gathers can be created to assist with velocity model building.
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3D Seismic-gravity Simultaneous Joint Inversion for Near Surface Velocity Estimation
Authors D. Rovetta, D. Colombo, E. Sandoval Curiel, R.E. Ley, W. Wang and C. LiangA novel 3D simultaneous joint inversion scheme for gravity and seismic travel time data is developed to solve for near-surface complex velocity distributions. The method incorporates industry-standard gravity and travel time inversion techniques while the joint inversion problem is solved by the introduction of various regularization functions about the model such as a-priori parameter distribution information, solution-space bounds, structural similarity via cross-gradient constraints and rock physics relations. The effectiveness of our joint inversion is demonstrated against a synthetic model representing a complex pattern of near-surface anomalies incorporating low and high velocity and density bodies. Results demonstrate the superiority of our approach where the shallow anomalies are better reconstructed by the joint inversion rather than that obtained by the single-domain inversions. The developed algorithm is tested with real data from Saudi Arabia acquired over a wadi structure. The results show a significant uplift of the time stack using the seismic-gravity joint inversion velocity model. The developed methodology is part of a multi-geophysics platform for near-surface velocity model building in complex geology scenarios.
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Integrating Resistivity and Seismic Surveys to Identify Shallow Hydrocarbon Reservoirs
Authors C. Badulescu, C. Iacob, M. Paduret, L. Tonita, N. Moga, C. Dutu and M. CohutDirect current resistivity survey is one of the oldest geophysical methods used for prospecting natural resources. When integrated in interpretation with seismic investigations, it usually offers high quality results, even in difficult investigation settings and complicated geology. The purpose of the study was to enhance seismic interpretation of the near-surface geology using high resolution direct current resistivity surveys, in order to confirm structures of potential shallow hydrocarbon reservoirs. Resistivity surveys have been executed in an area in Romania where seismic data gave poor results for the near surface geology delineation. Either by constraining inversion or interpretation, seismic and resistivity surveys proved to be an efficient couple of geophysical methods for delineating near-surface geology even in high noise and complicated geological settings. Geological models could be built for the studied area and structures with potential for hosting hydrocarbons have been identified.
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Guided Waves - Inversion and Attenuation
Authors D. Boiero, C. Strobbia, L. Velasco and P. VermeerGuided waves contain significant information about the near surface in seismic data, but prove difficult to remove through conventional velocity discrimination methods. Here, we analyse guided waves jointly with ground roll to characterise the near-surface properties and then we remove them by modelling methods. We begin by retrieving guided wave and ground-roll phase velocities. Then, we jointly invert them to build S- and P-wave velocity models using a robust multimodal inversion algorithm. Finally, we use the inferred guided wave and ground-roll properties to model the dispersive coherent noise and subtract it while preserving the signal.
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Imaging Unknown Faults in Christchurch, New Zealand, after a M6.2 Earthquake
Authors D.C. Lawton, M.B. Bertram, K.W. Hall, K.L. Bertram and J. Pettingaity of Christchurch, New Zealand, following a devastating Mw 6.3 earthquake on February 22, 2011 that caused the loss of 185 lives. The goal of the seismic program was to map previously unknown faults in and around the city for hazard assessment and to assist in the post-earthquake recovery effort. Seismic data were collected along six 2D lines, two of which were within the Christchurch metropolitan area and four were in rural areas west of the city. Recording conditions were challenging within the city, but good quality images were obtained along all of the seismic lines, with events interpretable to a depth of approximately 1.5 km. Numerous faults were imaged along the lines and these were interpreted in two groups – older faults that showed clear offsets in deep (> 1 km) reflections and younger faults that showed displacement in shallow reflections. Some faults in the latter group were interpreted to be directly associated with hypocentres of shallow after-shocks in the region. These interpretations are now being incorporated into a risk assessment for further possible shallow earthquakes in the region.
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Felt Induced Seismicity Associated with Shale Gas Hydraulic Stimulation in Lancashire, UK
More LessShale Gas exploration in the UK began in 2010 with five stages of hydraulic fracturing of the Bowland Shale. Hydofracturing in Cuadrilla Preese Hall-1 used 5 couplets of mini- and main-fracs with c. 10,000 bbl per stage, hydraulically isolated from each other during injection. An earthquake of 2.3 ML, a 1.5ML and a series of smaller events occurred from 31st March through May 2011 when work ceased for detailed analysis. Seismicity commenced after stages 2, 4 and 5; the largest occurring 10 hours after stage-2 shut-in. Seismological analysis using observations from seismometers emplaced after the 2.3 event, and regional data detected 55 seismic events from ML-2 to ML2, with 14 between ML 0.2 to M2.3, indicating a low B-value of 0.8 (+/- 0.3) suggesting an unusually small number of weaker events. The timing was highly correlated with injection with the largest events (stages 2 and 4) preceded by weaker events (ML0 and ML1.4) all located near the Preese Hall well, c. 500 meters south of the injection interval. The observed seismicity is almost certainly induced by hydraulic fracturing of the Preese Hall well. Future mitigation based on microseismic monitoring during hydrofracturing should reduce the likelihood of inducing further felt earthquakes.
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Induced Seismicity at Preese Hall, UK - A Review
Authors T. O‘Toole, J.P. Verdon, J.H. Woodhouse and J.M. KendallHydraulic fracturing activities during shale gas exploration at Preese Hall, near Blackpool, UK induced a series of anomalously large microseismic events, including two that were felt at the surface. The unexpected nature of this seismicity meant that microseismic monitoring of these operations was limited, with only two surface stations being deployed after a M_l 2.3 event was detected by the British Geological Survey's regional seismic network. The small size of the available dataset means that we must try and extract as much information from it as possible if we wish to fully understand what went on, and how risks might be reduced in future. Towards this end, we have used waveform inversion to determine moment tensors for a number of the induced events; events predominantly occurred with a pure strike-slip mechanism. To investigate the state of stress in the sub-surface from a different direction, we have also performed shear wave splitting analyses of the available dataset. These two techniques yield principle horizontal stress orientations that are in good agreement with each other, and that are also consistent with in situ measurements.
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Correlation Between Spatial and Magnitude Distributions of Microearthquakes during Hydraulic Fracture Stimulation
Authors J.P. Verdon, A. Wuestefeld, J.T. Rutledge, I.G. Main and J.M. KendallSeismic and microseismic event populations can be characterized by their spatial and magnitude distributions. The magnitude distribution is described by the Gutenberg-Richter b value. The spatial distribution can be described by the two-point correlation coefficient, Dc. We observe spatio-temporal variations in both b and Dc during hydraulic fracture stimulation in the Cotton Valley tight gas reservoir. We find that during the initial stages of the fracture stimulations, b is high while Dc is low, implying that deformation is dominated by smaller events clustered at points near the injection well. As injection progresses, b decreases and Dc increases. We investigate correlation between b values and Dc, which can be indicative of the style of fracturing. Initially, b and Dc are negatively correlated, consistent with mechanical weakening during the early stages of fracturing. As the stimulation progresses, b and Dc become positively correlated, consistent with later stabilization of the fractures and mechanical hardening when the induced fracture intersects a pre-existing fault. The mechanism of hardening may be crack blunting while propagating into a pre-existing fractured or damaged zone.
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Monitoring the Evolution of Fracture Compliances During Hydraulic Stimulation Using Passive Seismic Data
Authors A.F. Baird, J.P. Verdon and J.M. KendallSeismic anisotropy is a useful attribute for the detection and characterization of aligned fracture sets in petroleum reservoirs. While many techniques to estimate anisotropy have been successful in inferring fracture density and orientation, they generally provide little constraint on the ability of the fractures to facilitate fluid flow. A potentially useful property to provide insight into this is the ratio of the normal to tangential fracture compliance (ZN/ZT). ZN/ZT is sensitive to many properties including: the stiffness of the infilling fluid, fracture connectivity and permeability, and the internal architecture of the fracture. Here we demonstrate a method to infer ZN/ZT using shear wave splitting measurements on two microseismic datasets from hydraulic stimulations. Both examples show apparent increases in ZN/ZT during the stimulation process. We suggest that this 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 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 effectiveness of stimulation strategies.
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Beyond the Dots - Microseismic Monitoring of a Fractured Reservoir During Steam Injection in Oman
Authors S. Mahrooqi, S. Busaidi, I. Ismaili, F. Clow, T. Urbancic, A. Baig and A. KassamA Microseismic network of 13 wells over a fractured heavy oilfield in Oman has accurately monitored a steam injection program and has also observed sufficient multi-well events to enable extensive SMTI analyses. The SMTI data has been analysed together with the steam injection progress to identify the source mechanisms and fracture planes in relation to the pre-existing fault network, and has indicated, in different areas, both re-activation of existing faults and new fracture activity.
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Towards Self-consistent Microseismic-based Interpretation of Hydraulic Stimulation
Authors M.J. Williams, J.H. Le Calvez and J. StokesThe primary role of microseismic interpretation in the context of hydraulic fracture monitoring (HFM) is to provide understanding of the geometry of a placed hydraulic fracture treatment to enable better completion design, reliable production predictions, and real-time operational decisions during the treatment itself. One aspect of this interpretation is the identification of microseismic events that are related to the fluid-filled fracture propagation directly as distinct from those events which are not. We present a method to statistically identify clusters of microseismic events whose spatial and temporal separation are consistent with the propagation of a hydraulic fracture system, according to various standard models of fracture propagation. We then apply this interpretation to a forward model of complex fracturing to obtain consistency with the pumped data. We subsequently review that complex fracture model via a finite element geomechanical simulation, interpreting via elastic-brittle failure analysis and plastic deformation, to understand the potential source of microseismicity. Our principal finding is that it is possible to obtain a self-consistent interpretation between both fracture propagation and earth stress simulation by following this approach.
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Pseudo-elastic Impedance Calibrated to Rock Physics Models for Efficient Lithology and Fluid Mapping from AVO Data
Authors P.A. Avseth, T. Veggeland, M. Christiansen, K.J. Ĺrdal and F. HornWe have shown how we can derive attributes similar to elastic impedances, which we refer to as pseudo-elastic impedances, directly from rock physics templates of AI versus Vp/Vs, without knowing the density. This allows us to calibrate the elastic impedances to local rock physics models, and to honour the curvature of rock physics models, normally associated with compaction trends. In areas with large variability in burial depth, it is important to honour the true variability of the background trend. The final regression models are easy to implement in quantitative interpretation workflows, and allow for quick mapping of fluid or lithology anomalies that are consistent with rigorous rock physics models. We have demonstrated the validity of this approach on well log and seismic data from a prospective area in the Norwegian Sea.
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Fluid Identification Based on P-Wave Frequency-Dependent Azimuthal AVO Method in Fractured Media
More LessSeismic anisotropy is relevant to frequency. When fractures in reservoirs are saturated with different fluid types (oil, gas and brine), the variation rates of anisotropy versus seismic frequency (which is called anisotropy dispersion) are different. Thus, anisotropy dispersion can be used to distinguish different fluid types in fracture. In this paper, P-wave frequency-dependent azimuthal AVO inversion algorithm is proposed and applied to real seismic data. The studies focus on the analysis of P-wave anisotropic dispersion response for different fluid saturation. Continuous wavelet transform is employed for spectral decomposition, aiming at well time-frequency localization. Real data application indicates that the magnitude of P-wave anisotropy dispersion of brine is far stronger than that of oil and gas in fractures. Our study firmly proves that the method aimed at calculating the P-wave anisotropic dispersion for different fluids saturation is valid and reliable, and this brings us the confidence to discriminate fluid by the calculated anisotropic dispersion values in fractured reservoir.
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Estimation of Gas Saturation Changes from Frequency-Dependent AVO Analysis
Authors X. Wu, M. Chapman, E. Angerer and X. LiSeismic attenuation is believed to be sensitive to many reservoir parameters of interest, but its routine application is inhibited by the lack of reliable quantitative relationships which can relate observed behaviour directly to rock and fluid properties. This paper calibrates a frequency-dependent rock physics model for a gas field in Austria, through analysis of well log-data and frequency-dependent AVO analysis of seismic reflection data. Analysis of the model reveals the potential to estimate changes in gas saturation which cannot be detected with the standard, single frequency approximation.
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How Seismic Anisotropy Improves the Reliability of Exploration DHI (AVO)
Authors M. Ferla, F. de Finis and R. BacenettiThe role of seismic anisotropy has dramatically increased over the past two decades due to advances in parameter estimation. Many seismic processing and inversion methods take into account anisotropy. The inadequacy of isotropic velocity models has been emphasized in prestack depth migration, which is highly sensitive to the accuracy of the velocity field. At the same time this study demonstrates how amplitude variation with offset analyses are sensitive to anisotropy. The correct integration between different seismic processing steps that incorporate anisotropic models represents a valid support for the identification of explorative targets. A new integrated workflow has been developed in terms of anisotropic Thomsen parameters estimation, velocity and AVO models. We present interesting results of modelling for special cases of exploration interest, highlighting the anisotropic effects for gas sands encased in shales. The analysis has been also performed on a real dataset, confirming the relevance of models that account for the seismic anisotropy. This could explain the inability of elastic synthetics to match the prestack amplitudes of field data in some cases. This considerations lead to more realistic reservoir models and hopefully answer some of the unexplained pitfalls in AVO interpretation.
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Non-Stretch Fourth Order NMO through Iterative Partial Corrections and Deconvolution
Authors E. Biondi, E. Stucchi and A. MazzottiSource to receiver distances employed in seismic data acquisition have been steadily increasing and it is now common to work with data acquired with more than 10 km of offset. Sub-basalt exploration and seismic undershooting are just two applications where long-offset reflections are valuable. However, such reflections are often subjected to muting to avoid NMO stretch artifacts, thus causing a loss of valuable information. It is therefore of interest to find ways to avoid the distortions caused by the standard NMO correction and to retrieve these portions of the recorded wavefield for a better use in the processing. To this end we develop a non-stretch NMO correction based on a wavelet estimation and on a iterative procedure of partial NMO correction and deconvolution. To drive the corrections we make use of 4th order traveltime curves, that further extend the offset range of usable reflections. Then we estimate time and space variant wavelets, by means of SVD along the sought traveltimes, that become the desired output for the deconvolution trying to retrieve the original shapes of the partially stretched wavelets. We test our method on synthetic data and we perform a blind test on real data simulating an undershooting acquisition.
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