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70th EAGE Conference and Exhibition incorporating SPE EUROPEC 2008
- Conference date: 09 Jun 2008 - 12 Jun 2008
- Location: Rome, Italy
- ISBN: 978-90-73781-53-5
- Published: 09 June 2008
61 - 80 of 556 results
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Automated Geomodelling a Nigeria Case Study
More LessA methodology for automated 3D seismic volume interpretation is presented. Stack of horizons are generated from the 3D volume along with the faults. These horizons can then be used to build a geological model and better visualization of the depositional model. PaleoScan is a new technology to fast track interpretation of large seismic volumes with an innovative geo-model computation method, based on image processing and optimization techniques. In this work we describe a case study in offshore Nigeria where PaleoScan has been effectively used to map stratigraphic features and reservoir distribution and also build a robust structural model based on the generated horizon and fault cube.
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Gleaning Meaningful Information from Seismic Attributes
Authors S. Chopra and K. J. MarfurtPoststack seismic data loaded on workstations for interpretation exhibit a certain level of noise. This noise could be of various sorts - acquisition related, processing artifacts or random. In this presentation we focus on conditioning our seismic data prior to attribute computation. We discuss the use of some of the procedural steps for noise filtering and dip-steering options for computation of some geometric attributes like coherence and curvature. In addition, we discuss the impact the choice of algorithm can have on the final results. All these factors ensure that the seismic attributes yield more accurate information for interpretation.
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Volume Blending with Directional Seismic Attributes
By A. E. BarnesVolume blending is a powerful tool for multi-attribute analysis, but its use is limited largely to coherence, reflection strength, and the original seismic data. The success of discontinuity in blending has overshadowed the potential of complementary attributes, especially directional attributes such as amplitude gradients, azimuth, directional dip, and seismic shaded relief. Directional attributes make seismic data appear like illuminated apparent topography, which aids intuition. Amplitude gradients have seen little use but should be explored as they can reveal details not seen in other attributes. Shaded relief is the most powerful directional attribute, but azimuth and directional dip are useful substitutes. Smoothing directional attributes improves them for blending.
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An Approach of Seismic Interpretation Based on Cognitive Vision
Authors P. Verney, M. Perrin, M. Thonnat and J. F. RainaudAdvanced seismic interpretation most commonly rests on transforming original data representations by considering more or less numerous seismic attributes, which bear no explicit relation with geology. For this reason, they hardly allow fully solving problems such as reassembling sparse geological surface elements or specifying chronological or topological relationships between surfaces such as unconformity, on lap, interruption by fault. The present work intends to make further progress in geology-based interpretation of seismic data by using artificial intelligence tools based on cognitive vision. We propose a cognitive vision workflow for seismic interpretation based on a visual ontology and on three associated module dealing for data management, visual characterisation and geological correlation. An example of results is given showing the possibilities of the method for easily merging disconnected reflectors within one stratigraphical horizon taking into account simple geological criteria (amplitude, thickness, dip, vertical distance between reflectors).
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Fundamental Principles of Isotropic Marine Source Design
Authors J. Hopperstad, R. Laws and E. KraghWe discuss how to design a marine source array that emits the same pulse shape in all directions. We do this by analysing the relative contribution of the three main components of the farfield source spectrum: the ghost response, the aperture smoothing function, and the monopole spectra of the individual source elements. Counter to intuition, reducing the lateral array dimensions, such that it approximates a point aperture, will not result in an isotropic source; it will create a dipole because of the sea-surface reflection. For single depth arrays, the lateral geometry must be tailored such that the aperture smoothing function offsets the angular variation of the ghost response, in particular at frequencies close to the first ghost notch. Multi-depth arrays can be made insensitive to the lateral geometry by combining the source depths, such that the first ghost notch is far beyond the highest frequency of interest. This results in smaller spectral variation with take-off angle than that of single depth arrays. There is no advantage in using more than two layers.
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Are Seismic Sources Too Loud?
Authors R. M. Laws, E. Kragh and G. MorganIf a seismic survey is performed twice in quick succession the two images will not be exactly the same. Part of the difference results from ambient noise and part is shot-generated. The shot-generated noise originates both from the previous shot and from perturbations to the acquisition, such as small differences in shot and receiver positions. Only if the ambient noise is higher than the shot-generated noise will increasing the source output improve the overall signal-to-noise ratio of the image. We used data from a repeated survey, together with ambient noise measurements, to determine the power spectra of the signal, the ambient noise and the shot-generated noise. We did these tests in both calm weather and poor weather. We show that, in this example, the noise in the seismic image is mostly shot-generated not ambient. This means that the signal-to-noise ratio of the image would be largely unaffected if the source power were reduced. In our test example, for most of the bandwidth, the seismic source is unnecessarily loud.
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Using Onboard Time Lapse Processing for Quality Control of Time Lapse Acquisition on the Norne Fields
Authors B. Osdal, T. Alsos, A. Høiås and H. A. AronsenThis paper will show how onboard processing is used during acquisition in order to quality control the 4D acquisition. The effect of not being on position with source and receivers and the effect of industry noise can be directly analysed using 4D difference data processed onboard. This information is then used in the decision process of reshooting a line or not.
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Improved Marine 4D Repeatability Using an Automated Vessel, Source and Receiver Positioning System
Authors J. O. Paulsen and G. BrownA new automated and integrated, vessel, source, and receiver control system has been developed to improve the accuracy and repeatability of 4D surveys. The new control system replaces operator intervention with automated updates to vessel, source, and streamer steering devices from positioning information from all in-sea equipment. This has lead to a step change in the accuracy of source repeatability (2.5-m repeat accuracy for 95% of shotpoints), and has also improved the ability to repeat receiver positions.
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Benefits of Two-boat 4D Acquisition - An Australian Case Study
Authors T. A. Ridsdill-Smith, D. Flynn and S. DarlingIn February 2007 Woodside acquired a two-boat, push reverse 4D monitor survey over the Enfield oil field in offshore Western Australia (Figure 1). Two-boat push reverse acquisition was used to maximize 4D repeatability and minimize 4D infill in a survey area known for strong, unpredictable currents. The source and receiver positional repeatability obtained in the monitor is good with a mean |Dsrc|+|Drec| of 27m at the target offset of 1900m. Survey infill is only 7% compared to a predicted infill of 30% to achieve the same 4D coverage with oneboat acquisition. Shortcomings of the two-boat push reverse technique include a large minimum near offset of around 500m and a receiver motion 4D error when matching to the conventional one boat baseline survey. An NRMS of 15% was obtained for the final processed data. Following the success of the 2007 Enfield survey, Woodside has proposed to acquire a second two-boat monitor survey on Enfield in early 2008.
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The Concept of Triple Over/Under Streamer Data Acquisition and Processing
More LessMarine seismic data acquired with towed pressure sensing streamers suffers from the interference of the upgoing wave with the downgoing wave reflected at the sea surface. The downgoing wave introduces time-variant perturbations due to the time-variant shape of the sea surface, especially in rough weather conditions, meaning that the sea-surface ghost becomes time variant. These problems can only partly be overcome if a single streamer is replaced by a pair of streamers such that one streamer is towed vertically above the other. The basic dilemma with twin-streamer data is that one has to make a data processing decision: either to attenuate the downgoing wave to reduce the rough-sea perturbations while introducing a pseudo ghost, still attenuating the signal energy at certain frequencies, or to estimate the upgoing wavefield, while leaving the rough-sea surface perturbations still affecting the output data. If one accepts to stay with pressure sensing towed streamers, the situation changes drastically if at least one further streamer is added vertically to the twin-streamer pair. With such an extended over/under streamer configuration one can derive an estimate of the upgoing wave at the flat sea surface, whatever the actual sea-surface state is.
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GeoStreamer, a Dual-sensor Towed Marine Streamer
Authors R. Tenghamn, S. Vaage and C. BorresenThis paper reports the successful implementation of particle velocity sensors in a new dual-sensor towed marine streamer and presents examples of acquired and processed data from that streamer. Properly combining the recorded signals from the particle velocity sensors with those from the hydrophones cancels sea-surface ghost reflections and removes the notches from the data’s frequency spectrum caused by those ghost reflections. As a result, the streamer can be towed deeper below the sea surface than is currently practicable, and the data are much less affected by weather noise, without compromising its bandwidth. In fact, the bandwidth of the recorded data is increased.
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Wave-height-corrected Seismic Data
Authors R. J. Goto, E. Kragh, R. Laws, W. G. Morgan and R. PhillipsThe rough sea surface perturbs, to first order, the receiver ghost arrival time and introduces a source array static. We discuss how to measure and correct for both the receiver and the source perturbations for seismic data acquired in rough seas. For the receivers, ultralow (sub 1 Hz) pressure variations recorded by the hydrophones are inverted for wave height profiles which are both time and space variant. Local (compact) deconvolution operators are then used to stabilize the receiver ghost. For the source, real-time kinematic (RTK) GPS measurements accurately track the height of the source floats and a static correction is made at the time of each shot. Previously published work has concentrated on measuring and validating these errors, particularly on the receiver side, but corrections have not been applied to seismic data. Here, we show application of such corrections to seismic data acquired in rough seas. The resulting data show improved reflector continuity and a stabilized wavelet. Data repeatability is improved and the resulting data are more suitable for high-resolution surveys and time-lapse reservoir monitoring applications.
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Data Driven Velocity Model Building - An Application to Egypt Nile Delta
Authors M. C. Tanis, T. P. Summers, P. Cook and J. KegginThe complex Messinian layer in the Egypt Nile Delta poses a serious challenge to the building of an accurate velocity model and to the imaging of structures that lie below this interval that consists of interleaved anhydrite, shale and other facies. Conventional prestack velocity analysis, depth imaging and tomography provide only a limited solution to this complex velocity modeling and imaging problem. While the recent acquisition of multi azimuth data over this area aimed to address both the imaging and illumination below the anhydrite, building a reliable velocity model for prestack depth migration still remains a challenge. Here we present a novel approach to model these complex Messinian anhydrite layers interleaved with other thin sediments directly from seismic data using a binary style velocity modeling approach. The method is based on exploiting the distinct seismic character of anhydrite and estimating it directly from inverted section of an optimized migrated stack. Application of this procedure to data from Egypt Nile Delta shows that the technique can provide a reliable high resolution starting velocity model that can be further improved by iterative migration velocity analysis.
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CRS-PreSTM/CRS-PreSDM - Noise Reduction in Seismic Imaging
Authors H. Trappe, G. Gierse, G. L. Eisenberg-Klein and J. PruessmannWith the CRS method the incoherent seismic energy (noise) can be strongly suppressed. The increased stacking/gather fold, and additional wavefield attributes used in the time and depth domain processing enhance the imaging quality in both domains. The CRS time domain processing provides the requisites for an enhanced prestack depth migration with high potential, especially for data with low acquisition fold or high noise levels, where the velocity model is not too complex. The additional wavefield attributes can be used to enhance the velocity model in a robust noise-insensitive procedure, and to design migration operators. The field data examples of the CRS-PreSTM and CRS-PreSDM work flow reveal the high potential of noise reduction for seismic time and depth domain imaging. The key for the improved quality is the combined usage of the CRS attributes for the time domain S/N enhanced processing and regularization, the enhanced velocity building especially for noise contaminated data, and the migration parametrization in combination.
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Staged Subsalt Derisking for Development of the Cleaver Bank High
Authors R. van Boom, B. Spaargaren, A. Fongers and R. WervelmanFor many years the west of 49/14b, offshore U.K. was poorly imaged due to a complex en-echelon system of salt ridges. These issues prevented the development of Carrack West. In 2006 a seismic survey with a different azimuth, and longer offset was acquired. During the first stages of the processing the interpretation started on the Post-STM. A Pre-STM was delivered together with a Kirchhoff Pre-SDM. The Kirchhoff Pre-SDM on the 2006 dataset was encouraging, it was also noted that the vintage seismic, shot under a different azimuth, showed more energy in certain areas and a dual azimuth processing was performed. At the same moment it was realized that the standard approach of single valued Kirchhoff did not cater for the complex ray-paths in parts of this setting. A WEM enabled us to improve the imaging even further. It is demonstrated that the 5 different datasets all have their own merits. The minute mapping that was needed for the development planning of the Carrack West discovery profited from the parallel use of all the products and only now we are confident on the risk level to take the development of the project further.
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Azimuthal Anisotropy Correction in Champion West 3D Survey - A Case Study
Authors J. Sun, Y. Yin, B. Hung, R. Dike, J. DePledge, Y. Liu and R. WombellIn this paper we present a case study describing the application of a correction for azimuthal velocity anisotropy on the Champion West 3D seismic survey, offshore Brunei. The survey consisted of conventional sail lines, plus regions of two-boat undershoot in the area of existing platforms. The undershooting resulted in very large azimuth variation, and irregular data coverage especially in near offset range. Coupled with the regional azimuthal velocity anisotropy, the large difference in trace azimuths caused time jitters in data, which in turn caused difficulty in imaging. In this study, an azimuthal differential NMO was applied to the data, after scanning for the anisotropy parameters on the basis of stack response. The correction made major improvement to the data, as seen both before and after migration. Before migration, jitters were moderated and stack response was better. After migration, the improvement was seen as better focusing and greatly reduced amplitude striping.
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Increased Resolution of Seismic Data from a Dual-sensor Streamer Cable
Authors D. H. Carlson, W. Söllner, H. Tabti, A. J. Day and M. WidmaierTraditionally, towed marine cables measure the seismic wave-field using only hydrophones. A new solid core dual sensor cable has been introduced that measures simultaneously the pressure using hydrophones and the vertical component of the particle velocity using motion sensors. The first of many advantages of measuring the dual wave-field is that the two measures can be combined to separate the wave-field into up-going and down-going components. This method may be used to derive the up-going pressure field, thereby eliminating the receiver ghost. This gives increased bandwidth for both low and high frequencies, which consequently enhances resolution and signal penetration. Another advantage, and the reason for the added low frequencies, is that, since the receiver ghost can be suppressed, the streamer can be towed at greater depth where it encounters reduced acquisition noise thus improving the ability to acquire data during rough weather. This paper illustrates these applications using 2-D field test data that was acquired concurrently with a survey that used a conventional (hydrophone only) cable.
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Deepwater Gulf of Mexico - Benefits of Integrating Seismic, Gravity and Magnetic Data in a Sub-salt Environment
Authors A. G. Blacque, K. Mohn, S. V. Yalamanchili, A. Morgan and V. EgorovTechnical innovation in the Gulf of Mexico has been pushing the frontiers of petroleum exploration, leading to the expansion of proven play concepts into new areas of the deepwater Gulf of Mexico. Operators are now targeting structurally controlled traps containing Paleogene (Wilcox) sands, beneath the Louanne salt canopy. Understanding these deepwater sub-salt opportunities requires a regional, structural and stratigraphic context for the Wilcox play. Sub-salt sediment distribution, salt geometry, basement architecture and basement composition are all crucial to this workflow. Although many studies have been undertaken to better understand this sub-salt petroleum system, the fundamental problem of sub-salt seismic imaging remains the key challenge, even with the latest acquisition and processing technologies available. Through integrating seismic, gravity and magnetic data, better understanding of the sub-salt environment has been achieved. As a result of detailed integration, a new basement map has been created from high-resolution aeromagnetic data and Pre Stack Depth Migrated (PSDM) seismic data. Both 2D and 3D earth modeling were then completed to predict spatial distribution of autochthonous salt bodies, basement density and susceptibility variations, location of open/closed salt feeders to the allochthonous canopy and the location of the continental-oceanic boundary.
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Microseismicity of Non-linear Fluid-rock Interactions - From Geothermic Stimulations to Fracturing of Barnett Shales
Authors S. A. Shapiro and C. DinskeBorehole stimulations by fluid injections are common for developments of hydrocarbon and geothermic reservoirs. Often they induce numerous microearthquakes. Spatio-temporal dynamics of such induced microseismic clouds can be used to characterise reservoirs. However, a fluid-induced seismicity can be caused by a wide range of processes. We propose an approach to describing a broad range of non-linear fluid-rock interactions. We show that a linear pore pressure relaxation and a hydraulic fracturing are two asymptotic end members of a set of non-linear diffusional phenomena responsible for seismicity triggering. Microseismicity of Barnett Shales shows signatures of a non-linear diffusional triggering with an extremely strong hydraulic permeability enhancement in a 3-D domain of the reservoir. This indicates a process of a volumetric hydraulic fracturing via reopening of compliant preexisting fractures.
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Nonlinear Pore Pressure Diffusion and Associated Microseismicity in Rocks with Pressure-dependent Permeability
Authors T. M. Mueller and N. HummelThe fluid transport properties of fractured porous rocks usually show a strong dependence on the effective pressure and thus on the pore pressure. Consequently the process of pore pressure diffusion initiated by fluid injection experiments is governed by the non-linear diffusion equation. The spatio-temporal evolution of microseismicity associated with the process of nonlinear pore pressure diffusion is analyzed. In particular, it is shown that the process of nonlinear pore pressure diffusion allows to define two triggering fronts. One corresponding to the triggering of microseismic events without changing the fluid transport properties significantly, another one that triggers the majority of microseismic events and that alters the fluid transport properties significantly. The applicability of this model for the interpretation of microseismicity observed during hydraulic fracturing treatments is discussed.
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