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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
41 - 60 of 556 results
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Complex Volcanic Reservoir Characterization - A Case Study for Southern Songliao Basin, Northeast China
More LessDiscovery of a gas accumulation in Lower Cretaceous YingCheng Group volcanic rocks was confirmed by an exploration well in the southern part of Songliao Basin, Northeast China. A workflow and accompanying modified methodologies were applied to compute gas in place (GIP), which innovated from a similar study for the northern part of Songliao Basin. There were 3 volcanic sequences identified in Yincheng formation with 3 major domes on top of the sequences. The lithology was dominated by effusive deposits of rhyolitic composition and interbedded with minor explosive tuffs and tuff breccias. Several vertical and lateral lithology distribution patterns were identified to allow reconstruction of the sequence of volcanic events. Porosity was vesicular, fracture and vuggy along with fracture in lava flows, and micropores, vuggy leached in the pyroclastics and in-situ breccias. Multiple well pressure data analysis indicated that there was an unified pressure system, and that saturations could be obtained from a saturation-height function above free water level and the capillary pressure curves. A reservoir model was built with semi-deterministic simulation and used to calculate reserves in-place.
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Sensor Technology Comparison Method and Case Study, Part I
Authors D. Yu, B. Maechler and F. MayoSensor performance is vital to the quality of seismic image and detailed comparative study between the velocity and acceleration types will enhance a deeper understanding. In the analytical approach in Part I, it is demonstrated that the time delay of analog geophone data can be as large as 0.12 ms, depending on input signal spectrum, the range of sensor variances, and the deconvolution process; but a typical accelerometer time delay is only 10 μs. It also discusses the issue of the low frequency attenuation by geophones, in contrast to an accelerometer’s flat response to DC. Further digital synthesis analysis in part II provides further quantifying evidences of performance differentiators by statistical measures in terms of time delay, correlation and coherence.
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Laboratory Comparison of the Performance of Analog Geophones and MEMS Accelerometers
Authors G. Hauer, M. Hons, R. Stewart and S. SpiewakA seismic sensor testing laboratory at the University of Calgary is evaluating the performance of analog geophones and MEMS accelerometers using precise mechanical excitation. It is important to characterize the errors in the experimental results as being due to the imperfection of the sensors in addition to the distortions of the testing equipment during excitation. Seismic grade geophones and MEMS accelerometers both exhibit excellent performance as compared to their theoretical models down to measured acceleration amplitudes of less than 10 micro-g.
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Accelerometer Versus Geophone Response - A Field Case History
Authors M. S. Hons, R. R. Stewart, G. Hauer, D. C. Lawton and M. B. BertramA method is derived for the calculation of ground acceleration from geophone data using a frequency-domain inverse filter and an empirical scaling constant. Acceleration-domain spectra from geophones and MEMS survey at Violet Grove, Alberta, Canada are compared. We find that the geophone and accelerometer data, over a band of 5-200 Hz, are very similar. The accelerometer amplitudes are larger than the geophones' below 5 Hz and there are some differences at very high first breaks are not observed on the accelerometer records at some stations.
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Electronic Gimballed Geophones for Ocean Bottom Seismic Systems
Authors T. Buettgenbach, M. Bolduan and K. SchleisiekToday's multi component seismic acquisition systems suffer from orientation problems if they are deployed in inaccessible environments like the deep sea (OBCs, OBSes and seafloor observatories). If MEMS are no viable alternative because of internal noise, power, or dynamic range constraints, 15 Hz geophones, or costly mechanical gimbal systems had to be used in the past. A "Full-Tilt" 4.5 Hz geophone will be presented as a cost effective alternative that operates in any orientation at low geophone noise and distortion levels. An integrated position sensor keeps the geophone's coil in its center position due to an electronic feedback arrangement that produces a compensation current, which is proportional to sine of the tilt angle. A tiny fraction of this compensation current is added to the seismic signal. Therefore, the Z-component of a 3 axis arrangement can be accurately determined by vector rotation in a first post-processing step automatically. Data of a study about the intrinsic noise will be presented. Also data from a measurement at the seafloor will be shown, comparing signals of Full-Tilt geophones to signals of MEMS accelerometers.
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Vibrator Data Quality Prediction from Remote Sensing-based Near-surface Analysis
More LessRemote sensing allows a spatially dense look into the near surface. By leveraging technology such as multispectral satellite data, experienced users are able to create maps that aid logistics planning for surface seismic acquisition and give an estimation of data quality that can be achieved. This paper draws upon examples from a seismic survey in a coastal area of the UAE. We demonstrate that the integration of Landsat 7 ETM imagery calibrated by selected surface geology sampling makes it possible to predict probable instances of wet sabkha, investigate the correlation between the multispectral remote sensing data, and correlate the vibrator sweep performance data with the near surface. The integration of the data is aided by a GIS database.
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Reducing Harmonic Distortion on Vibrators - Stiffening the Vibrator Baseplate
By Z. WeiDue to non-linear effects, the ground-force signal transmitted into the subsurface by vibrators is contaminated by harmonics. The lack of ideal rigidity of the baseplate contributes to the overall harmonic content of the vibrator output. The baseplate flexure exacerbates harmonic distortion, especially the 2nd harmonic distortion. Meanwhile, the ground-force approximated by the weighted-sum method is limited to a narrow frequency bandwidth due to the low baseplate rigidity. A new baseplate design significantly increases the stiffness of the baseplate and dramatically reduces its flexure. The new baseplate is 2.5 times stiffer than the standard baseplate while only suffering a 5% increase to its weight. Field testing demonstrates that with the new baseplate the validity of the weighted-sum ground force in a frequency range up to 160 Hz is achieved. Meantime, the suppression of the even order harmonics contained in the ground force is observed. Very significant harmonic distortion reduction is achieved as the vibrator is on hard and/or uneven ground.
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Independent Simultaneous Sweeping
Authors D. Howe, A. J. Allen, M. S. Foster, I. J. Jack and B. TaylorIt is well known that higher fold and increased sampling is a sure path to improved data quality, and that vibroseis productivity rates are often the limiting factor on land 3D surveys. We want to find ways to use multiple sources simultaneously, so as to make a step change in both sampling density and productivity. In 2006 BP carried out a test of a very simple simultaneous vibroseis method, which we call ISS (Independent Simultaneous Sweeping). In this method all vibrators worked independently without any attempt to synchronise their activity, and the underlying principle of this test was that all interference between sources should be treated as ‘noise’. The noise was randomised as much as possible, and a standard suite of noise removal tools was used during processing. There was no requirement for specialised wavefield separation techniques. In this simple and robust approach, there is no apparent limit to the number of sources that can be operated simultaneously, there is no waiting time for any vibrator, and very little central control or communication is required. It is very well suited for work in many environments including difficult terrain, where vibrator manoeuvring is slow, or where radio communication is challenged.
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Simultaneous Sourcing without Compromise
Authors C. E. Krohn and R. NeelamaniWe show that iterative methods can improve the separation and remove noise for seismograms recorded with multiple vibrators operating simultaneously with a single sweep. Interference noise arises because multiple seismograms need to be recovered or separated from a smaller number of vibroseis field records. This noise represents a compromise made for high efficiency acquisition compared to methods, such as HFVS (High Fidelity Vibratory Seismic), in which there are as many sweeps as there are vibrators. The iterative methods start with an initial separation to as many seismograms as there are vibrators. Then updated seismograms are produced by adjusting them to match the survey data and to satisfy characteristics of noise-free seismograms. The iterations can be repeated until the measure of error is satisfactory. The match to survey data involves reconvolving parts of the initial seismograms with an estimate of the vibrator signatures and stacking. The characteristics of noise-free seismograms can include the fact that signal decreases with time but noise does not. We show improved separation and reduced noise for both pseudo-random sweeping and for long segmented sweeps (Continuous-HFVS). With these iterative methods, the full efficiency of simultaneous sourcing can be obtained without a noise compromise.
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Converted-wave Imaging in the Caspian Sea - A View of Mud Volcanoes
Authors R. G. K. Johnston and P. ParamoBP's field developments in the Caspian Sea are being enabled by ocean-bottom seismic (OBS), almost exclusively through wide-azimuth P-wave imaging. Mud volcano-related features, once thought to act like diffuse gas clouds to P-waves, complicate the propagation of P-to-S (PS) converted waves and are not invisible to them. Following initial data processing of two OBS surveys in 2003 and 2004, an internal study into data quality and propagation issues associated with PS waves focused on understanding how much of the poor imaging was fundamental to the data, and how much could be improved through data processing and more-refined model building. Processing trials supplemented by forward modelling of plausible scenarios for the internal geometries of the mud volcanoes begin to explain the range of PS image quality within each OBS survey, and between surveys, as well as the conundrum of poor PS image quality where P-wave imaging is not overly compromised. This suggests discrete zones of poorly-consolidated, possibly fluidised, material which have a significant impact on PS propagation. So far this has proved as much of a challenge in many areas as the effects of distributed gas on PP propagation.
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Improved Imaging Using Ocean Bottom Seismic in the Snøhvit Field
Authors G. Ronholt, H. A. Aronsen, M. S. Guttormsen, S. Johansen and L. KlefstadThe StatoilHydro-operated Snøhvit field is currently in the final preparatory stages of gas production. A thin oil column is present below the gas in the area. Poor seismic imaging in the western part of the field, due to shallow gas, has led to uncertainties in both seismic interpretation and hydrocarbon volume calculation. To reduce these uncertainties, ocean bottom cable seismic (OBC) data were acquired in late 2006. The OBC data were processed by CGGVeritas in 2007. A complex 4 component (4C) processing sequence was implemented along with extensive shear-wave statics work and imaging through anisotropic compressional (P reflected to P) and converted (P converted to S) wave pre-stack depth migrations (PSDM). The resulting PSDM OBC data images in the area beneath the shallow gas are significantly better than the images from streamer data. The PP images show that multi azimuth imaging, to a large extent, solves the illumination problems beneath the shallow gas. The PS images do not show signs of the high attenuation effects we see in the PP images. The combination of P and S velocity model building also allowed us to solve the shallow gas in the velocity model.
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Using 3D4C Data to Image and Interpret the Seismically Obscured Area over the Tommeliten Alpha Discovery
Authors G. Iversen, B. Lyngnes and T. StrandA high density 3D4C seismic survey was acquired over the Tommeliten Alpha gas condensate discovery in 2005 to image a seismically obscured area caused by complex velocity and density variations in the overburden. Both full azimuth PP and PS data were used and pre-stack co-depth migrated. The result was an improved image of the previously obscured area, especially in the PS dataset. The size of the Seismically Obscured Area (SOA) was reduced also in the pre-stack depth migrated PP data, but in this outer part of the SOA the PP data gave a different image compared to the PS dataset - making the interpretation challenging. Although the resolution of the PS data is lower at the target level, and the main Ekofisk Fm reservoir target (21-24m thick) is below PS seismic resolution, the PS dataset proved very useful in mapping the structural shape and faults inside the SOA, leaving only a small central area obscured.
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Depth Imaging beneath a Seismic Obscured Area (SOA) and Analyzing Its Character - Eldfisk Field Experience
Authors O. Knoth and N. D. WhitmoreTo overcome the challenge of imaging beneath a previously seismic obscured area (SOA) ConocoPhillips(CoP) and its partners used depth imaging utilizing OBC data with converted waves on the Eldfisk field. The imaging challenge is caused by overburden effects predominantly in the Miocene interval. Laterally compact low velocity streaks combined with compartmentalized and overpressured thick shales distort the wave field traveling through it. In 2006/2007 CoP and its co-ventures reprocessed the 2001 OBC data with emphasis on improved multiple attenuation, converted wave preprocessing and depth imaging to uplift the reservoir image within the SOA area. This resulted in a significant reduction of the SOA area (conservative ~30%) for the pressure data and a much improved converted wave image of the reservoir compared to previous results. The velocity field used in depth imaging was consistent with the expectations of a sharp lateral transition and exhibited both extremely low velocities (1450m/s at around 1.8 km depth) and sharp lateral transition (up to 30% reduction for 1000m lateral distance). The workflows and techniques were novel with respect to the role detailed interpretation played in the constraint tomography and how synthetic modeling helped to understand the imaging problem at hand.
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Unconventional 3D Seismic Land Acquisition in Abu Dhabi
Authors G. Ajlani, O. Suwaina and M. Al-KaabiAbu Dhabi National Oil Company (ADNOC) recently acquired a land vibroseis 3D seismic survey in high dunes area where through extensive pre-survey modeling of reservoir requirements and matching with survey design and acquisition parameters, excellent data quality has been achieved at target levels with successful and highly efficient operational performance. Traditionally, land 3D seismic data acquisition in Abu Dhabi has been characterized by large source and receiver arrays, many long high frequency sweeps, large migration aperture area and limited number of active channels caused by equipment limitations. Over the years the survey geometries have evolved to the use of smaller source and receiver arrays and smaller source arrays with single sweeps. In this project, the near-surface conditions in the form of 300 meters high and steep sand dunes interrupted by numerous sabkhas coupled with target reservoirs which are significantly different and much deeper than the rest of the province, presented significant challenges. This led to the need for different set of parameters for a fit-for-purpose seismic survey for reservoir characterization requirements at a certain formation at a specific depth range in a difficult data area. Additionally, project duration and cost requirements dictated the need to optimize migration aperture and patch layout.
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Field Test of the Point Receiver Land Acquisition Technology Performed in Timimoun, Algeria
More LessAn acquisition field test was performed in Algeria to assess the benefits of point receiver recording for enhancing the resolution and signal/noise ratio of land seismic surveys. Recording point receivers (or point sources) instead of the conventional receiver or source arrays enables pre-processing (such as intra-array static and amplitude corrections, surface noise attenuation) to be applied before Digital Group Forming (DGF). Point receivers were simulated by bunching at the same location the six geophones of each string and a dense strip of 4 lines of 6 km of "point receivers" was deployed with a fine sampling of 6.25 m X 6.25 m. These additional test lines were simultaneously recorded with the regular 3D receiver spread. Causing a minimal delay to the 3D operations, a point receiver 3D dataset of 2 km X 9 km was then recorded and compared to a 3D reference dataset of equivalent fold and geometry. Equivalent but independent processing sequences were applied to both datasets, except the above pre-processing applied to point receivers before DGF. The results are encouraging for the use of point receiver technology in improving signal/noise and bandwidth in this area.
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Comparison of Conventional Geophones Versus Digital Sensor - Preliminary Results from 2D Field Test Exercise
Authors F. Mancini, J. M. Mougenot and J. SuiterIn recent years digital single sensors (MEMS) have become a viable option to conventional geophone arrays for land acquisitions, both operationally and economically. Different advantages and disadvantages of single sensors are mentioned in the literature, such as broadband response, linear phase and amplitude response, lower weight, but also lack of array power for noise attenuation and higher costs. TOTAL decided to compare the two receiver systems during an acquisition feasibility test consisting of two 2D lines acquired in parallel with the same source. Results showed that the noise complexity of the data was better handled by geophone arrays. In order to get comparable results with single sensors, a dense distribution is required which could be difficult for a large 3D study. We also learned that single sensor data require specific processing to compensate for the lack of array filtering capability.
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Enhancing the Low-frequency Content of Vibroseis Acquisition - A Case History from the Minagish Field
Authors C. Bagaini, A. El-Emam and A. ShabrawiSeveral recent advances in land seismic acquisition equipment and software have encouraged the exploration geophysicists to reconsider the potential benefits of energetic low frequencies for geophysical applications such as imaging beneath high velocity and highly absorbing formations, deep imaging, inversion of surface waves to characterize the near-surface elastic properties, velocity model estimation in geophysically complex areas and acoustic impedance inversion. We briefly review the more relevant advances in land seismic acquisition and present their application to a recently acquired 3D seismic survey over the Minagish field in Kuwait.
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Utilizing Multi-azimuth Streamer Data for Reservoir Characterization and Uncertainty Analysis in the Nile Delta
Authors R. Marten, W. Rietveld, M. Benson, A. Khodeir, J. Keggin and P. CookMany authors have written about the imaging benefits of multi-azimuth seismic data, extolling the virtues of this type of acquisition as a way to defeat multiple diffractions, improve illumination and produce a better stacked image. Consequently, multi-azimuth streamer seismic data holds out the promise of producing a seismic image much richer in signal and resolution than its single azimuth equivalent. This acquisition strategy in the offshore environment could ultimately allow a much clearer picture of the subsurface where one had not existed before. A number of studies have borne this out, showing the benefits for interpretation of structure and seismic stratigraphy. Beyond these well understood benefits of multi-azimuth seismic for imaging, these data may also offer a unique opportunity to further characterize the reservoir based on multiple independent measures of the overburden and reservoir. The purpose of this paper is to present a case study at the Raven discovery utilizing multi-azimuth data for reservoir characterization and uncertainty analysis.
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Instantaneous Isochrons, Volume-flattening and a High Resolution View of Sedimentation Rate
Authors J. E. Rickett, J. Lomask and J. ClarkThe instantaneous isochron attribute is defined as the change in seismic time with respect to relative chronostratigraphic time. It can be computed by taking the derivative of a horizon volume used for volumetric flattening. In an example from Angola, the horizon volume is computed automatically with a global algorithm that picks every horizon in a 3D dataset. The instantaneous isochron attribute derived from this provides a dramatic view of the geomorphology of a turbidite sequence.
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High Frequencies Attenuation and Low Frequency Shadows in Seismic Data Caused by Gas Chimneys, Onshore Ecuador
Authors F. G. C. Brouwer, A. Welsh, D. L. Connolly, C. Selva, D. Curia and A. HuckHydrocarbon reservoirs can be sources or sinks for migrating gas, hence gas chimneys have an inherent correlation with HC reservoirs. If frequency effects show a strong correlation between reservoirs and gas chimneys, observing attenuation zones above reservoirs can be considered an equivalent indicator. Loss of seismic data quality is usually the main characteristic of gas chimneys but it is hoped that in the future we can use high frequency observations with CWT as an alternative method to detect gas clouds correlating with reservoirs, even in cases where there is no apparent loss of data quality. In this case study, using spectral decomposition techniques, zones of high frequency attenuation can be observed in seismic data above onshore sandstone oil reservoirs in Ecuador. This attenuation correlates directly with gas chimneys seen in the seismic volume. The amount of attenuation is dependent on the vertical extent of chimneys. Low frequency shadows of reflectors at reservoir depth can also be observed and correlated laterally with the extent of shallower gas chimney zones directly above. Intrinsic attenuation of seismic waves through gas chimneys is proposed as a likely cause of these frequency effects.
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