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First EAGE Workshop on Borehole Geophysics
- Conference date: 16 Jan 2011 - 20 Jan 2011
- Location: Instanbul, Turkey
- ISBN: 978-90-73781-95-5
- Published: 16 January 2011
44 results
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Acquisition and Processing Challenges for Very Large Offset 3D VSP in the Arabian Gulf, Saudi Arabia
Authors J. Owusu, N. Palacios, J. Musser, F. Amry, S. Rahati, T. Maghrabi, T. Alsheha, Y. Latif, S. Ahmed, A. Campbell and L. NuttIn 2009, Saudi Aramco acquired three large 3D VSP surveys in the Arabian Gulf, Saudi Arabia. These surveys were not only the first of their kind in Saudi Aramco, but each was also the largest, single well offshore 3D VSP worldwide. The primary objective of the survey was to improve the vertical and lateral resolution of signals from the sand stringers near and away from the well to properly place multi-lateral horizontal wells. A 100-level 3–component geophone tool was deployed in each well over a depth range of 1600 to 6600 feet. Each survey consisted of 32,000 shot points and 9 million traces. It was designed to optimize the subsurface coverage of mode-converted P-to-SV waves (C-waves) at the target using a maximum offset to depth ratio of 2.5. The surveys were completed in the shortest possible time to minimize drilling rig time. The objectives, acquisition design, QC procedures and logistical planning are described, which led to three successful offshore 3D VSPs.
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3D VSP Feasibility Workflow using 3D Wave Equation Modelling and Reverse Time Migration
Authors C.B. Barberan, A.B. Bon and J.G. GuilbotAccurately imaging steep dip reflectors and salt boundary are very important tasks for exploration in deep water sub-salt environments where reserves are often trapped underneath overhangs of massive salt bodies. Pre-stack depth migration for surface seismic imaging has been proved to be essential for seismic exploration in such complex areas. Although they require high computing power, full two-way solutions of the wave equation -suited for such complex environments- have been used in the past few years more intensively not only for surface seismic imaging purpose but also for pre-survey modelling. With the recent development of long downhole geophone arrays, VSP techniques have emerged to complement surface seismic with improved resolution, better high dip structure definition and to fill in "imaging holes" in complex sub-salt plays where surface seismic is blind. And the recent step-change advances in computing power have made the use of full two-way solutions commercially viable as well for large VSP projects. We propose a complete VSP feasibility study workflow based on 3D wave equation acoustic modelling and reverse time migration imaging illustrated by a case study from West Africa.
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Guided Target-Oriented 3D VSP Survey Design
Authors H. Djikpesse, M. Khodja and M. PrangeIn many situations, 3D VSP data are acquired to improve subsurface interpretation based on surface seismic data. However, this surface seismic prior information is often underutilized when designing the 3D VSP surveys to acquire. A Bayesian methodology is described for designing experiments or surveys that will optimally complement all previously available information. This methodology uses strong prior information to linearize the problem, and to guide the design toward maximally reducing forecast uncertainties in the interpretation of the future experiment. The prior information could possibly be correlated among model parameters or the observation noise. With no prior information this approach reduces to the fast recursive implementation of the D-optimality criterion. Synthetic geophysical tomography examples are used to illustrate the benefits of this approach.
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3D VSP Modeling and Imaging using the SEAM Model
Authors J. Gulati, X. Li, A. Campbell and S. AliSynthetic spiral 3D VSP data are generated using an isotropic, acoustic finite-difference scheme and the SEAM velocity model. Imaging the synthetic data using reverse-time migration demonstrate the ability to illuminate steeply dipping sediments in complex geological environments using 3D VSP data. With increased computing power, it is now feasible to have an iterative flow of survey design, synthetic data simulation and imaging for optimum placement of source and receivers in a 3D VSP survey.
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A methodology to estimate maximum offset and reflection imaging radius for 2D/3D VSP survey
Authors M. Lou, R. Williamson and D. QuinnWe have developed a straightforward and accurate ray-tracing based methodology to estimate both maximum offset and reflection imaging radius for multi-layered velocity models, which can be used for 2D/3D VSP survey design. Through numerical examples, we demonstrate that the presence of a high-velocity layer above a target zone will significantly reduce maximum offset and reflection imaging radius. In addition, our methodology more accurately predicts the VSP imaging radius than one estimated by the conventional straight-line based approximation. The conventional approximation often produces an incorrectly large reflection imaging radius.
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Successful Sidetracking of a Well Onshore Germany by Integration of 3D VSP Technology - A Case Study
Authors S. Frehers, H.G. Rueth, F. Ogliani and T. Ter BurgAfter drilling a low-performing well onshore North Germany the borehole was used to conduct a 3D VSP. Main purpose was to improve the seismic image of the zone in the vicinity of the well in order to determine a new landing point for the envisaged sidetrack. This turned out to be the right choice despite difficulties arising from surface restrictions during acquisition and the complex geological setting. A thorough pre-survey planning is essential in order to achieve the set goals. The quality of the seismic image of the zone in the vicinity of the well was highly increased. This allowed a reliable prediction of a less-faulted acreage and suitable facies conditions. Furthermore, enhanced structural mapping workflows helped to provide improved interpretation results in order to minimize the risk of failure.
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High Resolution 3D VSP Imaging: A Case Study from an Offshore Field in Qatar
Authors J. Gulati, Z. Djebbar, R.R. Chanpong, H. Cao and L. NuttA 3D VSP was acquired over the Bul Hanine field in Qatar to evaluate the improvement in reservoir characterization using high resolution images. VSP data were imaged using both single and three-component processing – median filter, plane-wave decomposition and VSP deconvolution being the main steps of the processing flow. The initial model was built using checkshot data from multiple wells and was calibrated using 3D traveltime inversion of the 3D VSP and ZVSP transit times. Imaging of the upgoing VSP data demonstrated the improved resolution compared to the existing surface seismic. Ant tracking of the VSP image cube revealed fracture patterns consistent with the known geology.
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Interval anisotropic parameter estimation in walkaway and 3D VSP data
By E. BliasThe purpose of this paper is to present a new method for interval VTI anisotropic parameter estimation above and within VSP array. Above receiver array, the method utilizes duplex downgoing multiples (multiples with two reflections: from the boundary and free surface) for estimation of a layered depth velocity model above the borehole receiver array. A three-term NMO approximation function is used to estimate NMO velocity and a non-hyperbolic parameter. Dix-type inversion is applied to the three parameters to invert them to interval Thomsen anisotropic parameters ε, δ above the receivers. Velocity gradient influence is analyzed analytically. It is shown that in many cases we can neglect its influence on anisotropy estimates. Within the VSP receiver array, the method creates a virtual horizontal reflector at each receiver depth by calculating reflected traveltimes as a sum of two first breaks from the symmetrical source points. For 3D or multi-azimuthal walkaway survey, this approach is generalized for a model with horizontal homogeneous orthorhombic layers.
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3D VSP in a Mining Context
Authors M. Humphries, C. Pretorius, K. Trofimczyk and E. GillotVSPs have an application across the mining lifecycle. The main advantage of VSP over surface seismic is the increase in resolution achieved by placing the downhole receiver below the highly attenuating weathered zone and recording the full seismic wave-train, including mode-converted shear waves. This presentation looks at a 3D VSP shot in conjunction with a surface seismic, specifically the improved resolution gained from mode-converted shear waves.
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Pre-stack VSP Elastic Inversion for Lithology Delineation in an Offshore Field of the Arabian Gulf, Saudi Arabia
Authors J. Ahmad, B. El-Marhfoul and J. C. OwusuWe present the results of a feasibility study on the application of pre-stack VSP elastic inversion to characterize the distribution of sand stringers in an offshore field of the Arabian Gulf of Saudi Arabia. Using amplitude preserving wavefield extrapolation techniques, the VSP data was converted into a pseudo surface seismic volume from which VSP common-midpoint (VSP-CMP) gathers were generated for the inversion analysis. AVO inversion was carried out using a standard surface seismic simultaneous AVO/AVA inversion workflow. Comparison with the surface seismic inversion showed that the VSP elastic inversion results were higher in resolution and showed a slightly better correlation with the well logs. Standard rock property analysis, cross-plots and body checking using multiple volumes can be carried out on the inverted VSP-CMP results. Overall, these preliminary results are very encouraging.
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An Early Look at a Time-Lapse 3D VSP
Authors A.J. Campbell, L. Nutt, S. Ali, K. Dodds, M. Urosevic, R. Pevzner and S. SharmaIn 2007 Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) started a project to evaluate the available technology for monitoring the movement of CO2 in an underground reservoir. In stage one of this project CO2 and methane are being produced in a nearby well, then the CO2/CH4 is being injected back down a second well into a depleted gas sand. The movement of the CO2 up-dip needs to be remotely monitored. One of the technologies being evaluated to monitor the movement of the CO2 is 3D VSP. The CRC-1 injector well is instrumented with 10 3C downhole accelerometers. A baseline 3D VSP survey was shot around this well in late 2007 and was followed by a repeat survey in 2010. We hope to monitor the movement of CO2 around the CRC-1 well by observing changes in the time-lapse signature.
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A shooting and receiving interchanging layout for crosswell seismic survey
More LessFor a conventional layout of a crosswell seismic survey, a downhole source and a receiver array are set in two wells respectively. A massive data set, in general, is acquired for velocity tomography and reflection imaging. We found that this conventional layout used in some areas in China led to some problems whether technically or economically making such surveys unattainable. In order to improve the situation, we propose a new observation configuration for crosswell seismic surveys which is titled “A shooting and receiving interchanging layoutâ€. The principle of the method is from the imaging of offset VSP. When a source is set in one well and a receiver array in another well, we obtain one half of the image between two wells. Then we change the former shooting well into the receiving well and former receiving into shooting well, to obtain the corresponding opposing half image. Imaging both halves constructs the full image. In comparison with a conventional layout, the new layout is oriented to acquisition of reflection signal instead of first breaks for tomography, which reduces the costs of field operation, shortens the cycles of data processing and interpretation.
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Crosswell Seismic Application in Unconventional reservoirs
Authors J. Khoury, P.L. Carrillo, B.W. Bryans and C.M. KolbThe combination of high resolution products derived from crosswell seismic technology provides operators the ability to not only assess the structural complexity of their unconventional reservoir but also assess the fluid saturation within it. We present a study using velocity, reflectivity and frequencies to optimize the imaging of an oilsand and a bitumen-saturated carbonate reservoir. The velocity tomogram, attenuation coefficient tomogram, and Q distribution values have been obtained for both reservoir types using tomographic methods. High resolution reflection images correlate with anomalies identified in both the velocity and attenuation tomograms. The extraction of accurate attenuation values could be useful for elastic and viscoelastic studies applied not only for unconventional resources, but also for all the stages of the reservoir lifecycle.
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Case study at area Well S7 with Borehole-Ground TFEM method
By Z.X. HeBorehole-Surface electromagnetic method (BSEM) has a long history in Russia, and has become an important mean for hydrocarbon exploration. It was introduced into BGP in the year 2000. On the basis of which, Borehole-Surface TFEM method has been developed: transmitting-receiving systems with 64 transmitting and receiving frequencies and up to 4 waveforms has been developed, accordingly, pseudo 2D constraint inversion and extraction of induced polarization anomaly have been achieved. In recent years, test and production for oil-water boundary identification have been conducted in LiaoHe oilfield, Tuha oilfield, JiLin oilfield, DaGang oilfield, ChangQing oilfield, Xinjiang and Saudi Arabia oilfield, etc. and achieved good results, deserving full recognition the Clients at home and abroad. Here we take the application in area Well S7 of the G basin in west China as an example to introduce this method and its application effects.
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Optimizing Salt Flank Locations Using Simultaneous Standard Salt-Prox Mapping and Interferomtric Imaging
Authors M. Karrenbach, V. Soutyrine and R. HeTraditionally standard salt proximity estimates and interferometric imaging have been used independently to obtain salt flank location estimates. We combine various standard and interferometric methods into an optimization algorithm that allows us to iteratively arrive at mutually consistent estimates of salt flank locations while also producing uncertainty estimates and trust regions.
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Seismic Side-Scan for Wide-Range High-Resolution Imaging around Boreholes
More LessStructural and lithological delineation is seen as an emerging mining application of seismic imaging. The general benefits expected from applying delineation techniques include: reduced economic risk, shorter project timelines, and more accurate resource evaluations. The structural aspect has been documented relatively well in the past decade and consists mainly of mapping faults, fracture zones, and dissolution features, bearing an impact on geo-technical models and designs. Mineral resource delineation by seismic techniques is a comparatively recent addition to the seismic application menu and is seen as a promising technique for the coming decades.
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Microseismic feasibility studies - identifying the challenges in assessing Probability of Success of monitoring projects
Authors W. Berlang, S.J. Oates and Y. FreudenreichWe present our approach to microseismic feasibility studies with the aim of promoting discussion of the methods used and assumptions made in assessing the Probability of Success (PoS) of a monitoring project. A feasibility study includes the following elements: assessment of the Value of Information (VoI); identification of hardware options; survey of analogue cases; modelling of event location errors and detectability (magnitude of the smallest detectable event) for candidate array geometries; generation of full waveform synthetics. A study comprising these elements provides the reference frame for discussions with the service companies, limiting misunderstandings and missed opportunities. To calculate event detectability the source corner frequency and medium Q factors are required: these are parameters with large associated uncertainties and the modelling results can be strongly dependent on the values chosen. Global earthquake compilations show a clear trend for corner frequency as a function of magnitude but it is unclear how this should be extrapolated to microearthquake magnitudes. It must also be decided whether the level of seismicity will be sufficient for imaging. We begin to consider whether expected levels of seismic activity can be estimated from easily available rock mechanical data to allow more confident assessments of the PoS of monitoring.
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Seismic While Drilling in the Grane Field
Authors H. Mathiszik, M. Cox, F. F. Bøen, S.A. Petersen, A. Sæbø and R. ComanUsing a standard surface seismic source and a set of acoustic receivers placed within the bottomhole assembly of a drillstring provides data equivalent to conventional wireline gathers, but also allows data acquisition in sections normally not accessible with cable based systems. In addition the data has the value of real-time availabilty. With this Seismic While Drilling (SWD) technique, geosteering decisions and formation characterisations can be made early during the drilling phase of the reservoir exploration especially in the long lateral sections typical for North Sea offshore production wells. A special benefit comes from the use of hydrophone data gathered during SWD deployments, not normally available in wireline VSP measurements. The potential of SWD will be demonstrated in an SWD case study from the Grane Field in the North Sea.
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Azimuthal Anisotropy Analysis of a Walkaround VSP: A Case Study from Saudi Arabia
Authors K. Al Dulaijan, J.C. Owusu and D.C. WeberWalkaround Vertical Seismic Profile (VSP) is useful for seismic azimuthal anisotropy analysis, and presents a unique opportunity to characterize the fracture distribution around a borehole. Saudi Aramco has recently completed the acquisition, processing and analysis of a walkaround VSP dataset to determine the orientation of fractures in an Ordovician sand channel in Northwestern Saudi Arabia. In this paper, the results of the azimuthal traveltime, polarization and shear-wave splitting analysis are presented and compared to fracture orientation derived from dipole sonic and image logs. The result of the azimuthal PS-wave traveltime analysis agrees very well with the dipole sonic log and indicates a NW fracture trend. The P-wave polarization analysis result shows significant scatter due to the poor data quality, but nevertheless indicates a NW fracture trend. Shear-wave splitting analyses suggest two possible fracture orientations, and one of them is consistent with the predictions from the polarization analysis results.
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Tight Sand Gas Prediction with Walkaway + Walkaround VSP
More LessFor lithologic traps, only 2D surface seismic survey cannot confirm the favorable growth directions of reservoir; 3D surface seismic survey, however, has an extremely high cost; therefore, VSP is considered an effective method to study the formation variation around the well, especially in the area where the structure is relatively simple. In order to identify the favorable directions of the developed gas-bearing sands in the SDG structural belt, VSP surveys are carried out on PC1 well in this area. Through zero-offset P and S-wave VSP for normal incident survey, Walkaround and Walkaway VSPs for azimuth and offset variant survey as well as the data processing on the high-quality data acquired, the geophysical parameters of the target layer are extracted, and therefore, the dominant reservoirs and their spatial extensions are described, which has provided a basis for the further horizontal sidetrack drilling of the well. The result shows that VSP exploration can effectively predict the favorable growth directions of reservoirs. This has revealed the potential of VSP in this area.
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3DVSP and cross-well surveys for deep EOR target imaging in South Oman.
Authors A.S. Al-Maamari and D. KiyashchenkoA geophysical surveillance pilot was started to identify the best monitoring solution for field-scale deployment. The field is being developed using steam-injection EOR methods. Baseline surface seismic, 3DVSP and cross-well surveys have been acquired and processed. Surface seismic is very challenging in the area due to complex near-surface conditions, multiple contamination and weak target formation reflectivity. On the other hand, 3DVSP and cross-well surveys allow us to get clear images of the target reservoir after application of the experimental processing approaches. For cross-well data, the use of sources and receivers below the targets and down-going reflections delivered the image of the interval of interest, whereas there was almost no use in sources and receivers above the targets. This was not expected originally and highlighted the importance of considering ’illumination from below’ as an option in pre-survey modeling. The extensive multiple presence in the field impacted the 3DVSP images created with the use of space-domain Deconvolution of up-going signal with a filter derived from downgoing waves. In order to suppress multiples more efficiently, we developed and applied ’interferometric deconvolution’ in the wavenumber domain. This deconvolution approach led to a cleaner image of the target.
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Application of 3D VSP's to Tight Gas Reservoir Characterization
Authors P. Mazumdar, T.L. Davis, J. Liu and R. BansalExploration and development of tight-gas sandstone reservoirs relies heavily on understanding the distribution of sandstone bodies in the subsurface. Shear-wave sourced 3D vertical seismic profiling (VSP) data are used for sandstone detection and fractured interval delineation. Anisotropic Kirchhoff 3D prestack depth migration was applied to the shear-wave reflection data. A geologically-constrained migration-velocity model was created by integrating the results from the virtual source method, well logs, interval velocity derived from surface seismic and the Dix-differentiation of the fast and slow shear-wave NMO ellipses. The resulting common image gathers (CIG) are flat, indicating that the reflection events can be stacked reliably. The depth migrated images tie with the well logs and the surface seismic data. The VSP images show subtle stratigraphic features involving sandstone intervals in the Williams Fork Formation and pinchouts in the coal beds within the Cameo Coal interval. Structural features, especially faults, previously unnoticed on surface seismic data are now clearly visible. The shear-wave VSP data can now be used as a complementary dataset to other commonly acquired data such as surface seismic, well logs and core data and microseismic information to improve reservoir characterization.
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A new Well-Seismic method to help Seismic-to-Well ties in complex environments. Example on the deep-offshore, Angola
Authors C.C. Cheyron, V.M. MARTIN, P.T. THORE and A.P. PAREThe development of PSDM processing enables interpreters to work in the depth domain. But as PSDM velocity is not the same as actual velocity, we wonder then whether the drilling trajectory is compatible with the PSDM data. The answer is certainly affirmative if the PSDM velocity is close enough to the actual subsurface velocity and negative if is not. To circumvent this difficult problem of the PSDM velocity, we devised a new method for computing a well trajectory that is fully compatible with the PSDM volume. Thus, even if the PSDM is biased, the relocated well trajectory will be shifted consistently. The events seen while drilling the well will be really crossed on the PSDM by the relocated trajectory and this way, even if the PSDM velocity volume is not clearly established, interpreters will be able to tie their PSDM more successfully. In this paper, we describe the principle of this new well-seismic method, called Multi Offset Relocation (MOR), which consists in intersecting wave fronts shot from several different VSP offsets. We show how to use it in practice, and present an example of its application in the deep-offshore subsalt domain in Angola.
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Optimization approach to automatic first arrival picking for 3C 3D VSP data
By E. BliasA new adaptive optimization method for accurate estimation of the 3C 3D VSP first breaks is proposed. It uses combination of different criteria to estimate first breaks through an objective function with adaptive parameters. First we pick first breaks manually for the first gather of the 3D VSP set, and adjust several coefficients to describe close-to-first-breaks wave-shape parameters. We calculate an objective function for a moving trace window and minimize it with respect time shift and the slope. This function combines four main properties that indicate first breaks on 3-C VSP data: linear polarization, signal/noise ratio, similarity in wave shapes for close shots and their stability in time interval after first break. We adjust coefficients as a combination of current and previous values, so this approach uses adaptive parameters to follow smooth wave-shape changes. The method utilizes three components by calculation objective function for the direct P-wave projection. Using adaptive multi-criteria optimization approach for several traces and three components makes this method very robust even for the data contaminated with high noise.
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3DVSP from interpreter's perspective
Authors M.C. Schinelli, E.A. Thedy, R.C. Sansonowski, O.F. Keller and P.R.S. Johann3D-VSP has clearly become a borehole seismic technique with strong impact over seismic reservoir characterization activities. Much attention has been given to operational aspects, new equipments and processing techniques, but the opinion of their interpreters is usually only superficially explored. We discuss here the interprets point of view, covering their experience in the use of three different 3D-VSPs surveys, one in marine and two others on land areas. Basic aspects of survey planning, acquisition and processing are revisited, with focus on interpretation by the perspective of learned lessons, highlighting benefits and obstacles in the use of those 3Ds, with suggestion to the future projects.
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3D VSP Processing Issues for Detailed Reservoir Characterization
Authors C.B. Barberan, E.C. Crouzy, A.M. Michot and P.W. WillemsenVertical Seismic Profile (VSP) was originally designed and is still primarily used to give us time-to-depth relationship for seismic-to-well ties. But the inherent 3D nature of the earth and a desire to obtain 3D reservoir information near the well has led to interest in acquiring 3D VSP data. Over the past few years, and with the development of long receiver arrays, an increasing amount of 3D VSP data have been acquired as a cost-effective technique by possibly giving a 3D high-frequency seismic volume around the well, and as an alternative to OBC acquisition. However the processing of large array 3D VSP remains a challenging issue. Not only 3D VSP images often suffer from limited acquisition aperture and limited fold, but processing techniques, including VSP migration, initially designed for 2D imaging have not evolved much. The consequence is to make the VSP images often difficult to interpret. We present a case study from a 3D VSP acquired offshore, where a workflow different than the conventional 3D VSP processing route has improved the results. Preserving fine interpretable details was finally obtained by improving the processing workflow, reducing the migration aperture and using a 3D mapping technique.
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Planning, Acquisition, and Processing with Modern Borehole Seismic Tools
Authors B. N. Fuller, L.A. Walter and J.M. SterlingNew technologies, such as large borehole seismic receiver strings challenge practitioners to explore and find new ways to optimize their use. Methods that have proved valuable in improving the outcome of 2D VSP, 3D VSP, and microseismic surveys are discussed.
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Large-Array 3D-VSP and Full-Azimuth 3D Surface Seismic Simultaneous Acquisition Survey Design
Authors C.W. Liu, G. Xu, Y.P. Li, K. Kang and H.L. LiangAlong with the VSP technique developing ceaselessly, especially with the advancing of manufacturing engineering of downhole geophone and the data processing level, 3D-VSP survey has become feasible. As a strong support for performing 3D-VSP survey, application of large-array downhole geophones will form a tendency. BGP and Daqing Oilfield Company in CNPC have performed a large-array 3D-VSP and full-azimuth 3D surface-seismic simultaneous acquisition test jointly aiming at the volcanic geologic body, through which, a study has been made in the view of geophysical parameter extraction, high-resolution imaging and reservoir description. This paper illustrates the design of this method taking the Xushen 21-1 in Daqing as an example.
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Downhole seismic sources for Reverse 3D-VSP surveys
Authors C. Naville, R. Ásmundsson, S. Thorhallsson, G. Asgrimur, J. Kolle and E. GillotAcquisition of Reverse 3D-VSP with Downhole seismic sources and large amount of surface seismic sensors is desirable in many instances. A non exhaustive overview of existing powerful prototype downhole seismic source devices is presented, as well as the main results obtained in a recent past, as well as the emerging technology which can contribute to industrialise the Reverse 3D-VSP method. Discussion with industry is welcome
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Vp/Vs ratio estimation from joint PP-PS inversion of 3D VSP seismic data
Authors T.L. Tonellot, J. Fitzmaurice, S. Khater and S. RahatiA workflow to derive Vp/Vs and acoustic impedance volumes from joint PP-PS inversion of seismic data is applied to 3D VSP data. PP and PS depth migrated VSP volumes, converted to PP time, are first sequentially inverted for acoustic and PS elastic impedance respectively. In a second step the main reservoir horizons are interpreted for each volume. The PS volume is then transformed in time so that the picked horizons coincide in the PP and PS datasets. The resulting data are jointly inverted for acoustic impedance and Vp/Vs. This workflow was applied to two 3D VSP datasets, recently acquired in the Arabian Gulf, to improve the delineation of thin sand stringers for optimized horizontal well placement. The 3D VSP inversion results were compared to the conventional elastic inversion of PP OBC data. When OBC data are good, no significant resolution improvement was observed on the 3D VSP results. In an area where the OBC data quality was low, the 3D VSP provided a superior image in the proximity of the well.
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A VFSA Method of Travel Time Inversion for Layer Interval Anisotropy Estimation Using Walk-away VSP
Authors D. Pei, B. Cornish, R. Zhou, D. Quinn and R. WilliamsonWe developed a new implementation of VFSA as an alternative method of travel time inversion for Thomsen’s parameters using walk-away VSP data. Velocity along the symmetry axis and TTI axis orientation must be known. Two synthetic tests show that the inverted parameters are within 10% of their true model values. The sensitivity analysis shows that ε is more dependent on ray paths in the horizontal direction or group angles close to 900 , while δ is more affected by ray paths with 550 incident angles. This implies that walk-away surveys designed for this purpose may require longer source-receiver offsets, e.g. the ratio of offset to depth may be equal to or greater than 1. Consequently, head and / or guided waves may interfere with accurate first arrival time determination and care is required in picking the proper event.
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3D VSP data RTM
More Lessit can accurately image the reflectors beyond 90o dipping angles. RTM uses the full wave equation to extrapolate the wave field. And a key issue is that it creates the low-frequency imaging noise if the Claerbout’s (1971) correlation imaging condition is employed. The modified imaging condition is that at reflection points, the arriving time of the incident wave equals to the taking-off time of the reflection wave. The imaging condition is suitable to be implemented in the local angle domain, which can completely attenuate the imaging noise. The high-order or compact finite-difference is commonly used to solve the full wave equation. The hybrid GPU/CPU high-performance computer system can rapidly finish the RTM of one shot gather. However, the absorbing boundaries are not suitable to be used on the platform because their finite-difference computation violates the GPU’s character. Therefore, the random scattering boundary condition is adapted (Robert, 2009). Reverse phase of the reflectivity at the same point on a reflector and Dependence on the high-wavenumber components in the migration velocity if multiples are imaged are two important issues for 3D VSP data RTM.
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Constrained viscoelastic full waveform inversion of North Sea offset VSP data
Authors M. Charara and C. BarnesFull-waveform inversion of VSP data requires simulation of the wave propagation in complex media in order to account of the complexity of the wavefield and therefore to extract more information from the data; the drawback is the increase of the number of the degree of freedom for a problem already underdetermined. Constraints consistent with the least squares formalism, i.e. through covariance matrices, on the data space and the model space can be introduced to significantly reduce the number of the degree of freedom. The data covariance matrix can be filled by using the polarization analysis to describe the multi-component data uncertainties and the model covariance matrix can be filled with spatial correlation functions. A visco-elastic full-wave inversion of offset VSPs data from the North Sea illustrates the impact of multi-parameter inversion when constraints help to regularize the inverse problem.
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Borehole seismic monitoring of CO2 storage within a saline aquifer at Ketzin, Germany
More LessThe borehole seismic monitoring in the in situ laboratory for saline aquifer CO2 storage (Ketzin, Germany) comprises crosswell tomography, offset-VSP, walkaway-VSP, and single level 3D-VSP. Time-lapse measurements of the crosshole tomography indicated a decreased P-wave velocity of the rock units affected by CO2 injection, the walkaway-VSP observed increased reflectivity from the sandstone layers of the formation. The offset-VSP and 3D-VSP provide structural geometry and characterization in the vicinity of the site. A particular challenge will be the time-lapse processing of surface-to-borehole data.
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Walkaround VSP for Fracture Detection in Northern Saudi Arabia: A Case Study
Authors D.C. Weber, K. Al Dulaijan and J.C. OwusuThis paper documents anisotropy analysis for fracture detection using data from a walkaround VSP that was acquired in a gas well in Saudi Arabia. The results are compared to dipole sonic, image log data and core analysis. Results from another walkaround VSP in a nearby well are included. Also covered is the VSP modeling and acquisition design process.
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A Brief History of Microseismic Mapping in Unconventional Reservoirs
By R.J. ZinnoMicroseismic monitoring has played a critical role in the success of oil and gas production from unconventional reservoirs over the last ten years. Several surveys spanning that period, including the first survey in a shale gas field, describe the evolution of that success. These case histories illustrate important lessons learned about influence of fine geologic structures on the fracture behaviour of these reservoirs during stimulation. These stimulation mapping examples chronicle the evolution of modern completion techniques to address and exploit these reservoir complexities; and describe the economic impact of applying microseismic monitoring to these operations.
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Elastic gaussian beam imaging of walk-away VSP data.
Authors M.I. Protasov and V.A. TcheverdaThe approach to true amplitude seismic imaging for walk-away Vertical Seismic Profile (VSP) multi component data is presented and discussed. This approach is migration procedure based on weighted summation of prestack data. True amplitude weights are computed with application of Gaussian beams (GB). The couple of beams is used in order to compute true amplitude selective image of the rapid velocity variation. The total true amplitude image is constructed by superposition of selective ones being computed for a range of available dip angles. Shooting from the bottom overcomes difficulties due to multi arrivals of seismic energy which are common for complicated velocity models (in particular salt intrusions). In addition, the global regularity of Gaussian beams permits one not take care of, is the ray field regular or not. The use of P- and S-wave Gaussian beams provides the possibility to handle raw multi component data without of their preliminary separation to PP and PS waves. The true amplitude selective images computed for a variety of opening angles gives possibility to implement AVO analysis.
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High-resolution seismic exploration system in boreholes - Seismic Prediction While Drilling (SPWD)
More LessSeveral borehole seismic methods are in use while drilling. Because the sought-after small-scale structures like thin local layers or faults encountered more and more in deeper regions the focus of exploration has shifted to methods with higher resolution. Combining a seismic source and receiver in a common device in the drill string would increase the seismic resolution. Within the project (S)PWD a first prototype of such a Seismic Prediction While Drilling (SPWD) device has been designed and tested in dry horizontal boreholes. With source signal frequencies up to 5000 Hz a resolution of about one meter is feasible. By focusing the seismic waves the energy can be maximized in desired directions. An increased radiation of seismic wave energy in direction of the borehole axis allows the view in areas to be drilled. If applied while drilling such a tool allows adjusting the drill path to minimize risks and costs of drilling. The next step in the project (S)PWD will be in 2010 the development of a wireline SPWD prototype for vertical boreholes with depths to 2000 m.
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Coda-wave interferometry analysis of Synthetic time-lapse VSP data at CO2SINK project site, Ketzin, Germany
More LessThe CO2SINK project is the first on-shore pilot CO2 storage study in Europe and Address a number of the Key Aspects related to geological storage of CO2 . Different Seismic Surveys Have Been Carried Out in the project , Such as time-lapse Seismic and surface moving source profiling ( MSP , Also known as walk -away VSP ) Surveys . We Have investigated the feasibility of coda -wave Interferometry Analysis for Monitoring of CO2 Injection Using time-lapse VSP synthetic data based on a project site CO2SINK elastic model . Our studies Suggest That coda -wave Interferometry Analysis Could Potentially Be A Reliable and Effective tool for monitoring Geological Carbon Sequestration .
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Borehole Hydrophone Aquisition - Some pitfalls and solutions
Authors A.J. Greenwood, J.C. Dupuis, A.W. Kepic and M. UrosevicHydrophones are highly sensitive broadband pressure sensors. They are slim-line, lightweight, rapidly deployable and do not require clamping. Strings of 24 – 48 receivers can be manufactured for the same cost as a single slim-line 3C shuttle. The passive hydraulic coupling and suspension within the water column employed by the hydrophones lead to specific acquisition issues due to noise sources related to cable and borehole seismic modes. With the use of a 24 channel hydrophone string, over several surveys in predominately mineral exploration boreholes, we have investigated suppression of these noise sources. Improvement of hydrophone coupling to the formation is achieved through higher viscosity drilling fluids. It is also encouraging that very high quality converted shear waves can be recorded with hydrophones. We show that due to easy deployment and rapid acquisition time it is possible to utilise very fine hydrophone increment of just 1m which enables a proper registration of tube waves and hence their effective removal. Finally we test our hydrophone array in a complex hard rock environment. Using a known geological model, through extensive elastic modelling we prove the validity of an ultra-high resolution VSP image constructed from hydrophone data.
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Characterization of an Uranium Deposit by 3D Borehole Seismic Techniques
More LessThe Millennium uranium deposit is located within the Athabasca Basin of Saskatchewan, Canada. The deposit, situated in metapelitic basement rocks, is controlled by multiple sub-vertical faults, and cross-faults and is overlain by ~500 m of intensely altered, porous Manitou Falls group sandstones. The rock quality directly surrounding the deposit is greatly reduced because of alteration and post Athabasca sandstone structures, which provide conduits for the migration of basin and meteoric fluids. This leads to significant risk for mine development and shaft sinking, because of the increased potential for water inflow into mine workings. To mitigate the risk involved with mining in such complex geology several projects were proposed as part of a pre-feasibility study. Of these, seismic methods were identified as the best tool to potentially identify alteration and structurally compromised zones. Subsequently, a comprehensive surface and borehole seismic program was completed in attempt to delineate these engineering hazards and to provide assurance of success of the shaft sinking and mine development. High resolution 3D, moving source profiling (MSP), vertical seismic profiling (VSP) and borehole side-scans were used at Millennium and their results were integrated, together with geological borehole logs into a comprehensive structural site model.
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Fiber Optic Geophones for Oil and Gas Field Applications
Authors B.N.P Paulsson, J.K. Andersen, E. Majer and P. HarbenWe are presenting a new fiber optic sensor system implemented as a Fiber Optic Geophone (FOG). We are presenting the design and experimental test results for the FOG and compare its performance with regular exploration geophones and geophones used for scientific investigations. We will demonstrate that the new Fiber Optic Geophone (FOG) has a significantly better performance than the current state of the art coil geophones in terms of noise floor, sensitivity and frequency response.
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VSP Imaging by Wave-field Extrapolation Technique
Authors B. El Marhfoul, J. Owusu and R. Al-ZayerImaging of Vertical Seismic Profile (VSP) data is routinely performed using Pre-stack Depth Migration (PSDM) processing. This approach requires an accurate subsurface velocity model, which is not always available, particularly in complex geological environments. In this work, we present a procedure to accurately image the subsurface by using common image gathers to update the velocity model. Using a wave-field extrapolation technique, the up-going and the down-going wave-fields from the VSP data are converted into a pseudo surface seismic dataset, while preserving the relative amplitudes and fidelity of the VSP data. Once transformed to the surface domain, the whole range of surface seismic imaging algorithms can be utilized to produce a more accurate subsurface image. Synthetic and real walk-away VSP data examples have shown significant improvements in the imaging quality when wave-field extrapolation is used compared to regular VSP data processing techniques. This technology will help to exploit the full potential of the VSP data.
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A 3D VSP Recorded in Abu Dhabi Yields New Understanding of a Complex Reservoir
Authors B.N.P. Paulsson, K.W. Müller, W.L. Soroka, S. Marmash, M. Al Baloushi and O. Al JeelaniA two well 3D VSP pilot project recorded in 2007 by the Abu Dhabi Company for Onshore Oil Operations (ADCO) was successful at demonstrating the value that can be gained from a high quality 3D VSP image. This second part focuses on interpretation and value addition. It is understood that for recovery to be optimized and by-passed resources to be minimized especially in later stages of field production more accurate models of a reservoir’s architecture and characteristics are needed. The results of this first 3D VSP survey was successful in characterizing details of the reservoir that could not be derived from the surface seismic or well log data alone. The higher quality images made it possible to map detailed stratigraphy and previously unknown important faults. The improved structural map and updated geologic model generated with information from the 3D VSP images were verified by wells drilled inside the 3D VSP image areas. Based on the successful results from this pilot project the 3D VSP technology is now considered a tool for projects that require detailed reservoir characterization in an area around a well within a diameter equal to the image depth, in this case 3,000 m.
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