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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
1 - 20 of 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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