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76th EAGE Conference and Exhibition 2014
- Conference date: June 16-19, 2014
- Location: Amsterdam, Netherlands
- Published: 16 June 2014
101 - 150 of 1028 results
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Microseismic Monitoring - Source Characterization and Physical Modeling
Authors O. Akbas, R. Stewart and N. DyaurSummaryThis research provides an overview of the physical modeling procedure using ultrasonic sources and receivers, describes and assesses the hypocenter and source mechanism algorithm, and illustrates the results. We developed a ray tracing and diffraction stack procedure to undertake locations and their evaluation.
Data was acquired using ultrasonic source and 3-component receivers on the natural sandstone. We evaluate how accurately variety of methods locate actual source positions. Location was performed using only P-waves, only S-waves and P-S waves and using traveltime residual, stacked energy, and energy/residual ratio. The most accurate location is obtained using energy/residual ratio. The relative errors of using only P-wave, only S-wave, and P-S waves are 0.8, 0.9, and 0.58 percent, respectively. Furthermore, location uncertainty increases when downhole receivers are left out for all cases. The relative error increased to 0.75 percent from 0.58 percent error when we exclude downhole receivers.
To increase the speed of the algorithm, CPU and GPU computing was implemented. Locating a single microseismic event with seven different methods takes 11.4 seconds on single core CPU, whereas, this number is decreased to 4.2 seconds using multi-core CPU computing. Further, implementing GPU computing decreases the total elapsed time to only 1.9 seconds.
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Quantifying Reservoir Stimulation Using Passive Traveltime Tomography
Authors J.W. Huang, J.M. Reyes-Montes, X.P. Zhao, F.D. Chu and R.P. YoungSummaryHydraulic fracturing stimulates reservoir and imposes stress changes in the surrounding rock that typically induce or trigger seismicity with a wide range of magnitudes. Seismic monitoring provides insight into the reservoir deformation and give critical feedback to the on-going stimulations. We have developed a passive seismic tomography technique adapted from earthquake seismology to jointly locate induced microseismic events and update the velocity of the reservoir illuminated by the microseismicity. We calculate travel-time based on the fast sweeping method to account for complex 3D distribution of velocity and use the adjoint method to transform the inverse problem to a forward problem which can also be solved by the fast sweeping method. In this paper, we apply our algorithm to a two-stage reservoir stimulation project and demonstrate the capability of the microseismic tomography in mapping the stimulated rock volume and in quantifying the reservoir degradation even in the absence of visible P-waves.
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Where is the Proppant? DFN Modeling, Proppant Placement, and Drainage Estimation from Microseismic Data
Authors C.W. Neuhaus, M. Ellison, C. Telker and K. BlairSummaryMicroseismic data for 17 hydraulically stimulated wells in the Marcellus Shale were used to conduct a proppant placement and drainage estimation evaluation in order to optimize the wellbore spacing, and in the future the completion design and landing depth of the wellbores. A calibrated discrete fracture network (DFN) was modeled onto the microseismic events taking into account the magnitude of the events, their source mechanisms, the rock properties of the stimulated reservoir, and the amount of injected fluid in order to distinguish between the total stimulated rock volume (SRV) where microseismic activity was observed and the part of the SRV that contains proppant filled fractures and will therefore be productive in the long term. Evaluating proppant placement in the perpendicular horizontal, parallel horizontal, and perpendicular vertical direction has significance for and direct impact on wellbore spacing, stage length and spacing, and the landing depth of the wellbores.
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Determining Resistivity from Towed Streamer EM Data Using Unconstrained Inversion - Tie to Well and Discovery Examples
Authors A.J. McKay, K. Flisnes Bergh and A.H. BhuiyanSummaryWe show that unconstrained anisotropic 2.5D inversion of Towed Streamer EM data in complex geological settings can produce resistivity models that are consistent with both interpreted log and seismic data, and known discoveries. We consider two cases from recent surveys in the Celtic and Barents Seas offshore Ireland and Norway respectively. In the Celtic Sea case we show an example where we have compared the results of unconstrained inversion to publically available log data. Not only is the overall depth trend recovered, but the main variation of the resistivity is captured as well as, in some intervals, comparable average interval resistivity. For the Barents Sea case we show an example resistivity and anisotropy section from one of eight survey lines that traverse the Skrugard discovery across its short axis (about 2km). While the resistivity section highlights that the sub-surface resistivity is complex, the somewhat simpler anisotropy section reveals an anisotropy anomaly that is co-incident with both the lateral, and depth, extent of Skrugard.
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Time Domain EM Soundings for the Delineation of the Fresh Water - Salt Water Interface in Eastern Canada Islands
Authors J. Giraud, M. Chouteau and A. BoucheddaSummaryA hydrogeophysical study involving electromagnetic and electric methods to map depths to the saltwater-freshwater interface into coastal aquifers was carried out. Time domain electromagnetic (TDEM) soundings were acquired along electric resistivity tomography (ERT) profiles perpendicular to the coastline in the Magdalen Islands (Eastern Canada). Departures from hydrogeological models allowed evaluating the influence of water pumping on the studied aquifers. As a result, except in zones closer to the pumping wells, a good agreement between the hydrogeological model and the position of the interface estimated by the coupling of TDEM and ERT is observed.
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First Results from an Electromagnetic Survey of a Gas Hydrate Vent Offshore Mid-Norway
Authors A. Swidinsky, S. Hölz, M. Sommer, M. Jegen, K. Weitemeyer and C. BerndtSummaryWe describe a transient electromagnetic survey to image the resistivity structure of the hydrate vent CNE03, offshore Mid-Norway which located approximately 10 km north of the Storegga Slide sidewall. The experiment is unique in that the electric dipole transmitter has two polarizations for each transmission station. Furthermore, the newly designed transmitter is deployed in a pogo-style acquisition, which is suitable for a detailed investigation of a small scale seafloor target. The geometry of the experiment makes conventional interpretation methods difficult so that we simplify the data by creating a single rotationally invariant quantity from the original four electric field measurements (two electric field measurements for two transmitter polarizations). This invariant is further reduced to an apparent resistivity, which is useful for rapid resistivity mapping of the seafloor. Results show that CNE03 is characterized by increased apparent resistivities which correlate well with increased P-wave velocities determined from ocean bottom seismometer measurements.
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3D Inversion of Vertical Dipole - Time Domain CSEM data
Authors T. Holten, M. Commer, G. Newman and S.L. HelwigSummaryThe EM code of Newman and Commer can be used for inversion of the time domain vertical dipole data of Petromarker. A depth gradient weight function has been calibrated to obtain the original target depth in the inversion. The main goal of the 3D inversion development is to combine all the measurements in an offshore survey so the data fit a resistivity model.
A synthetic data example is presented where the inversion use forward modelled data from a reservoir case. The inversion recovers the depth and horizontal size of the reservoir.
The inversion code has the potential to improve subsurface imaging by geometrical averaging over all transmitter/receiver combination in a survey, and to include seabed topography and known stratigraphic horizons.
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Integral Equation Method for Anisotropic Inversion of Towed Streamer EM Data - Theory and Application for the TWOP Survey
Authors M. Zhdanov, M. Endo, D. Yoon, M. Cuma, J. Mattsson and J. MidgleySummaryWe introduce a 3D inversion methodology for towed streamer EM data that takes into account anisotropy and includes a moving sensitivity domain. Our implementation is based on 3D IE method for computing the responses and Fréchet derivatives, and uses re-weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. Interpretation of the towed streamer EM data is a difficult problem because the data are acquired over large areas with huge number of moving towed streamer EM system positions. We overcome this problem by exploiting the concept of moving sensitivity domain, which is implemented using the IE method. In the framework of this concept, for a given transmitter-receiver pair, the responses and Fréchet derivatives are computed from a 3D earth model that encapsulates the towed EM system’s sensitivity domain. The Fréchet matrix for the entire 3D earth model is then constructed as the superposition of Fréchet derivatives from all transmitter-receiver pairs over the entire 3D earth model. This makes large-scale 3D inversion a tractable problem with moderate cluster resources. We present a case study of 3D anisotropic inversion of towed streamer EM data from the Troll West Oil Province.
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Anisotropic 2.5D Inversion of Towed Streamer EM Data from Three North Sea Fields Using Parallel Adaptive Finite Elements
Authors K. Key, Z. Du, J. Mattsson, A. McKay and J. MidgleySummaryWe present anisotropic inversion results from towed streamer electromagnetic (EM) surveys of the Bressay, Bentley and Kraken (BBK) heavy oil fields in the North Sea. The BBK discoveries pose several challenges to conventional controlled-source EM surveying since the relatively shallow water dampens the anomaly magnitudes due to airwave coupling, and the reservoirs are in close proximity to other resistive features. The 160 m spacing of the 44 receiver bipoles on the towed streamer offers much higher data density than is typically achieved with conventional seafloor receiver surveys. We tested the resolving capabilities of the towed-streamer by inverting the survey data using a new code based on a 2.5D parallel goal-oriented adaptive finite element method and a modified implementation of the Occam inversion algorithm. The inversion successfully images the 1–2 km wide Bressay and ∼5 km wide Bentley reservoirs, illustrating that the high data density of the towed streamer offers improved resolution over sparsely sampled nodal seafloor receiver data. The results also demonstrate the importance of allowing for anisotropy when inverting data from this region. Whereas anisotropic inversion clearly recovers the lateral edges of the known reservoirs, isotropic inversion results in inter-bedding of resistive and conductive layers that conceal the reservoirs.
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3D Anisotropic Inversion of Towed Streamer EM Data over the Mariner Field in the North Sea
Authors J. Mattsson, M.S. Zhdanov and M. EndoSummaryTowed streamer electromagnetic (EM) data over the Mariner oilfield in the UK sector of the North has been inverted using a fast and efficient 3D anisotropic inversion code. The electric field data were acquired with a single vessel using a horizontal bipole source and sensors housed in a towed streamer in a densely sampled grid over the subsurface volume of interest. The inversion algorithm is based on the 3D contraction integral equation method and utilizing a re-weighted regularized conjugate gradient technique to minimize an objective functional. This inversion method is proven to be fast and efficient for large data sets and is here shown to be suitable for towed streamer EM data from complex geological environments such as the Mariner area. In this case, the final 3D resistivity cube after inversion and with a corresponding misfit of 6.4 %, agrees well with the expected structure from seismic data and well logs. In particular, the 3D cube contains a resistive anomaly of 8–10 Ωm corresponding to the Maureen and Heimdal reservoirs on top of the resistive chalk and basement.
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Cableless Seismic Acquisition Technology Evaluation and First Full Field Implementation in France and Uganda
Authors J.M. Mougenot, C. Cardama, P. Faure and R. SanchezSummaryThe use of cableless recording units for land acquisition has been widely increasing in the last 10 years with almost 900 000 units produced at this time. During recent years Total has been investigating how this new technology could facilitate its land acquisition operations and what are the most appropriate systems. There are more than 10 vendors competing for this market with systems with different functionalities. After a market evaluation and field tests of some systems, Total selected two different systems to acquire its first full-field 3D cableless acquisitions in Uganda and in France. This experience gave very encouraging results in term of potential of this technology to reduce acquisition environmental footprint and recording downtime, manpower and logistics.
The presentation will focus on the evaluation of this technology and the reasons why two cableless systems with very different functionalities were selected to meet the specific constraints of these two surveys.
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Cableless Sesimic Acquisition to Reduce Environmental Footprint in Murchison Falls National Park Uganda
Authors C. Cardama, R. Sanchez and J.M. MougenotSummaryIn the quest of increasing hydrocarbon reserves and oilfield development, it has been necessary for the oil industry to work in frontier and more sensitive areas than ever before. New developments in equipment and methodologies provided the means to tackle some of these problems. Seismic data acquisition in the Murchison Falls National Park in Uganda (the largest protected area in the country) proved to be a demanding project in terms of seismic acquisition methodology, biodiversity care and operational planning. The use of cable less technology, small efficient servo hydraulic vibrators were the pillars on which the acquisition strategy was based. This equipment provided the possibility of minimizing the environmental foot print in the Park and obtaining very high quality data that will be instrumental in the development of the fields discovered in the EA-1 block in northern Uganda
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Allowing Seismic Surveys to Boldly Go Where No Surveys Have Gone Before
Authors S. Baris, N. Badel, H. Ribeiro, F. Layan, P. Herrmann, O. Lamerain, A. Jacques and P. FaureSummarySingle Source - Single Receiver wireless systems reduce acquisition to its most elementary components, allowing high flexibility to adapt to all environments. Small to medium size single sources can be used in any type of terrain, with minimal to zero footprint. Cableless single receivers adapt easily to any topography or natural, agricultural, or urban organization. Their reduced weight also means highly efficient deployment. The advent of these new solutions enables a shift from equipment-driven and access-driven to image-driven survey planning. In the Lussagnet-Izaute case study, we illustrate how such advanced systems lead to highly efficient operations to produce high resolution 3D seismic volumes.
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Quantifying Noise in a Land Seismic Image - Stealth Acquisition Experiment
Authors E. Kragh, M. Daly and J. QuigleySummaryWe wish to quantify the relative contributions of the different components of noise in the land seismic image. We acquired a repeat 2D land seismic line. The first pass used a conventional acquisition technique. The second pass was acquired in exactly the same manner, except that once the vibrator pads were down, the vibrators remained stationary with no pad vibration for the duration of the sweep. We call this “Stealth” acquisition; we record no signal, but we record all the noise that we introduce by the process of acquiring the data (except noise introduced while actually putting signal into the ground). A third acquisition recorded an equivalent time of background noise with no source activity. Analysis of the resulting data shows that we generate a considerable amount of noise simply by the process of acquiring the data, some 10–20 dB above the background noise level below 50 Hz, in the raw data. Our processing workflows can deal with this additional noise very effectively and, in the final image, this is reduced to only a few dB above the background level. While this might still seem considerable, it is out-weighed by the noise introduced when the vibrator trucks shake at full force.
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Field Measurements of the Distortion Resulting from the Use of Tilted Geophones
Authors T. Dean, P. Claasen and B. McCarthySummaryThe moving coil geophone is still the most commonly used sensor for land seismic surveys despite the introduction of other sensors. Modern geophone development has reached the stage where the signal recorded is of very high quality, but it can still be affected by the geophone being placed on an angle relative to the vertical (‘tilt’).
This paper describes the acquisition and analysis of field measurements using tilted geophones. It is shown that the critical angle of the 10 Hz geophones used for this test is 55°. For data recorded using vertically placed geophones separated by only 10 cm the perturbation level (the difference between the data recorded by adjacent geophones) averaged 8% and increased to more than 50% at tilt angles of 40°. The level of perturbation is heavily dependent on the orientation of the tilt angle relative to the source-detector axis, for example, for a geophone tilted at an angle of 30°, the perturbation varied between 14% and 48%. The obvious solution to these issues is to record data using sensors that have been planted extremely carefully or to use other sensors, such as digital accelerometers or 3C sensors that can have the effect of tilt removed during processing.
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Field Measurements of Land Seismic Ambient Noise and Their Implications for the Effectiveness of Geophone Arrays
Authors T. Dean, J.C. Dupuis and R. HassanSummaryHistorically, arrays have been used to attenuate ambient noise under the assumption that the level of attenuation is directly proportional to the square root of the number of sensors in the array. Given the availability of high channel-count point-receiver systems and the cost associated with laying out large arrays this assumption of ‘spatial randomness’ requires further analysis. Using measurements of ambient noise made at various sites in Perth, Australia with closely spaced geophones we show that ambient noise is strongly correlated over distances of up to 10 m. This correlation reduces the signal-to-ambient-noise performance of an array considerably. The correlation coefficient can be modelled using an exponential function and the correlation-distance used to determine the efficient geophone spacing. The optimum geophone spacing on days with a low wind speed (< 10 km/h, observed on 27% of days in the area) is 15 m. For days with a very high wind speed (> 80 km/h) the optimum spacing is 2.5 m, although this wind speed is very uncommon, occurring on average less than once each year. For more than 90% of days the wind speed is such that the optimum geophone spacing required for ambient noise suppression is 7.5 m.
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Simultaneous Mini Sources for Simultaneous Infill and Land Acquisition
Authors C. Berron, E. Forgues, J.B. Laffitte and M. LeclercqSummaryWe present and compare a new alternative technology that can emit simultaneously several tens of small seismic sources alongside a standard land acquisition. This solution fit the infill acquisition purpose where we cannot use standard large vibrators or explosive sources (environmentally sensitive zones, urban areas and so on). This technology is based on continuous mono-frequency emission, allowing many of these Mini Piezo Vibrator sources to emit together.
Various sources are compared on seismic data recorded for TOTAL and TIGF during the 2013 Lussagnet survey (France) with no perturbation on the simultaneous standard seismic acquisition. We show that we obtain an acceptable signal above 20 Hz with our Mini Piezo Vibrator after one day of autonomous vibration (approximately nine hours). The signal-to-noise ratio above 50 Hz is even comparable to a Mertz 22. We conclude that this technology is promising and additional work would be required to confirm the ability to image deep reflectors and to optimize low-frequency emission by an adapted vibration sequence.
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Exploring the Extent to which Streamer Separation Can Be Relaxed when Using a Multimeasurement Streamer
Authors S. Mahat, D. Saputra, J.R. Crompton, M. Vassallo and P.A. WattersonSummaryHerein, we explore how we can enable increased efficiency in marine seismic acquisition by using the data recorded by a multimeasurement streamer to allow the relaxation of the usual constraints on crossline streamer spacing - thus, obtaining the desired resolution while towing the streamers farther apart.
Pressure and pressure-gradient measurements acquired by multimeasurement streamers are used to reconstruct the wavefield between the streamers; this is achieved using the generalized matching pursuit (GMP) algorithm. GMP performs wavefield reconstruction and full 3D receiver-side deghosting using multimeasurement data from the streamers, as described by Robertsson et al. (2008) .
Although these scenarios can be modelled using synthetic data, it is important to evaluate real seismic data to understand the impact of variables such as acquisition conditions.
In this work, we analyse the results from a field test acquired in the North Sea using multimeasurement streamers. In the field test, a line of data was acquired using a 75m cable separation and then repeated using a 100m streamer separation.
We consider to what extent we can increase streamer spacing and acquisition efficiency without compromising the spatial resolution, concluding that this experiment supports the case for reconstruction at 100m streamer spacing.
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Assessment Performance Tracking of Multiple Target Prospects - A Statistical Approach
Authors G. Martinelli and C.B. StabellSummarySystematic tracking of exploration results relative to pre-drill predictions is challenging, but important. It is a means both to motivate assessment quality and to improve the assessments produced by the exploration team. Unbiased, accurate and consistent assessments are key for effective exploration decisions. This paper deals with the case where a single well targets multiple zones, compartments or reservoirs in a prospect. Most companies handle the situation as a case of multiple distinct targets. However, while simple, this approach ignores to what extent assessment has handled estimation of risk dependencies and volume correlations between targets. A key challenge for tracking assessments of multiple target prospects is tracking the estimates of risk dependencies between the targets: single well results do not give clear feedback on this estimate. We present an exploration program-level statistical measure of the quality both at the level of aggregate risk dependency estimates and at the level of the individual chance (risk) factor dependency estimates. The procedure is demonstrated with a hypothetical, but illustrative drilling program. Implementation of multiple target prospect assessment performance tracking should both improve assessments of this class of exploration ventures and stimulate more robust, accurate and transparent estimation of prospect-scale risk dependencies.
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Enhanced Seismic Imaging of Permian and Intra-Carboniferous Sections to Define the Dinantian Play Fairway - UK North Sea
Authors K. Rodriguez, N. Hodgson, R. Wrigley and H. NichollsSummaryIn Autumn 2013 the first gas production from the Breagh Field heralded the coming of age of the Dinantian (Early Carboniferous) play on the northern margins of the Southern Gas Basin and threw down the gauntlet to explorers to chase this new play into open acreage within this mature and infrastructure-rich basin.
In 2013 a dense 2D seismic programme was acquired to define the Dinantian play fairway that extends over a large swath of open acreage. Innovative seismic processing techniques resulted in high quality seismic imaging allowing for the first time both intra-Carboniferous and pre- Permian unconformity potential to be mapped with confidence, at the same time high-grading the prospectivity of these plays previously unimaged on poor legacy seismic and providing a new tool for explorers to gain a detailed regional understanding of this new play.
With over 750 BCF of resources discovered to date in this play-fairway, the “Yet To Find” based on Spectrum’s preliminary structure mapping of the new dataset is over 1TCF. A yet unquantified Triassic prospectivity is a secondary target. The identified potential dwarfs that of the established Carboniferous and Permian plays to the south.
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Old play, New Prospectivity - the Revival of the Basal Rotliegend Play, Offshore Netherlands and Germany
Authors J. Corcoran and S.F. LunnSummaryDirect seismic mapping of the Havel sandstone clearly demonstrates by way of isopachs and impedance maps the interplay of relict topography, sand development and resultant sand package onlap onto regional highs. Proprietary mapping of basal Rotliegend reservoir development sweet spots into regional megastructural closures assisted by an improved high resolution stratigraphic framework reveal the potential for tcf scale prospectivity in acreage which has been overlooked for a decade.
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A New Chance for the Zechstein-2 Carbonate Play in the Mid North Sea High Area?
Authors B. Jaarsma, J. Schneider, S.P. Tolsma and J.E. LutgertSummaryThe Zechstein-2 Carbonate play is under-explored in the Mid North Sea area. A review of regional well and seismic data combined with a comparison to analogues onshore Denmark and the Netherlands has put a positive light on the reservoir potential of the Zechstein-2 carbonates in the area near the Mid North Sea High. Petrophysical evaluation and 2D/3D seismic interpretation has given clear indications for the development of Zechstein-2 carbonate platforms with good reservoir quality in the Dutch Northern offshore in a larger area than previously thought. Combined with a better understanding and prediction of the presence of mature source rock and the presence of a working seal, this could lead to new exploration opportunities. The study incorporates the findings of a regional mapping project covering the entire Dutch northern offshore, which has given more insights into the structural evolution of the MNSH area.
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Revisiting a Mature Exploration Area - Insights from the Integration of Quantitative Seismic Analysis and Basin Modelling
By A.W. KayatmoSummaryExploration in a mature area presents both challenges and rewards. On the one hand, an abundance of data is available, but on the other hand, these data have typically been mined and exploited within familiar exploration contexts. The challenge is to see these data within a new context or applying new methodologies to get a new and better understanding of the area. The combination of detailed quantitative geophysical analysis, at one end of the spectrum, and a broad basin modelling context, at the other end, has provided a novel insight into a mature exploration play in the greater Ula area, Norwegian Southern North Sea. This synthesis not only accounts for the results of past exploration, but opens up possibilities for identifying overlooked potential in one region that was thought to have been maturely explored.
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Exploraiton Potential Karpinsky Ridge Rostov Dist, Russia
Authors J.E. Battie, E. Lorange and T. HaugenSummaryThe Karpinsky Ridge, Rostov Dist. Russia has been under explored partly due to a earlier belief that below the Base Cretaceous only metamorphic rocks were found. This premise has been dispelled through the acquisition of longer offset seismic that indicated deeper structures and the drilling of several wells below the Base Cretaceous. The source rocks for the discoveries in Carboniferous of the Ukraine and in Kamykia District and the Caspian Sea area expected to be present in the in the Karpinsky Ridge, Rostov Dist. With the advent of the longer offset seismic data and improved processing techniques quite large structures have been identified within the area and along the periphery of the Karpinsky Ridge Fields have been discovered. Exploration is still in at an early stage and as more seismic is acquired more targets are identified and refined. A number of leads and prospects have been identified. The total potential resources for OE hydrocarbons is estimated to exceed 600 million scm.
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Regional Prospectivity of Offshore Namibia and the Angolan Namibe Basin
More LessSummaryThe underexplored offshore deepwater basins of Angola and Namibia hold tremendous potential for hydrocarbons. Recently acquired broadband 2D seismic from the Namibe Basin (offshore Angola) and offshore Namibia has allowed a detailed imaging of syn-rift and post-rift structures, enabling more confident identification and mapping of prospects analogous to those so prolific in the South American conjugate margins.
Horst and graben structures that formed during rift phases are now visible in the depth-migrated seismic data beneath 3–6 km of post-rift sediments. Thick salt, which is well known to the north, extends into the northern Namibe basins. Post-rift channel and fan structures can be identified as high amplitude anomalies throughout the area.
Cobalt’s recent discoveries in Angola have proven the great potential of pre-salt sediments and fuelled the search for prospects at this level in other nearby basins. In Namibia, a new phase of activity over the last few years includes wells by HRT and Chariot which have proven the existence of mature middle Cretaceous source rocks. Several further wells are planned for 2014
The improved imaging and resolution provided by modern broadband seismic significantly de-risk exploration in a frontier deepwater area where well costs are high.
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Plays, Prospectivity and Analogues, NE Greenland
Authors P.W. Bradbury, A. Checconi, R. Masotti, K. Hernon, R. Dirkx and F. WinterSummaryThe license area offshore northeast Greenland represents a vast frontier exploration province. The aim of this work has been to develop a better understanding of the prospectivity in the region within the current structural and stratigraphic framework. The study comprises 4 elements 1) regional structural interpretation, 2) regional sequence stratigraphic interpretation, 3) 1D temperature modelling and 4) identification of conjugate margin analogues.
The structural and stratigraphic interpretations conducted during this study have generated a regional framework for prospectivity analyses. Additional studies into thermal maturity of likely source rocks, the creation of a stratigraphic column and the identification of conjugate margin analogues have facilitated the development of a regional understanding of NE Greenland. The database of potential seismic analogues for NE Greenland identified during ongoing seismic interpretation on the Norwegian Shelf continues to grow and as further tuning of this knowledgebase is possible with the addition of new data (particularly seismic) in coming years will in time provide powerful predictive tools for exploration in NE Greenland.
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Maximizing the Ultra-low Frequency Output from Air Guns
Authors M. Landro and L. AmundsenSummaryThe new generation of broadband seismic offered by the seismic industry today has led to increased focus on how to generate reliable seismic signals containing frequencies below 5 Hz. In this paper we demonstrate from simple air gun modeling theory and field measurements that the bubble generated by a large air gun generates a low frequent signal with a dominant frequency around 0.1–1 Hz. By exploiting this, we think it is possible to enhance and utilize seismic frequencies as low as 1 Hz for exploration purposes. The strength of this ultra-low frequency signal depends on source depth, gun volume and potentially also the shape of the water surface. We think that our approach can be used to construct low-frequency models of basins. In addition, such data could be used to find good starting models for full-waveform inversion as well as for imaging below basalt.
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Broad-band Well Tie
Authors R.E. White and E. Zabihi NaeiniSummaryBy extending the low-frequency content, broad-band seismic data lessens the dependence of seismic inversion on a background model. Despite that, the merging of the inverted seismic with a very low frequency background model poses a new problem: the duration of a well-log synthetic seismogram is often seriously inadequate for defining the sharp spectral decay to zero frequency. To address this issue, a practical approach based on multi-taper spectral analysis is proposed to determine the low-frequency spectral decay of the seismic well tie wavelet. Furthermore the impact on seismic inversion is demonstrated.
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Improved Interpretability via Dual-sensor Towed Streamer 3D Seismic - A Case Study from East China Sea
Authors S. Rongfu, C. Hua, W. Yun, Z. Yabin, M.T. Farouki, D. Benfield, H. LeGluet and P. LeeSummaryThe geophysical objectives in the exploration for hydrocarbons in the East China Sea require 3D seismic data of sufficiently high quality to facilitate accurate structural and lithological interpretation of the deep reservoir plays (typically at a depth of 2.5 – 5.5 km). Conventional ‘narrow bandwidth’ 3D seismic which contains the sea surface ghost lacks the required resolution of seismic structure and penetration of signal in the deep section which is characteristic of broadband seismic obtained via an acquisition de-ghosting solution.
A 3D broadband seismic survey was commissioned in 2012 by the operator CNOOC using dual-sensor towed streamer acquisition system specifically designed to eliminate the sea surface receiver ghost. Comparison of data quality with an adjoining 3D conventional survey acquired in 2011 shows improved imaging of the highly developed fault systems, the associated deep reservoir plays, and basement. In particular, the more prominent amplitude response, and the better delineation of the different facies character, facilitates more accurate interpretation of the seismic data.
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Broadband Uplift on Seismic Impedance Inversion - With Examples from Offshore Angola
Authors E. Sadeghi, C. Deplante and P. Dos-ReisSummaryWith development of broadband technology which provided seismic signal with broader bandwidths, one of the main questions has been the impact of this technology on impedance inversion and the improvements that it can bring in terms of seismic reservoir characterization. This paper tries to illustrate these aspects using examples on recently acquired broadband data from offshore Angola. 3D elastic inversion results of broadband data are compared with similar inversions on conventional high resolution data on the same zone.
The main uplifts from broadband data illustrated with examples on ROSA field can be grouped into: (1) less constrained a priori model for inversion thanks to acquired low frequencies with broadband data, leading to more accurate and reliable inversion results, (2) The extra recorded signal on the low frequency end of spectrum also provided cleaner image and inversion results which are less affected by side-lobes and easier to interpret, and (3) better delineation of reservoir bodies and particularly thick sandy intervals as well as improved well to seismic tie which all together lead to improved seismic reservoir characterization as a tool for a more efficient field development.
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Broadband for Everyone - Increasing Notch Diversity Using Variable Streamer Profiles
Authors P.E. Dhelie, J.E. Lie, V. Danielsen and A. MyklebostadSummaryThis paper presents a case-study across the 2D benchmark line on the Haugaland High in the North Sea. The aim was to test various cable depth profiles, using a hydrophone only cable, to better understand the impact of notch diversity on the final deghosted image. A total of 11 passes with different configurations were tested, all using the same conventional source. The data were then processed with their appropriate deghosting algorithms and results compared. Amongst the profiles tested – one new novel sinusoid snake streamer test showed excellent notch diversity and compares well to other more common slanted profiles.
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Increased Streamer Depth for Dual-sensor Acquisition - Challenges and Solutions
Authors M. Lesnes, A. Day and M. WidmaierSummaryThe towing depth applicable to dual sensor streamer acquisition has hitherto been limited by operational challenges associated with maintaining the fronts of the streamers at a deeper tow position, which creates additional drag, and noise recorded by the vertical particle velocity sensor. These restrictions have limited 3-D acquisition to a maximum towing depth of 20 m whilst 25 m towing depth is routinely used for 2-D acquisition. In July 2013, a field trial was performed with a slanted streamer, from 15m depth at the front to 30m at larger offsets. Since the front of the streamer is deployed at a depth routinely used for dual sensor streamer acquisition, such a slanted streamer profile is no more operationally difficult to achieve and has comparable noise performance to a horizontal streamer. Wavefield separation can be performed for arbitrary streamer profiles and the up-going wavefield output at a horizontal datum, thereby presenting no additional difficulties for subsequent processing steps. The benefit of deploying a substantial proportion of the streamer at greater depth is increased low frequency signal-to-noise ratio (less than 16 Hz). This uplift was demonstrated by comparing the data acquired using a slanted streamer profile to that obtained using a horizontal streamer.
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Acquisition Considerations for Broadband from Fixed Depth Hydrophone Streamers
By P.M. FontanaSummaryIn the last few years there have been several processing techniques developed for producing broadband data from conventional single depth hydrophone-only streamer surveys. These techniques rely on there being sufficient signal-to-noise at ghost notch frequencies to allow for the derivation and application of stable and effective de-ghosting operators.
Based upon some simple fundamentals of air gun physics and source and receiver side ghosting, we are proposing guidelines concerning source and streamer depth relationships to optimize the signal-to-noise at ghost notch frequencies to take full advantage of the new processing techniques.
These guidelines have been tested in the field and empirical results will be shown. In addition, we will also address some secondary issues relating to streamer tow depths to allow mitigation of weather downtime within the constraints of safety to crew, vessel, and in-water equipment.
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Optimizing the Generation and QC of Low-Dwell Sweeps
Authors N. Tellier, G. Ollivrin and D. BoucardSummaryLow-dwell sweeps are now currently used in production, as a result of mechanical enhancements in vibrators and custom sweep methodologies. Nonetheless, the low-dwell ramp-up required to preserve the target force at low frequencies increases sweep duration and can impact crew productivity. The ramp-ups must be carefully fine-tuned to respond to this new paradigm, thanks to vibroseis controllers and QC computation evolution.
At low frequencies, the classic ground-force zero-crossing feedback loop becomes indeed obsolete while system non linearity and the associated distortion increase. Vibroseis controllers need to perform a higher rate ground-force command, and be adapted to provide optimum gain and phase to accurately fit the sweep ground-force to the desired pilot.
Low-dwell sweeps and vibrator limitations can be simulated prior to production thanks to adapted tools that take into account factors such as the initial phase. Such simulations show that two identical sweeps, differing only on the initial phase, can exceed or not the vibrators’ physical limitations, consequently allowing the sweep optimization and validation. While rarely done in practice for low-dwell ramp-ups, QC is essential to properly monitor sweeps. Computation methods evolve: the use of an appropriate window and normalized QC values results in accurate and exploitable QC data (Patent pending).
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Integrated 4D Inversion Approach in Danish Chalk Fields
Authors U. Micksch, I.H. Herbert, A.J. Cherrett, H. Roende, M.A. Calvert and J. ZaskeSummaryThis paper presents an integrated 4D inversion approach applied in a two vintage regional and multi-vintage field specific 4D processing project during 2013. The most recent regional monitor survey was recorded in 2012. At first we give an overview of the area and available data volumes and introduce a chalk rock physics model to show which 4D changes are to be expected.
The 4D inversion workflow is based around a deterministic geostatistical inversion scheme. A time strain inversion is used to obtain a time aligned monitor survey and aligned differences between the 4D processed vintages. In addition, the time strain information is used as prior model information when inverting for 4D impedance changes. Legacy data volumes used in earlier 4D projects were re-inverted with the same deterministic geostatistical inversion algorithm for comparison with the 2013 reprocessed datasets. Altogether thirty main inversion projects were executed within 14 months.
We compare inversion results for several processing outputs and also compare adjacent fields. The quick turnaround made possible by the fast inversion software allows the asset teams very early on to interpret and include these data into the daily workflow, well planning or well interventions.
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Calibration of a Weak 4D Seismic Signal for Fluid Pressure Assessement
Authors D. Rappin, P. Dos-Reis, P. Thore and C. HubansSummaryTime lapse (4D) seismic monitoring technology is now a proven tool to monitor fluid movement and/or pressure changes and to distinguish between drained and undrained portions of a reservoir. Its ultimate goal is to quantitatively improve reservoir models, particularly their predictive capability.
Calibrating the 4D signal at the well with information obtained from production data is essential for it to be used quantitatively. In this paper, we present the application of a model-based inversion method to estimate the changes of elastic parameters in the reservoir due to production at the well.
The scheme used in the study is based on the observation that the production flow is driven by the “dynamic properties”. of layers, and therefore a layered model should be used to parameterize the inversion. To discriminate between possible physical phenomena connected with production (depletion or injection), we also need to use a petroelastic model.
We present results on a real data example on a 4D calibration of a turbidites field. The methodology was applied for a standard case and, more originally, to help interpreting an unexpected weak 4D signature.
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Analysis of Shallow Gas in the Ekofisk Area
Authors K. Haavik, K. Eidissen and M. LandrøSummaryAnalysis of shallow gas in Quarternary sediments in the Ekofisk area has been done using time-lapse amplitude interpretation and AVO analysis. A major underground blowout occurred in this area when drilling the 2/4–14 well into a Jurassic target in 1989, and huge amounts of gas leaked into shallow layers. The result from the time-lapse show that a new amplitude anomaly is present around the 2/4–15 relief well in the 2005 dataset, 15 years after the blowout, indicating that gas is still flowing. Furthermore, AVO analysis has been performed on a larger amplitude anomaly, thought to be due to naturally occurring gas. The result shows a clear separation between the areas that we assume are saturated with water and gas respectively.
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Repeated Time-lapse Seismic Data Quantify Amplitude Tuning at the Sleipner CO2 Injection Site
Authors A.K. Furre and O. EikenSummaryCO2 has been injected into the saline aquifer Utsira Fm at the Sleipner field since 1996. A seismic base survey and eight monitor surveys have been acquired to monitor the CO2. These show that the CO2 is still contained in the subsurface, with no indications of leakage. This extensive seismic dataset provides an opportunity to perform detailed analysis of seismic signal variation. In this paper we will show evidence of amplitude tuning development as layer thickness increases with time. We also see the geometric effect of signal pull-up for thin layers.
Repeated time-lapse seismic data have enabled us to quantify amplitude tuning curves for individual layers at the Sleipner field. Time pull-up and the amplitude together provide an improved estimate of layer thickness. This work also shows how thin layer tuning effects, which are actually of a geometric nature, will interfere with actual time-lapse changes. The thin layer time pull-up will interfere with the velocity induced time-shift, and accounting for this effect will enable a better analysis of time-shift effects at internal reflectors.
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Estimation of Critical and Maximum Gas Saturation Using Multiple 4D Seismic Surveys
Authors D. Obidegwu and C. MacBethSummaryEstimates for critical and maximum gas saturation are obtained using time-lapse seismic signatures from multiple surveys shot during gas liberation and dissolution in a producing hydrocarbon reservoir. To aid this process, hydrocarbon gas properties and behaviour are studied, and their relation to the fluid-flow physics is understood using numerical simulation and seismic modelling. It is concluded that for seismic surveys repeated at time intervals of six months or more, the gas saturation distribution during either liberation or dissolution exists in two fixed saturation conditions defined by the critical and the maximum gas saturation. This understanding is then used to interpret seismic data from a turbidite field in the North Sea, which has surveys repeated every 12 to 24 months. We find a critical gas saturation of between 0.6 and 4%, but that the maximum gas saturation is relatively unconstrained. These low critical gas saturation values are consistent with the range of measurements from other similar fields in the open literature.
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Initial Results of a Pilot 4D Seismic Monitor in a Difficult Shallow Offshore Carbonate Context - Dynamic Connectivity and Implications for Development
Authors A.A. Adeyemi, E. Brechet, C. Hubans, C. Euriat, J.F. Le Roch, T. Pigeaud and M. EmangSummaryWhereas Timelapse seismic is well known as a robust reservoir monitoring tool in deep offshore turbidites, this monitoring technique is less ubiquitous in fractured carbonate contexts. In our case, a prior 4D seismic feasibility was realized in such difficult context and predicts that the main 4D effects are linked to pressure changes (as first order effect) and then saturation effects. A pilot monitor survey was thereafter acquired over areas seen as the most “4D” sensitive zones 16 years after baseline acquisition and 10 years after production start-up. The pilot objective was to verify if 4D signal linked to depletion effects and water sweeping are detectable in the generally <5m thick fractured carbonate reservoir units. This paper shows in spite of seismically noisy context, validated 4D signals are present in the seismic monitor acquired and is interpretable. Through an integrated multi-attribute analysis of all available static and dynamic data; relative velocity/impedance changes enables the delineation of depleted zones and mapping of pressure and saturations boundaries. This has helped in the characterization of the fracture corridors, internal reservoir heterogeneities, understanding the main flow directions and identification of the swept and unswept zones which could help for future infill well placement.
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4D Seismic for Carbonate Reservoir Management and Monitoring - Proof of Concept from the Giant Al Shaheen Field, Offshore
Authors O. Jorgensen, A. Cherrett, A. Uldall, J.R. Henderson, H. Roende, V. Zampetti and M. EmangSummaryA proof of concept approach has been taken for the Al Shaheen Field before implementing a 4D surveillance program into the field management and surveillance. The proof of concept is not only to fully demonstrate that a 4D signal is present, but also to be able to quantitatively understand it and to derive information about the recovery processes from it.
Two overlapping surveys, one acquired in 1993 (before production) and the other in 2006 showed that 4D signals could be observed in the Al Shaheen Field. In order to assess such potential a cross-discipline workflow was applied that integrates the forward modelling of seismic synthetic time changes for the two main reservoir intervals combining the in-house rock physics model with reservoir simulation models and the use of the in-house time-strain inversion algorithm.
Comparison of synthetic and measured time thickness attributes showed good correspondence with regard to intensity, size, and shape of the production affected areas suggesting that the forward model successfully reproduces the actual cause of the changes.
4D seismic signals in the two main reservoirs can be related to fluid and pressure effects and consequently flood front position can be derived from seismic data.
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4D Seismic Warping in Two Steps for Overburden and Reservoir - Example of a Compacting Carbonate Field
Authors J. Fiore, C. Hubans and E. BrechetSummaryConventionally, the 4D warping is only performed over the reservoir interval, assuming that all 4D effects are located in the reservoir. In the case of a compacting reservoir, the overburden is affected by subsidence, hence by time shifts. For this particular situation, we present a workflow in two steps to optimize separately the 4D warping of the overburden and the warping of the compacting reservoir.
This two step approach has two advantages: First, the warping parameters can be optimized independently for each interval. The second advantage is to apply a time shift correction at top reservoir before the reservoir warping, to avoid any contamination of the reservoir warping by the overburden time shift.
This workflow is illustrated by a case study over an offshore carbonate gas field buried under a silicilastic overburden. The 1st warping step reveals that the overburden is affected by translation but only the western part is affected by vertical stretching (geomechanical arching effect). The 2nd warping step, focused on the reservoir, reveals a tilted rise of the gas-water contact, associated to a decrease in P-wave velocity.
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Traveltime Sensitivity Analysis for Walkaway VSP Anisotropy Survey Design
Authors R. Zhou, B.E. Cornish and D. QuinnSummaryComprehensive pre-survey modeling is a key to the success of a walkaway VSP anisotropy survey. Quantitative measures are required to guide the design of survey configuration and assess the uncertainty of anisotropy estimation. A model-based traveltime sensitivity analysis was performed to investigate optimal angle (or offset) ranges that have high sensitivity for interval anisotropic parameters delta and epsilon in a horizontally layered VTI medium. The results show that resolution and uncertainty of the anisotropy estimation depends on the selection of shot offset range, layer thickness, magnitude of anisotropy, and wave mode. Adding reflection data can reduce the uncertainty in anisotropy estimation, particularly for interval parameters in a layered medium. The sensitivity analysis can provide quantitative guidance for the design of a walkaway VSP anisotropy survey.
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A Methodology to Estimate Residual Source Statics from 2D/3D VSP Data
Authors M. Lou and F. DohertySummaryThe estimation and correction of residual source statics is critical to the accurate imaging of land WVSP (walkway VSP) and 3D VSP data. We present here a new methodology to estimate residual source statics from WVSP and/or 3D VSP data. We adapt the Fomel and Stovas’ traveltime function originally developed for surface seismic reflection moveout corrections to fit the first-break (FB) times in WVSP or 3D VSP data with large source offsets, for any anisotropic (VTI) layered velocity model. We employ a global optimization algorithm called simulated annealing (SA) to automatically search for the four parameters in the Fomel and Stovas’ traveltime function. After the best-fit FB time function is solved, the residual source statics are straightforwardly derived by calculating the deviation of the FB time of each source from the fitting function curve. We demonstrate the accuracy and robustness of our methodology using synthetic model data and a field WVSP dataset. The end result of the application of accurate residual statics is better migration focusing and subsequent improvement in the image quality of the VSP data.
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Demonstrating Seismic While Drilling As a Powerful Tool to Reduce Depth Uncertainty During Drilling
Authors R.S.H. Pettersen, J. Bailey, D. Moody and S. SoulasSummaryIn 2012, BP executed their first Seismic While Drilling (SWD) operation in the UKCS on an exploration well West of Shetland. One of the well’s main HSE risks was to unintentionally enter the main reservoir target at Base Cretaceous before having set the 9 7/8″ casing due to the associated risk of potential hydrocarbons in or just above it. Thus the main objective of the SWD operation was to help mitigate the risk by acquiring checkshots in real-time as input to depth prognosis updates and gradually reduce the predrill depth uncertainty during drilling. Not only was this SWD operation BP’s first in the UKCS, it was also a BP Global pilot running the Baker Hughes SeismicTrak™ tool. After a comprehensive and highly multi-disciplinary operations planning process appreciating the harsh offshore environments in mid-winter at this remote location, the SWD job was executed with zero HSE incidents as an integrated part of the overall drilling operations. During drilling, the depth prognosis updates were carefully shared with the operations team allowing it to drill ahead safely. With SWD technology we reduced the initial predrill depth uncertainty range of Base Cretaceous from 1200 feet to 100 feet at the final prognosis update.
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Reflection Salt Proximity
More LessSummaryKnowledge of salt boundary locations at the salt flanks and base is critical important for updating velocity models, getting better sub-surface images, estimating a reservoir size, and minimizing drilling risks for exploration and development wells in and near the salt. We have developed a Reflection Salt Proximity (RSP) method to delineate the salt-sediment boundary. This method utilizes the reflected P waves off the salt face to calculate the Salt Reflection Points (SRP) by measuring arrival times, azimuths, and vertical angles of the reflected PP waves with a sediment velocity model. This approach is a complementary to the salt proximity migration, sediment proximity and conventional salt proximity methods. It is a very useful tool when and where the traditional salt proximity is not suitable. An example in the Gulf of Mexico is used to demonstrate the method and to present the calculated salt reflection points, which are consistent with those results from the conventional salt proximity surveys.
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Vertical Seismic Profile S-S Imaging with Vertical Vibrators
Authors B.E. Cornish, B.A. Hardage, W. Palacios, L.N. Torres, R. Zhou and D. QuinnSummaryThe vertical vibratory source is ubiquitous in land P-P seismic acquisition. However, the source may be observed to produce a rich field of other (S-wave-like) modes that can be identified and processed to exploit the characteristics of the S wavefield at no additional acquisition cost. S-waves lack gas sensitivity and have complementary characteristics when compared to P-wave data in response to the same stratigraphy. Sh modes are decoupled and tend to have less interference, enhancing their S/N and facilitating processing. Direct S-wave arrival times can be inverted for velocity, leading to the construction of Vp/Vs and other attributes. This paper presents an example of combined P- and source-generated S-wavefield processing through imaging for zero-offset and offset vertical seismic profiles (VSPs).
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Two-dimensional VSP Elastic Reverse-time Migration with Attenuation of the Ambiguity Artifacts
Authors V.G. Baydin, L.E. Dovgilovich and I.L. SofronovSummaryIt is known that elastic reverse-time migration (RTM) for borehole seismic data suffers from various artifacts, among which the most stable and strong are ambiguity artifacts. These appear when the seismogram registered in the borehole is used in the back-propagation stage without preliminary separation into P- and S-waves. To compute back-propagation with the desired wave type, we change the objective function in the adjoint-state RTM formulation. Instead of a conventional wavefield misfit, we introduce the misfits of wavefield divergence and curl. This approach drastically improves PP, PS, SP and SS images both from the attenuation of the ambiguity artifacts and by the increased contrast in some details on the images. We report on the theoretical background of the proposed approach and on the examples of test cases.
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Imaging of Blended VSP Data Using Full Wavefield Migration in the Common-receiver Domain
Authors A.K. Soni, D.J. Verschuur and A.J. BerkhoutSummaryBlended source and/or simultaneous source acquisition in both surface and borehole seismic exploration has been a subject of interest to the oil and gas industry recently. The simultaneous or blended source acquisition, especially for large VSP surveys, can prove significantly beneficial in terms of cost efficiency. Henceforth, the subject of imaging and inversion of blended data is also an important area of research and development. In this paper, for imaging blended Vertical Seismic Profiling (VSP) data, we propose the Full Wavefield Migration (FWM) approach, which does not require an active deblending as a processing step. The FWM algorithm is performed in the common-receiver domain and is formulated in terms of a constrained least-squares inversion scheme. The inversion scheme enables to explain the subsurface reflectivity, where the incident wavefield contains all multiples and which may also be blended. We will illustrate the imaging of blended VSP data using relatively simple synthetic examples. The constrained inversion helps to reduce the crosstalk noise in the image obtained, due to wavefield interference in blending with high blending factors.
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Reservoir Characterization by Crosswell Seismic in Dom João Field, Brazil
Authors M.C. Schinelli, A. Bassrei, B. Bryans, S. Sharifian Schill, C. Kolb and H. DenaclaraSummaryEach borehole seismic technique has its advantages in imaging the subsurface, but the high resolution obtained using crosswell seismic imaging allows for a detailed survey of the reservoir. These results are enhanced when acquiring multiple profiles permitting a view of the entire field in two and three dimensions.
Petrobras supported a research project, with the technical participation of UFBA to evaluate for the first time in Brazil, the different aspects of crosswell seismic through surveys carried by Schlumberger in Dom João (5 profiles) and Miranga (1 profile) fields, respectively, in October and November 2011. The target of this project was to investigate the response of Sergi and Catu formations to crosswell method, and secondarily provide, on Miranga field, a baseline velocity image to monitor a future CO2 sequestration project. This study focuses on the processing steps that allowed for the production of crosswell seismic images for the five profiles acquired in the Dom Joao field and its preliminary interpretation.
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