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75th EAGE Conference & Exhibition - Workshops
- Conference date: 10 Jun 2013 - 13 Jun 2013
- Location: London, UK
- ISBN: 978-90-73834-49-1
- Published: 10 June 2013
1 - 100 of 138 results
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A Counter-current Heat-exchange Reactor for the Thermal Stimulation of Hydrate Bearing Sediments
Authors J. Schicks, E. Spangenberg, R. Giese, M. Luzi-Helbing and B. Beeskow-StrauchSince huge amounts of methane are bounded in natural gas hydrates occurring at all active and passive continental margins and in permafrost regions, the production of natural gas from hydrate bearing sediments becomes more and more of interest. Three different methods for the release of methane gas from destabilized hydrate are discussed in principle: thermal stimulation, depressurization and chemical stimulation. This study focusses on the thermal stimulation using a counter-current heat-exchange reactor for the in situ combustion of methane. The heat of the flameless, catalytic oxidation of methane was used for the decomposition of hydrates in sand in a pilot plant scale within a large reservoir simulator (LARS). The promising results of the latest reactor test for which LARS was filled with sand, and ca. 80 % of the pore space was saturated with methane hydrate are presented in this study. The data analysis showed that 15 % of the methane gas released from hydrates would have to be used for the successful dissociation of all hydrates in the sediment using catalytic combustion of methane.
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3-D Numerical Modelling of Gas Hydrate Accumulations at the Alaska North Slope
Authors M. Haeckel, E. Pińero, W. Rottke, T. Fuchs, C. Hensen and K. WallmannThe petroleum systems modelling software PetroMod has been extended by a new module simulating the basin-wide accumulation of methane hydrates under marine and permafrost conditions. This new feature includes kinetics for the production of methane by microbial degradation of organic matter at low temperatures and predicts the evolution of the thermodynamic stability zone of gas hydrates through geologic time. In combination with the proven capabilities for calculating oil and gas generation and flow paths for their migration based on the input geological information, PetroMod is now also able to predict the formation of gas hydrate reservoirs.The 3-D geological model of the Alaska North Slope developed by the USGS and Schlumberger is used to test the new PetroMod gas hydrate module. Model results are compared to published well-log and geochemical data from drill sites at Mt. Elbert and the Ignik Sikumi field test site, which are part of the Eileen gas hydrate accumulation. The simulations indicate that today’s gas hydrate accumulations started to form ~0.5 Ma ago with highest amounts near the major faults. The predicted extent and thickness of the gas hydrate stability zone matches well with the observations from the 148 wells in the area.
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Mapping Gas Hydrates in the Gulf of Mexico Using Marine CSEM
Authors K. Weitemeyer, S. Constable and K. KeyGas hydrates are a geohazard and a potential energy resource, yet the estimated global volume varies by over four orders of magnitude. This is because mapping hydrates with seismic methods alone is difficult, showing only the edges of the hydrate stability zone. Another technique, marine controlled source electromagnetic (CSEM) techniques, are more apt in estimating hydrate saturations than seismic methods. In 2008, an extensive CSEM data set was collected in the Gulf of Mexico (GoM) aimed at developing the CSEM technique to map marine gas hydrates. Preliminary 1D apparent resistivity pseudo-section results of the data show lateral variations in resistivity that may be attributed to gas hydrate, deeper salt bodies, carbonates, and water saturated channel sands. However, the exact resistivity values and geometries of hydrate emplacement within the surrounding sedimentary architecture requires 2D or even 3D modeling and inversion before final interpretations of the CSEM data and volume estimates of gas hydrate saturation can be made. Here we apply a newly developed parallel goal-oriented adaptive finite-element modeling algorithm (referred to as MARE2DEM) for efficient 2.5D imaging of the CSEM data and present preliminary inversion results for the GoM survey.
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Reservoir Modelling and Numerical Analysis of the Iġnik Sikumi Gas Hydrate Field Trial
Authors B. Anderson, N. Garapati and P. McGuireOn May 5, 2012, ConocoPhillips, in a cooperative effort with the Japan Oil, Gas, and Metals National Corporation and the U.S. Department of Energy, completed the first field program designed to investigate the potential for CO2-CH4 exchange in naturally-occurring methane hydrate reservoirs. The field team, operating from a temporary ice pad constructed in the Prudhoe Bay Unit on the Alaska North Slope, achieved incident- and injury-free operations. The program met its primary goal of conducting a controlled scientific experiment in a field setting to further our understanding of the response of gas hydrate reservoirs to gas injection. The project team is currently checking and validating the field data, after which the final quantification of time-series downhole pressures and temperatures synchronized to gas and fluid flow rates and chemistries will be available. These data, in conjunction with the various associated pressure and temperature datasets from sensors installed in the well, will then be analyzed to interpret the nature of the various processes that occurred during the test. Current efforts are focused on the interpretation of the data from this field trial using NETL’s hydrate reservoir simulator, HydrateResSim and will be discussed in this work.
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Seismic Characterization of Gas Hydrates in Northern Gulf of Mexico
More LessWhether gas hydrates are considered as hazard or resource, we need to find it first. We developed an integrated, seismic-based, five-step workflow (Dutta et al 2010; Dutta and Dai, 2007) to delineate and quantify gas hydrates using an approach very similar to finding hydrocarbon. The approach is primarily based on seismic characterization, geologic analysis and seismic inversion that is constrained with rock physics principles. These are performed within the gas hydrate stability zone. The introduction of gas hydrates in the shallow unconsolidated sediments tends to enhance both the stiffness and rigidity of the hosting rocks. Gas hydrate drilling worldwide has indicated that the increases in the stiffness and rigidity are somewhat proportional to the concentration of gas hydrates in the porous space of the sediments. This provides basis for the gas hydrate characterization and quantification using seismic information. In this presentation, we will review the technology, and demonstrate its application, using multiple examples from the Gulf of Mexico (GOM). Based on our models for gas hydrate exploration, a set of wells were drilled by the Consortium of USA – DOE and several oil companies in the deep water GOM (e.g. Green Canyon, Walker Ridge, Atwater Valley) and the model
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Rock Physics Models for Gas Hydrate Exploration - Where Are We?
By S. ChandGas hydrate exploration technology has developed from an interpretation of Bottom Simulating Reflections (BSRs) on seismic data to more advanced target oriented acquisition of seismic and other geophysical data. The rock physics modeling also developed in parallel to keep up with a demand for interpreting the observed anomalies and also the joint inversion of geophysical data such as controlled source electromagnetic and seismic. The simplest models of using empirical methods have given way to more sophisticated methods of incorporating rock microstructure into the inversion to minimize the uncertainties involved in predicting gas and gas hydrate saturations associated with gas hydrate prospects. Models combining rock property measurements and host medium grain to grain interactions gave good control on background rock properties about the host medium and hydrate microstructure. Self consistent approximation - differential effective medium (SCA/DEM) approach gave an opportunity to incorporate the microstructure through statistical assumptions. Visco-elastic effects of fluid flow within the gas hydrate microstructure and frequency related interface properties on seismic attenuation is incorporated into a new rock physics model which can explain contrasting acoustic observations from gas hydrate zones from different geological settings.
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A Two-steps Stochastic Inversion to Estimate Gas Hydrate Grades at the Mallik Field
Authors C. Dubreuil-Boisclair, E. Gloaguen, G. Bellefleur and D. MarcotteIn the last decades, permafrost gas hydrates have beneficiated from an increasing attention among researchers and industries around the world. However, little work has been done on characterizing this resource at the reservoir scale. In this study, we used cutting edge stochastic inversion software and we developed a cascade stochastic Bayesian algorithm to simulate the gas hydrate grade (product of porosity and gas hydrate saturation) on a 3D seismic cube at the Mallik field, in the Mackenzie Delta, Canada. Firstly, the 3D seismic data are stochastically inverted for acoustic impedance leading to multiple high-resolution AI realizations conditioned to the seismic and the well-log data from wells 2L- and 5L-38. Secondly, a petrophysical inversion is performed in a stochastic Bayesian framework using gas hydrate grade logs as hard data, and randomly selected AI scenarios as secondary data. For the later inversion, an in-situ petrophysical relationship linking gas hydrate grades to acoustic impedance is built using upscaled well data. The results are thus multiple 3D gas hydrate grade realizations conditioned to all available data, and reflecting a great part of the model uncertainty. These models allow calculating the total gas volume with its associated uncertainty for the studied region.
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3D Seismic and Multidisciplinary Investigations of Cold Seeps along the Hikurangi Margin, North Island, New Zealand
By J. BialasGas hydrate occurrences are known from continental margins world-wide. Mainly Methane gas is bound in solid phase in the sediments when stability conditions of temperature and pressure are given. Unusual but often observed exceptions within the gas hydrate stability field are gas migration pathways that enable gas transport through the hydrate stability field. Such migration pathways are known as cold seeps and indicated by active gas expulsion at the seafloor and / or by carbonate precipitations if they did exist over long time periods. The accretionary Hikurangi Margin hosts a large number of active cold seeps along the ridges of the accretionary wedge. Multidisciplinary marine investigations have been undertaken to analyse lifetime, activity and internal structures.
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3-D Images of a Large Pockmark from the Chatham Rise, New Zealand
Authors I. Pecher, K. Waghorn and J. BialasPockmarks have been observed on the Chatham Rise east of New Zealand over an area of >20,000 km2. Their formation has been linked primarily to gas hydrate dissociation during glacial-stage sealevel lowstands. We here show first images from a recent high-resolution 3D seismic images targeting the sub-surface structure of a giant pockmarks. Results show in particular that the pockmark may be underlain by a large gas chimney.
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Site Selection and Formation Evaluation at the 1st Offshore Methane Hydrate Production Test Site In the Eastern Nankai Trough, Japan
Authors T. Fujii, S. Noguchi, T. Takayama, K. Suzuki, K. Yamamoto and T. SaekiIn order to evaluate productivity of gas from marine gas hydrate by the depressurization method, Japan Oil, Gas and Metals National Corporation is planning to conduct a full-scale production test in early 2013 at the AT1 site in the north slope of Daini-Atsumi Knoll in the eastern Nankai Trough, Japan. By the pre-drilling campaign conduced in early 2012, we confirmed methane hydrate concentrated zone (MHCZ) with 60 m of gross thickness, which is composed of lobe/sheet type sequences in the upper part of it and channel type sand sequences in the lower part. This MHCZ is thicker than that of confirmed at previous wells in 2004 (45 m) located around 150 m northeast, indicating that the prediction provided by seismic interpretation and inversion analysis was reasonable. Well-to-well correlation between two monitoring wells (AT1-MC and MT1) within 40 m distance exhibited that lateral continuity of these sand layers (upper part of reservoir) are fairly good, which representing ideal reservoir for the production test. The XPT fluid mobility measurement results showed approximately 0.1 to several mD of water permeability in both MHCZ and seal formation, although there are some variations in measured values.
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Geochemical and Geophysical Assessment of Deep Sediment Methane Hydrate Deposits
Authors R.B. Coffin, W.T. Wood and I.A. PecherThis study provides an overview of geochemical assessment of deep sediment methane hydrates in several locations around the world. The U.S. Naval Research Laboratory has been involved in methane hydrate exploration of the coasts of New Zealand, Chile, Canada, Beaufort Sea on the Alaskan Shelf and in the Gulf of Mexico. These studies have shown a wide variation in the distribution and concentration of deep sediment methane hydrate deposits within and between geographic locations. General findings of these studies are not always consistent with a simple model of methane flux, suggesting the need for more thorough geochemical evaluation.
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Arctic Gas-Hydrates on the Svalbard Margin and in the Barents Sea
More LessLarge gas hydrate accumulations occur on the continental margins surrounding the Svalbard archipelago and in the Barents Sea. Their occurrence may be linked to seabed emissions of methane gas, however, a clear causative link is missing. The range of geological settings in this region includes shallow gas hydrates at the outcrop limit of the gas-hydrate stability zone, deep-sea hydrates in areas of considerable subsurface leakage and shelf-sea hydrates fed by vast petroleum systems. A better understanding of these accumulations with respect to their sensitivity to climate change in this environmentally challenged part of the Arctic region is globally relevant and forms one of the working premises for the newly established Centre of Excellence (CoE) for Arctic Gas Hydrate, Environment and Climate (CAGE) at the University of Tromsř. The CoE is funded by the Norwegian Research Council for a decade to study methane release from hydrates beneath the Arctic Ocean in an effort to understand potential impacts on marine environments and global climate systems.
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Seismic Studies of Giant Pockmark-like Features in Southern Chatham Rise, New Zealand
Authors J. Bialas, I. Pecher, B. Davy, C. Papenberg, F. Gross, J. Hillman, S. Koch and K. KroegerMethane is one of the most aggressive greenhouse gases driving climate change. Occurrence of marine gas hydrates depends on temperature, pressure, available gas and fresh water. Therefore changes in pressure and bottom water temperature will influence the formation or dissolution of gas hydrates. In general both parameters vary slowly and hence changes do not result in large Methane contribution to the atmosphere. An exception could be a sudden dissolution of larger quantities of gas hydrate with a related expulsion of Methane. Such focused fluid flow appears as funnel-shaped depressions at the seafloor, so called “pockmarks”. Typical dimensions are within a few hundreds of meters. However, five to twelve kilometre wide “giant pockmarks” (GP) are known as well. The mechanism of formation of GPs is not fully understood. New 2D and 3D multichannel seismic data have been acquired across two fields of giant pockmark-like features to better understand the formation of these depressions and potential for catastrophic methane release in the past.
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Thermo-hydro-geomechanical Simulations of Methane Gas Production from Deep Sea Methane Hydrate Formations
More LessA new constitutive model called Methane Hydrate Critical State (MHCS) model was conducted to investigate the geomechanical response of the gas-hydrate-bearing sediments at the Nankai Trough during the wellbore construction process. The strength and dilatancy of gas-hydrate-bearing soil would gradually disappear when the bonds are destroyed because of excessively shearing, which are often observed in dense soils and also in bonded soils such as cemented soil and unsaturated soil. In this study, the MHCS model, which presents such softening features, would be incorporated into a staged-finite-element model in ABAQUS, which mainly considered the loading history of soils and the interaction between cement-casing-formation. This model shows the influence of gas-hydrate-bearing soil to the deformation and stability of a wellbore and the surrounding sediments during wellbore construction. At the same time, the conventional Mohr-Coulomb model was used in the model to show the advantages of MHCS model by comparing the results of the two models.
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Advances in Testing of Natural Hydrate-Bearing Sediments: PCATS Triaxial
Authors J. Priest, M. Druce, P. Schultheiss, J. Roberts and C.R.I. ClaytonThe lack of correlation between test data on laboratory synthesised hydrate-bearing sediments has raised questions as to the interaction between the hydrate and the host sediment. The development of the Pressure Core Analysis and Transfer System (PCATS) has enabled the recovery of pressurised cores in liners and their subsequent transfer into other pressure vessels. Building on this development,, an advanced geotechnical testing apparatus has been developed to provide data on the behaviour of natural, intact, hydrate bearing sediments under in situ pressure and temperature conditions. This presentation describes the capabilities of this apparatus and presents some initial test results on pressurised cores.
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Investigating a Potential Link between Seafloor Pockmarks, Gas Hydrates and Offshore Hydrocarbon Reservoirs in the Canterbury Basin, New Zealand
Authors J. Hillman, A.R. Gorman, I. Pecher, J. Bialas and K. WaghornNumerous large pockmark structures found on the slope of the Chatham Rise and Canterbury Bight off the east coast of New Zealand have been examined. Using a variety of techniques, we test a hypothesis of pockmark formation that involves the dissociation of hydrates on a glacial timescale (Davy et al. 2010), estimate the timing of pockmark formation, and investigate the possible link between gas hydrates and hydrocarbon reservoirs in the Canterbury and Great South basins.
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Numerical Study on Hydrate Bearing Sediments during Gas Production
More LessThe fully coupled methane hydrate model developed in Cambridge was adopted in this numerical study on gas production trial at the Eastern Nankai Trough, Japan 2013. Based on the latest experimental data of hydrate soil core samples, the clay parameters at Eastern Nankai site were successfully calibrated. With updated clay parameters and site geometry, a 50 days gas production trail was numerically simulated in FLAC2D. The geomechanical behaviour of hydrate bearing sediments under 3 different depressurization strategies were explored and discussed. The results from both axisymmetrical and plane-strain models suggest, the slope of the seabed only affects mechanical properties while no significant impact on the dissociation, temperature and pore pressure. For mechanical deformation after PT recovery, there are large settlements above the perforation zone and small uplift underneath the production zone. To validate the fully coupled model, numerical simulation with finer mesh in the hydrate production zone was carried out. The simulation results suggest good agreement between our model and JOE’s results on history matching of gas and water production during trial. Parameter sensitivity of gas production is also investigated and concluded the sea water salinity is a dominant factor for gas production.
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P-Cable High-Resolution 3D Seismic Imaging of Gas Hydrates and Shallow Gas
By S. PlankeHigh-resolution 3D seismic data allows detailed mapping of gas hydrates and shallow gas on continental margins and shelves. New P-Cable 3D seismic data suggest the presence of gas hydrates and shallow gas in the epicontinental Barents Sea. A gas mix with higher order hydrocarbon gases are likely required to form hydrates in this region.
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Detailed Analyses of Seismic Anomalies Associated with Gas Hydrates in the Southern Hikurangi Margin, New Zealand
Authors A. Gorman, D.R.A. Fraser and I.A. PecherThe Hikurangi Margin, east of the North Island of New Zealand, contains a significant gas hydrate province. However, the distribution, concentration and dynamics of hydrate accumulations in the southern portion of the margin (the Pegasus Sub-basin) off the northeastern coast of the South Island are poorly constrained due to a lack of data. In late 2009 and early 2010, a seismic dataset consisting of approximately 3000 km of long-offset 2D seismic data was collected in the Pegasus Sub-basin. Bottom-Simulating Reflections (BSRs) are widespread in the data, and they are supplemented by other features that may indicate the presence of free gas and gas hydrates in zones of high concentration. We present the results of reprocessing and analysis of the data that has focussed on the identification of such specific gas-related features.
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Advance Gas Hydrate Reservoir Characterization Using Rock Physics-driven Quantitative Seismic Interpretation
Authors Z. Zhang and D. McConnellOne of objectives for studying gas hydrate deposits is to evaluate their potential for exploitation as an energy resource. The success of gas hydrate wells depend on the seismic techniques employed to select proposed drilling locations. We are developing prospection techniques of rock physics-driven seismic that not only identify potential gas hydrate accumulations, but also have some ability to better delineate and characterize what can be complex systems and reservoirs. We analyse macroscopic seismic lithological strata and microscopic hydrate behaviours in pore spaces, which are applied into our quantitative interpretation.
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Multicomponent Seismic for Gas Hydrate Monitoring
Authors E. Asakawa, T. Hayashi, H. Tsukahara and H. TakahashiThe first offshore methane hydrate production test will be conducted by METI and JOGMEC/MH21 in early 2013 in the eastern Nankai Trough, Japan. The test aims to prove the applicability of the depressurization technique for MH production. The flow test intervals will be at shallow sediment depths but at deep water depths. JOGMEC plans to carry out a multi-component seismic survey to monitor the dissociation of MH by the depressurization technique in the flow test scheduled in early 2013. JGI, Inc. has developed deep-water OBC system (Deep-sea Seismic System, DSS) with OCC Corp. and deployed the DSS close to the production test well in August 2012. After deployment, the DSS system was buried by ROV. Then we carried out the baseline 2D/3D multicomponent seismic survey. The results show the good quality to delineate the methane hydrate concentrated zone. After the baseline survey, we have left the DSS on seafloor and remains there until the monitoring surveys scheduled in April and August 2013, because the fixed location of the sensors is crucial for time-lapse monitoring survey.
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A New Approach for Quantification of Gas-hydrates in Fracture Shale in KG basin, India from MCS Data
More LessThe coring of NGHP has recovered gas-hydrates in fracture shale in KG basin, India. Higher resistivity and sonic logs at site 10 indicate gas-hydrates between 30 to 150 mbsf. The simple Archie's formula to resistivity log overestimates gas-hydrates when compared to pressure core measurements. This is due to its isotropic assumption, as fractures cause anisotropy. Considering gas-hydrates as part of matrix and brine in pores, we establish a relation between normalized resistivity (NR) and velocity of sediments with and without gas-hydrates respectively from log data, and estimate gas-hydrates by modified Archie's formula to NR with the saturation exponent (n) that matches with pressure core measurement by incorporating effect of anisotropy. We derive seismic velocities from MCS data along 6 in-lines and 6 cross-lines (each of 8 km long) around the drilling site in KG basin covering an area of ~14.7 sq km. The modified Archie's formula to velocity-resistivity transformed data show gas-hydrates between 5 to 50% along different lines. Average volume saturation is calculated as 12.28% using 60% average porosity observed in density-porosity log. Total volume of gas within 168 m average thickness (indicated by high velocity anomaly) of hydrate-bearing sediments is calculated as 51.5 BCM in the study area.
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Sources Methane and Gas Hydrate in Okhotsk Sea
By A. ObzhirovMultidisciplinary surveys were conducted to investigate gas seepage and gas hydrate accumulation on the Northeastern Sakhalin Continental Slope (NSCS), Sea of Okhotsk, during joint Korean–Russian–Japanese expeditions conducted from 2003 to 2012 (CHAOS and SSGH projects). About 500 gas seeps were detected in a 2,000 km2 area of the NESS (between 53º45'N and 54º45'N). Active gas seeps in the gas hydrate system on the NSCS were evident from features in the water column, on the seafloor, and in the subsurface: well- defined hydroacoustic anomalies (gas flares), side-scan sonar structures with high backscatter intensity (seepage structures), bathymetric structures (pockmarks and mounds), gas- and gas-hydrate-related seismic features (bottom-simulating reflectors, gas chimneys, high-amplitude reflectors, and acoustic blanking), high methane concentrations in seawater (10-20mkl/l), and gas hydrates in sediment near the seafloor.
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Conversion of Natural Gas Hydrate into CO2 Hydrate – A Win-win Situation
By B. KvammeInjection of carbon dioxide into in situ natural gas hydrate sediments will lead to an exchange in which carbon dioxide will primarily occupy the large cavities of structure I under the release of the original methane in these cavities. The exchange can go directly as a solid state exchange or it can be a result of a hydrate formation dissociation sequence. In the latter case new hydrate from injected carbon dioxide will release heat for dissociation of the in situ methane hydrate. In this case the heat transport is fast and the rate controlling mass transport is liquid transport and as such orders of magnitude faster than the slow solid state mechanism. The conversion is discussed in detail in terms of state of the art multi scale modelling and experimental work. The role of entropy changes in driving the exchange process is discussed in detail. Some highlights of results from a recent pilot plant test of the concept in Prudoe Bay Alaska are also discussed. Finally some remaining challenges, and possible strategies, for the future development of hydrate as an energy resource are discussed.
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A Common Framework to Calculate Uncertainties of Microseismic Events from Downhole and Surface Recordings
By U. ZimmerThe results of microseismic surveys are almost exclusively presented as “dots” in three-dimensional space. Although it is well known that each of these microseismic event locations has an individual level of accuracy, the location accuracy is rarely quantified. In cases where the location uncertainty is represented by bars around the “dot”, they often represent only the precision, i.e. repeatability, of the event location. For the purpose of interpretation it is much more useful to specify the accuracy, i.e. the difference between the calculated and the true location. By using a methodology based on probability density functions (PDFs) it is possible to calculate meaningful confidence intervals around the most probable solution, i.e. the “dot”, which are much more representative of the overall location accuracy. In addition, this methodology allows similar calculations for the two main types of localization algorithms used throughout the industry, i.e. migration-type algorithms and algorithms requiring explicit arrival time picks. This allows creating a common framework that can be applied equally well to recordings from surface and downhole arrays. With this approach it is possible to include the individual location uncertainties in the overall interpretation of the microseismic map.
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Simulation of Microseismic Deformation During Hydraulic Fracturing
Authors S. Maxwell and X. WengModeled geomechanical deformation associated with hydraulic fracture stimulation of a complex hydraulic fracture provides context for interpretation of microseismic deformation. Partitioning of modeled strains into shear and dilatational components allows relative comparison of the appropriate displacement mode with observed cumulative microseismic moments. A workflow is described where the input parameters of the simulation are varied to match both the footprint and deformation of the microseismicity, which then results in an estimate of the complete fracture network volume and proppant placement. In this way the effective stimulated volume can be assessed and used as input to a reservoir simulation to investigate well performance and reservoir drainage.
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Characterization of Unconventional Reservoirs Through the Use of an Integrated Rock Quality Index
More LessUnconventional reservoirs require hydraulic stimulation to be commercially productive. Recently, distinctions have been made between reservoir quality vs. completion quality (Cipolla et al. 2012), emphasizing the importance of both elements for production. There are many sources of variability in reservoir quality; in this paper I examine several fundamental reservoir properties in detail and combine them in a new way: the Rock Quality Index (RQI). Through the definition of a geomechanical model and corresponding mechanical stratigraphy, those factors having a substantial effect on reservoir quality became apparent. Two fundamental categories; compositional variation and fabric variation, are used to characterize overall reservoir variation. Burial, compaction, hydrocarbon generation, diagenesis, and tectonics all affect the mechanical character and in-situ stress state of the reservoir. The Rock Quality Index (RQI) is an effort to understand how composition and fabric relate to stress anisotropy, fracturing, and rock properties, and ultimately aid in defining the best zones for exploitation. Therefore, this Rock Quality Index (RQI) is vital for the defining the second element of unconventional reservoir success; completion quality. Without a reservoir framework to drive the completion design, high completion quality will be harder to achieve.
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Nonlinear diffusion-based Interpretation of Hydraulic Fracturing Induced Seismicity
Authors S.A. Shapiro and N. HummelWe present a microseismicity-based approach to characterise the hydraulic transport during borehole fluid injections. This approach allows to reconstruct the enhanced permeability evolution inside of the stimulated rock. We apply our approach to seismicity recorded during a hydraulic fracturing treatment in the Barnett Shale. The spatio-temporal analysis of induced events reveals a significant nonlinear fluid-rock interaction. To explain this, we consider an effective poroelastic medium representation. We suppose a nonlinear diffusional relaxation of the pore-pressure perturbation with a power-law pressure-dependence of permeability. We generate synthetic seismicity distributions whose spatio-temporal features are similar to the ones which we observe in Barnett Shale. Our results indicate that the fluid transport during the hydraulic fracturing treatment can be described by a nonlinear diffusion based on a power-law dependent permeability.
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Microseismic Monitoring of Reservoir Stimulation: Case Studies and Implications for Improved Geomechanical Characterization
Authors M. Bohnhoff, G. Kwiatek, P. Martinez-Garcon and G. DresenMicroseismic monitoring is a well-established but still widely underestimated technique to characterize reservoir geomechanical parameters and processes related to stimulation using fluid injection. We present two case studies in which we applied state of the art seismological methods investigating data sets of induced seismicity. Physical processes occurring in the reservoir during massive fluid injection can be determined with substantially refined resolution if appropriate effort is undertaken to acquire high-quality microseismic recordings. While the presented results were obtained for geothermal reservoirs the techniques can be applied to any type of geo reservoir.
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Strategies for Field Evaluation: How Proper Instrumentation Deployment Is the Key to Unlocking the Limitless Value of Microseismic Data
Authors A. Baig, T. Urbancic and E. von LunenMicroseismic data are traditionally restricted to locations of hypocentres and have been of great utility in outlining the geometries of the stimulated regions for hydraulic fracturing and other injection programs. However, there is much more information to be gained from careful analysis of the microseismic data, the key to unlocking this potential rests not only in careful analysis, but also in recording the data from properly calibrated arrays, with a heterogeneous distribution of sensors to allow for wideband recording, and with geometries that enable such higher-order processing techniques. The wideband recording ensures that the magnitude range can be accurately calculated for the large magnitude events that potentially saturate the 10 or 15 Hz instruments normally deployed for microseismic monitoring. By recording the seismic waveforms from a number of azimuths, techniques like seismic moment tensor inversion (SMTI) can be employed to gain an understanding of the fracture network that is being activated during these treatments, and the fracture modes (tensile, shear, shear-tensile) for each event, giving a clear picture of the dynamics of the discrete fracture network during hydraulic stimulations.
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Seismic Hazard Associated with Shale-Gas-Fracking? The Bowland Shale Case Study
By S. BaischThe phenomenon of micro-earthquakes occurring in the course of shale gas fracking is well known and has been extensively used for fracture mapping. The aspect of a potential seismic hazard resulting from these earthquakes is fairly new and there exist only few documented cases of fracking induced seismicity strong enough to be perceptible on the Earth’s surface. Recent examples are two felt micro-earthquakes induced by fracking operations in the Bowland Shale (UK). Here we present the results of a geomechanical study explaining the circumstances under which such exceptionally strong earthquakes may occur. From observation data we find evidence that the treatment well in the Bowland Shale intersects a critically stressed (natural) fault or similar plane of weakness. Micro-earthquakes are induced on the existing fracture plane by fluid pressure diffusion and co-seismic stress redistribution. Based on these physical processes we have developed a numerical model of the induced seismicity yielding results which are consistent with observation data. Using the numerical model we estimate the maximum magnitude that could potentially occur by similar hydraulic treatment operations in the Bowland Shale. The resulting magnitude is considered to be too small to cause damage at the Earth’s surface. To account for parameter uncertainties, we have developed an additional traffic light system for controlling future fracking operations.
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Bowland Basin Shale Operations Past and Present, Micro-seismic Monitoring and Induced Seismicity Mitigation Protocol
By H. ClarkeShale gas exploration commenced in the Bowland basin of NW England with the Preese Hall 1 discovery well in August 2010. Two more vertical wells and a 100km2 3D seismic have since been completed and an OGIP resource estimate of 200 TCF. Following the drilling of Preese Hall-1 a multistage hydraulic fracture program commenced in spring 2011. Associated with this fracturing activity, two felt seismic tremors occurred on April 1st (ML=2.3) and May 27 (ML=1.5). Hydraulic fracturing operations were paused and a detailed geomechanical study concluded that seismicity at Preese Hall-1 was induced by injection of fluid into a fault zone within the proximity of the well bore.
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Shale Rock Properties - Extending the Scope of Rock Physics
By F. KetsTraditionally, the emphasis of rock physics in the Petroleum Industry has been on the elastic properties of rocks, with the major area of application in the description and characterisation of reservoirs. The search for unconventional resources does not only lead to an extension of the discipline to the description of less porous and quite often anisotropic media, but also to a reconsideration of inelastic properties which are critical for the deliverability of the reservoir. Questions arise whether experimental techniques proven useful in rock physics calibration can be applied with equal confidence in unconventional reservoirs.
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Monitoring Fracture Network Stimulation Using Micoseismic Data
Authors M. Kendall, J.P. Verdon, A.F. Baird, A. Stork and P. UsherThe successful exploitation of many reservoirs requires fracture networks, sometimes naturally occurring, often hydraulically stimulated. Microseismic data acquired in such environments hold great promise for characterising such fractures or sweet spots. The loci of seismic events delineate active faults and reveal fracture development in response to stimulation. However, a great deal more can be extracted from these microseismic data. Inversions of shear-wave splitting data provide a robust means of mapping fracture densities and preferred orientations, useful information for drilling programs. They can also be used to track temporal variations in fracture compliances, which are indicative of fluid flow and enhanced permeability in response to stimulation. Furthermore, the frequency-dependent nature of shear-wave splitting is very sensitive to size of fractures and their fluid-fill composition. Here we discuss a range of methods for extracting spatial and temporal variations in sub-seismic scale fractures.
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Modifying the Seismic Source Array Spectrum to Reduce the Risk of Injury to Marine Mammals
By R. LawsAcoustic injury criteria for marine mammals are well defined and they are applicable to seismic sources. Several studies have suggested that marine seismic sources emit more high-frequency energy than is required for imaging. I show how the extent of the injury zone can be reduced by using a source array with an output spectrum that is partially attenuated at higher frequencies while probably not compromising image quality.
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Does Seismic Surveys Impact Fish and Fisheries
By I. GauslandThe presentation will discuss studies on impact of seismic operations on fish and fisheries, and review some shortcomings in studies frequently used in Environmental Impact Assessments and as a basis for governmental regulations of seismic operations. The difference of opinion between the parties may lead to unnecessary conflict and significant increased cost for all operations.
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Monitoring and Mitigating Bioacoustic Impacts from Seismic Surveys - The Australian Perspective
By C. ErbeIn Australia, proponents of seismic surveys and other operations emitting underwater noise have to prepare an Environmental Plan (EP) for approval by the regulator before operations can commence. In support of the EP, large amounts of data are often collected, such as baseline data on the marine soundscape, the physical and biological environment, operational noise characteristics etc. More data (on underwater noise, animal abundance and behaviour etc.) is sometimes collected during operations as part of monitoring and mitigation plans. Australia’s offshore oil & gas industry has recently come together to share data from bioacoustic environmental impact assessments, yielding charts of baseline soundscape characteristics, catalogues of anthropogenic source characteristics, improved models for sound field prediction, and growing databases of animal behaviour, abundance and migration. This data sharing will ultimately help to streamline the environmental approval process.
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Mitigating the Potential Effects of Underwater Sound on Marine Mammals - A Global Overview
By R. ComptonConcerns regarding the potential impacts of underwater sound upon marine species such as whales, dolphins and porpoises has led to the implementation of mitigation measures during marine seismic operations. These measures include the use of personnel and passive acoustic equipment to monitor for species within a given safety zone around the source, delay and shutdown to the source during operations. These procedures have become a common part of marine seismic surveys and represent best practice with regard to environmental responsibility.
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The Perceived Impacts of Seismic Surveys on Marine Life - Are They Really a “Big Issue” or Simply “Big Business”?
By J. HughesIn keeping with the title of this workshop, I have chosen my title to be even more provocative! I will summarise the main concerns put forward as being representative of this “big issue,” such as “the increasing levels of sound discharged in the oceans (by the petroleum industry) may prevent cetaceans communicating (masking effect) and hence have a negative impact on their life functions” or even “the oil industry’s activities may have a very significant impact in harming and even killing cetaceans and fish in the marine environment. It could even lead to their extinction!” Note the devious use of the words “may” or “could” in these claims made by those who most vocally express opposition to seismic surveys. In parallel, I will outline the facts and observations that surely point to such claims being conjecture (I provocatively call them “myths!”). Finally, I will explore what I consider to be the most likely motives that a variety of generic organisations (such as environmental NGOs, political parties, various levels of government and even sections of the petroleum industry) have for ignoring the facts, making the claims, spreading the misinformation and propagating the myths in order to make this issue “big business.”
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24/7 Automatic Detection of Whales Near Seismic Vessels Using Thermography
Authors O. Boebel and D.P. ZitterbartDetecting whales at sea by visual observation for mitigation purposes is inherently difficult and personal intensive while restricted to daylight hours. These caveats are overcome by the system described herein, which employs a state-of-the-art thermographic infrared scanner in conjunction with a learning computer algorithm to automatically and reliably detect whale blows. The stand-alone system provides detection, verification and documentation of each ship-whale encounter, allowing a retrospective review of every mitigation decision taken aboard. The system has been developed over the course of 5 years and was thoroughly tested in polar waters during 7 expeditions to the Arctic and Antarctic, accumulating 5871 hours of operation. Of these, 3472 hours, were analyzed with various learning automatic detection algorithms, which discovered about 4500 whale blows in this data. Direct comparisons of auto-detections with concurrent whale sightings by visual observers (double blind setup) confirm a very high degree of detection reliability within 2-3 nautical miles from the ship in subpolar and polar environments. The system, when used as a an “assistant”, allows a single marine mammal observer to efficiently monitor the ships entire surroundings and to take instantly and retrospectively verifiable decisions regarding the use of airguns, as all relevant data is automatically stored.
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Assessing Behavioural Responses of Small Cetaceans to Seismic Surveys
By P. ThompsonUncertainty over the extent to which small cetaceans are displaced by air-gun noise can constrain seismic survey activity within or near sensitive habitats. This paper outlines how recent exploration in the Moray Firth, NE Scotland, was managed to ensure compliance with the EU Habitats Directive, and describes the research programme that was undertaken to assess responses of harbour porpoise to a 10 day 2-D seismic survey in 2011. Passive acoustic monitoring and digital aerial surveys were used to detect changes in occurrence over a 2000 km2 study area. Although fine scale behavioural responses were detected, animals returned within a few hours and there was no evidence of larger-scale displacement.
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Using Modelled Far-field Signatures of Airgun Arrays to Estimate Impact on Marine Mammals
Authors A. Goertz, J. Ali, J.F. Wislřff and C. StrandWe present a workflow to assess the risk of excessive sound exposure on marine mammals based on forward modelling of the far-field signatures from airgun arrays. The employed physical model takes into account the hydrodynamic effects around an airgun including the air bubble, directivity effects of realistic airgun arrays, and the interaction of the primary signals with the surface ghost reflection. Based on such signatures, we are calculating sound pressure and sound exposure levels as a function of distance and direction from the source, taking into account different geometrical spreading models. We also consider the frequency-dependent hearing threshold for different cetacean species. The workflow allows the estimation of a variety of different properties and scenarios in line with most recent regulatory requirements. Results can be used as a base for the planning of mitigation measures, such as exclusion radii around sensitive areas and soft-start procedures.
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Effects of Sounds from Seismic Air Guns on Fish Behaviour and Catch Rates
By S. LøkkeborgScientific studies on the effects of sounds from seismic air guns on fish behaviour and commercial fisheries are reviewed. The results of these studies provide clear indications that the fish reacted to the sound of the air guns. There are species-specific differences in the responses, which may lead to both increased and reduced catch rates depending on the type of fishing gear and fishing ground, and on the hearing ability and behaviour of the exposed fish species.
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Highlights of the Joint Industry Program on Sound and Marine Life
By R. GentryThis talk summarizes seven years of research results by the Joint Industry Program on Sound and Marine Life (JIP). The results are intended to provide scientific facts for risk assessment of, and regulations for offshore E & P operations. Topics to be covered include: 1. Companies participating in this JIP, 2. Acoustic output to 25 kHz of a full airgun array, single guns, and typical gun clusters; 3. Improving existing propagation models for arrays (Gundalf, Nucleus) using new acoustic output data, 4. Hearing abilities of species considered most susceptible to airguns (minke whale, marine turtles), 5. Effects of airgun shots on animal hearing (dolphins, arctic seals), 6. Modeling the effects of airgun shots on tissue damage in fish, 7. Effects of airguns on animal behaviour (naďve and experienced humpback whales), 8. Marine mammal monitoring (PAMGuard to aid observers, calculating population density from recorded animal calls), 9. Mitigation (efficacy of ramp-up/soft-start, an active source for safe startup under low visibility conditions), and 10. A possible alternative to airguns (comparing the possible effects on animals of airguns and marine vibrators). The JIP seeks other partner companies to expand this important program.
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Experiences from Using WhaleWatcher™, a Marine Mammal Monitoring System Based on Single Sensor Seismic Streamers
By H. GrønaasSeismic contractors follow IAGC guidelines and local regulations that require the presence of marine mammal observers (MMO) onboard seismic vessels. To aid these observers during periods of limited visibility, passive acoustic monitoring systems have been proposed. Such systems must have a large enough aperture to obtain high resolution and a high number of receivers to achieve sufficient signal-to-noise ratio. Developing, deploying, and operating such systems reliably therefore poses significant technical, operational, and logistical challenges. This is especially true when these systems must be towed in parallel with the seismic in-sea equipment. By having the system integrated into the streamer spread, the operational complications are avoided and accurate localization performance is achieved. In this paper, we give an overview of the system, and share some experiences from using it in the field. Examples include both the detection and localization of an underwater sound speaker, as well as real whales.
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On the Terminology and Metrics of Airgun Sounds and Their Use in Regulation and Criteria
Authors M. Ainslie and R.P.A. DekelingThe need for and ongoing development towards an International Standard for underwater acoustical terminology are described in the context of airguns, with particular attention to the implications of recent European legislation.
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Marine Vibrator JIP
Authors M. Jenkerson, G. Johnson, J.-M. Mougenot and B. RosenbladtThe marine seismic prospecting industry has been interested in the potential geophysical and environmental benefits of marine vibrators for many years. However the airgun array has remained the dominant marine seismic source in commercial use. Recently a Marine Vibrator Joint Industry Project was formed to focus again on the development of commercial marine vibrators.
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Predicted Relative Effects of Marine Vibroseis Versus Airguns on Marine Mammals
Authors W.J. Richardson and W.T. EllisonMarine vibroseis (MarVib), if effective for geophysical purposes, might (relative to airguns) have reduced effects on marine animals. With Joint Industry Programme sponsorship, we prepared an Environmental Assessment (EA) of this possibility. Given the near-absence of specific data on biological effects of MarVib, most conclusions were based on indirect evidence. This presentation focuses on marine mammals, but the EA also addressed fish, turtles and invertebrates. With MarVib, higher frequency sound components (e.g., >100 Hz) should be reduced, benefitting species sensitive to sounds >100 Hz. Signal duration would be longer than for airguns, which could achieve similar source energy with lower peak pressures, thus reducing auditory and perhaps disturbance effects. Signal processing techniques applicable to MarVib might allow lower source energy per "shotpoint". There may also be more flexibility in the depth in the water column where a MarVib system could be operated. However, the higher duty cycle with MarVib may cause increased acoustic masking in species that rely on low-frequency sound. Also, with respect to behavioral disturbance, the higher duty cycle with MarVib may partly offset the benefits of the lower source level. Overall, use of MarVib rather than airguns would probably result in a reduction in some (but not all) types of impacts; controlled testing is needed. No extended abstract available.
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Toward Data-domain Waveform Inversion of Reflected Waves
Authors R. Brossier, S. Operto and J. VirieuxFull Waveform Inversion is becoming a powerful tool for quantitative seismic imaging from wide-azimuth seismic data. The method is today mainly used as a high-resolution tomography in the Earth zones sampled by both reflected and diving events. The classical formulation however prevents the reconstruction of the low-part of the wavenumber spectrum of the velocity model, from reflection-only data. The used of reflections in waveform inversion is a crucial challenge for deep imaging. This study, through a simple canonical example, first analyses the issue of classical waveform inversion formalism when used with reflection data. It is then shown that if a prior knowledge of reflectivity can be known, reflected waves can be be exploited in a data-domain approach to retrieve the long wavelength of the velocity, in particular when we used appropriate misfit measurement as the cross-correlation that overcome cycle-skipping issues, and inversion domain as the pseudo-depth domain that preserves the invariant property of zero-offset time.
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Developing angle-domain wave path tomography for velocity analysis in complex settings
Authors P. Bakker and S. GerritsenSeveral alternatives are discussed for the calculation of finite frequency sensitivity kernels for angle-domain wave path tomography. Starting point is an approach based on angle gathers from slant-stacked subsurface offset gathers and band-averaged frequency domain kernels. Then, to improve efficiency, shortcuts are discussed, both with respect to the angle gathers as well as the sensitivity analysis of the picked image depth. It is shown that these approaches produce satisfactory results for a 2D synthetic model updating exercise using the Sigsbee model. However, for 3D application on a realistic scale, further efficiency gains are required. One step in that direction is the estimation and usage of stationary shots.
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Wavefield Tomography without Low Frequency Data
More LessWaveform inversion (FWI) requires a good starting model and/or data at very low frequency (< 1Hz) for convergence. However, this is not a necessary condition, but an artifact of the objective function defined using differences of observed and simulated data. Image-domain tomographic methods using the same wavefields and wave-equations can converge to a reasonable solution from poor starting models and without long offset and/or low frequency data. Cascading image-domain and data-domain wavefield tomography eliminates the need for extremely low-frequency in the acquired data.
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Ongoing R&D in Ray-based Tomography - Well Worth the Effort
By P. HardyOver the past 20 years, ray-based post-migration reflection tomography has remained largely unchallenged as a velocity model building tool for seismic depth imaging. While other, more mathematically attractive methods have been proposed, they have yet to be adopted into standard workflows. The decisive elements for such tools are: accuracy, resolution, flexibility, ease of use, and turnaround time. The success of depth domain tomography comes from the good balance it offers between those elements. This paper aims to illustrate with a few practical innovations in post migration tomography, that ray based approaches are still making progress and are nowadays finding applications far beyond their expected range of validity.
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A Real-data Example of Automatic Migration Velocity Analysis with Extended Images in the Presence of Multiples
By W. MulderMigration velocity analysis for the two-way wave equation based on focusing extended migration images at zero subsurface offset are sensitive to the presence of multiples. Even after thorough multiple suppression, remnant multiple energy can lead to conflicting events that will focus at different velocities, one for the primary and another for the multiple. An automatic algorithm will produce a velocity in between these two. If the multiples are mainly caused by the presence of a water layer, a bias of the algorithm towards higher velocities may help. Here, an application of this approach to a marine data is presented and the result is compared to a well log.
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Differential Waveform Inversion - A Way to Cope with Multiples?
Authors H. Chauris and R.-E. PlessixIn the context of velocity estimation, we investigate the differential waveform inversion method. The approach is formulated in the data domain and consists of two main steps. First, for a given shot, we derive the optimal reflectivity section. From the result, we then compute the next shot gather. The new formulation measures the differences between the predicted shot and the observed shot at the nextposition. The main interest of the Differential Waveform Inversion strategy resides in the possibility to take into account surface-related multiples. The key point is the iterative migration.
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Wavefield Tomography Based on Local Image Correlations
Authors F. Perrone and P. SavaSeismic imaging includes the estimation of both the position of the structures that generate the data recorded at the surface and a model that describes the propagation in the subsurface. The waves recorded at the surface are extrapolated in the model by solving a wave equation, and they are crosscorrelated with a synthetic source wavefield simulated in the same model. Reflectors are located where the source and receiver wavefields match in time and space. If the velocity model is inaccurate, the reflectors are positioned at incorrect locations. We propose an objective function in the image space that does not require common-image gathers (CIGs). We consider pairs of images from adjacent experiments and reformulate the semblance principle in the physical space, instead of the extended space at selected CIGs. We use penalized local correlations of two images to estimate shifts in the image space.
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High-definition 3D Anisotropic WEM-tomography
Authors O. Holberg, E. Pedersen, J.K. Lotsberg and S.K. FossWe review the challenges and trade-offs involved in coupling shot record WEM with high-definitionanisotropic tomographic inversion, and demonstrate successful application of a 3D WEM-tomography process to improve the imaging of formations partly obscured by shallow velocity anomalies and salt.
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Seismic Reflection Tomography with 3D RTM Angle Gathers
Authors C. Zhou, S. Crawley, D. Whitmore, S. Lin, S. Frolov, Z. Liu and N. Chemingui3D angle domain common image gathers (ADCIGs) from reverse time migration (RTM) provide accurate representation of residual moveout in complex media, especially in the presence of salt. Thus, tomography with residual moveout picked from RTM angle gathers allows us to build accurate models for seismic imaging. However, RTM angle gathers bring some challenges too. Among them are spatial under-sampling and insufficient angular sampling in deep parts of the image. In this paper, we present the approaches to overcome such problems and our complete work flow to build models with 3D RTM angle gathers.
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Tomographic Full Waveform Inversion (TFWI) by Successive Linearizations and Scale Separations
Authors A. Almomin and B. BiondiTomographic Full waveform inversion (TFWI) provides a framework to invert the seismic data that is immune to cycle-skipping problems by extending the velocity model. However, this extension makes the propagation considerably more expensive because each multiplication by velocity becomes a convolution. We provide an alternative formulation which computes the backscattering and the forward scattering components of the gradient separately. The inversion setup includes two steps that maintain the high resolution results of TFWI. First, the linearized residual are updated in a nested inversion scheme. Second, the two components of the gradient are first mixed and then separated based on a Fourier domain scale separation to allow for a fully simultaneous inversion of model scales.
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Waveform Inversion via Nonlinear Differential Semblance Optimization
Authors D. Sun and W.W. SymesThis article describes a nonlinear differential semblance approach to waveform inversion. Nonlinear differential semblance optimization combines the ability of full waveform inversion to account for nonlinear physical effects, such as multiple reflections, with the tendency of differential semblance migration velocity analysis to avoid local minima. It borrows the gather-flattening concept from migration velocity analysis, and updates the velocity by flattening primaries-only gathers obtained via nonlinear inversion. We describe the underlying idea and formulation of this algorithm, and present a layered 2D acoustic inversion excercise for which standard full waveform inversion fails, whereas nonlinear differential semblance succeeds in constructing a kinematically correct model and fitting the data rather precisely.
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Tomographic Full Waveform Inversion (TFWI) by Extending the Velocity Model along the Time-lag Axis
Authors B. Biondi and A. AlmominThe extension of the velocity model along the subsurface-offset axes enables to define a tomographic full waveform inversion that converges robustly towards high-resolution velocity models. However the resulting numerical algorithms are extremely expensive and their performance is uncertain when transmitted events (e.g. diving waves) are present in the data. We discuss the advantages of extending the velocity model along the time-lag axis instead of the subsurface offsets. This extension leads to a linearization of the wave equation that overcomes the well-known limitation of the first-order Born approximation to model large time shifts in the data.
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About ADCIG Residual Moveout
Authors J.-P. Montel and G. LambaréThe increasing usage of wave equation and reverse time migration has motivated investigations about new types of common images gathers. Several types of angle domain common image gathers have then been proposed. While they should have the same aspect (up to specific artefacts) when the exact velocity model is used it appears that they exhibit significant differences in curvature when the velocity model is erroneous. This observation is analysed here by a theoretical analysis in the frame of high frequency asymptotics. This analysis is performed for several types of angle domain common image gathers revealing much more complexity than expected. It demonstrates that some well admitted statements such as the validity of Snell Descartes law for tomographic rays or the idea that the tomographic ray tracing could be limited to lower layers may not be satisfied for all type of angle gathers.
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The Challenge of Short-wavelength, High-contrast Velocity Anomalies
By J. EtgenConventional velocity estimation, based on post migration residual moveout, works very well when the velocity structure of the Earth is smooth enough in the lateral directions. This is true even when the initial velocity model is far from correct. Stated another way, long wavelength lateral velocity errors are not a significant challenge anymore. However, once the velocity varies significantly in the lateral directions at scales approaching or less than a few wavelengths, residual moveout estimation plus reflection tomography will often fail to improve velocity models.
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Unmanned Aircraft Systems for Rapid Near Surface Geophysical Measurements
More LessThis paper looks at some of the unmanned aircraft systems (UAS) options and deals with a magnetometer sensor system which might be of interest in conducting rapid near surface geophysical measurements. Few of the traditional airborne geophysical sensors are now capable of being miniaturized to sizes and payload within mini UAS limits (e.g. airborne magnetics, gamma ray spectrometer). Here the deployment of a fluxgate magnetometer mounted on an UAS is presented demonstrating its capability of detecting metallic materials that are buried in the soil. The effectiveness in finding ferrous objects (e.g. UXO, landslides) is demonstrated in a case study.
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Successful Execution of Remotely Piloted Ocean Vehicles to Conduct METOC and Turbidity Pre-site Survey
More LessThis is to present case studies conducted for Chevron’s Environmental Technology Unit off the coast of North West Australia. Two sorties, first a METOC survey followed by a detailed Turbidity study was conducted over 2 months (Dec 2012 and Jan 2013) to collect baseline data as pre-site survey prior to the start of a full scale dredging operation to lay a pipeline.
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Analysis and Separation of Surface and Body Waves in Onshore-seismic Dual Fields
Authors F. Poletto, A. Schleifer, B. Farina and L. PetronioDual-field seismic data are achieved recording particle velocity and pressure/stress at a same receiver-device location. The dual-field method makes it possible to separate different components of the propagating wavefields activated by a single source. We analyze the results of an onshore seismic application using dual-field measurements in the shallow ground. We acquired the seismic data by reciprocal source-receiver geometry that makes it possible to limit the number of dual-receivers installations and to record dual-field data by a source activated along a seismic line. The data obtained by shallow stress sensors buried in the ground and surface geophone are studied for the characterization of vertically propagating body waves, and of surface waves propagating in the horizontal direction with amplitude decaying in depth. The analysis shows that the surface and body waves can be separated combining the dual signals without needing of multi-dimensional processing. This technique may represent a new approach useful for wavefield analysis in near-surface applications, and not only, easily achievable with reciprocal geometry in shallow seismic acquisition surveys.
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Fusing Multi Source Remote Sensing Data – Selected Aspects on Geometric and Thematic Accuracy
Authors M. Pregesbauer, R. Michel and A. AltenbergerRemote Sensing data is widely used in geosciences for all kind of applications. The user thereby relies on, for his purposes, sufficient accuracy and well calibrated data. Beside standard processing methods geometric accuracy and radiometric calibration can be increased with the aim to enhance the thematic accuracy. With the focus on airborne remote sensing, different strategies for laserscanning and hyperspectral data processing with the focus on the subsequent analysis and interpretation are going to be discussed and an assessment on the possible accuracy is given.
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Seabed Sampling for Stratigraphy and Seep Studies Offshore Greenland and Norway
Authors S. Polteau, S. Planke, R. Myklebust and G. HickmanGravity coring and dredging has been used to obtain extensive seabed sampling of escarpments and potential hydrocarbon seep sites offshore Greenland in the northeast Atlantic. Sampling of sub cropping strata and thin overburden sediments provide consistent information on ages of the strata and the nature of potential active hydrocarbon systems. Our results are the first to document active hydrocarbon systems in the Baffin Bay, the northeast Greenland shelf, and on the southern Jan Mayen Ridge.
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Mapping Palaeorivers from Satellite Radar and Surface-wave Velocities
Authors A. Laake and C. StrobbiaSatellite radar data have revealed the existence of extensive palaeodrainage patterns below sand dunes in the eastern Sahara. So far, studies have focused mainly on the lateral mapping of these so-called radar rivers. We use seismic surface-wave data to estimate the penetration of satellite radar into the ground on a regional scale. We developed a scheme for the classification of radar backscatter intensity that allows the discrimination of direct surface backscatter in areas with hard rock at the surface from areas where the radar data penetrate the surface and provide information about palaeorivers in the subsurface. The estimated radar penetration depth is calibrated with seismic surface-wave velocity profiles. Corendering both data sets in 3D shows the high degree of correlation. The method is demonstrated on an area of about 200 x 200 km in the hyperarid desert of southwest Egypt. A 200-km high-resolution seismic section provides the shallow seismic data. The seismic surface waves confirm the existence of two phases of palaeovalleys in the area: wide valleys of several km in width from the Tertiary and Pleistocene rivers, the channels of which are mapped by the satellite radar data.
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Surface Deformation Monitoring with InSAR Data for Oil & Gas Applications
Authors A. Rucci, A. Ferretti and F. RoccaApart from the environmental impact of subsidence and uplift phenomena induced by fluid injection and/or extraction, recent reservoir optimization techniques ask for timely information about many geophysical parameters, both downhole and on the surface. In particular, surface deformation measurements are lately gaining increasing attention within the reservoir engineer community, which is searching for new monitoring tools to complement seismic surveys. In the last decade, a new remote-sensing technology called PSInSAR™- based on the use of satellite radar data - is receiving an increasing attention, thanks to its capability to provide accurate, large-scale surface deformation measurements. The main advantages of PSInSAR™ data, compared to conventional geodetic networks, are essentially related to the spatial density of measurement points, the temporal frequency of the observations, the precision and the limited cost, at least for the monitoring of large areas. Since the number of radar-mounted satellite platforms is increasing, as well as the accuracy of radar measurements, this source of information will probably become more and more common in projects related to reservoir monitoring and optimization. More in detail, ground deformation measurement can provide valuable information to better understand the fluid/gas flow in the subsurface or to calibrate the geomechanical model of the reservoir/overburden, if properly interpreted and integrated with more conventional data.
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Outcrop Structural Mapping from Hi-res DEM and Worldview Satellite Imagery
Authors B. Baugh and K. NavulurRecent trends in commercial remote sensing imagery point to increased spatial, spectral resolution, and agility of satellites. Satellites are now planned at 1foot pixel resolution allowing for new applications. The global capacity of satellites at these high spatial resolutions has signalled a new market for geological applications. Similarly, the number of spectral bands is evolving from traditional bands to specialized bands designed for specific applications. DigitalGlobe has recently announced that Worldview-3, the next generation satellite, will have 17 bands ranging from VNIR to SWIR region of the spectrum that are suited of variety of geological applications. These spectral bands can detect specific phenomenology of outcrops that are indicators of subsurface minerals. The advent of CMG’s allow for backward and forward scanning, allowing for unprecedented agility for collection of stereo imagery that can be used for creating elevation models. Several research papers show accuracy of DEM’s derived from satellite imagery are around 2 meters LE90. Combing the spectral capabilities of Worldview satellites with elevation models, can be indicators of outcrops for geological mapping. This papers talks about specific phenomenology that can be exploited for outcrop structural mapping from hi-res DEM and Worldview satellite imagery.
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Geoid Determination in the Western Part of Egypt from Gravity Data and Gps/Leveling
By A. RadwanPrecise geoid determination is one of the main current geodetic interests in Egypt. The method of Least-squares collocation (LSC) is used in this research for the computation of geoid in Egypt, combining a geopotentail model complete to degree and order 360 in addition to gravity and GPS/leveling data. No topographic information was taken into account on the area under study.
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Reactive Transport in Carbonate Reservoirs: Seismic and Transport Properties
Authors J.P. Nunes, L.G. Rodrigues and R.R. GuérillotThe recent surge in CO2 rich gas injection activities for both Capture Carbon and Sequestration (CCS) and Enhanced Oil Recovery (EOR) projects have led the oil industry and the academia to explore the implications of rock-fluid interactions at full scale development projects. Some of the main questions are: Do reactions occur? Are they relevant for CO2 capture and/or oil recovery? How to monitor them? To illustrate some of the ways the industry may tackle these questions reactive transport simulations have been made to evaluate the impact of rock-fluid interactions in the seismic, mechanical and flow properties in an outcrop based 3D geocellular model. Changes in elastic properties are evaluated using a flow coupled petroelastic model based on an effective medium approach. Current challenges to develop pore-to-reservoir reactive models will also be addressed.
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Dual Mineral Matrix and Organic Pore Textures in Mature Niobrara Formation, Rocky Mountains, USA – Implications for Tight-Oil Carbonate Reservoir Modeling
Authors C. Laughrey and R. RubleProspective tight-oil reservoirs in the Upper Cretaceous Niobrara Formation B Chalk in the northern Denver basin include elements of hybrid/interbedded and porous mudstone fine-grained systems. We recognize three reservoir/source rock lithologies: 1) argillaceous, organic-rich, pelletal impure chalk; 2) organic-rich pelletal chalk; and, 3) highly organic-rich pelletal impure chalk. Geochemical parameters indicate the rocks belong to Organic Facies B (oil-prone), and are thermally mature with respect to oil generation. We recognize both mineral matrix pores and organic-matter pores in the Niobrara reservoirs. Mineral matrix pores include interparticle and intraparticle voids. Intraparticle pores dominate storage capacity in the rocks and consist of nano- to micro-scale fabric-selective and not-fabric-selective carbonate pore textures preserved within compacted pelletal allochems. Organic porosity is well developed in alginite and liptodetrinite, but is absent in bituminite, and thus may be related to the original kerogen structure. Complex intraparticle pore geometry is readily modeled by 3D FIB/SEM segmentation. Laboratory NMR measurements of the modeled samples appear to discriminate discrete pore and fluid distributions in the tight reservoirs.
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Bi-Directional Interactions between Fluids and Geomechanics in Fractured Reservoirs
By G. CouplesPopular ideas used to explain geomechanical-fluid interactions in fractured reservoirs are inadequate, or wrong, as they are based on assumptions that are incorrect. Poro-elasticity considerations show that predicted aperture effects are highly sensitive to the local boundary conditions, which themselves may change during reservoir operation. The more-general approach is to consider fully-coupled systems of matrix blocks bounded by fractures, and the porefluids. Simulations based on such systems reveal significant insights, as well as surprising outcomes, with effective permeabilities that can vary by more than 1000x due to small changes in conditions. Application of these concepts within reservoir models can be used to improve the results of reservoir flow simulation.
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Carbonate Rock-typing – Diagenesis and Multi-scale Heterogeneity
Authors C. van der Land and Z. JiangCarbonate rock typing methods aim to associate texture with petrophysically similar rocks. Here, we use 2D thin sections of carbonate reservoir rocks to create 3D statistical pore-size distributions and extract pore network models from these to obtain multiphase flow properties. This novel rocktyping method is applied to synthetic depositional rock textures which are progressively altered by a succession of typical diagenetic processes. For each stage in the paragenetic sequence, flow properties were calculated. Secondly, by comparing experimental results with our modelled predictions, we validate our methodology of pore reconstruction, network extraction, stochastic network generation and creation of multi-scale networks.
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Calculating Vertical Stress in Chalk
More LessWhen effective stress is calculated with reference to the grain cross sectional area as defined from Biot’s coefficient, it is simple to illustrate zones of chalk where the stress on the mineral grains is high, and pore collapse would be a challenge during production of hydrocarbons.
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Quantification of Depositional and Diagenetic Geobody Geometries for Reservoir Modelling, Hammam Fauran Fault Block, Sinai Peninsula, Egypt
Authors C. Hollis, H. Corlett, J. Hirani, D. Hodgetts, R. Gawthorpe, A. Rotevatn and E. BastesenOutcrop data has traditionally been used to constrain conceptual models during subsurface reservoir characterisation and geocellular modeling, but published data of depositional and diagenetic geobodies in carbonate systems is lacking. Furthermore, few studies address how these diagenetic bodies, which often cross-cut sedimentary bedding, can be captured in reservoir models, even though diagenetic modification is likely to impart a significant influence on flow behaviour. This paper presents a case study from the differentially dolomitised pre-rift Eocene Thebes Formation on the Sinai Peninsula. It documents the size of depositional and diagenetic geobodies and demonstrates how these data have been incorporated into a 3D geocellular model. The results can be used as input parameters or templates for reservoirs in which fault/fracture controlled dolomite bodies have been described, whilst the workflow could have broader applications to other carbonate reservoirs.
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Modeling and Validation of Fluid Flow-Geomechanics of Mauddud Reservoir in Sabriya Field
More LessIntegrating fluid flow and geomechanics is becoming increasingly important in the oil and gas industry to improve production history matching and forecasting. This is especially the case in carbonate reservoirs due to the heterogeneous nature of the rock and flow properties within the pores and sometimes fractures. During production, carbonate reservoirs are subject to mechanical, hydraulic, and, in some cases, thermal processes. Coupled modeling between these processes helps to better understand the behavior of a carbonate field. When hydrocarbon is produced, the pore-pressure change within the reservoir gives rise to accompanying stress changes, which cause rock deformation that will either reduce or enhance permeability and porosity. A multidisciplinary approach is necessary to integrate geology, petrophysics, fluid flow, and geomechanics within a coupled reservoir simulation. 3D fluid-flow geomechanical modeling was conducted for a carbonate field in order to improve the history match of a reservoir model, and we examined the impact of permeability change and enhancement on well performance. The results showed significant improvement of the reservoir production history matching by including coupled reservoir simulations. The geomechanical changes brought on by production will influence other field-development operations such as in-fill drilling. Geomechanical modeling of the reservoir can be extended to optimize wellbore stability through a 3D mud-weight cube for field-scale in-fill drilling optimization.
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Challenges in Brazilian Pre-Salt Reservoirs Geophysical Characterization
By P. JohannThis paper focuses on the impact that reservoir geophysics has had on the production development of the Brazilian marine carbonate pre-salt fields starting from the first oil discovery in 2005 to 2011. The evolution of three main knowledge areas of reservoir geophysics technology will be detailed, namely: acquisition, processing and interpretation, all oriented towards reservoir characterization and monitoring. Seismic acquisition technology has experienced an increase in “information density” (seismic traces per square kilometer). In the seismic processing domain, the improvement of seismic algorithms and methodologies has allowed for better seismic data quality, resolution and imaging. In particular, the algorithms/techniques of 3-D multiple suppression and 3-D depth migration have significantly evolved in recent years. In the seismic interpretation area, geological context-oriented seismic attributes algorithms/methodologies have made possible better reservoir characterization in the deep and ultra deep-water Brazilian offshore basins. No extended abstract available.
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Land Vibroseis Source Advances towards Low Frequencies
Authors Z. Wei and T.F. PhillipsExtending the frequency bandwidth towards low frequencies using the Vibroseis method has gained a lot of attenation recently. The source (vibrators) becomes one of the obtacles in the success of recording low frequency seismic signals. How do we increase the vibrator ground force at low frequencies (< 10 Hz)? Can the vibrator control electronics effectively supress harmonic distortion at low frequencies? This paper attempts to provide a fresh look at these questions.
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The Impact of the Field Array on Temporal Bandwidth of Land Seismic Data and Its Mitigation in Processing
By R. FerberThe field array is the group of seismic sensors connected to a single recording channel. The impact of field arrays on the spatial bandwidth of the recorded seismic data is described by the array wavenumber amplitude transfer function. In this paper, I discuss, however the impact of the field array on the temporal bandwidth of the recorded data. A requirement for this is an accurate knowledge of the field array geometry. I also propose a novel technique to correct the corresponding signal amplitude distortions in data processing.
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The End of Unblended Acquisition
Authors G. Berkhout, G. Blacquičre and E. VerschuurWe show that deblended shot records have a better S/N than shot records in unblended surveys. We also show that a further improvement in S/N is obtained in the migrated domain. Both improvements increase with increasing blending fold and decreasing survey time. An interesting consequence of this remarkable property is that blended surveys can be carried out under more severe noise conditions than unblended surveys. We conclude with the observation that unblended seismic acquisition may evolve into a technology of the past.
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Land Broadband Seismic from the Receiver Perspective
By D. MougenotThe broadband capability of land receivers is reviewed. On the low-frequency side, two more octaves have been added to the signal using low dwell sweeps. These low frequencies can be recovered below the resonant frequency of the geophones by compensating for their attenuation. However, this inverse filter works well so long as there is adequate signal-to-noise ratio. To avoid amplifying instrument noise, new geophones have been developed with a higher sensitivity and a lower resonant frequency. MEMS accelerometers that display a linear response down to DC seem to be the ideal sensor to capture very low frequencies. Their limitations relating to the instrument noise may be compensated by a high spatial sampling. On the high-frequency side, progress is limited by the absorption that occurs at shallow levels and during propagation. The spurious frequency of geophones occurs above the high-cut used in most of the surveys. MEMS accelerometers benefit from a broadband response, higher sensitivity and lower instrument noise at high frequency. Therefore, the main issue is not related to the receivers but to the possibility of enhancing the signal-to-noise ratio at high frequency.
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On the Use of a Seismic Sensor as a Seismic Source
Authors D.F. Halliday, T. Fawumi, J.O. A. Robertsson and E. KraghWe investigate the use of seismic sensors as small seismic sources. A voltage signal is applied to a number of geophones, forcing the mass within the geophones to move, generating a seismic wavefield that is recorded on a separate array of geophones. We observe higher-frequency (25 Hz and above) surface and body waves propagating from the geophone source at offsets of 10s of metres.
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Interpretation of Single-sensor Data - a Practical Perspective
Authors V. Aarre, A. Poole, B. Mitchell, S. Tan and G. BusanelloLand broadband seismic technology involves not only high-technology acquisition equipment, sophisticated mathematics, and near-surface challenges. Much more than that, the technology provides fit-for-purpose measurements of the subsurface. We present how geological survey objectives dictated a novel broadband single-sensor survey design. We also show how those measurements provided important geological insights that had a material impact on economics and risks, and hence, further development plans for a prospective area in central Australia.
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Onshore Low Frequency Acquisition for Full Waveform Inversion – from Field Trials to State of the Art Production Surveys
Authors G. Baeten, F. ten Kroode, S. Rawahi and S. MahrooqiFollowing a series of field tests, low frequency acquisition and processing and Full Waveform Inversion techniques have been applied successfully in 3D, long offset, Wide Azimuth surveys from the Middle East. The field testing evolved from downhole testing of vibrator sweeps to an extensive high channel count 2D line test. The development of low frequency acquisition techniques has been accompanied by testing and maturation of Full Waveform Inversion techniques, eventually leading to high quality velocity model estimates for state of the art onshore data.
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Is Broadband Land Seismic as Good as Marine Broadband?
Authors M. Denis, M. Denis, V. Brem and F. PradalieTo evaluate the potential of modern land seismic capabilities and further improve the seismic value for the mining industry, AngloGold Ashanti (AGA) has acquired a prospect area of 35 km2. The geological objective of this test is to image the formations above and below the Ventersdorp Control Reef (VCR) and down to the carbon leader reef, for depths ranging from 2.7 to 3.8 km. This test was the opportunity to illustrate how dense broadband seismic can significantly improve land seismic imaging. The benefit of a dense acquisition with a broadband source is illustrated on the AGA prospect. We compare a conventional acquisition (SLI 420m, SI 70m, RLI 300m, RI 50m; 10-90 Hz) with a dense acquisition (SLI 50m, SI 50m, RLI 100m, RI 50m; 10-90 Hz) and a dense acquisition with a broadband source (SLI 50m, SI 50m, RLI 100m, RI 50m; 3-160 Hz). The expectations from dense acquisition are confirmed by an outstanding imaging quality for all depth levels, simultaneously for ultra-shallow and ultra-deep targets. This in turn leads to significantly improved depth velocity models for a simultaneous optimal focalization and positioning of the seismic reflections. This leads to a textured image with unprecedented stratigraphic details available for the interpretation.
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Putting It All Together – Broadband, High-density, Point-receiver Seismic in Practice
Authors J. Quigley, D. Holmes and K. O’ConnellLand seismic technology is advancing rapidly on many fronts. There are challenges in assimilating all the changes and new opportunities. A technical viewpoint may be to analyse each of the component technologies, both acquisition and processing, on their separate merits, but how are we to put it all together in the most technically effective manner, while maintaining cost-effectiveness and, ideally, reducing the overall time to which business decisions are made from the data? Technical development and tighter integration across the full design-to-interpretation workflow should go hand-in-hand and the more we improve our technical capabilities in multiple areas, the greater the imperative to act to integrate those technologies for maximum gain. The presentation will discuss WesternGeco’s several years of practical experience with integrated land seismic acquisition and processing technology delivering high-density, broadband full-azimuth data as a key enabler for improved reservoir characterisation.
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Interpretation of Broadband Land 3D Data from Saudi Arabia
Authors L. Giroldi and B. WallickA broadband, high-density, full azimuth 3D land survey was acquired in 2011 over a producing oil and gas field in the Eastern Province of Saudi Arabia. The survey was designed to test a combination of point receiver sensors and a specially designed low frequency sweep for use in improved reservoir property prediction. Interpretation of this broadband, high density, full azimuth land 3D survey shows the beneficial impact of the increased bandwidth and improved signal- to-noise ratio on the interpretation of seismic data. The recovery of low frequency signals has dramatic implications for seismic inversion, as less reliance can be placed on the initial model. Moreover structural clarity and resolution can be achieved in the interpretation of both shallow and deeper levels. Results from this work indicate that a more accurate representation of the wave field is achieved with this broadband, full azimuth dataset, with the recovery of low frequency signal being one of the most notable aspects. This has a dramatic impact on both the interpretation and the seismic inversion.
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Improving Business Decisions by Evaluating Subsalt Structural Uncertainty and Illumination with Modeling and Re-migration
Authors J. Sinton, P.A. Valasek, J. Zhu, S.K. Towe, S.M. Carlson, T. Kendricks and M. GreenwoodBuilding a salt velocity model is usually an iterative process. Based on seismic character alone there is often a large range of alternative salt models. The iterations can often take on the form of trial-and-error in trying to understand the geologically most likely model. We present a solution to allow integration of geological insight in the velocity model building. The solution consists of a seamless system of flexible model editing and fast migrations. This creates an arena for the geophysicist and the geologist to work together. The crucial component is the interactivity, or the ability to work in the same time frame. This means that several salt models can be created and resulting images analyzed during the course of a working day.
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Subsalt Target-oriented Salt Interpretation in the Gulf of Mexico
By Y. AgnihotriThe conventional method for salt interpretation begins with the search for salt-sediment interfaces, starting with the shallowest depths and progressively moving deeper. This search is typically conducted on intermediate seismic products like sediment flood volumes, salt flood volumes, and overhang sediment and salt flood migrations. However, with this approach, poorly imaged subsalt areas become known only after spending considerable time interpreting intermediate salt features. In this paper, I present a new methodology where a reference salt geometry is obtained early in the salt interpretation process. Having a reference seismic volume helps identify poorly imaged subsalt targets much sooner. A geologic model-based interpretation is performed in these identified areas, and changes in salt geometry are assessed based on their impact on subsalt imaging.
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Houston, We Have a Problem: Geologic Ambiguities Persist on GoM Subsalt Seismic Data
Authors E. Ekstrand and B. Hartled out, and the industry necessarily transitions to less obvious and/or more complex subsalt objectives.Our talk describes three generic subsalt trap styles that comprise a significant portion of remaining play inventory, but which carry increased geologic risks, thereby requiring wholesale improvements in subsalt seismic fidelity if the industry is going to maintain its current favorable success rate: 1) Low relief anticlines situated in the down dip lower slope salt trend. 2) “Bowl weld” truncation traps of the middle to upper slope. 3) “Ribbon truncation closures” that are ubiquitous across the subsalt play fairway. We illustrate the critical velocity sensitivities and imaging challenges associated with each of these trap geometries, and discuss how interpretational ambiguities that endure on current subsalt seismic datasets impede the commercial assessment of these remaining exploration opportunities.
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Interactive and Integrated Salt Model Building
Authors S.-K. Foss, E. Karlsen, A. Osen, M. Rhodes, J. Mispel, D. Michel, N. Kotava, D. Merten, B. Lehnertz and N. EttrichBuilding a salt velocity model is usually an iterative process. Based on seismic character alone there is often a large range of alternative salt models. The iterations can often take on the form of trial-and-error in trying to understand the geologically most likely model. We present a solution to allow integration of geological insight in the velocity model building. The solution consists of a seamless system of flexible model editing and fast migrations. This creates an arena for the geophysicist and the geologist to work together. The crucial component is the interactivity, or the ability to work in the same time frame. This means that several salt models can be created and resulting images analyzed during the course of a working day.
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Keynote 1: Review of the Geology of Deformed Salt Bodies and the Implications for Seismic Imaging
By I. DavisonThis paper reviews new data on the internal structure and external geometry of salt bodies and their interaction with surrounding sedimentary strata, followed by a discussion of teh implications for seismic imaging.
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Workshop Introduction: Pitfalls in Subsalt Imaging
Authors I. Jones and I. DavisonThe physical behaviour of most earth materials is fairly straightforward, both in terms of their deposition and subsequent deformation. Consequently, the geometries to be imaged and interpreted are likewise usually well understood. Salts, however, do not conform to the usual behaviours of earth materials due to the ductile nature of the material. Consequently, in both imaging and interpretation of salt province data, special care needs to be taken. In this work, we review various considerations for velocity model building, migration, and subsequent interpretation of complex salt bodies.
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Advanced Seismic Processing Reveals the Structures Hidden beneath Salt Diapirs on Legacy Data from the Onshore Netherlands
Authors R. Wervelman and J. Pi AlperinBy means of two recent sub-salt imaging projects, using rather old short-cable narrow-azimuth and low-fold seismic surveys acquired two decades plus ago, some new/emerging model-building and imaging technologies will be discussed: - shallow subsurface model-building by 5D data regularization, before migration and by a joint tomographic inversion of deep reflection data and first-break picks; - enhanced structural interpretation of complex salt structures by intermediate RTM’s. The two projects enabled an extensive geophysical comparison between various depth imaging tools: standard and extended Kirchhoff, 1-way WEM and Reverse Time Migration. For one of the projects examining multiple salt-diapir scenarios, depth-imaging using RTM, led to reduced subsurface uncertainties especially regarding the risk of depletion of the target block (towards already producing neighbouring fields to the North and to the East); the imaging project consequently aided plans for a development well planned to spud Q2 2013. In the other project a large reduction in the subsurface uncertainty led to a confident decision not to drill the prospect.
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Challenges in Quantifying Subsalt Structural Uncertainty
Authors K. Osypov, N. Ivanova, Y. Yang, A. Fournier and D. NikolenkoSubsalt deep-water exploration and production (E&P) is one of the most challenging and risky businesses. In particular, drilling in the deep-water environment is becoming increasingly complex and expensive. Uncertainty is inherent in every stage E&P business, and understanding uncertainty enables mitigation of E&P risks. Therefore, quantification of uncertainty is beneficial for decision-making, and uncertainty should be managed along with other aspects of business. For example, decisions on well-positioning subsalt should take into account the structural uncertainties related to non-uniqueness of the velocity model used to create the seismic depth image. Even with our best efforts to combine all available data, there is still intrinsic ambiguity in our velocity models, i.e., multiple feasible models can fit the data equally well. The velocity model ambiguity exacerbates significantly in the presence of salt. The paper discusses the challenges for structural uncertainty analysis for subsalt targets.
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The Impact of Reverse Time Migration on the Evaluation of Mature Southern North Sea Assets
By S. FletcherA case study is presented on the impact of Reverse Time Migration (RTM) on the derivation of a geologically realistic velocity model in a depth imaging sequence applied to a field development in the Southern North Sea. Exceptionally complex salt overhangs in the area make imaging of the Rotliegend target particularly challenging and despite repeated applications of the depth imaging technique since the mid 90’s, with the most recent one carried out in 2008, it has remained challenging to define the reservoir compartmentalisation and reconcile mapped GIIP volumes with well production profiles. The present interpretive processing workflow, with the application of state-of-the-art RTM in the velocity model building stage and a close cooperation of interpreters and depth imagers, has improved the imaging particularly below salt overhangs, yielded a geologically plausible model of the reservoir structure and offered new options for development well placement.
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The Effects of Marine Data Acquisition and Velocity Model Building Practices to Imaging Complex Geological Setting - Modeling Study
By M. CvetkovicInterpreting seismic images in complex geological settings still remains a challenging task. Significant academia and industry effort in the last decade has been devoted to addressing imaging issues in complex marine
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A Decade of Advances in Subsalt Imaging from Exploration to Development on Kaombo
Authors P. Williamson, V. Martin, A. Douillard, L. Lemaistre and V. ClergetAbout two thirds of Block 32, offshore Angola, is covered with allochthonous salt bodies that complexify seismic wave propagation thereby tending to make illumination irregular and accurate imaging difficult. In the early years of exploration of this block, this prevented a good estimation of the resources and their distribution in several subsalt areas. However, in the last decade the industry has seen a dramatic improvement in subsalt seismic imaging capabilities thanks to the significant uplift in illumination offered by wide-azimuth towed-streamer (WATS) data and the improved simulation of seismic wave propagation used in Reverse-Time Migration (RTM), enabled by the increased computing power now available. In this talk we review the evolution of the imaging of Block 32 during this period: we show that, as our images improved, so did our understanding of the geology, which fed back into the model-building, yielding further improvements; and we look at the most recent advances due to full waveform inversion and image post-processing using RTM surface-offset gathers. Cumulatively, these have transformed our vision of, and confidence in, the subsalt reservoirs and allowed the Kaombo multifield development to be launched.
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