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74th EAGE Conference and Exhibition - Workshops
- Conference date: 04 Jun 2012 - 07 Jun 2012
- Location: Copenhagen, Denmark
- ISBN: 978-90-73834-28-6
- Published: 04 July 2012
1 - 100 of 156 results
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A short course in modern inversion techniques - Copenhagen 2012 v2
By N. PillarWith the change in the SEC rules in 2010 regarding the booking of Proven Undeveloped Reserves (PUD) geophysical techniques that can be proven to be reliable and repeatable will have a large potential impact on the valuation of oil companies in the future. One of the more reliable techniques is seismic inversion. In recent years with the improvement in data quality, both seismic acquisition and processing and the use of partial and angle stack data has allowed the conversion of seismic reflectivity data to rock property information, from band limited acoustic impedance to petrophysical properties such as Vshale, porosity and water saturation. The precursor to any inversion is an analysis of the rocks themselves. This is required so we can gain an insight into their possible spatial variation in the area of interest. This comes from analyzing the rock properties using well data. Comparing the trends seen with existing rock physics models, empirical or theoretical, is an important part of the process. This will highlight any differences in a particular formation that might be related to depth, some form of alteration, cementation etc. After fluid substitution plotting the different elastic properties for each fluid case against each other will allow an understanding as to whether discrimination between lithology and or fluid is possible.
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Exploration and Development of the North Sea Chalk Low Permeability System
By Joan MegsonThis talk characterises the North Sea Chalk hydrocarbon play as a low permeability system. The low vertical and lateral permeabilities are responsible for phenomena which include tilted oil-water contacts and non-structural trapping mechanisms, as seen for example in the Halfdan Field. Predicting the petroleum system requires an understanding of hydrocarbon entry points, the permeability within the Chalk at the time of hydrocarbon emplacement, and the factors controlling lateral migration, which occurs at geological time scales. It is this slow lateral migration that results in dynamic and non-structural traps. An additional consequence of low permeabilities is low recovery rates in the absence of significant natural fracturing or induced fractures.
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The Dynamic Chalk Sea: The Depositional Effects of a Collapsed Shelf-break Front, Erosional Contour Current Systems and Dry Borderlands
By Finn SurlykIn the Late Cretacous NW Europe was covered by the deep, epeiric Chalk Sea (e.g. Surlyk et al., 2003). Oceanic conditions spread across the continent as revealed by deposition of up to several kilometres of coccolithic nannofossil ooze of the Chalk Group. This was probably due to the break-down of the oceanic front located at the shelf-slope break due to very high eustatic sea level. The chalk is generally considered as deposited by a gentle rain of coccolith debris in the form of zooplankton pellets or marine snow. Local downslope redeposition by slides, slumps, debris and turbidity flows took place mainly adjacent to major fault zones which were reactivated by inversion tectonics in several episodes in the Late Cretaceous. However, recent work based on reflection seismic data shows that this paradigm is in need of major revision.
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Ecology of Lower Cretaceous Chalk
More LessChalk sediments, that is here biomicrite with a porosity of 10 – 50%, are widespread in the late Cretaceous of the Boreal Realm. Chalk, however, is uncommon in the Early Cretaceous where clays and mudstones dominate for example in the Lower Saxony Basin (LSB), northeast England (Speeton) and elsewhere. The Tuxen and Sola Formations (late Hauterivian – Aptian) of the southern North Sea (Danish Central Graben) represent this unusual Early Cretaceous chalk system.
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Chalk as a Reservoir
More LessReservoir properties of chalk depend on the primary sediment composition as well as on subsequent diagenesis and tectonic events. Chalks of the North Sea almost exclusively have mudstone or wackestone texture. Microfossils may have retained their porosity where degree of diagenesis is low, or be partly or fully cemented where diagenesis is more pronounced. It is a chalk characteristic that permea bility is controlled by the porosity and internal surface of the mud matrix, whereas the larger pores play an insignificant role. Cemented microfossils may take up a significant volume in a wackestone, and the best reservoir properties are typically found in mudstone intervals.
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Facies Change and Sequence Expression Along a Proximal to Distal Transect in the Lower Cretaceous Chalk of the Tuxen and Sola Formations (Danish Sector North Sea Basin)
More LessThe Tuxen and Sola Formations represent the earliest boreal chalk deposits in the North Sea Basin and are oil-bearing in the Danish sector of the Central Graben. These formations vary in thickness at the large scale, and show a decimetre to metre scale bedding pattern in the cored sections. The core display will show key stratigraphic surfaces and a lateral variation in the facies expression, factors that are likely to influence reservoir characteristics.
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Reservoir Facies Variation in the Upper Cretaceous and Danian Chalk of the Danish Central Graben
More LessThe Upper Cretaceous and Danian Chalks in the North Sea contain reservoir facies which are part of a world-class hydrocarbon system. The examples shown highlight some of the paradoxes of these microporous fine-grained, high-porosity, low-permeability carbonate reservoirs. Total porosity in typical chalk reservoirs is in the range of 25% to 35% with the best examples up to 45%. Permeability is typically low at 0.001mD to 10mD, but is enhanced by natural or induced fractures. Most chalk reservoirs have much better porosity than is expected from burial depths. The better than expected preservation of porosity is most likely related to an early onset of overpressure in the chalk. Apparently during burial, the overlying Tertiary mudstones provided a seal through which the pore fluids of the chalk were not able to escape. The microporous nature of the pore network is related to the size of coccoliths, the calcareous nanofossils which constitute at least ninety percent of the rock. The majority of pores are found in between the coccoliths and in coccospheres.
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Upper Campanian − Maastrichtian Holostratigraphy of the Stevns-1 core, Denmark
More LessThe Upper Cretaceous chalk of the Danish Basin has been interpreted as a major contourite complex on the basis of high-resolution seismic data. The sea floor had a pronounced topography with kilometre-wide ridges and valleys up to almost 200 m deep, interpreted to have been formed by contour-parallel bottom currents. Only few ancient contourite systems have been recognised, mainly based on sedimentary facies and only rarely on architecture and morphology. Two cored boreholes, Stevns-1 and -2, 443.3 m and 345 m deep, respectively through the Danish chalk contourite complex offer a unique possibility to compare seismic and sedimentary facies. The contourite chalk is completely bioturbated except for thin intraclast conglomerates and a few thin levels, showing possible primary lamination. In terms of lithology and trace fossils the contourite chalk is similar to horizontally bedded pelagic chalk uninfluenced by bottom currents. Published contourite models cannot normally be used for the chalk due to the very fine grain size, generally complete bioturbation, and lack of any vertical trends in grain size on a millimetre- to centimetre scale. It is thus only rarely possible to document the influence of bottom currents on the basis of facies analysis alone and this can only be inferred by architectural analysis of seismic-scale outcrops.
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The Chalk of the Ekofisk Field, Norwegian North Sea – An Example From the Cores of Well 2/4-X 32
More LessWith original recoverable reserves estimated to be around 717 x 106 m3 o.e., the Ekofisk Field is the largest hydrocarbon discovery within the Upper Cretaceous–Lower Palaeocene chalk successions of the North Sea. In the Ekofisk Field, the reservoir intervals consist of overpressured and naturally fractured chalk sediments belonging to the Maastrichtian Tor and Danian Ekofisk Formation.
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Comparing Core Facies with Image Log Data, a Tool to Better Understand the Fractures Distribution in the Reservoir - An Example from the Wells 2/7-B 9A and 2/7B 7A in the Eldfisk Field, North Sea
More LessThe Eldfisk Field is third largest field in the Norwegian Continental shelf when it comes to remaining resources. The reservoir rocks of the Eldfisk Field consist of overpressured chalk sediments principally belonging to the Maastrichtian Tor Formation and the Danian Ekofisk Formation. The chalk matrix in the Eldfisk field is characterised by high porosity and low permeability, but as with other naturally fractured reservoirs, effective permeability is enhanced by fractures. Thus, mapping of fractures plays an important role in characterization ofthe dynamic behaviour of the reservoir.
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Advanced Microseismic Data Analysis and its Application to Unconventional Reservoir Characterization.
More LessThe current upswing in unconventional reservoir production, what some are calling the “shale gale”, is not a geology play. The tight rocks being turned into producers are millions of years old and have been known to be petroliferous for decades. Rather, this is a technology play. The technologies that have enabled this storm are 3: horizontal drilling, hydraulic fracturing, and microseismic monitoring. Horizontal drilling extends the pay intersection to allow for economic levels of production. Frac’ing creates the connections with the hydrocarbon molecules that give them free pathways to the wellbore and thence to the surface. Microseismic monitoring illuminates and informs as to how well the frac propagated and just what part of the reservoir has been contacted by the frac in a way that predictive models can’t.
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Novel Workflow for Shale Gas Reservoir Characterization
Authors Lars Sonneland, Geir V. Dahl, Martin Haege and Hilde BorgosShales may contain significant volumes of organic matter and constitutes the key source rock for hydrocarbon reservoirs. Today these types of organic rich shales are being economical exploited in situ and have become recognized as unconventional reservoirs. The current reservoir characterization technology has been evolved over decades focusing on sandstone - and carbonate – rocks. This technology is not directly applicable for shale reservoir characterization. One important difference is that permeability and porosity in shales are order of magnitude different from sandstone - and carbonate – rocks, another that we are describing the source - rock it. We will in this paper propose a new procedure for characterization of shale reservoirs. The workflow is highlighted in figure 1. Input data is typically cores, well-logs and surface seismic data in addition to micro-seismic data .The core, well-logs and seismic data are integrated in a hierarchical fashion that allows characterization of key features in the shales. The total organic content (TOC) in the shales is an example of such a feature. Løseth et. al has in a recent paper that acoustic impedance (AI) in organic-rich claystones decreases nonlinearly with increasing TOC percent. Further claystones mixed with low-density organic matter have significant higher intrinsic anisotropy than otherwise similar non-organic claystones.
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Geomechanics for Unconventional Resources
More LessIn this paper, we discuss the effect of geology on the resulting heterogeneous distribution of rock mechanical properties, and use this information to define key drivers of geomechanical problems of well construction and completion design. The paper explores the relationship of rock fabric on anisotropic elastic properties and on anisotropic strength, and provides examples of reasonable values across well known facies of North American organic-rich mudstone plays. The paper proceeds by defining the effect of mechanical anisotropy on nearwellbore stress concentrations, fracture width, breakdown pressure, solid production potential, and minimum horizontal stress. By discussing requirements of near-wellbore completion quality and far-wellbore completion quality and by providing means of evaluating these important sets of properties, we provide an effective workflow for hydraulic fracture design. Finally, the paper discusses important implications of time dependent behavior to organic-rich mudstone plays, including Biot’s poroelastic coefficient and creep. Principal conclusions of this work are that organic-rich mudstone plays are geologic plays, and understanding the geologic drivers of their vertical and lateral heterogeneity in mechanical and reservoir properties is of highest importance for exploration and production. Their content of stiff mineral constituents, compliant organic constituents, and compliant pores has a strong effect on the mechanical behavior of organic-rich mudstones. Thus, differentiating between the stress supporting constituents of the rock, and the non-stress supporting detrital minerals, is of highest importance to characterize their behavior. Confusion of these two constituents usually leads to misrepresentation of the shale facies. Organic-rich mudstones are also strongly anisotropic and their behavior cannot be approximated using isotropic models. For example, the near-wellbore stress concentrations in anisotropic rocks depend on the rock elastic properties. Thus, they are different for different rock types, and result in significantly different stress concentrations, breakdown pressures and fracture widths. As an additional example, changes in breakdown pressures and fracture width are strongly dependent on the ratios of elastic modulii (Eh/Ev and Gv/Eh) in the directions parallel and perpendicular to bedding. In addition, the far-field horizontal stress also depends on the anisotropic elastic rock properties, and thus changes from rock type to rock type. Finally, we found that the Biot’s poro-elastic coefficient in organic-rich mudstones is in the range from 0.2 to 0.6. These are low values and are in line of those expected for stiff rocks.
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Reservoir Simulation for Unconventional Reservoirs - Advances and Future Challenges
More LessDevelopment of unconventional resources is accelerating, and the technology for the recovery of unconventional gas and oil is rapidly advancing, being in part responsible for continuing low price of the hydrocarbon-based energy. The term “unconventional” can include a variety of resources, such as coalbed methane gas, tight and shale gas, CO2 injection based enhanced recovery, thermal recovery (Cyclic steam stimulation (CSS) and Steam/Solvent Gravity Assisted Drainage (SAGD)), gas from hydrates, in-situ coal gasification, etc. Of these, the main commercial impact for oil production comes from the CSS and SAGD operation, and for gas (and condensate) production from tight, and recently shale, plays. Since shale gas reserves are geographically more pervasive compared to heavy oil and bitumen deposits, this presentation will deal primarily with the unconventional gas resources and the methods for their simulation analysis and recovery optimization. While commercial applications of tight gas date back to 1970’s, continued improvements in exploration/imaging tools, horizontal drilling, and advances in reservoir engineering interpretation techniques in the following decades have made tight gas a significant part of the current North American gas production. Kuuskraa (2006) estimated that in 1994, unconventional gas (including tight gas, CBM and shale gas) already accounted for 40% of US natural gas production. Recently the multi-stage fracturing technology for horizontal wells spurred unprecedented increase of production from shale gas and oil deposits in North America and today the trend is expanding worldwide.
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Developments in Strategic Decision-Making for Water Management in Unconventional Gas Plays
More LessEnvironmental issues in unconventional gas plays are dominated by the water supply chain, including supply, competition with other water uses, transportation, storage, reuse, recycling, treatment, and disposal. The limiting factor within the supply chain varies from play to play, so no single solution is the right one for every geographic location. Application of well-reasoned decision-making tools that assess and quantify the risk-benefit comparison between water management options throughout the water supply chain, results in more cost effective solutions for operators, that are at the same time compliant with regulations and minimize impact on the local community and environment. Simulated case studies will be used to demonstrate decision-making tools ranging from regulatory register risk evaluation to triple bottom line modelling to incorporate social and sustainability aspects of technology selection.
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DS3 Recording for Faster Seismic Acquisition Rate Without Data Compromise
More LessThe main challenge for seismic data acquisition today is to design a survey which accurately images the subsurface in line and cross line, despite the presence of a variety of noise contaminants. This challenge must encapsulate the raising of data quality and the provision of adequate seismic areal coverage, whilst keeping unit acquisition costs low and ensuring no harm to people or the environment. Recent developments in acquisition technologies, like simultaneous recording (Bouska, 2008), and dramatically increased channel counts, help to facilitate high productivity and thus allow the acquisition of properly sampled data in both source and receiver domains, over large areas with fast turn-around.
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Broadband Full-Azimuth 3D Land Seismic Acquisition - Managing 100,000 Channels, Over 20,000 Vibrator Source Points, and 24 Terabytes of Uncorrelated Data per Day
Authors Peter I. Pecholcs and John QuigleyOn land, broadband simultaneous vibroseis acquisition methods and high-channel count recording systems provide key technologies for the acquisition of full-azimuth 3D seismic surveys. This paper focuses on the experience gained in the acquisition and processing of two high density source (1,280 and 3,200 VPs/km2) and receiver (640 stations/km2) surveys, and reviews a new high-productivity field test for optimal de-noising.
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Acquisition and Processing of Dithered Vibroseis Data: A Case History
Authors Claudio Bagaini, Mark Daly and Ian MooreWe present a 2D and a 3D case history of acquisition and processing based on a simultaneous vibroseis acquisition technique we call dithered vibroseis acquisition. This technique is designed to achieve high productivity whilst maintaining the prestack amplitude fidelity required for beyondimaging applications of the seismic exploration method. A land seismic acquisition system with a very large channel count was used to acquire the data for the 3D example. We compare the processing results obtained with conventionally acquired datasets and those obtained with the dithered datasets using three different processing routes. We observe that the poststack results are not substantially deteriorated using this high productivity acquisition technique, particularly in the case of the 3D example, even if the standard processing route is adopted. However, the prestack separated data are significantly better if the processing route that includes algorithms specifically designed for a dithered dataset is adopted.
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Simultaneous Shooting, Today and Tomorrow
Authors Jim Keggin and Ray AbmaSince 2006 in North Africa and the Middle East, BP has faced the problem of having to evaluate and meet drilling commitments in several very large onshore exploration licences within the relatively short timeframes set by the terms of exploration license agreements. Historically, time and cost considerations would have precluded the use of 3D and large exploration areas would have been evaluated using 2D data. Our goal was to acquire simultaneous seismic source data as good as or better than conventional data while reducing the cost and time of the operation. Moving from conventional to simultaneous source acquisition has required significant changes to the way we think about seismic acquisition. For example, our usual approach of discontinuous recording of shots separated in time needs to be replaced with continuous recording with careful scheduling and recording of the location and times of all shots (Howe, et al. 2008). Reduced field effort is offset by increased processing effort and computational costs, and new processing approaches and mathematical techniques, especially those of sparse inversion and compressive sensing, will become more important in dealing with simultaneous source data. Between 2008 and 2012, BP acquired around 25,000 square kilometers of high fold wide azimuth 3D in the desert regions of North Africa and the Middle East. In every case, simultaneous source technology resulted in better data at less cost. Furthermore, less time in the field has meant a significant reduction in HSE exposure. Whilst the bulk of these wide azimuth onshore data had fold in the 500-1500 range, high channel-count crews and simultaneous source techniques now make it possible to acquire super-dense 3D data with regular ‘carpets’ of sources and receivers on a 50m*50m grid (or less). This approach resulting in 20,000 fold data, has been tested over a small focussed area for field development purposes (Howe, et al. 2009). Onshore success naturally suggests the potential for application offshore. Certain challenges need to be overcome before simultaneous source acquisition becomes more than a simple compromise between cost and quality. These new methods of acquisition have and will continue to require modifications to traditional methods of acquiring and processing seismic data.
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Lessons Learned from Commercialization of Marine Simultaneous Sources
More LessSimultaneous source acquisition in marine environments has been much slower to develop than it has for land. Land has several key enablers not found in the marine environment such as a relative ease of adding extra sources, use of complex source signatures as well as a history of related highproductivity acquisition schemes. Marine acquisition also has a long history now of simultaneoussource studies but has only recently seen the first fully commercial marine simultaneous source survey acquired. This paper will examine the key issues, barriers and solutions that have led to this significant step and speculate on future simultaneous source marine activity.
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A Synthetic Simultaneous Source Wide-azimuth Acquisitions Study over a Complex Marine Environment
Marine blended source acquisition is becoming increasingly important in the seismic industry due to the possibility of reducing costs through higher productivity and improving seismic data quality through denser source sampling. One major drawback of marine blended acquisition is the crosstalk noise generated by the nearly simultaneous firing of the source arrays. It is essential to understand the characteristics of this type of noise interference and identify proper acquisition techniques and processing workflows to reduce its effects on image quality. We start by 3D finite difference modelling of a complex subsurface. We then combine the synthetic shots to simulate a four boat, wide azimuth (WAZ) marine seismic survey comprised of two streamer vessels with sources, and two additional source vessels located between the streamer spreads and off the tails of the streamers. All four sources fire nearly simultaneously with a randomized time lag of up to 500 milliseconds between sources. Overall, the gain from near-simultaneous source firing versus a conventional four-vessel WAZ marine design is an increase of approximately 2.67 times in terms of both source density and fold. This type of blended acquisition survey can also be acquired in about the same amount of acquisition time as a conventional four vessel WAZ survey. In general, most of the processing techniques for the simultaneous source blended data rely on the fact that crosstalk noise exhibits coherency in the shot domain, but appears random when viewed in a different data domain such as common channel, offset and midpoint domains. A number of processing techniques are applied in order to optimally deblend the data. These techniques are tested in several different domains and also in a cascaded manner. The results and effectiveness of each technique is evaluated and compared against the original non-blended synthetic data. Direct comparisons between the processed blended data and the single source non-blended data reveal comparable seismic images in both the prestack and post-stack domains.
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Compressive Sensing in Marine Acquisition and Beyond
Authors Felix J. Herrmann and Haneet WasonSimultaneous-source marine acquisition is an example of compressive sensing where acquisition with a single vessel is replaced by simultaneous acquisition by multiple vessels with sources that fire at randomly dithered times. By identifying simultaneous acquisition as compressive sensing, we are able to design acquisitions that favour recovery by sparsity promotion. Compared to conventional processing that yields estimates for sequential data, sparse recovery leads to significantly improved results for simultaneous data volumes that are collected in shorter times. These improvements are the result of proper design of the acquisition, selection of the appropriate transform domain, and solution of the recovery problem by sparsity promotion. During this talk, we will show how these design principles can be applied to marine acquisition and to other problems in exploration seismology that can benefit from compressive sensing.
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Seismic Full-waveform Inversion Using a Source-receiver Compression Scheme
Authors Aria Abubakar, Tarek M. Habashy and Guangdong PanIn seismic full-waveform inversion (FWI), one usually deals with a large size data set. This is one of the major bottlenecks for inversion, particularly if a significant part of this data set is lacking sensitivity (redundant) to the unknown model parameters. Such redundancy is usually the result of collecting data employing a non-optimally designed survey. A large number of sources in the survey contributes to a large computational cost in running the forward simulator a number of times corresponding to the number of these sources. On the other hand, a large number of receivers contributes to the computational cost of constructing the Jacobian matrix (the derivative of the simulated data with respect to the unknown parameters), as well as in inverting the Hessian matrix in a Newton-type inversion approach. To deal with these issues, the simultaneous-source encoded FWI approach was proposed to reduce the number of sources used in the inversion, see Krebs et al. (2009). In this approach, a large number of physical sources are converted into one simultaneous source or several simultaneous sources by summing the individual physical sources using a phase-encoding technique. Another approach to reduce the computational time and memory usage of an inversion algorithm is the so-called source-receiver compression scheme, see Habashy et al. (2010). This approach constructs compressed simultaneous source and receiver arrays that have minimum redundancy and maximum sensitivity to the unknown model parameters. The synthesized simultaneous source array has a reduced number of sources that decreases the number of forward model simulations required to carry out the inversion. In addition, the synthesized simultaneous receiver array has a reduced number of receivers that further decreases the size of the Jacobian matrix. Hence, this compression approach significantly reduces the computational time and memory usage of any inversion method. Moreover, because this approach removes the small eigenvalues in the data, the effects of noise are also suppressed. In this paper, we apply the source-receiver compression approach for solving three-dimensional (3D) acoustic FWI for obtaining the compressional (P-wave) velocity. The forward problem is formulated using a frequency-domain finite-difference (FDFD) approach with fourth-order accuracy. For the inversion method, we employ the Gauss-Newton framework described in Habashy and Abubakar (2004) combined with the multiplicative-regularization technique described in van den Berg and Abubakar (2001). As a demonstration, we show inversion results of 3D SEG/EAGE salt model.
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Multisource Least-squares Migration of Marine Streamer with Frequency-division Encoding
Authors Yunsong Huang and Gerard T. SchusterThe computational cost of conventional migration can be reduced by multisource migration of supergathers, and the accompanying crosstalk noise can be reduced through least-squares inversion. This approach is not directly applicable to marine streamer data, however, due to the mismatch between the limited number of live traces/shot recorded in the field and the pervasive number of traces generated by computer simulation. This leads to a strong mismatch in the misfit function, and results in strong artifacts (crosstalk) in the migration image. To eliminate this noise, we present a frequency division multiplexing (FDM) strategy with iterative least-squares migration (ILSM) of supergathers. The key idea is, at each ILSM iteration, to assign a unique frequency band to each shot gather. This assignment ensures that there is no spectral overlap among shots, and therefore their crosstalk is zero. Our results in applying this method to 2D marine data for the SEG/EAGE salt model show better resolved images than standard migration computed at about 1/10 the cost. We conclude that multisource migration for a marine geometry can be successfully achieved by a frequencydivision encoding strategy, as long as crosstalk-prone sources are segregated in their spectral content. This is both the strength and the potential limitation of this method.
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Analysis of a Coherency-constrained Inversion for the Separation of Blended Data: Discovering the Leakage Subspace
Authors Kenneth Bube, Panagiotis Doulgeris, Gerrit Blacquière and Gary HampsonSo-called blended or simultaneous source techniques are interesting because they offer the potential for better acquisition sampling, improved illumination and greatly enhanced computational efficiency. The procedure of retrieving data as if they were acquired in the conventional way is called deblending. Several inversion techniques have been proposed for solving this ill-posed problem with the iterative estimation and subtraction algorithm being one of them. This particular method uses a dedicated iteration that integrates a coherency-pass filter. However, fundamental questions regarding the convergence and limitations of this method have not previously been addressed. The notion of the leakage subspace is hereby introduced to aid in this quest.
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Separation of Multiple Interfering Sources in Wireless Communications from a Geophysical Perspective
More LessRedundancy in array processing enables the separation of multiple correlated signals. In wireless communications, the adoption of multiple antennas has increased the spectrum efficiency by enabling the transmission of high data-rate over limited spectrum allocations. Over the last decade, one of the most disruptive innovation in wireless communications has been the revisiting of multiple antennas communications as Multiple Input Multiple Output (MIMO) system (Paulraj et al, 2004). There is such a strong parallelism between the array processing adopted in wireless communication systems and seismic acquisition and processing that in depth investigations of the communalities can only be beneficial. Seismic acquisition for simultaneous sources can be modelled as MIMO system when accounting signals from multiple receivers. In this paper we introduce the MIMO modelling and processing algorithms to show that MIMO processing tools can be transferred and tailored for acquisitions of simultaneous sources with minimal efforts in the model abstractions.
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Submarine Slope Instabilities – Key Questions and Natural Field Laboratories
More Less, impact, risk assessment). Following a brief introduction on the – typical – three-phase landslide development, we address a number of issues at stake (e.g., gas, hydrate, excess pore pressure) that should be addressed in more detail in future research activities within this field. These include in situ measurements, but also the advanced use of geophysical methods to derive soil properties in the shallow sub-surface. Finally, we highlight activities conducted to develop the Finneidfjord area (northern Norway) as a natural field laboratory for submarine landslide investigations. When it comes to smaller-scale landslides, high lateral and vertical resolution is paramount to understand such landslides, which still can have devastating consequences.
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Seismic Characterization of Landslides and the Emergence of Seismic Monitoring
Authors Stphane Garambois, Denis Jongmans and Agns HelmstetterAfter a short introduction on the main problems that need to be better understood in order to better model landslide dynamics, this contribution focuses on how geophysical methods and in particular seismic methods can help in better characterizing the geometry and heterogeneities of landslides. Indeed, these parameters constitute fundamental inputs that need to be precisely addressed before any numerical modelling attempt. We will notably show the sensitivity of shear wave velocity to the variation of shear strength in soft landslide. In a second part, a focus is done towards the emergence of seismic monitoring methods which are now used for different purposes. They include seismic activity assessment and characteristics (location, magnitude, classification), which permit to study the correlation of this activity with other observables (meteorological, displacements) in order to better understand hydro-mechanical coupling at a large scale on landslides. Seismic observatories also enable characterizing the dynamics properties of seismic site effects due to the presence of disturbed materials. Again, this property must be taken into account in any numerical attempt that aims at evaluating the potential sensitivity of the landslide stability to different earthquake scenario. Finally, the use of correlation of continuous ambient seismic noise recorded by at least two different sensors can permit to detect very weak shear wave velocity changes between these sensors. Strong variations of shear wave velocity might be a precursor to rupture.
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Geotechnical Characterisation of Submarine Slope Movements, Example from the Area off the Nice Airport (SE France)
Authors Sbastien Garziglia and Nabil SultanIn order to present the geotechnical approaches and tools developed and deployed in IFREMER, light is shed on the shelf and upper slope off the Nice airport. The complex problem of submarine mass movements is split into two distinct aspects that are the mechanical behaviour of sediments and the movement stages. It is hence shown that the area off the Nice airport remains predisposed to failure due to fresh water infiltration and gas occurrence in the sediments. The steep continental slope is recognised as a factor predisposing both failure and subsequently large and rapid movements of sediments. Given the high probability for a 30-m thick slide to occur due to the occurrence of shear zones, conclusions point toward the need to deploy an array of sensors for the monitoring of changes in pore pressures and potential pre-failure movements.
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Electrical Imaging for Landslide Monitoring: State-of-the-art and Future Challenges
Authors Vincenzo Lapenna, Angela Perrone and Sabatino PiscitelliIn this paper we briefly resume the current start-of-the-art of the electrical imaging techniques for monitoring hydro-geological instability phenomena. An overview of the more interesting results obtained in the Southern-Apennine chain (Italy), that is one of the Mediterranean areas affected by large and diffuse landslide events, is presented and discussed. Furthermore, we focus the attention on novel and challenging applications of the electrical imaging. To-date, combining robust techniques for tomographic data inversion and new distributed sensor network for field data acquisition is possible to obtain 3D high-resolution electrical images and time-lapse 4D tomographies able to follow the spatial and temporal dynamics of electrical parameters (i.e., resistivity, self-potential) in landslide bodies.
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Marine Drilling/Construction Hazard Site Investigations - Current Practices and Techniques in the Oil E&P Industry
More LessThis presentation will provide an overview of the current practices and techniques used in the oil exploration and production (E&P) industry for mapping and assessment of constraints and hazards to marine drilling and construction operations. This presentation will also provide an overview of the various types of hazards and the equipment and techniques that are used to identify and map them and include a brief overview of industry standards and guidelines for drilling and construction site investigations.
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The Role of Sediment Composition in the Flow Behaviour of Submarine Landslides
More LessLaboratory experiments on subaqueous mudflows at St. Anthony Falls Laboratory, the University of Oslo and NGI/ICG showed that the flow regime undergoes a remarkable transition as the slurry’s cohesion is varied. Flows of highly cohesive slurries exhibit plug flow in the head, hydroplaning, and rapid stretching, but create only a feeble turbidity current. Flows with low clay content generate a turbulent head and a pronounced turbidity current. Their body is fluidized, with the sand fraction settling out progressively. A simple model indicates that low-to-intermediate cohesion mudflows are a candidate for explaining extended and massive deposits of clean sand that have been found in abyssal plains far from their possible origin. The viability of this proposal depends crucially on the scaling properties of the system, which are discussed in some detail. Adapting the definition of the Reynolds number to flows of cohesive materials, the impact forces of debris flows on structures like pipelines can thus be described in a simpler and more universal way than hitherto, elegantly combining aspects of solid and fluid behaviour. Time permitting, the implications for numerical modelling of such flows and the reasons for the striking difference with sub-aerial debris flows will be briefly touched upon.
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Assessment of Unstable Slopes in Mountain Regions Using Geophysical Methods
Authors Hansruedi Maurer, Christian Hauck and Thomas SpillmannAs transport routes, power networks, communication lines and human habitats expand in mountain valleys, the risks associated with landslides are growing on a yearly basis. To minimize these risks, there is a need to identify and characterise mountain slopes that are susceptible to failure. We assess the potential and limitations of geophysical techniques for investigating potentially unstable mountain slopes. Our assessments are based on two case studies involving mountain permafrost in form of a rock glacier and a steep rock slope well below the occurrence of permafrost. The internal structure of the rock glacier is delineated with a combination of seismic refraction tomography, geoelectrical tomography and ground penetrating radar measurements, whereas the steep rock slope is characterized with 3D seismic and ground penetrating radar measurements in combination with microseismic investigations.
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Seismic Full-waveform Inversion and Data Fusion on Clayey Landslides
Authors Gilles Grandjean, Anouard Romdhane and Adnand BitriFrom numerous studies, geophysical methods based on seismic surveying appear to be well-adapted to investigate the morpho-structure of landslides and to progress in understanding the related mechanisms. Indeed, these methods allow direct and non-intrusive measurements of acoustic (Vp) or shear (Vs) wave velocity, two important physical parameters for estimating mechanical properties of reworked moving materials. Different processing techniques and inversion strategies were applied on the La Valette and Super-Sauze mudslides (French South Alps) as well as on the Ballandaz landslide (Savoie, France) to retrieve these parameters. On each of these sites, measurements were recorded along 2D profiles of several hundred meters length, with sensor spacing from 2 to 5m (of about few meters). A first approach, based on first breaks acoustic inversion for estimating Vp distribution on the Super-Sauze and La Valette sites was carried out; then, SASW (spectral analysis of surface waves) was performed to image Vs distribution on the same site. In order to produce a more geotechnical diagnosis of these sections, a fuzzy logic fusion was used to assimilate both of these parameters into a highest level of interpretation. This approach has (also) the advantage to take into account the resolution and accuracy of each individual method. Finally, a 2D elastic full-waveform inversion test was carried out on a synthetic seismic data set computed from a conceptual Super-Sauze velocity model. This test highlights the difficulty to manage highly contrasted media in terms of velocity but also of topography. Nevertheless, the integration in the inversion process of the whole seismic signal produce a more coherent model in terms of Vp and Vs distribution compared to abovecited conventional techniques.
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Characterization of Clayey Landslides Using Multidisciplinary and Multiscale Dataset
Authors Grgory Bivre and Denis JongmansSlow-moving landslides frequently affect gentle slopes made of clayey formations, with volumes which can range from a few m3 to several tens of millions of m3. These landslides frequently exhibit sudden acceleration phases and flows, which can be triggered by changes in the stress field or modifications in the soil characteristics. We present here an integration of multidisciplinary and multiscale data. The combined interpretation of continuous geophysical data acquired over the whole Avignonet landslide (OMIV observatory on landslides in the French western Alps) with punctual measurements (boreholes and geotechnical tests) allowed building a robust 3D morpho-structural model of the slide down to a depth of around 300 m. It has been possible to characterize the main lithological features along with the location of slip surfaces. We propose here that, in complement with shear-wave velocity which gives information on the mechanical effect of the slide, resistivity variation observed at sub-surface can be linked to grain size variation which seems to partially control the landslide kinematics via the regulation of overpressure. Combining geophysical data allows to take into account both lithological variations and landslide activity to account for surface displacements.
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Quantifying the Effect of Fluids and Mechanical Weakening of Fractures and the Implications for the Rupture of Large Landslides
Authors Benoit Derode, Frdric Cappa and Yves GuglielmiLandslides are sensitive to fluid pressures generated by rainfall and snowmelt. Correlations between landslides accelerations and seasonal infiltrations have been widely observed, and models have shown that fluid pressurization in fractures is a dominant driving factor. Nevertheless the precise mechanisms relating fluid pressures and rupture are poorly constrained. Here, we quantified in-situ the effect of fluids and strength loss related to pressure increase and deformation in fractures. Then, using hydromechanical modeling, we analyzed these data and explained the evolution of permeability with deformation. Finally, based on these novel observations, we used improved hydromechanical simulations to model the behaviour of the Schilienne landslide in France.
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Geophysical and Geotechnical Investigation of a Complex Offshore Foundation Ground Affected by Glacial/Postglacial Sedimentation Processes (German North Sea)
Authors Wenfang Fan, Hanno Keil, Stefan Kreiter, Volkhard Spiess and Daniel Hepp and Tobias MrzThe complexity of the North Sea sediments resulting from the Quaternary glacial history requires a thorough assessment of the local depositional environment to determine appropriate foundation dimensions or to even exclude specific areas from construction. In this study geophysical and geotechnical methods were combined to assess a potential offshore foundation ground in the southern German North Sea. Due to a dense net of multichannel seismic lines the very heterogeneous subsurface structures were mapped and prominent glacial buried valleys were identified in the study area. Geotechnical methods applied at selected locations were used to investigate the bearing capabilities of the soil. The results show, that the infill types of glacial buried valleys have to be taken into account regarding the suitability of foundation ground.
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Monitoring the knes Rock Instability, Norway
Authors Lene Kristensen and Lars Harald BlikraThe knes rock instability at Storfjorden, western Norway, has the potential to form a major rock avalanche and create a tsunami with devastating impact to the settlements around the fjord. The most feasible mitigation measure to this hazard is monitoring as large rock avalanches are usually forecasted by an acceleration of displacement. The knes/Tafjord Early Warning Centre is responsible for monitoring of knes rock instability. Here we present the monitoring concepts, measurements of displacement and the early warning system.
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A Monitoring System for Time-lapse Electrical Resistivity Tomography (ERT) and Time Domain Reflectometry (TDR) Measurements
More LessA permanent monitoring system for time–lapse acquisition of 2D Electrical Resistivity Tomography (ERT) and Time Domain Reflectometry (TDR) measurements was installed in a test site affected by a landslide in Basilicata region (southern Italy). Time-lapse acquisition allows us to monitor in real time the variation of resistivity values in the first layers of the subsoil, closely connected with variation of the water content and potentially associated to the reactivation of the landslide. A weather station installed in the test site gives information about the rainfall intensity and frequency and suggests the acquisition time interval. Resistivity trend analysis was carried out in different ways: at first acquiring many sets of data that were inverted by the software RES2DINV, in order to obtain a series of ERT representative of the resistivity distribution in the subsoil during different periods of the year; then, considering resistivity data coupled with TDR ones and rainfall, realizing a statistical study in the time and space. Statistical study has been made by using a MATLAB script specifically created for this work. The installed system and the results are presented.
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Geophysical Characterization of the Furggwanghorn Rock Glacier, Switzerland
Authors Kaspar Merz, Lasse Rabenstein, Thomas Buchli and Hansruedi MaurerDegradation of alpine permafrost due to changing mean annual air temperatures can act as trigger for landslides and other ground instabilities. For a better understanding of the underlying thermo-hydromechanical processes an interdisciplinary research project has been set up. An extensive geophysical and monitoring campaign was carried out on rock glacier in the Turtmann valley, Canton Valais, Switzerland over the last two years to investigate its internal structure. We employed seismic refraction tomography, electrical resistivity tomography and ground-penetrating radar. Additionally seven boreholes were drilled to a depth of 25m and equipped with temperature sensors and inclinometers. Results from the seismic tomography show a lateral very heterogeneous zone below an active layer of 3-4m thickness. The bedrock depth could not be detected over large parts of the profiles. On the electrical tomograms we can clearly distinguish between ice-free zones at the front and the flanks of the rock glacier and an ice-rich zone in the central part. Several internal shear horizons could be identified on the radar profiles. Most of them could be tracked over several profiles. Deformation measurements in a nearby borehole show that the horizon at about 15m depth is currently active.
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Quick-clay Landslide-prone Grounds in Norway and Sweden:A Complex Problem Requiring a Combined Geophysical and Geotechnical Approach
Quick-clay sliding occurs in formerly glaciated coastal areas in, e.g., Norway, Sweden and Canada. The soil was originally deposited in shallow marine environments which emerged following isostatic rebound and fall of the relative sea level since the last glacial maximum. Long-term leaching of salt, due to groundwater flow and percolating surface water, affects clay-particles bonding and makes the soil highly susceptible to failure when disturbed. We review the properties of quick-clays in order to define a suitable, integrated and multi-disciplinary approach to improve identification and mapping of quick-clay areas. Though electrical resistivity tomography is actually the geophysical method of choice, it is paramount to combine a range of geophysical and geotechnical approaches for a better assessment of a given quick-clay site. The discussed integrated approach is here presented for 2 Norwegian and 1 Swedish quick-clay sites. The collected data and preliminary site characterization will illustrate the high diversity of quick-clay grounds as well as the complexity related to an integrated approach.
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A Multidisciplinary Study of Near-shore Landslides in the Trondheim Harbour, Mid Norway
The geological and historical record in the bay of Trondheim, mid Norway, illustrates that landslides are recurrent phenomena. Recent and ongoing development of the area, including land reclamation and extension of harbour facilities, have increased concerns about the stability of the shoreline slopes and highlighted the need for better understanding of these mass-movement processes. Herein, we summarize previous and ongoing work in the area that shed new light on the origin and development of mass wasting processes in the bay of Trondheim. Intergration of geotechnical and geophysical data (including shear wave reflection profiling) from both on- and off-shore shows that the presence of softer and more sensitive laminated clay-rich beds facilitates translational slope failure, by acting as slip planes. Additional pre-conditioning factors promoting instability include the loading of weaker clay-rich beds by delta progradation, over-steepening from erosion and/or sediment accumulation and artesian groundwater pressure at different stratigraphic levels. For the more recent landslides, anthropogenic factors like embankment fillings and vibrations from construction work are considered the most important triggering mechanisms. Finally, the results presented illustrate the importance of detailed morphological analyses, combined with a geological model including the physical/geotechnical characteristics of sediments on- and off-shore, in order to perform proper shoreline slope stability assessment.
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Ray Based Approaches for Velocity Model Building: Past Present and Future
More LessBy mid 2000’s emergence of full waveform inversion (Virieux and Operto, 2009) has somehow occulted the progresses and perspectives of ray based velocity model building. Whatever by now velocity model building in industry remains still largely done using ray based approaches. Moreover, if breakthroughs have not been strongly put forwards for these techniques, they provide now solutions with unexpected resolution and accuracy. My aim here is to review this approaches and to investigate their future. The success and power of ray based approaches is strongly connected to a theoretical and numerical frame, which has been built around ray theory, and which offers a so powerful frame for the physical understanding of velocity model building. I first recall them before reviewing the various ray based velocity model building tools. Finally I discuss their connection with full waveform inversion approaches and discuss the perspectives.
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Objective Functions for Full Waveform Inversion
More LessSeismic full waveform inversion as conventionally formulated imposes very strict constraints on data acquisition: high signal-to-noise at very low frequencies and/or very long offsets. In this workshop presentation, I will explain how some of these constraints arise in the mathematics of wave propagation, and describe some alternative optimization formulations of seismic inversion, which may allow fitting of less constrained data. The extreme heterogeneity of the earth’s sedimentary crust still poses many unresolved challenges for inversion.
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Challenges and Value of Applying FWI to Depth Imaging Projects
Authors Laurent Sirgue, Bertrand Denel and Fuchun Gaol Waveform Inversion (FWI) has now been established a few decades ago (Lailly 1983; Tarantola 1984; Pratt et al. 1996). Until recently however, the application of the technique on full scale 3D seismic was made impossible due to the computational requirements of modeling thousands of shot. Recent advances in high performance computers along with the development of efficient 3D modeling algorithms has lead to the first successfull full scale application on field data (Plessix et al. 2009, Sirgue et al. 2009, Vigh and Star 2009). However, the added value of performing FWI to Prestack Depth Imaging Projects is yet to be clearly defined by the industry. This task is particularly challenging since the type of seismic acquisition largely varies from one asset to the other; along with the geophysical environment (presence of salt, water depth etc…). In this workshop, we will present a wide range of application of FWI to streamer data in both deep offshore and shallow water environments. We will show that FWI may be applied to a wide range of seismic data: from an isotropic application in the Caspian Sea to more complex application such as deep offshore of Gulf of Guinea.
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Full-waveform Inversion Aided Depth Imaging
By Denes VighFull-waveform inversion based on the finite difference approach was originally introduced in the time–space domain (e.g.Tarantola, 1984, Pica et al., 1990, Sun & McMechan, 1992). Inversion can also be implemented in the frequency-domain (Pratt et al., 1998, 1999, Ben-Hadj-ali et al., 2008). The advancement in hardware over the past few years has allowed us to execute 3D FWI on real datasets in marine (Plessix,2009, Sirgue at al.,2009,Vigh et al.,2009,2010) and in land (Plessix,2010) environments. They demonstrate that FWI can be used for velocity updates if the acquired data has adequate low frequency signal and long offsets. The shallow section of the model could be significantly enhanced by the use of FWI, which can result in a much improved depth image over all. One of the challenges with FWI is converging to the local minima, which makes the technique very sensitive to the starting velocity model especially when 3D is considered. To lessen the sensitivity of the initial velocity field, the implementation of FWI at low frequencies using long offsets is required (Bunks et al., 1995, Pratt, 1999).
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A Priori Model Estimation for FWI from Constrained Kinematic Inverse Problem
Authors Christophe Barnes and Marwan ChararaExploration inverse methods such as Monte Carlo could provide the general solution for the fullwaveform inversion (FWI) inverse problem allowing incorporating complex a priori information and data with arbitrary noise distributions. Such an approach applied directly to the FWI problem is computationally not tractable. The alternative is to solve the highly non-linear part of the problem with a fast forward problem such as traveltime with a constrained random exploration. The obtained a priori models and uncertainties help greatly in solving the FWI. This approach was successfully applied to an offset vertical seismic profile (OVSP) case.
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Investigating the Differential Waveform Inversion
Authors Herve Chauris and Ren-douard PlessixIn the context of velocity estimation, we propose the Differential Waveform Inversion approach defined in the data domain. For reflected data, it first consists of migrating a single shot. From the updated velocity model, we then compute the data at the next shot position and compare it with the observed data. The minimization of the misfit appears to be equivalent to the Differential Semblance Optimization approach formulated in the depth migrated domain, at least for reflected data.
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Traveltime Inversion, an Integration Tool in Complex Structures: Lessons from Case Studies in Thrust Belt Setting
Authors Anne Jardin and Karine BrotoWe have developed a traveltime tomography software whose flexibility enables to tackle complex subsurfaces. Applications of this software in thrust belt setting are presented. The role of geology and seismic data integration to validate the final model is analyzed.
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Sensitivity Analysis of Joint Diving Wave+Reflection Tomography in Anisotropic Media
Authors Jacopo Panizzardi and Nicola BienatiIt is more and more evident that the transmitted component of the wavefield recorded by reflection acquisitions does contain precious information about velocity. Indeed, the results obtained from firstarrival traveltime tomography, and in general those obtained by the various works recently presented on Full Waveform Inversion are motivating an increasing interest for transmitted waves, i.e. refractions and diving waves.
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Wavelet Estimation and Multiple Modeling in Full-Waveform Inversion
Authors Ivan Chikichev, Ke Wang and Spyros LazaratosFull-waveform inversion (FWI) has the potential to extract information not only from primary reflections, but also from multiples. We show that accurate modeling of multiples provides strong constraints on the amplitude, frequency spectrum and phase of the seismic wavelet. Thus the method presented here leads to a very robust estimation of the wavelet without relying upon well control. As a consequence, it is applicable to and could be particularly beneficial to the early stages of exploration.
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Resolution in Seismic Inversion- Spectral Gap or Spectral Overlap, Which is Harder to Handle?
By Dave NicholsEarly methods that combined kinematic inversion with amplitude inversion have a gap in the resolved wavenumbers between those resolved by the amplitudes of the scattered field and those resolved by the kinematics. This gap required us to add extra constraints to the problem to recover a model that spans the full wavenumber spectrum. Three advances have brought us to a situation where that gap is often closed. 1) Modern seismic acquisition techniques with lower frequency sources and wider offsets have broadened the spectrum of wavenumbers resolved by single scattering inversions. 2) Full wave inversion using the two way wave equation data provides an accurate treatment of multiply scattered data and can accurately treat the low frequency part of the spectrum. 3) Improved tomography techniques now provide kinematic inversions that resolve more detail of the smooth background. This closing of the gap provides us with a new challenge. We have multiple measurements contributing to resolution of the same parts of the model. We must be careful to combine them in a way that honours the accuracy of each of the measurements.
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Waveform Inversion Using Blocky Parameterization in the Laplace Domain
Authors Changsoo Shin and Hong LeeThe Laplace-domain waveform inversion yields realistic smooth velocity models. By exploiting the inversion’s merits, we developed a cost-effective inversion algorithm by reducing the number of inversion parameters. To develop the idea, we adopted the blocky parameterization method. Then, we applied the Gauss-Newton method combined with the CGLS method to accelerate and stabilize the convergence process. Through numerical tests, we confirmed that the simultaneous inversion of both velocity and interface was feasible, and the smoothed inversion results are comparable to those of the conventional Laplace-domain inversion. The resolution of the inverted velocity models depends on the block size. Therefore, the block size should be carefully determined by tessellating the rough subsurface structure. In our tests on synthetic and field data, the total number of inversion parameters was reduced to less than one-hundredth of the conventional Laplace-domain inversion. The proposed algorithm maintains the robustness of the Laplace domain inversion, and the results are acceptable for use as initial velocity models for consequent inversions, such as the frequency-domain waveform inversion.
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Challenges in the Full Waveform Inversion Regarding Data, Model and Optimisation
Authors Jean Virieux, Romain Brossier, Ludovic Mtivier, Vincent Etienne and Stphane OpertoFull waveform inversion has been proposed in the early eighties and we now find various illustrations of this high resolution seismic imaging technique on both synthetic and real data. We investigate the different issues one may address regarding the three elements of this technique. The optimisation formulation should move towards more complete Newton-like methods. The hierarchical data sampling strategy should prevent the local optimisation approach to be trapped into a local minimum. The model description should keep the number of degrees of freedom as low as possible while prior information should be integrated into a regularisation term moving the full waveform inversion from a data-driven approach to a more balanced data and model contributions when available.
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Full Waveform Inversion by Iterative Depth Migration and Impedance Estimation Using Well Control
Authors Gary F. Margrave, Robert J. Ferguson and Chad M. HoganWe relate full waveform inversion (FWI) to processes familiar to practicing geophysicists. A key theoretical result behind FWI is that a linear update to a migration velocity model is proportional to a prestack reverse-time migration of the data residual (the difference between the actual data and data predicted by the model) where the proportionality factor must be estimated. We argue that in most real-world cases this factor will be frequency dependent, or in the time domain, it will be a convolutional wavelet. The estimation of the velocity update from the migrated section and the common process of impedance inversion are analogous, and we view FWI as a practical cycle of data modeling, migration of the data residual, and "calibration" of this migration to deduce the velocity update. The calibration step can be accomplished like a conventional impedance inversion where the migrated data residual is tied to the velocity residual (the difference between actual velocity and migration velocity) at a well. As there are a great many established algorithms for impedance inversion, so there are a plethora of possibilities for calibration. We present an extended example using the Marmousi model in which we use wave-equation migration (e.g. depth stepping) of the data residual and a simple least-squares amplitude scaling and constant phase rotation, determined at a simulated well, to calibrate the migration. We find that our approach produces a much improved velocity model in only a few iterations.
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Understanding Uncertainty and Managing Risk with Geophysics
More LessUncertainty in Geophysics starts with uncertainties in the measurements. It continues with the uncertainties in the models, explicit or implicit, that are used for processing. Modelling, calibration, optimisation and interpretation are mixed to produce numerical models of the subsurface that are the basis for decision making. All these aspects are illustrated in several examples.
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Incorporating Fault Uncertainties Into the Reservoir Model and Evaluating their Impact on the Fluid Flow
Authors Cecilie Otterlei, Oddvar Lia, Judithvan Hagen, Paul Gillespie and Signe OttesenSeismic imaging can be very challenging for some reservoirs, like sub-salt and deep structures, and also for structurally complex fields there can be significant uncertainty associated with the seismic interpretation. A workflow that incorporates the fault uncertainties into the reservoir model has been created, and their impact on the fluid flow is evaluated. Uncertainties in compartmentalization, fault location, fault displacement, sub-seismic faults and fault sealing have all been considered. The workflow is fully automatic and contains the complete chain from structural modelling to flow simulations. It can be extended to also take uncertainties in horizons and properties into account. The field case application is a sub-salt structure with poor quality seismic, and where pressure and PVT data indicate that the reservoir is compartmentalized. The economics are marginal, but large in-place volumes provide a high potential for the field. The uncertainties related to the faults are believed to be among the most important factors contributing to the total uncertainty in the in-place volumes and the recovery factor, and the uncertainty workflow allows a robust field development plan to be created that reduces the investment risk.
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Quantifying Petrophysical Uncertainty to Help Reduce Risk
More LessThe goal of formation evaluation is to identify the nature and volume of fluids contained in a given formation. Traditionally, single values for these parameters are presented though in reality each is subject to uncertainty. If we quantify the uncertainty associated with our analysis, our evaluations become more useful in the process of deciding whether or not a field is an economic prospect. Here we will consider the nature of petrophysical uncertainties and discuss approaches that can be used to quantify, understand and reduce them.
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Decision Making in the Presence of Uncertainty
By Peter KingUncertainty is intrinsic to all aspects of modelling reservoirs and their performance. It arises from several sources i) many key data have to be interpreted from data using incomplete or imperfect physical models ii) data are only taken at very sparse intervals which have to be interpolated between iii) the intrinsic non-linearity of the flow makes forward prediction inherently unstable and therefore non-deterministic. It is a widely held view in the industry that gathering more data, such as production history, will reduce uncertainty. However, history matching (inferring reservoir properties from production history) is an inverse problem which is inherently unstable so good history matches do not necessarily produce good forecasts. Moreover reservoir modelling is carried out to support reservoir management decisions. Decision making in the presence of high levels of uncertainty can be complicated. The aim of this talk is to highlight the sources of uncertainty inherent in reservoir modeling and to indicate some modern ways by which optimal reservoir management decisions can be made in the presence of such uncertainty.
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Integration of Stimulation into Field Development Planning
By Kevin MauthField development planning has occasionally included stimulation treatments as a contingency rather than as a key component of the plan, if stimulation was included in the planning process at all. Except in tight reservoirs, stimulation has been considered a means of last resort to establish or maintain production from wells that produce less than expectations. By leaving stimulation as a contingency, operators have missed opportunities to collect the data needed to enable successful stimulation. Often, completed well architechture requires compromise in stimulation practices, which makes optimization impossible. As the quality or accessibility of new reservoirs available for development continues to decline, the importance of incorporating stimulation and the associated data collection requirements into field development planning has never been greater. This presentation will discuss some common pitfalls associated with considering stimulation as a contingency as well as the integration of new disciplines to the planning process which increase the probability of success for stimulation treatments. Field examples will also be provided to emphasize the benefits of an expanded field development plan (FDP) workflow for different types of reservoirs.
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Tight Chalk Reservoir Stimulation as a Field Development Tool
Authors Franz Marketz, Maryvan Domelen, Sara Kofoed and Simon WherityFor the development of very tight chalk reservoir in the Danish Sector of the North Sea stimulation is a key value driver. Stimulation and Completion techniques have therefore been screened as early as the “Assess” and “Select” phases the hydrocarbon maturation process. In this way a well concept tailored to the reservoir has been developed and tested before executing full field development.
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Cemented, Multi-stage Ball Drop Completion Field Trial in the North Sea
More LessStimulation is a necessity for optimum production in many fields. This is becoming more and more important in order to make certain projects economically feasible. There are many stimulation techniques and technologies available. The challenge is to find the optimal stimulation solution that matches the drilling/completion design (casing size, zonal isolation type etc.) and does not drastically extend the stimulation time. One success story can be seen through the experience of one North Sea operator. They have been through an evolution on stimulation techniques on their long producing chalk field. Initial stimulation technique was traditional perforation in clusters throughout the entire reservoir section bullheading with 28% HCL. Post production analysis showed that this resulted in poor acid distribution and uneven production. Liner deformation around the clusters also resulted in costly operations.
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Practicalities of Stimulation in Tight Gas Reservoir
More LessRemaining development opportunities in the Southern Gas Basin of the North Sea are mostly confined to lower permeability sandstones and pose significant technical execution challenges and cost hurdles. Field operators have increasingly had to rely upon the adoption of multiple fractured, horizontal wells to achieve commercial rates, deploying completion practises that historically evolved in chalk oil fields in Denmark and Norway and materials honed in the recent technology-enabled opening up of shale plays across North America. The presentation will summarise activities of the most active operator in recent years in the Southern Gas Basin.
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Shale Developments: Use of Modern Data Mining Methods to Interpret Similarities and Differences Between Gas and Oil Completion/Stimulation Strategies
Authors Randy La Follette and William D. HolcombThe presentation and discussion will focus on results of statistical analysis of Barnett Shale gas and Bakken Shale oil production result drivers. Large data sets including key reservoir parameter proxies, well architecture information, completion variables, and stimulation data were compiled, quality controlled, and analyzed to identify key production influences. The analysis and interpretation took into account both controllable influences on well productivity, e.g., well length, azimuth, completion type, stimulation size and materials, along with uncontrollable influences, e.g., fracture bounding bed presence / absence, fracturing into unknown Geohazards. The most obvious production driver is well location, a fair proxy for reservoir quality in the “shale” plays. Well architecture, including azimuth, length, and drift angle may or may not be a major determinant of productivity. It is apparent from both the Barnett and Bakken data sets that longer well lengths do not produce proportionately more hydrocarbons. Specific completion and stimulation parameters, e.g., use of coarse-mesh proppants are also important productivity drivers in certain circumstances.
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Monitoring and Modelling Hydraulic Fracture Stimulation: Future Directions
Authors Quentin J. Fisher, J- M. Kendall, J. P. Verdon and A. Baird and M. HudsonMultiple hydraulic fracturing along horizontal wells has proved to be a game changer that has led to the economic recovery of a vast amount of natural gas from shale resource plays in the USA. Optimization of hydraulic fracture stimulations has generally been achieved using a trial-and-error approach; although the microseismic monitoring of event locations has over the last decade proved to be a key enabling technology. Reductions in gas price, combined with the push to exploit resource plays in highly populated areas without a well-developed supply chain, mean that there is increasing pressure to optimize hydraulic fracture stimulations. Use and integration of advanced microseismic monitoring and geomechanical modelling offers the potential to make a step change in the optimization of hydraulic fracture stimulation. In particular, interpretation of microseismic attributes such as the magnitude and frequency dependence of shear wave splitting can be used to track temporal and spatial changes in fracture density, compliance and potentially size. Geomechanical modelling of the stress distributions prior to and following fracture stimulation can potentially help optimize the spacing and sequencing of individual stages of a fracture treatment as well as identifying the optimal time to conduct workovers (i.e. refracturing).
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Interpretation and Application of Microseismic Images
More LessMicroseismic monitoring (MSM) of hydraulic fracture treatments is routine in North America and has added significantly to our understanding of fracture growth. The interpretation of microseismic images is advancing steadily, extracting more information from event patterns, temporal evolution, and acoustic waveforms. The increasing amount of information from MSM provides significant opportunities to improve stimulation designs, completion strategies, and field development. However, the applications of microseismic interpretations are many times ill-defined, overlooked, or not applied properly. The integration of microseismic images, fracture modeling and reservoir simulation is required to determine the effective stimulated volume. One of the most common misapplications of microseismic interpretations is the assumption that larger stimulated volume (SV) will automatically result in increased well productivity. Characterizing propped and un-propped regions of the hydraulic fracture is critical when evaluating well performance and estimating drainage area and hydrocarbon recovery. This abstract highlights the interpretation and application of microseismic images using excerpts from SPE 152165 (Cipolla et al 2011a).
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New Deterministic Calculation Regime for the Estimation and Characterization of the Stimulated Reservoir Volume (SRV)
More LessA new deterministic calculation regime for the estimation and characterization of the Stimulated Reservoir Volume (SRV) exists. It is based upon a combination of ultra-fine-scaled measurements of induced surface deformation morphology (on the micrometer scale) and a new, two-pass, geomechanical inversion technique. The approach overcomes instabilities and questions of uniqueness with inverted solutions of reservoir strain distributions. The technique was deployed concurrently with the more conventional, High-node-count, highfrequency, downhole, offset microseismic mapping in an exploratory horizontal completion located in an Eagle Ford horizon where low deviatoric stresses made dual and tri-modal complex fracture networks likely. Passive microseismics and microdeformation techniques respond to fundamentally different mechanical processes associated with hydraulic fracturing. This pilot application merged the results from these two mapping diagnostics to explore the potential for integrated geophysical/geomechanical information to facilitate a more accurate and comprehensive understanding of treatment performance.
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Advanced Drilling and Completion Solutions for Unconventional Shale Gas
More LessNatural gas and oil production from shale reservoirs has reshaped the petroleum industry in North America and is posed to have a major impact in other locations around the world. Traditionally shale has been seen as a source rock, a trap or a drilling hazard within the petroleum industry. Today, however, many of these source rocks have proven to yield commercial production of hydrocarbons when the correct technologies are applied to understand the reservoir potential of a given formation and proper drilling and completion techniques are used. This presentation will examine some of the basic geological requirements that need to be assessed to determine if a particular shale reservoir has good production potential. It will then look at drilling and completion solutions have proven successful in different shale reservoirs. Guidelines to optimize the completion design based on specific reservoir properties will be discussed. New validation technologies, including microseismic fracture mapping will be discussed showing their significance in maximizing the productive capacity from a given well and improving our understanding in the production mechanisms. New technology in the area of stimulation fluids and equipment development will also be discussed.
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Completion Based Stimulation Technology; When Fracturing Just Doesnt Fit
Authors Thomas Jrgensen and Rune FreyerReservoir stimulation is required for efficient production in many fields. But traditional fracturing requires much preparation and design. Rock mechanics in small fields require significant data capture and it is not possible to develop as good understanding by trail and error like in the large North American plays where hundreds or even thousands of wells are stimulated before “cracking the code”. Challenges in small fields can be to understand stress fields, water or gas intervals, formation damage or other field specific issues.
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Surveillance Field Trial to Identify Thief Zones in MFF-09B, a well with a Controlled Acid Jetting (CAJ) Liner
Authors Hansvan Dongen, John Davies, Kerem Yuksel and Edo BoekholtzThe Controlled Acid Jetting (CAJ) well design was developed by Maersk Oil for the development of relatively thin but aerially very extensive oil accumulations in low-permeability Chalk formations in the North Sea. These relatively low-cost long horizontal wells (up to 30,000 ft Total Depth) enabled the development of the Dan West Flank and Halfdan oil fields, which would otherwise have been uneconomic. Building on the favourable experience from the North Sea, Maersk Oil has also applied CAJ wells for the cost-effective development of the Al Shaheen oil field in Qatar. The initial development decision for CAJ wells to develop these waterflooded oil fields has effectively resulted in a zero (below Coil Tubing reach) well intervention policy in terms of inflow/outflow profile surveillance and subsequent treatment of any thief zones and/or high-skin intervals encountered. The initial field developments have met expectations with respect to development costs and initial oil production rates. A significant number of waterflood patterns now show faster than forecasted watercut development, which indicates the presence of non-conformances (i.e. natural and/or induced fractures acting as thief zones). When thief zones are present in water floods, oil ultimate recovery is lower than forecasted, which has triggered the need to develop new CAJ well intervention technologies for inflow/outflow profile surveillance and subsequent treatment of thief zones. A dedicated Long Wells Conformance Control (LWCC) team was set up in 2010 to accelerate the development and implementation of such new technologies for the North Sea and Qatar oil fields.
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Temperature Dependence of Ultrasonic Velocities in Shales – Can We Use It For Interpreting Time-lapse Seismic?
Authors Andreas Bauer, Rune Holt, Audun Bakk, Erling Fjr and Jrn StenebrtenThe temperature dependence of ultrasonic velocities in shales show significantly larger temperature sensitivities than predicted by the Gassmann fluid-substitution model, which can be attributed to temperature dependent velocity dispersion. We expect the temperature dependence of velocities to be frequency dependent, resulting in different temperature sensitivities at seismic, sonic and ultrasonic frequencies.
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A Study of Geomechanical Effects on Time-lapse Seismics
Authors Giorgio Cassiani, A. Brovelli G. Vignoli and B. PlischkeTime-lapse seismics is known to be a very effective monitoring technique for the subsurface fluid movement and saturation changes, as well as for geomechanical phenomena [Snieder et al., 2007]. The integration of seismic and reservoir engineering is now becoming state-of-theart [Boutte, 2007] while the number of applications is steadily increasing [Staples et al., 2006]. Among the future challenges to the use of time-lapse seismics is the integration with geomechanics [Landrø, 2006]. The improvement of time-lapse seismic technology [e.g. Tang et al., 2007, Aarre et al., 2007] allows for better and more accurate data acquisition, that in turn allows to “see” effects previously difficult to detect. The effects of geomechanics on time-lapse seismic data have been described in detail by a number of publications [Hatchell and Bourne, 2005; Sayers and Schutjens, 2007; Cox and Hatchell, 2008; Kristiansen and Plischke, 2010]. The overall impact of reservoir exploitation on the changes in seismic response includes the following aspects: (1) Fluid saturation effects, that are based upon: (a) dependence of density on fluid saturation; and (b) dependence of bulk moduli on fluid saturation (Gassmann, 1951). This is the key effect sought in time-lapse seismics, as it allows remote monitoring of the fluid migration in the reservoir. Mainly, two effects are sought in data hopefully depending on the above saturation changes, i.e.: - time shifts, i.e. changes in reflector location in time as a consequence of changes in velocity, and mainly: - impedance changes, i.e. reflectivity changes, as impedance is the product of velocity and density, both changing with fluid content. (2) Pressure (effective stress) effect: this is the first, well known geomechanical effect, often referred to in the literature as pressure effect, but it is actually a dependence on effective stress. It is generally observed that the velocity decrease is very strong in presence of effective stress decrease (expansion), while velocity increase is relatively mild under stress increase (compaction) [e.g. Hatchell and Bourne, 2005]. This asymmetric behaviour is often explained in terms of crack opening under stress release conditions.
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Drilling in Depleted Fields - From Surprises to Surprises?
More LessAs a result of depletion, the reservoir rock generally compacts and thus leads to stress changes both within the reservoir and in its overburden. In turn these changes affect drilling operations. The presentation will describe those changes and introduce a series of field cases illustrating their impact on drilling. Most of these field cases are based upon the analysis of tens of wells and all of them showed behaviours, which surprised the authors at the time of their study. A brief overview of each case is given below. A few years ago, Geomec analysed the stress changes due to depletion – i.e. reservoir stresspath – for a series of over thirty fields, as part of a large Joint Industry Project (Figure 1). The presentation will give a brief overview of the project’s results, insisting on those, which were not expected at its onset.
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Workflow for Coupled Geomechanical and Reservoir Problems – Recent Experiences
More LessDevelopment of unconventional resources requires solving difficult reservoir engineering problems, many of which have some geomechanical component of the analysis. Geomechanics is coupled with the reservoir problem to varying degrees, ranging from problems that can be solved sequentially to those requiring full coupling. In an overall workflow of solving such problems, the selection of the method to solve them, and the degree of coupling which they demand, are some of the most important decision points. The use of the coupled flow and geomechanical models has become more commonplace in recent years, but there are still somewhat specialized, and the best (and most efficicent) workflow for coupled simulation depends critically on the type of the problem being investigated. In this presentation, we will give a survey of this part of the workflow and discuss several examples that demonstrate the differences of the possible approaches.
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Solutions for 3D Coupled Geomechanical and HF Modelling in UG Reservoirs
More LessWith growing worldwide activity in exploration and development of ultra-low permeability unconventional reservoirs the O&G industry has become increasingly dependent on efficient and effective horizontal wells drilling as well as hydraulic fracture completions to increase surface area and promote gas migration. Integrated geomechanical workflow allows encompassing rigorous 3D stress modelling provided by VISAGE* system, hydraulic fracture modelling by P3D model, near wellbore analyses and drillling optimization techniques. Solutions span from data screening, throughout data integration and analysis and finally to well design support covering various scales. They rely on a more accurate 3D stress field characterization, reflecting the structure, heterogeneity/anisotropy, pressure, temperature effects from well to reservoir scale.
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An Acquisition System Using Complementary Components to Achieve Robust Broadband Seismic
Authors Stian Hegna and Gregg ParkesIn a conventional marine acquisition system there are several components that have limitations in terms of bandwidth. However it is possible to re-design or re-arrange several of these components to produce complementary responses providing broadband seismic. The basic components of an acquisition system consist of sources and receivers. On the receiver side the main limitations are related to the sea surface reflection (receiver ‘ghost’). On the source side the limitations are related to the sea surface reflection and the responses of the airgun arrays. The induced responses of these effects are all known or can be measured, so why can’t they simply be removed? The problem arises because most of these responses contain deep notches, which make their removal very unstable in any practical sense. Now all these effects can be related to specific components of the acquisition system. It is then possible to re-design the parameters of those components to produce complementary responses that negate the effect of the notches. Once these components are in place the seismic data can be corrected in a very robust way to produce optimal broadband seismic.
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Deep Interpolated Streamer Coverage - Broadband Seismic Data Offshore South Africa
More LessDeep interpolated streamer coverage is a seismic acquisition technique based on 3D over/under towed-streamer acquisition (Kragh et al. 2009). The technique is designed to provide broadband seismic data and deploys two receiver spreads: a shallow spread primarily for high-frequency information and a deep spread for low-frequency information. The cable separation between over cables is chosen for high temporal and spatial resolution and is typically 50 - 100 m. The cable separation for the deep spread is designed to record the low-frequency component and is much coarser, typically 300 m. In the processing phase, data from the deep (under) cables are interpolated to match the crossline sampling of the shallow (over) cables. Wavefield separation is then used to combine the high-frequency response of the upper cables and the low-frequency response of the interpolated lower cable data set to give a broadband seismic data set that is suitable for detailed structural interpretation and amplitude inversion. In addition to the technical benefit of improved low-frequency response, there is also an operational advantage. The deep cables that provide most of the low-frequency component are towed in a seismically quieter environment than the shallow cables, and hence, are less susceptible to swell noise. This operational advantage is particularly important in areas such as offshore southern South Africa, which has a short season for seismic acquisition and is notorious for high levels of swell in both good and bad weather.
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What to Expect from Variable-Depth Streamer Data
Authors Dechun Lin, Yves Lafet and Ronan SablonVariable-depth streamer acquisition is emerging as a key technique for providing wide-bandwidth seismic data. This technique allows us to obtain a usable bandwidth from 2.5 Hz up to the source notch. It has consistently produced high-quality images in terms of seismic resolution, layer stratigraphy and low-frequency penetration. Seismic interpretation and inversion becomes easier and more robust.
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Increasing Spatial and Temporal Bandwidth with Multi-component Streamer Data
Increasing bandwidth is not only about temporal frequencies but also about spatial wavenumbers, in particular those which are poorly sampled in the cross-line direction with streamer separations of 16 to 24 times the inline sampling interval. In this talk, we present results from a test with a mini-3D array of prototype 4C marine streamers in which we use, in addition to the pressure, the vertical and crossline gradients of the pressure wavefield in order to reconstruct and 3D deghost the wavefield at arbitrary points within the aperture. From the experimental 3D survey, we show examples of spatial and temporal enhancement of wavefields reconstructed using a generalised matching pursuit algorithm, comparing pressure-only and multi-component reconstructions. We find that multicomponent reconstruction is able to de-alias high wavenumber diffractions, that are completely missed by a pressure-only matching pursuit algorithm with priors, and generate broad-band unmigrated timeslices with excellent resolution.
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Increased Temporal Bandwidth Using Hydrophone Only Recording and Conventional Airgun Arrays – Why Not?
More LessUsable bandwidth is determined by the signal to noise ratio rather than just signal. Modern streamers have superior noise performance compared to older versions and this reduction in noise seems to have been overlooked in the search for greater temporal bandwidth. Above approximately 2Hz the noise floor is determined by environmental issues such as swell noise and cable jerk rather than noise inherent to the equipment. Tests show that a usable temporal bandwidth of at least 3-90Hz can be obtained using a conventional airgun array and modern hydrophone only recording when a) the sea surface is not a perfect mirror and b) the streamer is towed in a deep, quiet environment.
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Analysis of a Broadband Processing Technology Applicable to Conventional Streamer Data
Authors Zhengzheng Zhou, Milos Cvetkovic and Bing Xu and Philip FontanaWe recorded 2D lines parallel to and in close proximity of one another, with streamers towed at different depths. We applied WiBand, GXT’s broadband processing method, to a deep tow line and recovered data free of receiver ghost notches. We find a good phase match between the WiBand result and a shallow tow line. The match validates the phase fidelity of the WiBand process.
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Preparing Data for Full Waveform Inversion: A Workflow for Free-surface Multiple Attenuation
Authors Jyoti Kumar, Adriana C. Ramrez and Suhail ButtWaveform inversion estimates a quantitative model of the subsurface by minimizing the differences (residuals) between observed and calculated seismic data. The success of waveform inversion depends on the complexity of the misfit function. If the starting model is not in the neighbourhood of the global minimum, it can cause the inversion to fail and converge into a local minimum (Sirgue et al., 2011). Since low-frequency data are more linear with respect to the model misfit than high-frequency data, most waveform inversion implementations adopt a strategy that proceeds sequentially from low to high frequencies. Therefore, data preconditioning for waveform inversion must preserve as much low frequency signal as possible. Traditionally, the bubble pulse generated by the source in marine acquisitions has been removed from the data. The bubble can generate undesired results in, e.g., data-driven multiple prediction algorithms such as SRME (Verschuur et al., 1991), where the auto-convolution of the bubble can generate long period artefacts, and requires long filters in adaptive subtraction step. It is difficult to constrain the adaptive subtraction to preserve the primaries untouched when a long filter is used. However, it has also been recognized that the bubble pulse contains valuable low-frequency signal that can benefit the quality of the velocity model estimated by waveform inversion. We propose a workflow for waveform inversion data preconditioning that preserves the bubble and low frequency signal while effectively attenuating the free surface multiples.
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Delivering Technical Limit Seismic Data: Nature Vs. Nurture
Authors Linda Hodgson, Daniel Davies, Thomas Hance and Mike SmithThe quality of the seismic product depends on three interacting elements: the fixed physical constraints of the location, the acquisition methodology, and the processing sequence. Recent developments in marine technology have enabled a step change in acquisition, but how much difference will this make to the final product? We compare examples of the new ‘broadband’ methods to ‘conventional’ data, to explore how much extra signal may be expected at different parts of the frequency spectrum. In the right circumstances, substantial gains are possible at both low and high frequencies. In other, more challenging settings, matters such as improved processing or better azimuthal coverage may have more impact.
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Assessing Frequency Bandwidth and Resolution Enhancement of Seismic Data:A Broadband Perspective
Authors Didier Rappin and Christian Deplante and Thierry CadoretImproved resolution and enlarged bandwidth are key direct expectations of broadband data, which stem from the definition of a broadband signal – which will be recalled. How should both be assessed and preserved or improved across seismic acquisition, processing and reservoir characterization, possibly including seismic inversion? Examples aim at stimulating thoughts on how our habits should evolve on these topics to adapt to modern high-bandwidth data. In a first part we will focus on how the concept of seismic resolution could be revisited when considering broadband data: event separability and detection issues which are mainly determined by bandwidth and signal/noise ratios will be discussed. Examples of both non-broadband and broadband signals will be used in order to study how these measurements should be assessed. Handling time-variant signals involves a bit of mathematical concepts to preserve relations between different variables such as temporal or spatial ones and frequencies or wavenumbers. It is especially when the bandwidth becomes much larger than the carrier frequency that these must be correctly taken into account throughout the design and use of signal processing tools. At this level, the topic of wavelet estimation will be a subject of particular attention knowing its importance for the reliability of reservoir characterization. Across various examples, some guidance for best practice will be proposed for discussion.
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Increasing the Reservoir Characterization Potential with Multi-component Streamer Data
Increasing the bandwidth, both horizontally and vertically plays an important role in seismic inversion and reservoir characterization. This talk will present an overview of a novel multi-component (MC) marine seismic acquisition system combined with advanced data processing techniques that use the pressure recordings and its associated vertical and cross-line spatial gradients to estimate the scattered subsurface wavefield with unprecedented spatial wavenumber and temporal bandwidth content. Results from an experimental test with a mini-3D array of prototype MC streamers will be presented and the discussion focused on the consequences of the overall temporal and wavenumber bandwidth enhancements onto migration and inversion processing and applications. Preliminary well-tie and inversion results will be discussed.
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High Frequency Losses – Stripping Various Causes
Authors Didier Rappin, Thierry Castex, Christophe Barnes and Kevin SamynThe quantitative use of the seismic amplitude information during the interpretation is a key point for many prospect evaluations and almost all reservoir characterization studies. The local amplitude information of interest is always affected by a series of signal attenuators along the propagation of the incident and reflected wavefield which are highly dependent upon the geological context, structural shape, lithologies and fluids. Also, incident amplitude effects might or might not have frequency dependence as well as phase or dispersion characteristics. These many causes of attenuation are often pragmatically treated by a combination of a few well-known tools: spherical divergence compensation, surface-consistent or volumic time and frequency-dependent compensations. Cases where severe amplitude attenuation effects cannot be treated using usual approaches are a serious issue, in particular for seismic characterization of reservoirs. In this paper, we present some ways to study the impact and the relevance of specific attenuation processes. Alternative mechanisms and tools for a quantitative assessment of these processes are proposed. Some results are shown from field case studies including VSP data and 3D surface acquisition, standard and broadband. We also attempt to point out the impact of the bandwidth in the identification and quantification of possible attenuation causes.
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Broadband Verses Conventional Marine Seismic: The Importance of Compensating for the Earth Filter
Authors Anthony Hardwick, Nick Woodburn and James WhittakerBroadband seismic enables us to move closer towards the ideal goal of imaging, which is to provide the true response of the earth. The potential of broadband seismic can only be fully realised if the effect of various earth filters are accurately compensated for, which often requires some attenuation estimation such as effective Q. This is particularly true in sub-salt and sub-basalt settings where locally strong attenuation of the seismic signal occurs. Here we evaluate the differences in spectral content between broadband and conventional marine seismic through simple synthetic earth models in an attenuating medium. Without compensating for effective Q, we demonstrate that the top and base of a thin layer at depth may not be resolvable in the broadband case. We then describe the application of the pre-stack Q inversion method to derive a spatially varying interval consistent effective Q field from data in the Faroe-Shetland Basin. Application of this field demonstrates a substantial uplift in resolution within the prospective intra-basalt Flett formation.
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Spectral Fusion – A Tool to Combine Low and High Frequency Datasets
More Less“Spectral fusion” is a new multi-trace filter which addresses the purpose of merging two seismic traces or datasets whose frequency spectrums modules have a partial overlap. Pillet et al. (2007) presented a specific application of pre-stack inversion, aimed at using a low-frequency dataset in a first inversion, in order to bridge the 7-16 Hz frequency gap in a second target inversion of a HR seismic. In parallel to this workflow which was carried forward to be used in operation, the geophysical team at Total’s Geosciences Research Center in Aberdeen took a different route and restated the problem directly from the seismic side. The idea was to find a generic way to combine two surveys which mainly differ by their respective bandwidths. When doing so, it is necessary to deal with the overlapping bandwidth appropriately. Spectral fusion directly enables to bridge the low-frequency gap of the high-resolution survey using the conventional, lower bandwidth streamer data. The method also improves the seismic well-tie. Spectral fusion was originally filed for patent in the UK on 9th May 2008 under GB 0808418.8.
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Value of the Broadband Seismic for Interpreter and Reservoir Geophysics
Authors Cyrille Reiser, Folke Engelmark, Euan Anderson and Tim BirdIdeally, geoscientists would like seismic to provide clear, objective information about the subsurface in terms of: identification of the main geological features and stratigraphic sequences, structural elements, elastic/rock properties, potential prospects and lithology-fluid content of potential reservoirs. 3D seismic has offered the greatest benefits to seismic interpreters and reservoir geoscientists in the last few decades, but historically, seismic images have stopped short of delivering on these requirements, as the seismic bandwidth was limited due to the conventional streamer design and acquisition method. Over the last few years, starting in 2007 (Tenghamn et al. 2007) with the introduction of the dualsensor towed streamer technology, new acquisition methods and technologies have been made available with the aim of providing broader seismic bandwidth without any compromise in data quality or tradeoffs in acquisition efficiency. On one side, the combination of two sensors in the streamer cable itself enables an effective removal of the sea-surface ghost by wavefield separation, allowing us to capture the full bandwidth of the upcoming wavefield. More recently, a time and depth distributed source enables the removal of the sea surface ghost on the source side (Parkes, 2011) expanding further the frequency bandwidth. Thus, interpreters and reservoir geophysicists can now have ghost free seismic enabling a significant broadening of the seismic frequency bandwidth on the low and high side of the spectra. Some results of this latest development will be presented with an end-user perspective.
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Variable Depth Streamer – Benefits for Rock Property Inversion
Authors Yves Lafet, L. Michel, R. Sablon and D. Russier and R. HanumanthaThe quality of an inversion depends on the seismic frequency content, the signal-to-noise ratio, the wavelet, and the low frequency model used to incorporate information outside the seismic bandwidth. In order to quantify the benefits of broadband seismic data for inversion, we compare pre-stack inversion results from conventional streamer and variable depth streamer data from NW Australia. The inversion results are combined with a Bayesian fluid classification scheme to map three rock facies and quantify associated uncertainty.
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4D Processing Between Variable Depth and Conventional Streamer Data
More LessProcessing data with variable-depth streamer acquisition has recently become possible through a new advanced algorithm called joint deconvolution (Soubaras, 2010). In this particular acquisition, the receiver depth increases non-linearly with offset and this allows for a wide diversity of receiver ghosts to be recorded. This acquisition and associated processing dramatically increases the possible frequency bandwidth, on both low & high frequencies sides, from 2.5Hz to the source notch. This particular broadband technique will be referred to as BroadSeis in this paper. While most acquisitions in the future will certainly be realized with broadband techniques, the question of 4D matching between conventional and BroadSeis data must now be addressed during an intermediate period when the baseline data is a conventional acquisition. This paper considers this challenge and demonstrates that a good 4D response can be obtained Compared to conventional flat streamer data, processing variable depth streamer data implies a major change: the receiver ghosts are rigorously taken into account, whereas they cannot be removed from the wavelet in conventional flat streamer processing. The variable receiver depths of BroadSeis give asymmetrical ray paths which are taken into account by the imaging process and by the proper summation of the up-going and down-going wave fields in the joint deconvolution. Typical cross-equalization in a 4D process aims at solving issues related to differences in the vintages acquisition (acquisition related time- and amplitude differences) and positioning (4D binning). The case between BroadSeis and conventional data has one more problem to solve: the difference in cable profiles. This problem can be handled by joint de-ghosting and re-ghosting processes. In the following sections, we will discuss all these topics: wavelet processing, time de-striping, 4D binning, regularization, imaging and final matching. The dataset used for this comparison is a dual recording acquired by Shell in a highly structured deep offshore play. Data Overview While shooting a conventional 3D survey offshore Gabon, Shell acquired an additional 430 sq km swath of BroadSeis data to evaluate the uplift brought by the broadband image. The first comparison on PSTM data was generated in the end of 2011 and is shown in Figure 1. It shows the overall improvement typically achieved by BroadSeis in terms of enhanced spectral bandwidth. The acquisition geometry consists of 10 cables, 8000m long. The conventional streamers were towed at 11m depth with a source depth of 9m, while the BroadSeis configuration was towed between 11m and 50m with a source depth of 7m. No specific repeatability of source positions was requested when the vessel acquired these swaths, as shown by the azimuth maps of the two acquisitions (Figure 2). With these limitations in mind, both volumes were processed in a 4D sense in order to assess any impact of the variable streamer depth on the 4D signature, which should ideally be zero in the common bandwidth.
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The Latest Sleipner CO2 Injection Monitoring Using Dual Sensor Streamer Technology
Authors Ivar Andreas Sand and Anne- Kari FurreCO2 is at Sleipner injected into the Utsira Fm, a shallow aquifer at 800-1100 m depth. Since 1996 more than 13 Mt has been injected (by 2012). In order to monitor the distribution of the CO2 in the sub-surface, a total of seven seismic monitor surveys have been acquired. These form, together with a base survey from 1994 (prior to injection start) a unique dataset (Chadwick et al 2004, Arts et al 2008). In addition, three gravity monitoring surveys give complementary information (Alnes et al 2011). In this paper we focus on the most recent seismic dataset, acquired using PGS’ Geostreamer dual sensor technology. These data can be redatumed from a deep tow depth to a shallower tow depth for comparison with previous monitor surveys, and are in addition expected to have broader frequency content than previous data ( which have non-optimal tow depths), enabling interpretation of finer details.
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Inverse Methods to Combine Geology, Geostatistics and Multiphysic Data
By Miguel BoschInverse methods are used to infer model parameters from observed data that are related via random or deterministic functions. Their application to Earth Sciences has expanded more recently to encompass the problem of data and information integration considering the combination of multiple geophysical data surveys, information of multiple properties distributed in space, their relationships, embedded object structure and scale issues. Modelling the complexity related with the multiple parameter subspaces and functions across them is priced by the coherency of the estimated results with the available information and the simplification of the posterior information due to modes and uncertainty reduction. The formulation of this problem is based on modelling the posterior probability density that combines the various components of the available information and data. Appraisal of the posterior information can be obtained parameter wise with calculation of probability distributions describing the posterior uncertainty, or globally via full model configurations corresponding to maximum posterior probability configurations or realizations from the posterior probability. We present examples of the applications of these methods to various problems in Earth Sciences, ranging from the description of the lithospheric structure of interacting plate boundaries to the characterization of hydrocarbon reservoirs.
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Monte Carlo Based Tomographic Full Waveform Inversion with Multiple-point a Priori Information
Authors Knud S. Cordua, Thomas M. Hansen and Klaus MosegaardIn a probabilistic formulation, the solution to an inverse problem can be expressed an a posteriori probability density function (pdf) that combines the independent states of information provided by data and a priori information. Here, we define an a posteriori probability density function that defines the solution to a tomographic full waveform inverse problem, which provides a means of obtaining an uncertainty estimate. Unfortunately, no explicit formulation of the solution to this problem can be defined. Therefore, the a posteriori probability density function has to be sampled. The full waveform inverse problem is known to be computationally very hard and is, traditionally, considered out of reach for Monte Carlo sampling strategies. We show that by means of informative a priori information this problem become tractable for a sampling strategy anyways. We outline the theoretical framework for a full waveform inversion strategy that integrates the extended Metropolis algorithm with sequential Gibbs sampling, which allows for arbitrary complex a priori information to be incorporated. At the same time we show how temporally correlated data uncertainties can be taken into account during the inversion. The suggested inversion strategy is tested on synthetic tomographic crosshole ground penetrating radar full waveform data using multiplepoint based a priori information. This is, to our knowledge, the first example of obtaining a posteriori realizations of a full waveform inverse problem.
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Integration of Information from Diverce Sources
More LessFrom a probabilistic point-of-view solving inverse problems can be seen as a way of combining states of information in form of probability density functions. Typically, the states of information are provided by a set of observed data and some a priori information obtained independently of the data. The solution to the inverse problem is then the combined state of information quantified by the a posteriori probability density function. Within this probabilistic framework we will discuss methods for combining information from diverse sources. Specifically we will discuss methods for combining information from pre-stack seismic waveform data, a priori geological structural information and information about the relation between rock physics parameters (such as permeability, and oil saturation). One approach is to solve such an inverse problem sequentially: Initially an elastic inversion of the seismic data is performed followed by a transformation of elastic properties to rock physics parameters. Another approach is to directly solve the inverse problem parametrised with rock physics model parameters. We will discuss the benefits and challenges combining these sources of information sequentially and directly using the probabilistic formulation of inverse problems.
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Subsurface Analytics: Operationalizing the Original “Big Data”
Authors Duncan Irving and Jamie Cruise“Big Data” has become a convenient short-hand for the exponential growth of data volumes across many industry sectors. This is nothing new in the subsurface domain but E&P DM practitioners can learn from “new” industries how best to deal with complexity and timeliness in their analytical ecosystems. We present an architecture that brings to bear the twin paradigms of massive knowledge discovery using Map-Reduce and “operationalized” decision support using a Relational Database Management System. We describe how this single data instance drives rigorous geological, geophysical and engineering insight into right-time integrated operations generally and allows data, and insight derived from it, to drive business decisions across the enterprise.
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An Interdisciplinary Study of the Physico-chemical structure of Earth's Mantle: Combining Geophysical Data Analysis with Mineralogy, Petrology and Geochemistry
By Amir KhanWe jointly invert local fundamental-mode and higher-order surface-wave phase-velocities for radial models of the thermo-chemical and anisotropic physical structure of the Earth’s mantle to 1000 km depth beneath the North American continent. Inversion for thermo-chemical state relies on a self-consistent thermodynamic method whereby phase equilibria and physical properties (P-, S-wave velocity and density) are computed as functions of composition (in the Na2O-CaO-FeO-MgO-Al2O3-SiO2 model system), pressure and temperature. We employ a sampling-based strategy to solve the non-linear inverse problem relying on a Markov Chain Monte Carlo method to sample the posterior distribution in the model space. A range of models fitting the observations within uncertainties are obtained from which any statistics can be estimated. To further refine sampled models we compute geoid anomalies for a collection of these and compare with observations, exemplifying a posteriori filtering through the use of additional data. Our thermo-chemical maps reveal the tectonically stable older eastern parts of North America to be chemically depleted (high Mg#) and colder (>200°C) relative to the active younger regions (western margin and oceans). In the transition zone the thermo-chemical structure decouples from that of the upper mantle, with a relatively hot thermal anomaly appearing beneath the cratonic area that likely extends into the lower mantle. In the lower mantle no consistent large-scale thermo-chemical heterogeneities are observed, although our results do suggest distinct upper and lower mantle compositions. Concerning anisotropy structure, we find evidence for a number of distinct anisotropic layers pervading the mantle, including transition zone and upper-most lower mantle.
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History Matching Under Uncertain Geological Scenario
Authors Hyucksoo Park and Jef CaersThe main interest lies in obtaining multiple history matched models under uncertain geological scenario.
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Geothermal Energy in Denmark – Potential, Policy and Progress
A new assessment of the geothermal resources in Denmark published by GEUS concludes that the Danish subsurface contains huge geothermal resources (Mathiesen et al. 2009). To rationalise administration the Danish Energy Agency (DEA) has established a new simple application procedure with a standard license period and work program. These initiatives and rising prizes on fossil fuels have together with public concerns related to climatic changes and increasing emission of CO2 to the atmosphere triggered the awareness of the large potential of the geothermal resources, which may contribute to a safe, sustainable, price stable and reliable supply of energy. It is thus expected that geothermal energy may play an important role in the future energy strategy in Denmark (Fenham et al. 2010; Nielsen et al. 2011).
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Shallow Geothermal Energy in Denmark – Current Status and Trends
Authors Claus Ditlefsen and Thomas Vangkilde- PedersenShallow geothermal energy is a renewable energy source with large potential for reducing CO2 emissions. The application in Denmark, however, is limited compared to e.g. Sweden and Germany. Preliminary estimates indicate that the energy extraction from a 100 m closed loop borehole may be up to 40% lower for an unfavourable geological situation compared to a favourable situation. More know-how and experience under Danish geological conditions is needed and the project GeoEnergy aims at paving the way for a wider use by providing knowledge, tools and best practice.
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