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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 - 20 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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