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- Volume 21, Issue 6, 2003
First Break - Volume 21, Issue 6, 2003
Volume 21, Issue 6, 2003
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Innovative non-conventional techniques for focused reservoir imaging
By B. ChmelaBill Chmela, general manager, READ Well Services, in Houston discusses advances in 3DVSP surveys and other emerging technologies designed to improve understanding of the reservoir at reasonable cost 3DVSP surveys and other non-conventional seismic techniques for focused reservoir imaging can be used to improve resolution and to deliver more accurate reservoir models for production enhancement. New techniques and methods are being introduced by small innovative companies and have emerged in some areas as a primary imaging process. New technologies and data acquisition techniques have been developed that allow very large scale 3DVSPs to be acquired while using less than 10% of the rig time than similar surveys would have used just a few years ago. In tough imaging areas, this may be the only way to obtain improved images. In the past, the biggest factor limiting the use of 3DVSP technology was the cost involved with tying up a rig for days. By significantly reducing the cost, these high resolution surveys are becoming more common. With many oil and gas fields moving into the declining stage of production, the industry is continuously looking for ways to improve the recovery of hydrocarbons from existing reservoirs. In older fields there is a wealth of high resolution data obtained at the borehole (well logs) and a much wider but low resolution view of the field from surface seismic measurements. Trying to get a better understanding of a reservoir typically revolves around attempts to improve the resolution of the seismic image and/or measure changes in the reservoir properties to uncover bypassed hydrocarbons. The focus of READ Well Services has from the start nearly 20 years ago been to develop technologies that reduce the cost of finding and recovering hydrocarbons. It has developed and worked with other companies to introduce a number of new technologies that fill the gap between the high resolution data inherent in well logs and the low resolution seen with surface seismic.
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Multifarious VSP methods for imaging of complex structures with three component data
Authors Y.P. Li, D. Faw, J. Jackson and D. DushmanYingping Li, Baker Atlas, Dorothea Faw, Ocean Energy, James Jackson, David Dushman, Fran Doherty, Baker Atlas provide a case study of the benefits of modern borehole seismic technology in the Gulf of Mexico In the US Gulf of Mexico, petroleum reserves are often trapped underneath overhangs of diapiric salt domes. Although the top of salt and sediment structures can be well imaged using current surface seismic migration techniques, the interpretation of the salt flank with irregularly shaped overhangs and the position of updip sands is often ambiguous. Therefore, vertical seismic pro-filing (VSP) surveys are usually requested for additional, independent information (Sparlin and Deri, 1989; Millahn and Manzur, 1990; Nosal et al, 1991; Barker and Guzman, 1992; Burch and Johnson, 1993). The area under study is located in Eugene Island Block 126. There was a deviated well drilled to 11 794 ft SS in the vicinity of a salt dome. A comprehensive borehole seismic survey was designed and acquired in this well in order to obtain a better image of the updip sands and the salt flank. The multifarious three component (3C) VSP data consisted of a rig-source velocity survey (VS), an offset VSP (OVSP), and a refraction salt proximity survey (SP). A map view of the VSP survey configuration is shown in Figure 3. The blue circles are source locations and the pink triangles represent the well trajectory. The survey source was a 500 in, 3 fourgun tuned airgun array. A DGPS navigation system was used to ensure that all sources were in the correct position. To reliably estimate arrival directions for the SP, a gyro tool was attached to the 3C geophone and was used to monitor the orientation of the horizontal geophone components. The rig-source VS was acquired to accurately measure the sediment velocities needed for processing the OVSP and SP data. The data were also subsequently used for ray-trace modelling. The 3C OVSP was shot at an offset of 5571 ft from the wellhead. Presurvey modelling indicated the optimal position of the source to produce reflection images for both the salt face and the updip reservoir sands flanking the salt. The source location of the SP was positioned over the salt top at an offset of 5404 ft from the wellhead. The SP source was positioned in order to utilize the refracted waves to define a salt face position by calculating salt exit points in 3D space. In this paper, we present VSP imaging results from the multifarious VSP survey in the Eugene Island field. The OVSP reflection image of the salt face closely matches the salt exit points calculated from the 3D SP. The determined salt boundary is also consistent with existing well control data. The sedimentary reflectors from the OVSP data terminate as they approach the salt face. Post-survey travel time modelling of the SP data and the salt lead from the VS data provided a further constraint on the shape of the salt flank. Both the refraction and reflection images can be integrated with other available information such as well control data, results from ray-trace modelling, and surface-seismic data, to produce a consistent image of the salt face and the target sand beds. Careful processing and interpretation of the various VSP data result in the precise definition of a diapiric salt dome and updip sands in the Eugene Island field.
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Risk of a major accident on the Norwegian shelf on the increase
The risk of a major accident occurring on the Norwegian shelf is rising, according to a report from the Norwegian Petroleum Directorate (NPD). The development in the number of undesirable incidents linked to gas leaks, kicks and damage to load-bearing structures provides the major cause for concern, according to the report Development in risk level on the Norwegian shelf, Phase 3 – 200, the third in a series which started in 1999/2000. Gunnar Berge, director general, NPD, said: ‘This report confirms a trend we have observed, specifically that the risk of major accidents is rising. This is disturbing. On the other hand, the changes are not dramatic, but the report shows that we have great challenges in store on the Norwegian shelf when it comes to improving safety.’ He indicated that NPD does not merely record incidents and near-misses, but also attempts to find indicators that can provide early identification of the risk of major accidents for an entire industrial sector. The NPD initiated the Development in risk level on the Norwegian shelf project in 1999/2000, and the results are presented in annual reports. The objective of the project is to develop and apply measuring tools that illustrate the development in the risk level on the Norwegian shelf. The methods used in the project are based in part on incident-related indicators and socio-scientific analyses.
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Marine seismic contractors take steps to improve safety environment
By A. McBarnetAndrew McBarnet reviews the involvement of the marine seismic industry in the UK offshore oil company initiative A Step Change in Safety, launched nearly six year ago and still being actively pursued. Good news has always received a bad press, or indifferent at best. That’s why accident free operations go unnoticed and health and safety only ever takes a high profile when something goes wrong. Hence it must have been with mixed feelings that the International Association of Geophysical Contractors (IAGC) found itself draw into the Step Change in Safety initiative launched by the UK Offshore Operators Association in 1997. As Jim Sommerville, chairman, EAME Chapter, IAGC, puts it ‘The marine seismic industry was quite advanced in cross-fertilisation with regard to safety issues. We had already worked together on the development of safety manuals in a way which saw full collaboration between competitors.’ It is also a fact that over the past 10 years, apart from a blip in 1998, the overall record in terms of marine seismic safety worldwide shows a low rate of incidents and an underlying trend of continuing improvement. That said, Sommerville acknowledges that the Step Change in Safety initiative did bring some renewed focus to the IAGC’s view of safety issues. Step Change in Safety has a bit of history behind it dating back to Piper Alpha oil production platform disaster in the UK sector of the North Sea on 6 July 1988 in which 167 people died. The accident was the result of a massive leakage of gas condensate on the Occidental operated installation which ultimately caused a series of explosions and fireball engulfing Piper Alpha in just 22 minutes.
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Imaging sub-basalt structures using locally converted waves
Authors M. van der Baan, T. Kerrane, J.-M Kendall and N. TaylorIn many parts of the world large-scale basalt flows associated with continental rifting have blanketed pre-existing sedimentary basins. In recent years considerable attention has turned to exploration beneath these extensive flows. A region of particular interest is the Atlantic margin of northwest Europe which experienced early tertiary magmatism associated with the interaction of the Iceland plume and the opening of the Atlantic Ocean (Barton and White, 1997). Imaging sub-basalt structures is notoriously complicated and there are several reasons why few successful case histories exist. The basalt flows are characterised by rough boundary topography and finely-layered internal structure. Therefore, scattering and attenuation are probably significant. Additionally, basalt can be considerably higher in velocity than the surrounding sediments. Hence, the point of critical P-wave reflection is reached at small incidence angle and only a small amount of P-wave energy penetrates the basalt layer. Finally, short-offset data is contaminated with multiples, again due to the high-impedance contrast with surrounding sediments. The Rockall Trough which lies in deep water west of Northern Ireland is the site of extensive intrusive and extrusive dolerites and basalts. Seismic exploration in this area presents both acquisition and processing challenges. We present a methodology for processing long-offset data recently acquired in this region. Our technique builds on the approach of Emsley et al. (1998) who use locally P-to-S converted waves recorded at far offsets to penetrate the basalt. The use of local mode conversions extends the range of useful offsets for imaging beneath interfaces with large velocity contrasts (Tatham et al., 1983; Tatham and Goolsbee, 1984). We improve upon the approach of Emsley et al. (1998) by also employing the τ-p transform to further isolate converted waves from multiples and head-waves and to increase the signal-to-noise ratio of the extracted waves. In addition, the τ-p transform enables us to isolate arrivals with specific incidence angles, thereby facilitating the distinction between converted-wave and pure-mode energy even further (Tatham et al., 1983; Tatham and Goolsbee, 1984).
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Applications of ground penetrating radar in the Three Gorges Project, China
Authors L. Zhangming and L. JijianDuring the Three Gorges Project (TGP) on the Yangtze River in China, a number of complicated engineering and geological problems had to be solved. A quick and high resolution non-destructive method was needed to find buried geological defects and engineering quality problems in order to guarantee the project schedule and engineering quality. Ground penetrating radar (GPR) was one technique which was effectively applied to detect the extension of inhomogeneous weathering of granite prior to excavation, to map the extent and attitude of large faults and weathering of alternative layers, and to check the engineering quality of concrete work for TGP during construction. In this paper, three case histories are discussed.nThe detection results proved consistent with the results of excavation with the performance of GPR obvious. As a large scale, high technology and difficult construction, The Three Gorges Project(TGP) is the largest and most ambitious infrastructure project in the world. Geological exploration and surveys have been conducted for more than 30 years providing a wealth of data prior to construction. Like other large hydroelectric engineering projects, the TGP was faced with many complicated engineering, geological and technology problems during construction, such as outlining inhomogeneous weathering layers in granite during excavation, determining the occurrence of large faults and weathering of alternative layers, detecting the position of seepage in the embankment, checking the engineering quality of the work, and so on. If these problems could not be rapidly solved during construction, they would delay the construction process and affect construction quality. As a high resolution non-destructive detection technique, GPR was able to rapidly and economically solve some of the problems encountered during TGP construction which other geophysical methods could not accomplish. Through test, research and practical application, successful results were obtained using GPR to solve the engineering problems mentioned. In this paper, three case histories are presented. The first shows the delineation of inhomogeneous layers in granite, the second illustrates the definition of faults and weathering of alternative layers, and the third demonstrates the checking of the engineering quality of work for the TGP. The detection results were confirmed by on-site inspection.
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Volumes & issues
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Volume 42 (2024)
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Volume 41 (2023)
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Volume 40 (2022)
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Volume 39 (2021)
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Volume 38 (2020)
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Volume 37 (2019)
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Volume 36 (2018)
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Volume 35 (2017)
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Volume 34 (2016)
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Volume 33 (2015)
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Volume 32 (2014)
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Volume 31 (2013)
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Volume 30 (2012)
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Volume 29 (2011)
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Volume 28 (2010)
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Volume 27 (2009)
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Volume 26 (2008)
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Volume 25 (2007)
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Volume 24 (2006)
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Volume 23 (2005)
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Volume 22 (2004)
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Volume 21 (2003)
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Volume 20 (2002)
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Volume 19 (2001)
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Volume 18 (2000)
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Volume 17 (1999)
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Volume 16 (1998)
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Volume 15 (1997)
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Volume 14 (1996)
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Volume 13 (1995)
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Volume 12 (1994)
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Volume 11 (1993)
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Volume 10 (1992)
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Volume 9 (1991)
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Volume 8 (1990)
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Volume 7 (1989)
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Volume 6 (1988)
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Volume 5 (1987)
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Volume 4 (1986)
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Volume 3 (1985)
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Volume 2 (1984)
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Volume 1 (1983)