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- Volume 17, Issue 5, 1999
First Break - Volume 17, Issue 5, 1999
Volume 17, Issue 5, 1999
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Well log stacking: a new technique to improve laterial prediction and sequence stratigraphic interpretation with wireline data
Authors David G. Quirk, Andrew E. Stocks and Fresenay MiskerThe correlation of wireline (well log) data forms an essential part of the interpretation of geological strata in sedimentary basins. The technique usually involves matching by eye the log curves produced from digitally sampled data in one well with those from one or more adjacent wells. Correlation is used primarily to access the lateral extent of rock bodies in the subsurface. However, it is a highly subjective process and is hampered by the difficulty in visually discriminating between non-geological noise, local geological trends and regional geological trends when comparing a number of logs. Variations in the thickness of an interval or differences in the data acquisition between wells compound these problems. Correlation is also constrained by the number and spacing of the wells in question. Thus it is hard to determine what is happening between widely separate wells whilst it is difficult to interpret more than a few log curves at one time due to their complexity. The solution is typically to concentrate on only a few obvious features such as a minimum or maximum peak or the diagnostic response of a specific rock type and to make some simple assumptions as to the lateral extent of other features. Even where additional geological data such as biostratigraphic or core information are available, it is often impossible to be sure that a log pattern in one well is not coincidentally similar to that in another well and regional geological trends will often be disguised by higher amplitude, shorter wavelength local variations. As a consequence of the problems mentioned above, the methodology used in correlating well logs is typically rather arbitrary and is unlikely to use all the data to full advantage. Hence it is difficult to objectively evaluate the validity or confidence level in correlations when assessing, for example, whether reservoirs are likely to be connected across a field or what the chance are of finding a known reservoir in a new well. The subject of this paper is a new automated tool which aims to improve on traditional manual correlation techniques by incorporating all equivalent data from every relevant well into a single log curve. The process suppresses the effect that local variations have on the log signal and enhances the regional response.
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Time-lapse seismic in Shell Expro — some examples and economic aspects
Authors John Verbeek, Matthias Hartung and Gjalt van der ZeeProduction of hydrocarbons from reservoirs may give rise to altered reservoir fluid density, bulk modulus and pressure and, consequently, to a change in fluid-filled reservoir rock properties. Under favourable conditions, the resulting changes in acoustic impedance can be detected on seismic. ‘Time-lapse seismic aims at extracting the pore fluid information. The conversion from impedance to fluid information is not unique and prior information, such as initial saturation, porosity and geological model, should be used to constrain the interpretation. The past years have seen a growing interest in seismic monitoring of production effects. The 1998 SEG sponsored Distinguished Short Course on ‘Time Lapse Seismic in Reservoir Management’ (presented by Ian Jack) covered a comprehensive overview of techniques applied, their strong and weak points and their, sometimes unexpected, successes. More recently, 4D examples, shown in a special issue of Leading Edge (October 1998, Vol. 17, no. 10), highlighted the requirement for a consistent description of seismic, static, dynamic and production information. With only a small number of examples with economical success, the track record of 4D remains short whilst the learning from each case history is still great. In order to avoid disappointment, picking the right order in which to carry out studies is an issue. Shell EXPRO prefers to first gain experience by addressing simple cases that can be handled with available technology. Then, in parallel, techniques are being developed to apply time-lapse seismic to more complicated fields. In order to prioritize potential candidates for 4D, a ‘value of information’ approach has been adopted. Over 40 fields,covering Tertiary, Jurassic and Permian reservoirs, have been addressed. The results of such an exercise depend on perceived technical risk and contribution to the business. Hence, they are time dependent. The ability to extract information that actually impacts the production management of a field controls whether time lapse seismic is attractive. It is therefore necessary, for each case, to assess - What information might technically be obtained, given the physical constraints, - What is the probability of successfully obtaining that information, given the data quality, - What impact such information would have on the field’s management. For the latter item, timing is an important factor. A decision tree can be drawn up to assess the economic benefit of time lapse seismic, taking into account the uncertainties and the alternative acquisition and drilling scenarios. Figure 1 illustrates that, for quite a large number of the fields that Shell EXPRO is involved in, the impact of time-lapse seismic is limited, despite technical feasibility, because no major change in development strategy would result. Fields are either too small, additional wells would be too expensive, or drilling based on the survey would come too late. In many cases, information on reservoir extent would become available through production itself, and time-lapse seismic would, at best, accelerate production. For a minority of fields (about one in four, with present-day technology), however, the situation is not so negative. Case histories for Gannet-C and Brent will show that, even with current-day technology, production effects are expected to be detectable that could impact the business. In the case of Gannet-C, the benefits were considered such that a dedicated seismic time-lapse survey has been acquired.
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Microseismicity — so what?
Authors John Dangerfield, Robert Paul Young and Shawn MaxwellPhillips licence 018 group is testing the use of microseismic activity to understand compaction and subsidence. Monitoring in 1997 in the Cl 1a well bore showed activity rates of 5 events per hour. If part of this activity is in association with waterflood fronts, then such a high event rate, combined with good event positioning, shows the potential to define the waterflood fronts inthe field. At least 16 oil and gas fields have been described with microseismicity which may be associated with hydrocarbon production and waterflooding, so this phenomenon could be more widespread than expected. At Ekofisk, positioning the microseismic activity has the strong added benefit that the faults interpreted in the crestal area are obscured from normal seismic view by gas in the overburden. Microseismic events have been monitored at Ekofisk with downhole geophones on a wireline in four separate short-term periods over the last 10 years. In all cases, rates of activity of between 5 and 15 events an hour were observed. Subsidence of the sea bottom has been continuing throughout that period at the average rate of c. 37 cm per year. It is significant that there was no microseismicity in the upper overburden because it indicated the absence there of seismically active faults. The 1997 work was the first to provide accurate event positioning. This was due to two elements: the tool was largely resonance free and the analysis, performed by the team from Keele University, used more sophisticated analysis tools.
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Airborne electromagnetic mapping of surficial deposits in Finland
Authors R. Puranen, H. Saavuori, L. Sahala, I. Suppala, M. Makila and J. LerssiAerogeophysical measurements hove been carried one by the Geological Survey of Finland (GTK) for the last 50 years. During this time the measurement techniques have been modified and improved several times (see Puranen 1963; Peltoniemi 1992; Poikonen et al. 1998). Airborne magnetic, electromagnetic (AEM) and radiometric surveys of Finland with a flight altitude of 150 m and a line spacing of 400 m were completed in 1972, after which surveys were restarted at a lower altitude (nominally 30 m) with a closer spacing (200 m). The low-altitude aerogeophysical mapping now covers more ~than 80% of Finnish territoty. Originally the aerogeophysical data were mainly used in exploration for base metal ores, but more recently the results of low-altitude aeromagnetic surveys have proved to be extremely useful in geological mapping of bedrock. During the past decade, tow-altitude AEM data have also been increasingly applied in environmental and engineering studies of surficial deposits. This is natural because most AEM anomalies in Finland are associated with clay dtposits, peatlands and lakes which cover about half of the country. Numerous applications have been presented for different AEM systems. In Austria AEM surveys have been used in a country-wide search for prospective clay deposits (Hubl et al. 1996). The AEM method has also proved to be useful in clay thickness mapping (Gamey et al. 1996), groundwater exploration (Paterson & Reford 1986) and saltwater intrusion mapping (Fitterman & Deszcz-Pan 1998) in the USA, where the method has further been tested in locating ordnance disposal (Irons 1989) and waste disposal sites (Nyquist & Beard 1996). In Alaska the AEM system has been used in measurement of sea-ice thickness (Kovacs & Holladay 1990)and in Canada the method was tested in sea-bottom profiling of coastal areas (Becker et al 1986). In Japan AEM measurements have been applied in landslide surveys (Konishi 1998) and in Australia the feasibility of AEM surveys for dryland salinity mapping is being tested (Coppa et al 1998). The AEM system of GTK has been used in investigations of various surficial targets in Finland. Soininen et al.(1998) examined the feasibility of the system for measurement of ice thickness in the Baltic Sea. The system has also been used in mapping and monitoring landfill sites to detect possible leakages (Jokinen & Lanne 1996). The areal extent of polluted soils around the waste pond formed by pulp mill effluents could be rapidly defined from AEM measurements (Puranen et al. 1996). AEM data have also been used to assess the occurrence of fine-grained sulphidic sediments (Astrom 1996), which can be oxidized to harmful acid sulphate soils (see Palko 1994; Dent & Dnwson 1999). The first tests of using AEM measurements for mapping of conductive overburden were made by Peltoniemi (1982). Thickness mapping of peat and clay deposits would be beneficial for peatland inventories and various construction works. However, for reliable AEM modelling of these deposits we need data of their in situ conductivity structures, which have been recently gathered by electric conductivity probing (see Puranen et al 1997). In this article we present conductivity data and relate them to modelling and interpretation of AEM data over surficial deposits in Finland.
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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)