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- Volume 53, Issue 2, 2005
Geophysical Prospecting - Volume 53, Issue 2, 2005
Volume 53, Issue 2, 2005
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4D technology: where are we, and where are we going?
More LessABSTRACT4D reservoir monitoring is now becoming established as a normal part of business in some companies, in some regions such as the North Sea, and for some applications, such as thermally enhanced oil recovery. Some spectacular technical successes have been reported, mostly in monitoring water floods. We are beginning to understand how to acquire and process surveys to achieve good results. There is still, however, much to improve in how we use 4D technology to capture its potential value in field management and still some way to go in the tool itself, to make it faster, cheaper and more sensitive to small production effects.
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Improved understanding of velocity–saturation relationships using 4D computer‐tomography acoustic measurements
Authors K. Monsen and S.E. JohnstadABSTRACTA recently developed laboratory method allows for simultaneous imaging of fluid distribution and measurements of acoustic‐wave velocities during flooding experiments. Using a specially developed acoustic sample holder that combines high pressure capacity with good transparency for X‐rays, it becomes possible to investigate relationships between velocity and fluid saturation at reservoir stress levels. High‐resolution 3D images can be constructed from thin slices of cross‐sectional computer‐tomography scans (CT scans) covering the entire rock‐core volume, and from imaging the distribution of fluid at different saturation levels. The X‐ray imaging clearly adds a new dimension to rock‐physics measurements; it can be used in the explanation of variations in measured velocities from core‐scale heterogeneities. Computer tomography gives a detailed visualization of density regimes in reservoir rocks within a core. This allows an examination of the interior of core samples, revealing inhomogeneities, porosity and fluid distribution. This mapping will not only lead to an explanation of acoustic‐velocity measurements; it may also contribute to an increased understanding of the fluid‐flow process and gas/liquid mixing mechanisms in rock. Immiscible and miscible flow in core plugs can be mapped simultaneously with acoustic measurements. The effects of core heterogeneity and experimentally introduced effects can be separated, to clarify the validity of measured velocity relationships.
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The 4D seismic signature of oil–water contact movement due to natural production in a stacked turbidite reservoir
Authors C. MacBeth, K.D. Stephen and A. McInallyABSTRACTCombined time‐lapse reservoir simulation and seismic modelling has been performed on both 1D and 3D models of a channelized turbidite reservoir. The models have been built using core, log, laboratory and seismic data from the Nelson Field (central North Sea) as a template. Oil and water movement in the main channels, channel margins and interchannel regions is investigated, with a particular focus being the effect of poor net‐to‐gross. The analysis confirms that saturation effects dominate the response whilst stress‐sensitivity effects play a minor role. The trough–peak signature in the seismic difference volumes formed by the sweep of the water can be continued and mapped slightly further than the channel margins. This characteristic 4D signature remains roughly intact, despite the complicated depositional architecture, and accurately delineates the area of moved fluid, but it needs additional calibration to combat the detrimental influence of the low net‐to‐gross. Signal strength is largely dependent on reservoir quality, but is also compromised by the net‐to‐gross, fluid distribution and, more critically, by the exact timing of the seismic survey. For example, a region of bypassed oil zone remains undetected as it forms early during the production. This work demonstrates that to understand fully the 4D signature at a quantitative level requires adequate knowledge of the fluid properties, but also, more critically, the geology.
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Rock physics analysis for time‐lapse seismic at Schiehallion Field, North Sea
Authors Mark Meadows, Don Adams, Rich Wright, Ali Tura, Steve Cole and David LumleyABSTRACTRock physics analysis plays a vital role in time‐lapse seismic interpretation because it provides the link between changes in rock and fluid properties and the resulting seismic data response. In this case study of the Schiehallion Field, we discuss a number of issues that commonly arise in rock physics analyses for time‐lapse studies. We show that:
- 1 Logarithmic fits of dry bulk (Kdry) and shear (Gdry) moduli vs. effective pressure (Peff) are superior to polynomial fits.
- 2 2D surface fits of Kdry and Gdry over porosity (φ) and effective pressure using all the core data simultaneously are more useful and accurate than separate 1D fits over φ and Peff for each individual core.
- 3 One average set (facies) of Kdry(φ, Peff) and Gdry(φ, Peff) can be chosen to represent adequately the entire Schiehallion reservoir.
- 4 Saturated velocities and densities modelled by fluid substitution of Kdry(φ, Peff), Gdry(φ, Peff) and the dry bulk density ρdry(φ) compare favourably with well‐log velocities and densities.
- 5 P‐ and S‐wave impedance values resulting from fluid substitution of Kdry(φ, Peff), Gdry(φ, Peff) and ρdry(φ) show that the largest impedance changes occur for high porosities and low effective pressures.
- 6 Uncertainties in Kdry(φ, Peff) and Gdry(φ, Peff) derived for individual cores can be used to generate error surfaces for these moduli that represent bounds for quantifying uncertainties in seismic modelling or pressure–saturation inversion.
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When 4D seismic is not applicable: Alternative monitoring scenarios for the Arab‐D reservoir in the Ghawar Field
More LessABSTRACTGhawar, the largest oilfield in the world, produces oil from the Upper Jurassic Arab‐D carbonate reservoir. The high rigidity of the limestone–dolomite reservoir rock matrix and the small contrast between the elastic properties of the pore fluids, i.e. oil and water, are responsible for the weak 4D seismic effect due to oil production. A feasibility study was recently completed to quantify the 4D seismic response of reservoir saturation changes as brine replaced oil. The study consisted of analysing reservoir rock physics, petro‐acoustic data and seismic modelling. A seismic model of flow simulation using fluid substitution concluded that time‐lapse surface seismic or conventional 4D seismic is unlikely to detect the floodfront within the repeatability of surface seismic measurements. Thus, an alternative approach to 4D seismic for reservoir fluid monitoring is proposed. Permanent seismic sensors could be installed in a borehole and on the surface for passive monitoring of microseismic activity from reservoir pore‐pressure perturbations. Reservoir production and injection operations create these pressure or stress perturbations. Reservoir heterogeneities affecting the fluid flow could be mapped by recording the distribution of epicentre locations of these microseisms or small earthquakes. The permanent borehole sensors could also record repeated offset vertical seismic profiling surveys using a surface source at a fixed location to ensure repeatability. The repeated vertical seismic profiling could image the change in reservoir properties with production.
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Repeatability enhancement in deep‐water permanent seismic installations: a dynamic correction for seawater velocity variations
Authors Alexandre Bertrand and Colin MacBethABSTRACTWith the increasing use of permanently installed seismic installations, many of the issues in time‐lapse seismic caused by the lack of repeatability can be reduced. However, a number of parameters still influence the degree of reliability of 4D seismic data. In this paper, the specific impact of seawater velocity variations on time‐lapse repeatability is investigated in a synthetic study. A zero‐lag time‐lapse seabed experiment with no change in the subsurface but with velocity changes in the water column is simulated. The velocity model in the water column is constant for the baseline survey while the model for the repeat survey is heterogeneous, designed from sea salinity and temperature measurements in the West of Shetlands. The difference section shows up to 80% of residual amplitude, which highlights the poor repeatability. A new dynamic correction which removes the effect of seawater velocity variations specifically for permanent installations is developed. When applied to the synthetic data, it reduces the difference residual amplitude to about 3%. This technique shows substantial improvement in repeatability beyond conventional time‐lapse cross‐equalization.
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4D seismic for oil‐rim monitoring
Authors Rob Staples, Paul Hague, Toon Weisenborn, Peter Ashton and Barbara MichalekABSTRACTIn the central North Sea ‘Gannet‐A’ field, a 50 ft oil rim is overlain by a gas cap of variable thickness. Oil is produced from horizontal wells which initially produced dry oil, but as the field became more mature, a significant water cut was seen in several wells. A dedicated 4D seismic monitor survey was acquired in order to assess the remaining distribution of oil reserves. By forward modelling the synthetic seismic response to parameters such as contact movement and residual saturations (using 2D and 3D wedge models), and comparing the results with real seismic data, we are able to decipher the contact movements across the field. It is shown that, in one part of the field, the increased water cut is caused primarily by the vertical displacement of the entire oil rim into the initial gas cap. This oil‐rim displacement produces a very different 4D seismic response from the case of a static gas–oil contact and rising oil–water contact (normal production). As a result of these observations, we are able to optimize field production by both re‐perforation of existing wells and by drilling sidetracks into the displaced rim: a brown‐field development opportunity that might otherwise be missed.
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4D signal enhancement using singular‐value decomposition: application to mapping oil–water contact movement across the Nelson Field
Authors Fiona J.L. Reid, Alexandre Bertrand, Alan T. McInally and Colin MacBethABSTRACTA new method for time‐lapse signal separation and enhancement using singular‐value decomposition is presented. Singular‐value decomposition is used to separate a 4D signal into its constituent parts: common geology, time‐lapse response and noise. Synthetic tests which demonstrate the advantages of the singular‐value decomposition technique over traditional differencing methods are also presented. This signal separation and enhancement technique is used to map out both the original and moved oil–water contacts across the Nelson Field. The singular‐value decomposition technique allows the oil–water contact to be mapped across regions which would have been missed using traditional differencing methods. In particular, areas toward the edges of the field are highlighted by the technique. The oil–water contact is observed to move upwards across the field, with the largest movements being associated, as anticipated, with natural production. The results obtained are broadly consistent with those predicted by the reservoir simulator model. Singular‐value decomposition is demonstrated to be a useful tool for enhancing the time‐lapse signal and for gaining confidence in areas where traditional differencing fails.
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Seismic preprocessing and amplitude cross‐calibration for a time‐lapse amplitude study on seismic data from the Oseberg reservoir
Authors E. Stucchi, A. Mazzotti and S. CiuffiABSTRACTThe cross‐calibration of different vintage data is an important prerequisite in attempting to determine the time‐lapse seismic effects induced by hydrocarbon production in a reservoir. This paper reports the preprocessing and cross‐calibration procedures adopted to modify the data of four seismic vintages (1982, 1989, 1992 and 1999) from the Oseberg field in the North Sea, for optimal conditions for a time‐lapse seismic amplitude analysis. The final results, in terms of time‐lapse variations, of acoustic impedance and of amplitude‐versus‐offset, are illustrated for selected data sets. The application of preprocessing to each individual vintage data set reduces the effects of the different acquisition and noise conditions, and leads to consistency in the amplitude response of the four vintages. This consistency facilitates the final amplitude cross‐calibration that is carried out using, as reference, the Cretaceous horizon reflections above the Brent reservoir. Such cross‐calibration can be considered as vintage‐consistent residual amplitude correction.
Acoustic impedance sections, intercept and gradient amplitude‐versus‐offset attributes and coherent amplitude‐versus‐offset estimates are computed on the final cross‐calibrated data. The results, shown for three spatially coincident 2D lines selected from the 1982, 1989 and 1999 data sets, clearly indicate gas‐cap expansion resulting from oil production. Such expansion is manifested as a decrease in acoustic impedance and a modification of the amplitude‐versus‐offset trends in the apical part of the reservoir.
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Cross‐matching with interpreted warping of 3D streamer and 3D ocean‐bottom‐cable data at Valhall for time‐lapse assessment
Authors Stephen A. Hall, Colin MacBeth, Olav I. Barkved and Philip WildABSTRACTLegacy streamer data and newer 3D ocean‐bottom‐cable data are cross‐matched and analysed for time‐lapse analysis of geomechanical changes due to production in the Valhall Field. The issues relating to time‐lapse analysis using two such distinctly different data sets are addressed to provide an optimal cross‐matching workflow that includes 3D warping. Additionally an assessment of the differences between the imaging using single‐azimuth streamer and multi‐azimuth ocean‐bottom‐cable data is provided. The 3D warping utilized in the cross‐matching procedure is sensitive to acquisition and processing differences but is also found to provide valuable insight into the geometrical changes that occur in the subsurface due to production. As such, this work also provides a demonstration of the use of high‐resolution 3D interpreted warping to resolve the 3D heterogeneity of the compaction and subsidence. This is an important tool for Valhall, and possibly other fields, where compaction and subsidence (and monitoring thereof) are key factors in the reservoir management since the predominant observed production‐induced changes are compaction of the soft, high‐porosity chalk reservoir, due to pore‐pressure reduction, and the resultant overburden subsidence. Such reservoir compaction could have significant implications for production by changing permeabilities and production rates. Furthermore the subsidence effects could impact upon subsea installations and well‐bore stability. Geomechanical studies that have previously been used to model such subsidence and compaction are only constrained by observed surface displacements and measured reservoir pressure changes, with the geological overburden being largely neglected. The approaches suggested herein provide the potential for monitoring and assessment in three dimensions, including the probable heterogeneity and shearing, that is needed for full understanding of reservoir compaction and the resultant effects on the overburden to, for example, mitigate well‐bore failures.
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Volumes & issues
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Volume 72 (2023 - 2024)
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Volume 71 (2022 - 2023)
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Volume 70 (2021 - 2022)
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Volume 69 (2021)
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Volume 68 (2020)
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Volume 67 (2019)
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Volume 66 (2018)
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Volume 65 (2017)
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Volume 64 (2015 - 2016)
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Volume 63 (2015)
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Volume 62 (2014)
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Volume 61 (2013)
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Volume 60 (2012)
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Volume 59 (2011)
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Volume 58 (2010)
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Volume 57 (2009)
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Volume 56 (2008)
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Volume 55 (2007)
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Volume 54 (2006)
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Volume 53 (2005)
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Volume 52 (2004)
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Volume 51 (2003)
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Volume 50 (2002)
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Volume 49 (2001)
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Volume 48 (2000)
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Volume 47 (1999)
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Volume 46 (1998)
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Volume 45 (1997)
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Volume 44 (1996)
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Volume 43 (1995)
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Volume 42 (1994)
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Volume 41 (1993)
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Volume 40 (1992)
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Volume 39 (1991)
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Volume 38 (1990)
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Volume 37 (1989)
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Volume 36 (1988)
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Volume 35 (1987)
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Volume 34 (1986)
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Volume 33 (1985)
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Volume 32 (1984)
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Volume 31 (1983)
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Volume 30 (1982)
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Volume 29 (1981)
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Volume 28 (1980)
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Volume 27 (1979)
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Volume 26 (1978)
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Volume 25 (1977)
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Volume 24 (1976)
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Volume 23 (1975)
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Volume 22 (1974)
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Volume 21 (1973)
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Volume 20 (1972)
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Volume 19 (1971)
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Volume 18 (1970)
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Volume 17 (1969)
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Volume 16 (1968)
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Volume 15 (1967)
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Volume 14 (1966)
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Volume 13 (1965)
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Volume 12 (1964)
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Volume 11 (1963)
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Volume 10 (1962)
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Volume 9 (1961)
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Volume 8 (1960)
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Volume 7 (1959)
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Volume 6 (1958)
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Volume 5 (1957)
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Volume 4 (1956)
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Volume 3 (1955)
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Volume 2 (1954)
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Volume 1 (1953)