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- Volume 11, Issue 3, 2005
Petroleum Geoscience - Volume 11, Issue 3, 2005
Volume 11, Issue 3, 2005
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Calibration improves uncertainty quantification in production forecasting
Authors Duane A. McVay, W. John Lee and Martin G. AlvaradoDespite recent advances in uncertainty quantification, the petroleum industry continues to underestimate the uncertainties associated with reservoir production forecasts. This paper describes a calibration process that can improve quantification of uncertainties associated with reservoir performance prediction.
Existing methods underestimate uncertainty because they fail to account for all, and particularly unknown, factors affecting reservoir performance and because they do not investigate all combinations of reservoir parameter values. However, the primary limitation of existing methods is that their reliability cannot be verified because the testing of an estimate of uncertainty from existing methods yields only one sample for what is inherently a statistical result. Verification and improvement of uncertainty estimates can be achieved with calibration – comparison of actual performance with previous uncertainty estimates and then using the results to scale subsequent uncertainty estimates.
Calibration of uncertainty estimates can be achieved with a more frequent, if not continuous, process of data acquisition, model calibration, model prediction and uncertainty assessment, similar to the process employed in weather forecasting. Improved ability to quantify production forecast uncertainty should result in better investment decision making and, ultimately, increased profitability.
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Uncertainty analysis of fluvial outcrop data for stochastic reservoir modelling
Authors A. W. Martinius and A. NæssUncertainty analysis and reduction is a crucial part of stochastic reservoir modelling and fluid flow simulation studies. Outcrop analogue studies are often employed to define reservoir model parameters but the analysis of uncertainties associated with sedimentological information is often neglected. In order to define uncertainty inherent in outcrop data more accurately, this paper presents geometrical and dimensional data from individual point bars and braid bars, from part of the low net:gross outcropping Tortóla fluvial system (Spain) that has been subjected to a quantitative and qualitative assessment. Four types of primary outcrop uncertainties are discussed: (1) the definition of the conceptual depositional model; (2) the number of observations on sandstone body dimensions; (3) the accuracy and representativeness of observed three-dimensional (3D) sandstone body size data; and (4) sandstone body orientation. Uncertainties related to the depositional model are the most difficult to quantify but can be appreciated qualitatively if processes of deposition related to scales of time and the general lack of information are considered. Application of the N 0 measure is suggested to assess quantitatively whether a statistically sufficient number of dimensional observations is obtained to reduce uncertainty to an acceptable level. The third type of uncertainty is evaluated in a qualitative sense and determined by accurate facies analysis. The orientation of sandstone bodies is shown to influence spatial connectivity. As a result, an insufficient number or quality of observations may have important consequences for estimated connected volumes. This study will give improved estimations for reservoir modelling.
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The effect of hydrocarbons on quartz cementation: diagenesis in the Upper Jurassic sandstones of the Miller Field, North Sea, revisited
Authors Nils Einar Aase and Olav WalderhaugVariable quartz cementation and porosity distribution in the Upper Jurassic Brae Formation deep-water sandstone reservoir of the Miller Field (UK, North Sea) is mainly controlled by the amount of coatings on the quartz grains. Oil was possibly present in the pore space for the last 40 Ma, but had no significant effect on preserving porosity in the oil leg relative to the water leg. Samples with anomalously high porosities, which commonly occur in the shallowest sandstone intervals, are microquartz coated, and it is a misinterpretation to explain these high porosities as due to hydrocarbons inhibiting quartz cementation. Porosity preservation due to microquartz coating is quite common in Upper Jurassic sandstones in the North Sea. Kinetic modelling of quartz cementation with the Exemplar program correctly predicts observed quartz cement volumes in the Brae Formation outside the microquartz-coated zones, when high quality petrographic data, including abundance of clay coatings, are provided as input. Significant overprediction of quartz cement volumes occurs only for samples where scanning electron microscope analysis shows microquartz coatings to be present. Failure to recognize grain coating as one of the major parameters controlling quartz cementation may lead to incorrect geological models of reservoir quality and selection of inappropriate exploration or production strategies.
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The structural geology of the High Central Zagros revisited (Iran)
Authors A. Bosold, W. Schwarzhans, A. Julapour, A. R. Ashrafzadeh and S. M. EhsaniThe main purpose of this study in the High Central Zagros area near the townships of Lordegan and Yasuj was to investigate the nature of the High Zagros Fault (HZF) and the structural styles in the adjacent foreland. A second task was to determine whether the prolific petroleum plays productive further out in the Zagros Foreland would extend into the study area and if new petroleum plays could be recognized.
Two selected cross-sections across and one just behind the HZF provide evidence for its overthrust nature. In some places the HZF exhibits a low-angle thrust plane, locally with a substantial amount of thrusting over the foreland. The interpretation is based on surface geological mapping, available well data, remote sensing from satellite data, aeromagnetic and high resolution gravimetry data, magnetotelluric, WARRP (wide angle reflection refraction profiling) and reflection seismic data. The integration of this unique set of newly acquired geophysical data complements recently published papers, which were based more on surface geology and rather limited subsurface data. The data were integrated to construct a regional balanced cross-section and to develop structural petroleum play concepts for the area investigated.
Play potential may exist both in the overthrust terrain of the High Zagros as well as in the subthrust. Analysis of the width of the surface anticlines (or more precisely: their frequency) gives a first indication of the depth of the décollement plane. Within the adjacent Zagros Foreland evidence was found for the presence of multiple detachment planes above the ubiquitous deep Hormuz level, i.e. at the level of the Kazhdumi and the Pabdeh–Gurpi Formations. These intermediate décollement planes are thought to be responsible for the generation of deeper structural plays unrelated to the structuring that can be observed at the surface.
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Exhumation of the Corrib Gas Field, Slyne Basin, offshore Ireland
Authors D. V. Corcoran and R. MecklenburghThe Corrib Gas Field contains in excess of 1012 SCF (28.3×109 Sm3) of gas in place in a Triassic fluviatile sandstone reservoir, sealed by late Triassic evaporites and trapped in a relatively simple, but underfilled, NE–SW-trending anticlinal structure. In contrast, the post-Triassic overburden manifests a more complex evolution with indications of flipped polarity, extensional faulting on a crestal detachment structure during the Jurassic, succeeded by alternate episodes of burial and exhumation from the Early Cretaceous onwards. Compactional, thermal and stratigraphic frames of reference have been used to assess the magnitude and timing of the principal exhumation episode, which occurred during the early Cretaceous. Large uncertainty is associated with exhumation estimates derived from individual techniques, but stratigraphic analysis and seismic interpretation support the conclusion that kilometre-scale (800–1700 m) exhumation and erosion of the Jurassic overburden has occurred in the Corrib area. A number of post-Aptian re-burial and exhumation events also occurred but are unlikely to have exceeded the earlier maximum burial of source, reservoir and seal rocks achieved during the Late Jurassic. Continued local extensional reactivation of the listric detachment structure, post-Aptian, has resulted in a pattern of heterogeneous exhumation within the field area. However, the severely attenuated post-rift stratigraphic record throughout the Slyne Basin suggests that long-wavelength processes, such as mantle hotspot activity and shoulder uplift linked to the rifting and thermal subsidence of the Rockall Basin, were the primary drivers of exhumation in the Corrib area during the Cretaceous-Tertiary period.
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Sequence stratigraphic-based analysis of reservoir connectivity: influence of depositional architecture – a case study from a marginal marine depositional setting
More LessThis case study of the Sunrise and Troubadour fields (offshore northwest Australia) concentrates on the impact of primary depositional architecture on reservoir connectivity via a sequence stratigraphic-based, 3D reservoir modelling approach. The marginal marine reservoir is composed of fluvial-dominated and wave-dominated depositional environments. The succession is divided into six sequences and 12 systems tracts. Each systems tract is subdivided into parasequences which form the basic building blocks of the 3D model. The connectivity of sandbodies within each parasequence, systems tract and sequence were calculated when the models were palinspastically restored to a depositional datum.
The findings indicate that depositional connectivity trends within a sequence stratigraphic framework are predictable. Connectivity trends can be related directly to depositional and stratigraphic trends and to position in the sequence stratigraphic hierarchy. Therefore, with a good understanding of depositional settings and high resolution sequence stratigraphic subdivision of strata, predictions of depositional connectivity trends at all stratigraphic hierarchical levels can be made. All connectivity trends at sequence and systems tract stratigraphic hierarchical levels remained the same when the area of the model was reduced by a factor of four and the volume of the model was reduced by truncation below the gas–water contact. Hence, the relationships between the stratigraphic trends and the connectivity trends for the thicker stratigraphic units can be said to be scale invariant.
Three-dimensional reservoir models currently provide the best means of quantitatively assessing and predicting reservoir connectivity. However, in low fault density settings, initial screening of reservoir connectivity can be made using connectivity indicators calculated from simple parameters derived from well data such as ‘Thickness divided by Sand/Shale ratio’.
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Sandstone porosity related to vitrinite reflectance
More LessVitrinite reflectance (VR) and the volume fraction of quartz cement in sandstones are both measures of thermal exposure. This paper shows how porosity reduction by quartz cementation can be related to VR for heating at a constant rate, using simple models for quartz cementation and VR. This relationship reveals that the heating rate is an explicit parameter, which means that there is no pure relationship between porosity (volume fraction quartz cement) and VR. However, the rate of quartz cementation and the rate of change of TTI (time–temperature integral) have a close exponential dependence on temperature. Porosity reduction by quartz cementation as a function of VR therefore nearly follows the same curve for very different temperature histories. This property of porosity reduction by quartz cementation is used to calibrate average specific surface parameters as a function of porosity. The calibration procedure is demonstrated on two datasets, where the average specific surface depends on porosity to the power of an exponent that is somewhat greater than 3.
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Volumes & issues
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Volume 30 (2024)
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Volume 29 (2023)
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Volume 28 (2022)
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Volume 27 (2021)
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Volume 26 (2020)
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Volume 25 (2019)
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Volume 24 (2018)
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Volume 23 (2017)
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Volume 22 (2016)
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Volume 21 (2015)
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Volume 20 (2014)
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Volume 19 (2013)
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Volume 18 (2012)
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Volume 17 (2011)
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Volume 16 (2010)
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Volume 15 (2009)
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Volume 14 (2008)
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Volume 13 (2007)
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Volume 12 (2006)
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Volume 11 (2005)
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Volume 10 (2004)
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Volume 9 (2003)
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Volume 8 (2002)
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Volume 7 (2001)
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Volume 6 (2000)
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Volume 5 (1999)
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Volume 4 (1998)
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Volume 3 (1997)
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Volume 2 (1996)
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Volume 1 (1995)