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Third EAGE Workshop on Rock Physics
- Conference date: November 15-18, 2015
- Location: Istanbul, Turkey
- Published: 15 November 2015
1 - 20 of 30 results
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Seismic Expression of Deep-water Slope Channel Complex & Frontal Splay Architectural Elements: Calibration with Outcrop & Sea Floor Analogues from Southern and Eastern Turkey & Various Modern Systems
By B. CroninSummaryFrontal splays and slope channel complexes make up the bulk of deep-water depocentre volume. Slope channel complexes, often fed through a series of gullies of upper slope/shelfal canyons, are well known and exceptional examples are exposed in turbidite-filled basins of southern and eastern Turkey. Frontal splays are located at or beyond feeder channel mouths, of variable dimension and character, they range in grain size from gravel to silt and yet appear to have a predictable and narrow range of geometry. Frontal splays in particular remain one of the most undersampled and unimaged deep-water sedimentary features, are often seen on seismic data where they straddle the resolution limits, and as a result there is no accepted general model for them. The terminology for deep-water depositional bodies that are not in erosionally based features remains highly inconsistent, with terms like ‘lobes’, ‘splays’, ‘frontal splays’, ‘mouth lobes’, ‘distributary lobes’, ‘channelised lobes’, ‘suprafan lobes’ etc all vying for widespread usage and promoting confusion across the study of modern and outcrop deep-water sedimentary systems and their reservoir equivalents. It is no surprise that general accepted principles of architecture, rock property distribution, body geometry, and even location on the sea floor, are not yet in place.
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Relative Rock Physics Templates in the Elastic Impedance Domain
More LessSummaryIn this paper we examine the relationship between rock properties and seismic data. We present a framework based on AVO principles which assists in the interpretation of seismic data. The method discussed here weaves together principles from the following:
- relative rock physics,
- rock physics templates, and
- elastic impedance,
The result is a relative rock physics template defined in terms of elastic impedance parameters. It provides a useful insight to seismic parameters in cross-plot space.
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Fluid and Rock Impedances: New Orthogonal Seismic Attributes that Honor Rock Physics
Authors P. Avseth and I. LehockiSummaryWe explore some new rock physics attributes that can be used to screen for reservoir sandstones and hydrocarbon pore-fill from seismic inversion data. We honor the physical properties of the rocks by defining attributes, in general referred to as fluid- and rock-impedances, that comply with calibrated rock physics models. Two different approaches are presented, both of which attempt to separate out the effects of fluids and rock stiffness. The first set of attributes include the fluid saturation sensitive CPEI (Curved pseudo-elastic impedance for saturation), and the rock stiffness/lithology attribute PEIL (Pseudo-elastic impedance for lithology), which are near orthogonal to each other in a Vp/Vs versus acoustic impedance (AI) domain. The second set of attributes includes a porosity attribute derived from shear impedance, and the fluid deviation attribute from a background trend in acoustic impedance. The latter set of attributes shows almost perfect orthogonality for any burial depth. We demonstrate the use of these attributes on well log and seismic inversion data from the Norwegian Sea, and successfully screen out reservoir rocks filled with hydrocarbons.
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Rock Physics Guided Quantitative Seismic Inversion
Authors R. Lubbe and M. El MardiSummaryThis study highlights the powerful application of RPTs in quantitatively describing the reservoir properties (e.g., porosity, mineralogical and fluid composition), using the inverted seismic data. Two very different case studies in Saudi Arabia have been presented. In case study 1, the RPT guided the seismic inversion to highlight areas of high effective porosity (i.e., reservoir). Case study 2 described how the RPT and seismic inversion were used to extract geologically reasonable geo-bodies. These geo-bodies were then transformed to a nett-pay map. The nett-pay map identified possible remaining pay zones in an already-producing onshore field.
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Improved Seismic Inversion and Facies Using Regional Rock Physics Trends: Case Study from Central North Sea
Authors A.V. Somoza, K. Waters and M. KemperSummarySeismic inversion is routinely used in the delineation of drillable leads and prospects. However, the algorithms and techniques that are most widely used today suffer from a number of shortcomings. In this paper we demonstrate how we can use per-facies compaction trends, derived from a regional rock physics study in Central North Sea, to perform joint inversion of seismic data from the Brenda and Forties fields. No wells from the Brenda or Forties field were used in deriving the per-facies compaction trends. The joint inversion results provide reliable estimates of seismic facies and corresponding absolute rock properties without the requirement for direct well calibration and model building.
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Impact of Low Frequency Broadband Land Seismic on Unayzah Fairway Characterisation: Exploration Perspective
Authors Ayman Suleiman, Luis Giroldi and Mukarram AhmedSummaryIn the early 1990s, Saudi Aramco embarked on an exploration strategy to find and develop significant, non-associated gas reserves for the Kingdom, which resulted in a number of major new fields. Many of these have been found in the Paleozoic Unayzah siliciclastic reservoirs, in the deep subsurface of the Eastern Province, specifically in the area surrounding the giant Ghawar structure. Standard quantitative seismic approaches in mapping potential hydrocarbon-bearing layers within the Unayzah fairways have been addressed, initially by Melvin et al. (2010) , as well as a combination of quantitative and qualitative workflows, with the same purpose ( Mustafa and Giroldi (2010) , Giroldi (2010) ), but were inadequate to fully understand the variations in reservoir fluid and mineralogy. Ahmed et al. (2013), presented an integrated workflow based on conventional seismic data, which yielded 3D lithofacies with associated hydrocarbon probabilities. Their methodology has proved to be encouraging in terms of providing a spatial understanding of the hydrocarbon-bearing clastic reservoir and also in reducing uncertainty in the placement of development wells. In the exploration context, there are still challenges to the proposed workflow, due to the limited well control, and hence this paper will focus on the impact of the newly acquired broadband land seismic, in exploration and characterization of the Unayzah fairways for exploration purposes.
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Porosity Concepts in Gassmann Fluid Substitution: A Simulator to Seismic Modelling Perspective
Authors H. Amini and C. MacBethSummaryIn this study, porosity concepts in Gassmann’s fluid substitution model are examined and their implications for quantitative 4D seismic studies are discussed. Three porosity models (total porosity, effective porosity, and a movable fluid model) are compared from a simulator to seismic (sim2seis) modelling perspective. From the three selected models, total porosity predicts the largest softening effect due to gas breakout and the smallest hardening effect from water-flooding; whereas the movable fluid model predicts the least softening due to gas breakout and the largest effects for water-flooding. Effective porosity predictions lie between the total porosity and movable fluid models. The differences between these models are due to the proportion of fluids in the mixture which are input into Gassmann’s equations. Sim2seis results based on different porosity models were evaluated against the observed 4D seismic. This comparison shows that the magnitude of the saturation-induced hardening and softening signals due to the movable fluid model is closer to the observed seismic. The total porosity model is in least agreement with the observed 4D seismic.
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The Rock Physics Model: A Key Element for Seismic Back-Loop from a Geological Model
Authors A. Nateganov, T. Cadoret, F. Pivot and S. AmoyedoSummaryWith the quality increase of seismic amplitudes it becomes often possible to use them to constrain the geomodel infilling in terms of facies or petrophysical properties. Their use can also be extended to the final stage of a geological model building study once the model is completed by checking if the infilling is coherent with the actual seismic response. This so-called Seismic Back-Loop (SBL) offers the opportunity to identify zones where the geological model infilling can be improved and how the petrophysical properties should be modified in order to improve the coherency with actual seismic response. Rock-Physics Model (RPM) is at the core of all computations done during SBL. It represents a quantitative link between the geomodel and seismic domains. RPM is also useful for qualitative and quantitative interpretation of observed mismatches between synthetic and actual seismic response. This imposes high requirements on the reliability of the seismic dataset chosen as reference and also on the accuracy of the RPM. The aim of this paper is to present a SBL case study and show the methodology associated to the use of an RPM during such workflow.
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Effect of CO2 on Rock Properties: Frio Crosswell Case Study
Authors M. Al-Hosni, S. Vialle, B. Gurevich and T. DaleySummaryA time- and –space dependent rock physics model using the constant cement model is used to interpret velocity changes observed from the CO2 sequestration site of Frio (Texas, USA). For this site, classical, purely mechanical rock physics models cannot explain the observed changes in both P-wave and S-wave velocities, suggesting changes in the rock frame itself. We introduce a non-elastic factor in the model in the form of cement percent change. The strategy was to estimate changes in the dry-frame from shear-wave measurements given by crosswell traveltime tomography, as the shear moduli is independent of the nature of fluids. Our approach allows variation of cement percent both in time and space, which effectively explains the time-lapse changes observed at the injection well and away from it. The model suggests that a small amount of cement is removed at grain contacts, consistent with minute dissolution.
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Quantitative Seismic Reservoir Mapping Using Rock Physics Templates: A Case Study From a Tught Onshore Reservoir in Saudi Arabia
Authors A. Alzahrani and R. LubbeSummaryA rock physics template (RPT) has been constructed using wireline data from six onshore wells penetrating a clean, tight, clastic, hydrocarbon-bearing reservoir of the (early) Permian age within Saudi Arabia. A multi-attribute inversion feasibility analysis suggested that a combination of both acoustic impedance and VP/VS would optimally describe the porosity and shale distribution within the reservoir. The rock physics template was designed to map the zones of higher porosity and variation in shale properties using these inverted acoustic impedance and VP/VS volumes.
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Geophysical Monitoring of CO2 Flow During Sandstone Flooding Experiments
Authors M. Soldal, J. Park, L.O. Lamech, T. Tran, G. Sauvin, Ø. Johnsen and N.H. MondolSummaryIn this study, liquid CO2 is injected into fully brine saturated reservoir core samples in the laboratory, and consequently changes in electrical resistivity, acoustic velocities (ultrasonic frequency) and their anisotropy are measured. To enhance the spatial resolution, a system enabling velocity and resistivity measurements at different points and in different directions along the specimen’s axial direction has been developed. Electrical resistivity and acoustic velocity and amplitude are all clearly influenced by pore-scale heterogeneity and fluid flow pattern and it is important to study this interaction. So far, focus has been related to CO2 geological storage, but the outline of the study is believed to also be applicable for CO2 injection for enhanced oil recovery (EOR). The study is still ongoing and some preliminary results are presented here and discussed.
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Study of Heterogeneity in Carbonate Rock Samples Using Digital Rock Physics
More LessSummaryCarbonate reservoirs are considered extremely complex due to their texture heterogeneity. Using new approaches in Digital Rock Physics (DRP) is possible to compute core plug sample properties and study the heterogeneity from digital image data generated from X-ray computed tomography (CT) scan. Some numerical methods are effective to calculate and analyze these samples properties. However, there is a limitation when the simulations are run at the scale of the whole core plug, especially for fluid flow simulation, due to the large amount of calculation and consequent large computer memory requirement. Therefore, simulations are often done only in subsamples of the core plug. To get the dynamic properties of the whole core plug samples and study their heterogeneity, we propose a combined approach of DRP. Digital imaging processing is used to run digital core models for the calculation of porosity and permeability. The Lattice Boltzmann Method (LBM) is used to simulate fluid flow and calculate the absolute permeability. Experimental measurements are used to compare with simulated results of subsamples which are selected from the whole core plug sample. By comparing and analyzing the results of subsamples, the dynamic properties of the whole core samples are obtained and the heterogeneity is studied.
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Application of Digital Rock Physics and FIBSEM Imaging to Unconventional Reservoir Characterization in the Najmah Formation
Authors P.K. Mishra and B.H. AkbarSummaryAn attempt has been made to characterize the reservoir properties of the low porosity carbonates of the Najmah Formation through digital rock studies and FIBSEM(focused Ion beam electron microscope) imaging to determine the Microstructural characters of mineral matrix, Organic matter distribution, Pore network and organic matter pore type, pore size and permeability variation in the kerogen rich Kimmeridigian stratigraphic interval. It is possible that intergranular clay pores in highly thermally mature rocks were originally filled with organic matter and that during progressive thermal maturation, transformation of organic matter to hydrocarbon s removed much of the porefilling organic matter. Segmented three dimensional focused ion beam electron microscope (FIBSEM) volume from Najmah Formation brings out the distribution of Connected porosity disconnected porosity and organic material. It has been observed that although the TOC content of the rocks are very good (>21%), and good maturity index (VR0>1)only few samples show good connected porosity within the organic matter. These intervals can be considered as the potential sweet spots in the Najmah Formation after integration with petrophysical and geomechanical parametres.
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The Influence of Pore Structure on the Velocity Dispersion Properties of the Tight-gas Sandstone
More LessSummaryThe pore structure of reservoir sandstones has significant influence on their elastic properties, and also determines the fluid-flow related wave dispersion and attenuation mechanisms. In the squirt fluid models, only the compliant pore or crack with fixed aspect ratio and concentration has been taken into account, and the fixed aspect ratio must be considered as the “average” one in the rock, which is not completely realistic. Based on an iterative procedure to add soft pore with different aspect ratio into the rock frame, the existing squirt fluid model was extended to consider complex pore structure of reservoir sandstones, especially when the aspect ratio has a relatively wide distribution. The new method can predict well the saturated velocities for experimental data at ultrasonic frequency by using the pore aspect ratio distribution. The calculated velocity gradually increases with frequency and finally approaches the high-frequency velocity limit. This observation suggests that the squirt flow may be still important even in the seismic frequency band, and can cause apparent velocity deviation from the predictions based on Gassmann’s equation.
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A Laboratory Study to Investigate Velocity Anisotropy of an Organic-rich Shale, Central North Sea, Offshore Norway
Authors M. Koochak Zadeh, O.Y. Ogebule, N.H. Mondol, J. Jahren and J. I. FaleideSummaryThis study investigates velocity anisotropy in a well-consolidated organic-rich shale drilled from the central North Sea, offshore Norway. Shales are known as anisotropic rocks showing vertical transversely isotropy. Quantification of anisotropy in shales attracts greater importance considering that shales comprise almost 70% of the buried rocks in sedimentary basins. Two core plugs were prepared perpendicular and at 45o inclination to the in-situ bedding planes in order to measure velocity variations in different directions by increasing the effective stress. The results show that anisotropy of measured shale is stronger for S-wave velocity. The anisotropy decreases with increased consolidation, but not significant, probably due to closing of pre-existing stress-induced fractures and microcracks in the tested sample. The outcomes of this study can contribute to indirect analyses of seal integrity of mudstones and shales from seismic velocity data.
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Rock Physics and Seismic Modelling
By A. DraegeSummaryThis study shows a new way of applying rock physics depth trends. In seismic modeling we want to preserve internal geological variations in a system, even if depth and temperature change and diagenesis progresses. We suggest a workflow for creating realistic depth trends for whole geological systems, while honouring the internal variations within the system.
A shale - sand interface can appear as soft, dim or hard, dependent on where in the diagenetic process the rocks are. This is also reflected in the rock physics approach described.
The vertical variation observed in a geological system will be honored when applying it as an analogue on a new location, to create a realistic representation of the subsurface geology in the prospect area.
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AVO Modeling and Analysis Constrained by Burial History: Barents Sea Demonstrations
Authors I. Lehocki and P. AvsethSummaryIn this paper we use combined burial history and rock physics modelling of sand and shale, in order to predict expected AVO signatures for a given burial history. The advantage with this approach is that we can extrapolate away from wells in areas with complex tectonics. We can also simulate expected AVO signatures as a function of geologic time. Finally, we can use burial history to create AVO pdfs for calibration and classification of AVO attributes in a given field. We demonstrate the use of these techniques on the Myrsildre and Skalle wells in the Barents Sea.
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Inversion of Rock Physics Properties From Seismic Attributes Using a Downscaling Method
Authors B. Dupuy, H. Balhareth, M. Landrø, S. Garambois, A. Stovas, A. Asnaashari and J. VirieuxSummaryAn underground gas blowout occurred in 1989 during a well drilling in North Sea. As a result, gas has migrated to shallower layers ( Landrø, 2011 ). Repeated 2D lines were acquired as part of a site survey. We ultimately aim at estimating the gas saturation and rock frame properties for a shallow sand layer (480 meters deep) where gas has accumulated. To serve this objective, we developed a two-steps inverse approach: first, we obtain the seismic P-wave velocities (before and after blowout) using seismic full waveform inversion (FWI). The acoustic FWI was applied in the time domain ( Balhareth and Landrø, 2015 ). We downscale these macroscale data to microscale porous medium properties. First, we estimate frame properties of the reservoir rocks using the baseline’s inverted velocity model and, then, we estimate the gas saturation change using the monitor’s velocity model. The Biot-Gassmann’s ( Biot, 1956 ; Gassmann, 1951 ) relations are the basis for the downscaling of poroelastic parameters. For partially saturated media, these equations are generalized using frequency-dependent moduli ( Pride et al., 2004 ; Dupuy and Stovas, 2014 ). We present the forward model equations which are the basics of the inverse downscaling method. Then, we show the data obtained by time-lapse FWI which are used in the downscaling process. Finally, the results of the downscaling for baseline and monitor models are presented.
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Sea Lion Field, North Falkland Basin: The Use of Seismic Attributes in Fan and Reservoir Definition
By R.J.W. BuntSummaryFan bodies in the North Falkland Basin, including those that comprise the reservoir in the Sea Lion field, have been defined seismically with the aid of several seismic attributes. This has provided an understanding of their geomorphology, potential reservoir variations and distribution, and has helped illuminate the full extent of some fans as well as the internal architectural detail of others. The attributes used include reflectivity attributes on full-stack and part-stack 3D seismic data, impedance attributes on inverted seismic data, isochron attributes and geometric attributes such as coherence, dip, curvature and spectral decomposition. The combination of spectral decomposition and high resolution visualisation techniques has greatly aided the identification and interpretation of some of the fans. Seismic attribute responses have been quantified with the integration of well data from the 18 wells within the 3D data-set. This has aided the detailed reservoir characterisation and definition of geological features within some of the fan bodies. This detailed use of seismic attributes on the North Falkland Basin 3D data-set has been of benefit for both the appraisal and development of the Sea Lion field, and has also helped to define future exploration targets and well locations within the basin.
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Joint Interpretation of Electric and Elastic Measurements in Frontier Exploration
Authors K. Senger, S. Fanavoll, E. Nerland, P. Gabrielsen, R. Svendsen, B. Kjølhamar and D. BaltarSummaryIn frontier exploration, explorationists often struggle with lack of information, particularly due to lack of well data. Exploration thus relies primarily on elastic (e.g., 2D/3D seismic) and electric (e.g., 3D controlled-source electromagnetic surveys) data sets between the few (if any) wildcat boreholes. Traditional exploration involving rock physics relies almost exclusively on elastic parameters derived from seismic inversion, in itself a challenging task without adequate well control. Inversion of CSEM data, on the other hand, is much more robust in the absence of well control and provides a geologically meaningful representation of sub-surface resistivity. Having two geophysical data sets sensitive to the same subsurface, but relying on different physics, reduces uncertainty in frontier exploration if these are integrated at the interpretation stage. Furthermore, universal rock physical relationships linking elastic and electric properties to various geological parameters (e.g., Hashin-Shtrikman bounds, Archie’s Law) can be used to relate the two measurement types to geological drivers, such as porosity and fluid trends. Here we present a simple workflow incorporating both elastic and electric data sets in frontier exploration, combining them using cross-plotting. We illustrate how the workflow can be used on a regional-scale case study where resistivity and velocity maps can be used together to provide a qualitative background porosity map.
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