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EAGE/SEG Research Workshop on Fractured Reservoirs-Integrating Geosciences for Fractured Reservoirs Description 2007
- Conference date: 03 Sep 2007 - 06 Sep 2007
- Location: Perugia, Italy
- ISBN: 978-90-73781-74-0
- Published: 03 September 2007
1 - 20 of 42 results
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Seismic Detection of Fractured Reservoirs: Progress and Challenges
By L.A. ThomsenSeismic anisotropy provides an indirect measure of fracture distributions. The interpretation of that seismic observation is a matter of rock physics, and requires an adequate accounting of rock-fluid interaction, and of fracture size and shape.
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Fractal Behavior of Permian Fault Systems and Seismic Facies Analysis Derived from 3D Seismic Data
Authors P.O. Thierer, H. Trappe, H. Endres, T. Lohr, C.M. Krawczyk, O. Oncken, D.C. Tanner and P.A. KuklaWithin the presented case-study different disciplines worked together improving the knowledge of the developement of the North German Basin. We focus the area of a producing gas field with special respect to the analysis of seismic and sub-seismic deformation which requires a very detailed fault detection to bridge the information gap between seismic data and well data. Advanced coherency algorithms as Shaded-Relief take into account local dip and azimuth distribution of coherent energy. This leads to detect additional tectonic lineaments than could have been inferred from conventional amplitude images. Beside fracture attributes like fault density by length or connectivity, we show the attribute Fractal Dimension, which is an index for the relation of long to short fractures. Results are displayed in a map showing the areal distribution of the attribute. Correlation between FMI/FMS Log Data and FD values is linear. Using this relation, we end up with a predictive FMI-map (only based on seismic data) showing the spatial distribution of fracturation for undrilled areas. Neural Network analysis was used to classify the seismic data at reservoir depth. Plotting the results yields to a map of the seismic classes, each representing the quality of the reservoir.
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Prestack P-wave Seismic Anisotropy Analysis for Fractures Detection and Characterization in Carbonate Reservoirs
More LessDynamic well data indicated a communication between two hydrocarbon producing Carbonate Jurassic reservoirs: Arab D and Hanifa, in an oil field in Saudi Arabia. Detecting and characterizing the existing fracture system(s) using 3D P-wave prestack seismic data is the main objective of this study. The azimuthal variations of the normal moveout (NMO) velocities and amplitude-variation-with-offset-and-azimuth (AVOA) have been analyzed for Arab D and Hanifa reservoirs, in addition to the upper Fadhili formation. The azimuthal NMO velocity anisotropy suggests the existence of two dominant populations of fracture systems orthogonal to each other: NE (~50-70 deg from N) and SE (~140-160 deg from N). AVOA anisotropy in particular further suggests that the NE trending anisotropy (~65-70 deg from N) is the most dominant fractures orientation among the two. These conclusions are consistent with dynamic data, and with the information obtained from image logs which indicates the dominance of a fracture system oriented ~N70E. The NMO velocity and AVOA ellipticity maps highlight the fractures induced anisotropy network and their intensities allowing more optimized well planning and reservoir development. This is the first successful application of 3D P-wave prestack seismic azimuthal anisotropy analysis for fractures detection and characterization in carbonate reservoirs in Saudi Aramco.
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Relating Surface Seismic Attributes to Borehole and Core Measurements: A Case Study in the Barnett Shale, USA
Authors K.E. Beeney, J. Rich and M. AmmermanThe use of 3D seismic attributes has expanded dramatically and shows substantial promise for current and future applications. Two of the more common attributes, seismic anisotropy and curvature, have been compared to well and core data to understand the correlation between small scale rock physics at the borehole and their manifestation on surface seismic. Surface azimuthal anisotropy directions are shown to correlate with the borehole sonic data. The magnitude of the surface seismic anisotropy is seen to be greater in the faster rocks. This is consistent with the sonic logs which show higher anisotropy in the faster, more fractured limestones. Curvature is seen to be related to anisotropy and can be explained by fractures opening or closing, depending on the curvature.
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Integrating Multi-scale Geological and Reservoir Simulations to Predict Fracture Productivity; Case Studies from Repsol
Authors M. Bonora, H. Araujo, J.L. Silvestro, T.R. Zapata, M. Zubiri, L. Villalobos Vencela and A. SerranoIn this paper we present three different case studies where a variety of natural fractures information are described, organised and integrated to build geologically realistic discrete fracture networks models (DFNs). These DFNs are then analysed and calibrated with dynamic well tests to aid both development well planning and reservoir evaluation.
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Seismic Characterization of Fractured Tight Gas Reservoirs, Piceance Basin, Colorado
Authors K.T. Lewallen, G. Chen and X. WuThe Piceance basin has large gas reserves stored in a thick section of tight clastic rocks. Fractures can significantly enhance production and the reliable pre-drill prediction of fracture anomalies is the key to economic success. An extensive test combining logging, core measurements, numerical modeling, borehole seismic and surface seismic data has been collected in the northern Piceance basin to determine the seismic characterization of fractures. Preliminary results to date include: • Fractures occur in sand intervals • Seismic anisotropy (P- and S-wave) has a positive correlation to fracture occurrence • Anomalies exist when anisotropic P-wave modeling is compared to isotropic results. This phenomenon may be enhanced for PS-converted waves. Remaining work will examine whether these observations may be observed on surface measurements.
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Geomechanics-Seismic Links for Fracture Identification
More LessGeomechanical simulation and seismic anisotropy detection have been integrated at the process level to improve identification of open fracture systems. The integration is accomplished via a cross-comparison in the damage domain, which reduces the inherent uncertainties of the individual methods. The geomechanical component can be simple, such as considering how pre-existing fracture networks might respond in a present-day loading arrangement, or can be more complex, addressing the evolution of rock properties during the tectonic evolution. The seismic anisotropy predictions are based on the parallel-fracture-set model. At present, these seismic anisotropy prediction methods represent a limiting factor because they are based on that convenient, but not always applicable, concept of fracture distributions. The geomechanical simulations can, in principle, be used to represent very complex fracture systems, and we hope to overcome the seismic anisotropy limitations to move both components forward. One of the main limitations in the geomechanical method is that it is difficult to identify a suitable validation. In this paper, we outline a potential validation case that has both experimental and field aspects. The link between geomechanics and seismic anisotropy is likely to prove very useful for investigating some types of natural fracture systems.
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Curvature and Fracturing Based on GPS Data Collected at Sheep Mountain Anticline, WY
Authors P.F. Allwardt, N. Bellahsen and D.D. PollardBedding curvature has often been used to predict areas of increased fracture intensity. We investigate this curvature-fracture relationship at Sheep Mountain Anticline, WY, USA, by coupling fracture mapping with structural mapping using high precision GPS. Comparison of principal curvature magnitudes with fracture measurements indicates that greater curvature correlates with greater dispersion of fracture orientations. Fracture intensities, however, do not correlate with curvature.
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CT-scan Images of 3D Natural Open Fracture Networks in a Porous Media; Geometry, Connectivity and Impact on Fluid Flow
Authors O.P. Wennberg, L. Rennan and R. BasquetComputed Tomography (CT) scan imaging techniques have been used on core samples to investigate the effect of natural open fractures on reservoir flow in the Snøhvit Gas Condensate Field. Firstly, CT scanning was used to describe the 3D geometrical properties of the fracture network including orientation, fracture density and fracture connectivity. Two types of fractures are observed: F1 fractures are short and stylolite related, and F2 fractures are longer crosscutting the core and without any obvious relationship to stylolites. Secondly, monitoring of single and two phase flow experiments on samples containing natural open fractures was performed under 20 and 90 bar confining pressure by using CT-scanning. 1-phase miscible flooding experiment show approximately 3 times higher flooding velocity in an open F2 fracture compared to the matrix. 2-phase flooding by gas injection into a 100% water saturated core gave early gas breakthrough due to flow in the fracture and thereafter very little water production. CT-scanning of core samples provides an effective tool for integrating geology and fluid flow properties of a porous fractured media.
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Structural Controls on Volcanic Reservoirs: Application of Borehole Images
Authors C. Shrivastva, N. Machin, S. Sinha and A. PalFaults and fractures play a very important role in occurrence of oil in the field. The fractures which strike perpendicular or oblique to the seismic-level faults were found to be contributing to oil production whereas the fractures running parallel to the seismic-level faults did not contribute to the oil production
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Fracture Systems Associated with the Menchaca Anticline, Sabinas Basin Gas Field (Northern Mexico)
Outcrop analogue studies can significantly improve the understanding of fracture distribution and their impact on fluid flow in fractured hydrocarbon reservoirs. In particular, outcrop analogues may reduce the uncertainties about some fracture parameters difficult or impossible to manage if one uses only subsurface data. In this paper, we document the results of the analysis and characterization of the fracture pattern in sedimentary rocks of Menchaca anticline (Sabinas Basin, Northern Mexico). This is a symmetric, evaporite-cored detachment fold developed during the Laramide Orogeny and is interpreted to be a perfect analogue of the gas-producing subsurface structures of the Sabinas Basin. Our work aimed to analyze the evolution of the fracture systems within the framework of the fold development, quantify their density, and identify the main factors controlling their variability. The results of our outcrop study were compared with those coming from the analysis of the micro-resistivity wellbore image logs acquired in two producing wells drilled in a gas field whose structural setting is similar to the Menchaca fold. In conclusion, the comparison and integration of the subsurface and outcrop data in the Menchaca fold shows the relevance of outcrop studies for geologic modelling and characterization of fractured reservoirs.
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Quantitative Descriptions of Faults and Microfractures from Surface Outcrops of a Fractured Dolomite Reservoir
By K. DeckerThe outcrop study focuses on the qualitative and quantitative assessment of fractures of different scale (subseismic faults, slickensides, microfractures) in Late Triassic dolomite of the Austroalpine Calcareous Alps. Outcrops are regarded surface analogues of deep oil and gas fields in the subcrop of the Vienna Basin with respect to lithology, lithostratigraphy and deformation history. Results include assessments of the fractured matrix (microfractures, joints) carrying most of the fracture porosity, faults, and fault rocks. The latter form marked discontinuities in the fractured host rock. All structures are quantitatively described with respect to their abundance, geometry and the properties of fracture fill and fault rocks. Depth extrapolation of the surface-derived fault data to reservoir depth (about 3 km) is enabled by a regionally consistent deformation model and the genetic classification of faults to well-defined and dated deformation events.
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Geological Characterization and 3D Modelling of North Oman Cretaceous Reservoirs’ Fracture Network
More LessThe presentation will describe the fracture network characterization at regional scale,field scale and well scale. Focus is on data integration and on understanding the main aspect of fracture that impact on production
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The Compliance of Macrofractures
More LessNumerical modelling studies of wave propagation through fractured media which require the normal and shear compliance of individual fractures to be specified are increasingly common, whilst it is generally acknowledged that laboratory and field fracture compliance data are extremely limited. Using a combination of analytical and numerical modelling it is shown that macrofractures, defined as having lengths of approximately a metre to a few tens of metres, might be expected to have compliance values of order no greater than 10-11 m/Pa. This would result in P wave anisotropy less than or equal to 1% ( =0.01). The normal compliance of water filled fractures is predicted to be an order of magnitude less than gas filled fractures. However, the compliance of water and gas filled fractures can be similar at exploration seismic frequencies if the rock matrix is sufficiently porous and permeable to enable fluid to diffuse in and out of the matrix material. The need for additional field estimates of fracture compliance is emphasised.
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Seismic Interpretation of Fracture Specific Stiffness
More LessExploring the subsurface requires an understanding of the relationships among physical processes that occur on multiple length and time scales. Fractures and other mechanical discontinuities have geometric length scales that can be altered over time through such physical processes as stress, fluid transport, geochemical interactions, etc. This paper examines effect of the geometric length scales of discontinuities and the alteration of these length scales on the interpretation of fracture properties from seismic measurements. For a given seismic frequency range, the predicted fracture specific stiffness as a function of frequency is a key diagnostic indicator of the spatial homogeneity or heterogeneity within a fracture.
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Aligned Vertical Factures, HTI Symmetry, and Thomsen Parameters
More LessThe Sayers and Kachanov (1991) crack-influence parameters are shown to be directly related to Thomsen (1986) weak-anisotropy parameters for fractured reservoirs when the fracture density is small enough. These results are then applied to seismic wave propagation in reservoirs having HTI symmetry due to aligned vertical fractures. The approach suggests a method of inverting for fracture density from wave speed data.
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Inversion of AVAZ for Characterizing Fractured Reservoirs with Multiple Fracture Sets
By N.R. BartonNatural fractures in reservoirs play an important role in determining fluid flow, and knowledge of the orientation and density of fractures is required to optimize production. Variations in reflection amplitude as a function of azimuth and incidence angle are sensitive to the presence of fractures, but current models used to invert the seismic response often make simplified assumptions that prevent fractured reservoirs from being characterized correctly. For example, many models assume a single set of perfectly aligned fractures, whereas most reservoirs contain several fracture sets. In this presentation, the variation in the reflection coefficient of seismic P-waves as a function of azimuth and offset is used to determine the components of a second-rank fracture compliance tensor. The variation in the trace of this tensor as a function of position in the reservoir can be used to estimate the variation in fracture density and permeability of the fracture network, and may be used to choose the location of infill wells in the field. The use of this tensor to estimate the anisotropy of the permeability tensor, the orientation of deviated wells, and the relative orientation of neighboring infill wells to ensure adequate drainage will be discussed.
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Two Models for Seismic Attenuation and Dispersion in Fractured Porous Reservoirs
Authors B. Gurevich, M. Brajanovski, R.J. Galvin and G. LambertNatural fractures in hydrocarbon reservoirs can cause significant seismic attenuation and dispersion due to wave induced fluid flow between pores and fractures. We present two theoretical models explicitly based on the solution of Biot’s equations of poroelasticity. The first model considers fractures as planes of weakness (or highly compliant and very thin layers) of infinite extent. In the second model fractures are modeled as thin penny-shaped voids of finite radius. In both models attenuation exhibits a typical relaxation peak around a normalized frequency of about 1. This corresponds to a frequency where the fluid diffusion length is of the order of crack spacing for the first model, and the crack diameter for the second. This is consistent with an intuitive understanding of the nature of attenuation: when fractures are closely space, the waves reflected/scattered by cracks interfere with each other, with the interference pattern controlled by the fracture spacing. Conversely, if fracture length is smaller than spacing, then fractures act as independent scatterers and the attenuation resembles the pattern of scattering isolated cracks.
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A Rock Physics Model for Fractured and Patchy-saturated Reservoirs
Authors M. Brajanovski, T.M. Mueller and J.O. ParraA rock physics model is presented for dispersion and attenuation of compressional waves in fractured porous rocks that are saturated by a mixture of liquid and gas. These two different types of heterogeneities are described by four parameters: The fracture spacing and fracture weakness characterizing the fractured medium; the correlation length and degree of saturation characterizing the fluid patches that are embedded between the fractures. This model is employed to explain the extremely strong P-wave velocity dispersion in a narrow frequency range found for a limestone reservoir. The results indicate that seismic attenuation and dispersion may provide additional constraints on relevant reservoir parameters.
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Frequency-dependent Anisotropy of Rocks with Complex Fracture Networks
By M. ChapmanFracture models which are commonly used to interpret seismic anisotropy tend to be more simple than those used by geologists to characterise fracture networks. Frequency dependence of seismic anisotropy provides a new avenue for constraining more complex fracture models from our seismic data. In this paper we present a new model for calculating the frequency-dependent anisotropic properties of rocks with complex fracture systems. Analysis of the model suggests attributes which can be related to important properties of the fracture networks, particularly scale-lengths, connectivities and fluid saturations.
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