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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
21 - 40 of 42 results
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Seismic Inversion for Azimuthally Anisotropic Models of Fractured Formations
More LessThis invited overview paper discusses recent advances in seismic data processing and inversion for azimuthally anisotropic media and application of kinematic and amplitude signatures of reflected waves in fracture characterization. Because of the complexity of fractured reservoir models, it is beneficial to acquire wide-azimuth, multicomponent data and combine the results of moveout and AVO (amplitude-variation-with-offset) analysis. Still, quantitative fracture characterization faces significant challenges, especially for reservoirs with multiple fracture networks. The transition from the effective parameters that control seismic signatures to fracture orientations and compliances requires using additional information (e.g., well logs, core measurements, and geologic data). The potential and pitfalls of using long-spread, wide-azimuth P-wave data in fracture detection is illustrated on a case study from Rulison field in Colorado. A comprehensive processing sequence designed for layered azimuthally anisotropic media helped to identify pronounced azimuthal AVO anomalies at the top and bottom of the tight gas sand reservoir. Geologic and well-log information indicates that these anomalies correspond to areas of intense fracturing. Estimation of the fracture parameters, however, is complicated by a poor correlation between the AVO and NMO (normal-moveout) ellipses.
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Characterization of Multiple Fracture Sets at Rulison Field, Colorado, USA
Authors V. Grechka and I. VasconcelosConventional fracture-characterization methods assume the presence of a single set of cracks in the subsurface. We relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets. Our technique relies on recent findings indicating that multiple, differently oriented, possibly intersecting cracks embedded in an otherwise isotropic host rock result in nearly orthotropic effective media. Here, we estimate the governing parameters of crack-induced orthotropy from 3D, wide-azimuth, multicomponent seismic reflection data acquired over the tight-gas Rulison Field in Colorado. We translate azimuthal variations of the normal-moveout velocities into interval crack densities, fracture orientations, type of fluid infill, and velocities of P- and S-waves in an unfractured rock. Our inversion procedure identifies one set of cracks in the western part of the study area and multiple, likely intersecting fractures in its eastern part. We validate both our underlying theoretical model and the obtained estimates by two independent measurements: the estimated fluid-infill parameter indicates dry cracks as expected for the gas-producing sandstones at Rulison; and the obtained crack orientations are supported by well observations. As a by-product of fracture characterization, we build an orthorhombic velocity model of the Rulison reservoir.
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Reconstruction of the Layer Anisotropic Elastic Parameters and High-resolution Fracture Characterization from P-wave
Authors R. Bachrach, M. Sengupta, A. Salama and N. DuttaIn this paper we show how layered based high resolution estimates of component of the subsurface anisotropic elastic tensor can be reconstructed by using wide azimuth P wave data. We combine the kinematic information derived from anisotropic velocity analysis together with dynamic information derived from AVAZ analysis of wide azimuth seismic data and derive interval P-impedance (Ip), S-impedance (Is), and anisotropic parameters associated with anisotropic fracture media. These parameters can be used to infer fractured rock properties using stochastic rock physics inversion. We characterize a fractured reservoir by quantitative interpretation of seismic interval (or layer) properties. We use three seismic inversion attributes – P-impedance (IP), S-impedance (IS), and an anisotropy attribute (a linear function the anisotropic parameters in the subsurface), obtained by integrating data from kinematics (traveltimes) and dynamics (reflectivities) Ultimately, we use rock physics model-based Bayesian inversion to derive porosity and crack density estimates along with their associated uncertainties.
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Fracture Properties Inversion from Azimuthal AVO Using Singular Value Decomposition
Authors I. Varela, S. Maultzsch, X.Y. Li and M. ChapmanWe develop a technique for inversion of fracture properties from azimuthal AVO. The method is based on the decomposition and reconstruction of the amplitude changes with azimuth in terms of principal components, which are calculated via singular value decomposition (SVD). We present the method and illustrate its application to surface seismic with a synthetic example of multi-azimuth common-offset gathers with different fracture densities.
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Reflection Response of a Fractured Medium: Physical Modelling Versus Numerical Simulations
Authors M.A. Alhussain, E. Liu, B. Gurevich and M. UrosevicAzimuthal variation of the AVO response (AVOaz response) of fractured reservoirs is usually modeled using equations for reflection coefficients obtained for plane waves. However, plane wave approximation can break down at long offsets where incidence angle approaches critical angle. Since AVOaz response is often more noticeable at large offsets, spherical wave effects must be carefully analysed and taken into account. In order to analyse these effects quantitatively we performed an AVOaz laboratory experiment and then numerically simulated this experiment. The AVOaz response of a physical model is studied in laboratory with finely layered Plexiglas simulating vertical fractures. Transmission measurements are performed to construct the elasticity tensor for the HTI model. This elasticity tensor is used as an input into numerical simulations which are performed using an anisotropic full-wave reflectivity algorithm. The comparison of the experimental data with simulations shows a very good match. The agreement is especially good for critical angles extracted by picking inflection points on AVO curves for each azimuth. This shows that (1) reflection measurements are consistent with the transmission measurements; (2) the methodology of picking critical angles on seismograms using the inflection point is robust, even in the presence of random and/or systematic noise.
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Analysis of Scattered Signal to Estimate Reservoir Fracture Parameters
Authors S. Grandi, M.E. Willis, D.R. Burns and M.N. ToksozWe detect fracture corridors and determine their orientation and average spacing based on an analysis of seismic coda in the frequency-wave number (f-k) domain. Fracture corridors have dimensions similar to seismic wavelengths which causes scattering. The distribution of energy in shot records in the f-k domain depends upon its orientation to the fracture strike. In the direction normal to fractures, energy reflected and transmitted at the reservoir level mostly propagates as scattered waves with slower apparent velocities than waves propagating along the fracture channels. The associated f-k spectral differences allow the identification of the preferred fracture orientation and spacing. We apply our technique to a fractured reservoir in the Lynx field, in the Canadian foothills. The estimated preferential fracture orientation is about N40°E, which agrees with regional stress measurements. The average fracture spacing is 75 m on the West side of the survey, while the fractures are more sparse on the East side. We also applied the Scattering Index methodology (Willis et al., 2006) to the same data, post-stack and pre-stack. This technique has higher resolution to map fracture distribution, intensity and orientation, and therefore complements the spectral method in providing an integrated description of reservoir fractures.
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Diffraction Imaging for Fracture Detection
Authors E. Landa, S. Fomel and T.M. TanerThe present way in fractures studies usually consists in using indirect indicators such as anisotropy, well log information etc. On the other hand, it is diffracted waves that contain direct and the most valuable information about fracture presents in the subsurface. Diffracted waves are fundamentally different from seismic reflections conventionally used in seismic method. Using synthetic and field data examples, we demonstrate the possibility of separating seismic diffractions in the data and imaging them with optimally chosen migration velocities. Our criterion for separating reflection and diffraction events is the smoothness and continuity of local event slopes. Our criterion for optimal focusing is the local varimax measure. The objective is fast velocity analysis in the pre-stack or post-stack domain and high-resolution imaging of small-scale heterogeneities.
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Fractured Reservoir Parameters from Borehole Image Analysis
Authors A. Etchecopar, H. Onda, J. Kheroubi, L. Souche, C. Demichel and Y. WangThe success of borehole imaging is widely due to the need, expressed by reservoir modelers, for fracture detection and quantification. The way geologists describe fractures through clustering in different sets, mainly addresses the timing of the structural evolution. This is not necessary the best approach for reservoir modeling that requests more quantification of the fracture network. As a borehole image is both a virtual outcrop, and a high resolution measurement of the resistivity, it can feed both approaches. In favorable cases images provide orientation, density, aperture and even an estimation of the fracture length and height. They also provide the relationship between fractures and bedding or between different fracture sets. These parameters directly constrain the fracture network properties at reservoir scale: density, permeability and connectivity. Images can also be interpreted in terms of structural models from bed boundary dips. These models provide valuable information on relationships between structures and fracture networks.
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Fracture-induced Sonic Anisotropy Characterization Using a Combination of Borehole Image and Sonic Logs
Authors R. Prioul, A. Donald, R. Koepsell, T. Bratton, C. Signer, A. Boyd and A. EtchecoparWe develop a methodology to model and interpret borehole dipole sonic anisotropy related to the effect of geological fractures using a forward modeling approach. We use a classical excess-compliance fracture model that relies on the orientation of the individual fractures, the compliances of the fractures, and the compliances of the host rock. We extract the orientation of individual fractures from borehole image log analysis. We validate the model using borehole resistivity image and sonic logs in a gas-sand reservoir over a 160-ft vertical interval of a well. We observe significant amounts of sonic anisotropy and numerous quasi-vertical fractures. We show that using just two adjustable fracture compliance parameters, one for natural fractures and one for drilling-induced fractures, is an excellent first-order approximation to explain the fracture-induced anisotropy response. We assumed equal normal and tangential compliances. Predicted fast-shear azimuth matches measured fast-shear azimuth over 130 ft. Predicted slowness anisotropy matches the overall variation and measured values of anisotropy for two anisotropy zones. The medium is mostly a horizontal transverse isotropic medium (HTI). We also show that the measured sonic anisotropy is caused by the combination of stress and fracture effects.
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Fracture Density Prediction Using Elastic Parameters of Rocks in Fractured Reservoirs
Authors P. Habig, L. Mattioni, L. Micarelli and A. FerrerDiffuse fracture characterization, in hydrocarbon producing reservoirs, is generally based on analysis of BHI logs. It is thus strongly dependent on quality and availability of these data. We propose a new method of diffuse fracture density estimation, which is based on relations between dynamic elastic parameters of rocks and fracture density. Based on RHOB and DSI Sonic logs (DTCO and DTSM) which permit calculating logs of dynamic elastic parameters, this method allows generating simulated fracture density logs by an empirical relation using parameter E/PR (Young modulus/Poisson ratio). The method has been applied to four fractured reservoirs worldwide. It gives some good preliminary results, especially for carbonated fields (limestones and dolomites) with diffuse fractures.
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A Pilot Multi-azimuth VSP Study for Fracture Characterization Using Seismic Anisotropy
Authors S. Maultzsch, R. Nawab, S. Yuh, B. Mouly, M. Idrees, F. Amraoui and B. FrignetWe present the analysis of a multi-azimuth VSP that was designed as a pilot study in one of our tight gas fields to evaluate the quality of seismic anisotropy measurements in this area, with the objective of using them for fracture characterization. We investigate various different anisotropy attributes, which are shear-wave splitting, azimuthal variation of P-wave traveltimes, and azimuthal AVO. The assessment of these attributes provides important guidance for the analysis of anisotropy from surface seismic data in the area. We find clear evidence of azimuthal anisotropy in the VSP data with the most robust measure being shear-wave splitting, but also P-wave traveltime anisotropy can be observed. Other borehole data from the same well suggest that no open fractures intersect the well itself. We resolve this apparent discrepancy between VSP and borehole measurements by seismic modelling with discrete fracture zones that do not intersect the well, but are within the VSP investigation radius.
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An End-to End-Approach to Naturally Fractured Reservoir Modeling: Workflow and Implementation
Authors L. Souche and M. RotschiFracture network models provide both qualitative and quantitative information about the reservoir. They can serve various, sometimes contradictory, purposes: understanding early water breakthrough, optimizing productivity or maximizing recovery. In order to optimize its predictive value, the model must account for all available data: seismic, well logs, well tests and production history. In this paper, we present a general workflow and implementation for building an easily updatable fractured reservoir model. It consists in (1) building a statistical representation of the fracture network, taking into account the spatial variability of the parameters that describe the model (fracture density, orientation, etc.), (2) generating stochastically a geologically consistent discrete fracture model from this representation and (3) computing equivalent fracture permeability, porosity and shape factor to feed the flow simulation model. This process is tightly integrated in the reservoir modeling workflow, so that model updating and uncertainty analysis can be automated.
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Fracture Network Model and Unsealing Analysis of Tight Sandstone Reservoir in Dongpu Depression Bohaiwan Basin, China
More LessFracture systems are considered to influence productivity significantly in tight sandstone reservoir. Understanding the in-situ fracture network is of importance in the exploration and development of natural gas reservoir. The paper illuminates that 3Dmove stress-strain analysis based on geologic process is a more credible prediction method. It can also estimate the unsealing of different fracture orientation. It is where the shoe pinches to establish reliable geological fracture network model and evaluate fracture unsealing extent by appropriate kinematics structural inverting arithmetic. This study showed the workflow of fracture modeling and the application in a sandstone reservoir using structural validation tools and interactive fracture simulations constrained with lithology, well log and core data. The fracture network model can help us understand not only what a fractured reservoir looks like, but also how the existing fracture network behaves. The presented method only show how we assess the fracture network in the area, every geological setting has its own peculiarities and needs a proper approach. In any case, we need to understand and constrain the geological history to assess the fracture network properly.
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Modelling a Fractured Dolomite Reservoir Without Substantial Information – a case study
Authors J.R. Püttmann and U. EickelbergA fractured dolomite reservoir of an alpine thrust nappe, influenced by several deformations, was modelled for simulation requirements. A three step workflow (data analysis - conceptual model - reservoir model) was initiated to deal with the limited data situation. Log, core and test data as well as 3D seismic were insufficient to describe the reservoir situation. An outcrop study extended the limited data set and resulted in 6 deformation cycles, several fracture and fault rock types. The rock types were grouped to 4 pseudo facies types (matrix, micro-fractured, fractured and fault rock) and log-calculated porosity was split up on these 4 pseudo facies. Using different synthetic phi/k functions permeability was calculated for each pseudo facies. A large number of production tests approved the calculated permeability. As a dual porosity / dual permeability approach was planned for simulation, the 4 pseudo facies types had to be combined in two sets of contrasting properties. Fault rock with very high permeability represents the simulation fracture set, whereas matrix, microfractures and fractures with poor to medium permeability act together as simulation matrix set. Both sets have their own phi and k properties. Without major adjustments a good history match was achieved.
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Fluid-induced Seismicity: Pressure Diffusion and Hydraulic Fracturing
By S.A. ShapiroIt has been already recognized that the passive seismic monitoring has a significant potential to characterize physical processes related to fluid dynamics in hydrocarbon reservoirs. One of its conventional applications is mapping of hydraulic fracturing. However, this is not the only application. Understanding of the spatio-temporal dynamics of microseismic clouds can significantly contribute to estimating important physical characteristics of fracturing processes and of reservoir properties. For instance, r-t-analysis turns out to be very useful. E.g., plots of distances from the injection source to event hypocenters versus event occurrence times show signatures of the hydraulic fracture opening, fracturing fluid loss and its infiltration into reservoir rocks as well as the diffusion of fluid pressure into surrounding formations. The back front of the induced seismicity is especially useful for characterization of fractured domains after terminating the injection.
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Delineation of Fracture Systems by Precise Locations and Mechanism Analysis of Microseismic Multiplets
Authors R. Kamitsuji and K. TezukaMicroseismic monitoring during a massive hydraulic injection experiment was performed in the Yufutsu gas field, JAPAN in May 2005. The reservoir is naturally fractured granite and overlaying conglomerate located at around 4 km depth. The monitoring network consisted of four deep stations and three shallow stations. 35 mutiplet groups were extracted from 3700 of microseismic events observed by the deep stations. We relocated these multiplets using the precise phase picking and the pseudo 3D velocity model. Location uncertainties of less than 20 m were small enough to delineate the fracture systems and the two dominant strikes of N-S and NE-SW were interpreted from the principal directions and the locations of the multiplet clusters. Furthermore, the focal mechanisms of the representative events of the every multiplet groups were constrained by the grid search technique performed on the observed P-wave polarity and S-wave polarization. The N-S and ENE-WSW dominant fracture systems are also estimated by the mechanisms. These dominant fracture orientations are consistent with the most likely orientations of shear slippages in the stress condition around the injection well, which is a strike-slip stress regime whose maximum horizontal stress axis is in NNE-SSW direction.
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Imaging Hydraulic Fracture Zones from Surface Passive Microseismic Data
Authors V.A. Kochnev, V.S. Polyakov, I.V. Goz, I.S. Murtayev, V.G. Savin, B.K. Zommer and I.V. BryksinMost of the current microseismic methods for hydraulic fracture imaging employ multi-level receiver systems deployed on a wire-line array in one or more offset wellbores. Microseisms induced by changes in stress and pore pressure around the hydraulic fracture created when high-pressure liquid and proppant injected into the rock then can be located using conventional techniques based on arrival-time approach. We developed and successfully applied a technique for imaging hydraulic fracture zones from surface passive microseismic data. The dynamics of the process of developing of hydraulic fracture and fracture orientation can be recovered from data and consequently visualized. We demonstrate the technique on several examples from West Siberia production sites.
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Fracture Quality From Integrating Time-Lapse VSP and Microseismic Data
Authors M.E. Willis, D.R. Burns, R. Lu, N. House and M.N. ToksozWe present a new methodology to characterize the quality of hydraulic fractures by extracting the amount of scattered seismic energy from the induced fractures. A 3D VSP is collected over the field before and after the hydraulic fracturing is performed. Microseismic recordings of the hydraulic fracturing treatment form the basis for imaging operators. These imaging operators are used on the time lapse (difference) VSP volumes to extract the amount and angular variation in amplitude of scattered energy. Model results show that compliant, open fractures produce larger amounts of scattering than stiff, closed fractures. It may also be possible to use the azimuthal variation in scattered energy as an additional indicator of fracture compliance.
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Effects of Fluid Saturation on Shear-wave Splitting in Multicomponent Seismic Data
Authors Z. Qian, X.Y. Li and M. ChapmanUnderstanding pore-pressure and saturation changes is important in mature reservoirs. Here we analyze shear-wave splitting in a 3D3C onshore survey from Shengli Oilfield, China, where the thin sand-reservoir has been undergone production through water-flooding which altered the fluid composition and the pore-fluid pressure. Dividing the data into orthogonal azimuthal sectors and processing each sector separately reveals significant shear-wave splitting. The amount of shear-wave splitting can be correlated with the degree of water saturation. Furthermore, the slow shear-wave component shows amplitude dimming in water-flooded areas, whereas the zone of original oil in place shows only weak shear-wave splitting. Rock physics modeling based on the evolution of microcracked rocks and anisotropic fluid substitution incorporating both saturation and pressure changes confirm the observations. The saturation changes have little effect on the P and the fast shear-wave as confirmed by core analysis in the laboratory. However, the substitution of water for oil changes the fluid viscosity that has a strong effect on the slow (quasi) shear-wave. Moreover, the fluid substitution due to water flooding also changes the pore-fluid pressure that modifies the crack aspect ratios, further enhancing shear-wave splitting. These observations reveal the potential of using shear-wave splitting for oil-water discrimination.
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Integrated Fracture Characterization of Rulison Field, Piceance Basin, U.S.A.
Authors R.D. Benson and T.L. DavisAn integrated study at Rulison Field has demonstrated the importance of fracture characterization in understanding reservoir productivity. Time-lapse (4D), Multi-component (9C) seismic data along with Vertical Seismic Profile (VSP), well logging, rock core, and formation outcrop data have been combined to understand the variable well productivity within the field. High productivity areas are identified, and are characterized by their low Vp/Vs ratio and high seismic anisotropy.
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