Azimuthal offset‐dependent attributes applied to fracture detection in a carbonate reservoir

Shen, Feng; Sierra, Juan F.; Burns, Daniel R.; Toksöz, M. Nafi

doi:10.1190/1.1468596

Cited by 49 publications

(21 citation statements)

References 27 publications

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“…The azimuthal AVO (AVOaz) properties of reflected PP waves have been used to identify fracture orientation (e.g. Shen et al, 2002). However, the effective media model (EMM) is only valid for media with fractures that are small relative to the seismic wavelength (e.g., Lynn, 2004, Liu et al, 2000.…”

Section: Introductionmentioning

confidence: 99%

“…Resulting seismic reflections from the top and bottom of a fractured reservoir will display amplitude variations with offset and azimuth (AVOA). In recent years much progress has been made analyzing AVOA effects (e.g., Sayers and Rickett, 1997;Perez et al, 1999;Jenner, 2002;Shen et al, 2002;Hall and Kendall, 2003;Lynn and Cox, 2003;Minsley et al, 2004).…”

Section: Introductionmentioning

confidence: 99%

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Characterization of in-Situ Stress and Permeability in Fractured Reservoirs

Burns¹,

Toksoz²

2005

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During the past six months we have adapted our 3-D elastic, anisotropic finite difference code by implementing the rotated staggered grid (RSG) method to more accurately represent large contrasts of elastic moduli between the fractures and surrounding formation, and applying the perfectly matched layer (PML) absorbing boundary condition to minimize boundary reflections. Two approaches for estimating fracture spacing from scattered seismic energy were developed. The first relates notches in the amplitude spectra of the scattered wavefield to the dominant fracture spacing that caused the scattering. The second uses conventional FK filtering to isolate the backscattered signals and then recovers an estimate of the fracture spacing from the dominant wavelength of those signals. Both methods were tested on synthetic data and then applied to the Emilio field data. The spectral notch method estimated the Emilio fracture spacing to be about 30 to 40 m, while the FK method found fracture spacing of about 48 to 53 m. We continue to work on two field data sets from fractured carbonate reservoirs provided by our industry sponsors-the offshore Emilio Field data (provided by ENI-AGIP), and an onshore reservoir from the Middle East (provided by Shell). Calibration data in the form of well logs and previous fracture studies are available for both data sets. In previous reports we showed the spatial distribution fractures in the Emilio Field based on our calculated scattering index values. To improve these results we performed a map migration of all the scattering indices. The results of this migration process show a very strong correlation between the spatial distribution and orientation of our estimated fracture distribution and the fault system in the field. We observe that the scattering index clusters tend to congregate around the fault zones, particularly near multiple faults and at fault tips. We have also processed a swath of data from the second data set (the onshore carbonate field). FMI data are available from a number of wells for comparison to our seismic scattering analysis results. The agreement is very good, providing confidence that these methods can be applied to land seismic data that do not have the ideal azimuthal coverage. I. Executive SummaryThe purpose of this project is to develop and implement large-scale numerical models to quantify the effects of fracture parameter variations on seismic reflection signals and in-situ stress variations on flexural modes in boreholes. These models will be used as the basis of data analysis and inversion routines for estimating the heterogeneous fracture distribution in fractured reservoirs from seismic and borehole field data.Fracture property distributions estimated from seismic data will be used to estimate the permeability tensor in the reservoir for input to reservoir simulators.During the past six months we have adapted our 3-D elastic, anisotropic finite difference code in two ways. First, we implemented the rotated staggered grid (RSG) method to more ac...

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Characterization of in-Situ Stress and Permeability in Fractured Reservoirs

Burns¹,

Toksoz²

2005

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“…A 3D finite difference method is used to simulate wave propagation through a simple fractured reservoir which is modeled as a single horizontal layer containing parallel, evenly-spaced fractures. Previous work has shown that the finite difference method is accurate over a wide range of scattering regimes and can account for all converted waves and their interactions (Shen et al, 2002). Synthetic seismograms are generated along transects perpendicular and parallel to the fracture orientation and spectral characteristics of the wavefield scattered from the fracture zones are analyzed.…”

Section: Introductionmentioning

confidence: 99%

“…Others have looked at shear wave data for fracture analysis (Lynn et al, 1995;Gaiser et al, 2002). Shen et al (2002) used numerical experiments and field data from a fractured reservoir in Venezuela to demonstrate the advantages of using attributes based on frequency variations with offset and azimuth (FVOA) in addition to AVOA based attributes to further constrain the interpretation of parameters related to the presence of vertically aligned fractures.…”

Section: Introductionmentioning

confidence: 99%

“…In particular, offset dependent attributes are useful in characterizing vertical fractures because of the variation in incidence angle of waves impinging on the fractures. Much effort has focused on measuring amplitude variations with offset (AVO) for seismic reflections from a fractured reservoir (Gray et al, 2002, and Shen et al, 2002. Amplitude variation with offset and azimuth has been widely used to identify the orientation of vertical fractures with variable levels of success (Ruger, 1998;Shen and Toksoz, 2000;.…”

Section: Introductionmentioning

confidence: 99%

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Characterization of in-Situ Stress and Permeability in Fractured Reservoirs

Burns¹,

Toksoz²

2003

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A 3-D elastic wave propagation finite difference model, including effects of attenuation, has been implemented and compared with other existing modeling codes for validation. Models of seismic scattering from discrete large-scale fractures as well as equivalent anisotropic medium representations of small-scale fractures have been generated and used to develop data analysis methods for applications to seismic field data. An inversion scheme has been developed to estimate fracture orientation and fracture density from amplitude variations with offset and azimuth (AVOA). The method has been tested on synthetic data and field data from an offshore fractured carbonate reservoir with promising results. Spectral characteristics of the numerical model data of the seismic wavefield scattered from aligned fractures with different spacing between fracture zones have been analyzed. Results indicate that the spacing of these large, open fracture zones can be estimated from the wavenumber spectra of the scattered wave amplitude as a function of offset in pre-stack data. Two approaches for converting seismically derived fracture parameters into fluid-flow parameters for use in reservoir simulators have been identified. The first is the numerical modeling of Stoke's flow in fracture networks, and the second uses a statistical model of a fracture distribution that allows for the calculation of the elastic properties and permeability tensor of the resulting equivalent medium. These approaches will be compared in the coming year. Multiple meetings have been held with our industry partner, Shell Oil, to identify a field test site for the project. We are focusing our efforts on a fractured carbonate field. The field application test site selection and data transfer will be completed in the coming year. I. EXECUTIVE SUMMARYThe purpose of this project is to develop and implement large-scale numerical models to quantify the effects of fracture parameter variations on seismic reflection signals and insitu stress variations on flexural modes in boreholes. These models will be used as the basis of data analysis and inversion routines for estimating the heterogeneous fracture distribution in fractured reservoirs from seismic and borehole field data provided by Shell. Fracture property distributions estimated from seismic data will be used to estimate the permeability tensor in the reservoir for input to reservoir simulators.During the first year of the project we adapted and implemented a 3-D elastic wave propagation finite difference model, including effects of attenuation. Computations have been compared with other existing modeling codes (e.g., from Lawrence Berkeley Laboratory) for validation. Models of seismic scattering from discrete large-scale fractures as well as equivalent anisotropic medium representations of small-scale fractures have been generated and used to develop data analysis methods for applications to seismic field data. Two seismic analysis methods have been investigated for the estimation of fracture paramet...

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Acoustic Reflectivity From Variously Oriented Orthorhombic Media: Analogies to Seismic Responses From a Fractured Anisotropic Crust

Malehmir

Schmitt

2017

JGR Solid Earth

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The variations in the strength of seismic reflections with both angles of incidence and azimuth are being used to constrain the orientations and degrees of mechanical anisotropy of geological formations. This information is often used in turn to infer the directions of fracturing and stress usually assuming simplified geological structures that in many cases are not realistic. In order to further understand this problem, the reflectivities from a set of variously titled samples cut from a single block of a common anisotropic composite material are measured with both angles of incidence and azimuth in the laboratory. Each sample is analogous to a formation in which the fracture sets increasingly dip. The observed responses differ from plane wave theory due to the finite nature of the experiment. However, the deviations are well accounted for by modeling the acoustic reflectivity using its bounded pulse. As expected, the reflectivities vary with the sample tilt, the angle of incidence, and the azimuth at which the measurements are made. However, some of these responses, such as the precritical reflectivities, do not appear to strongly vary with azimuth for a given sample. Critical angle phenomena, too, must be used cautiously as the observed reflectivity peaks do not lie at the exact critical angle. These results suggest that reflectivities could be an important source of information with regard to the spatial orientation of anisotropic layer.

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Azimuthal offset‐dependent attributes applied to fracture detection in a carbonate reservoir

Cited by 49 publications

References 27 publications

Characterization of in-Situ Stress and Permeability in Fractured Reservoirs

Characterization of in-Situ Stress and Permeability in Fractured Reservoirs

Characterization of in-Situ Stress and Permeability in Fractured Reservoirs

Acoustic Reflectivity From Variously Oriented Orthorhombic Media: Analogies to Seismic Responses From a Fractured Anisotropic Crust

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