Transcript Azimuthal anisotropy analysis to estimate fracturing

AASPI
2015 Workplan for “VVAz” Analysis
of Prestack Migrated Data
Jie Qi and Kurt J. Marfurt
(The University of Oklahoma)
AASPI
Outline
• Introduction
• Motivation
• Challenge
• Methodology
• VVAz
• AVAz
• Azimuthal crosscorrelation
• Application
• Geology background
• Azimuthal attributes
• Conclusion
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Motivation
• Azimuthally limited or vector-tile gathers are part of the
wide-azimuth processing workflow
• Can we implement interpretation tools to provide residual
AVAz analysis capabilities?
• If the data were migrated using isotropic velocities, these
residuals are a measure of VVAz
• The application of such a tool would increase vertical
resolution and precondition the data for subsequent AVAz
analysis
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Horizontal Transverse Isotropy (HTI) medium
(Left): No HTI anisotropy results in equal travel time paths in all
azimuths;
(Right): In an HTI anisotropic media with aligned vertical fractures
the travel time is azimuth dependent and is not equal in all
directions.
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(Courtesy of Close et. al., 2010)
Shear wave spitting in anisotropic medium
3-C
Receiver
(Courtesy of Ed Garnero)
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Fracture:
Woodford shale
Marcellus Shale
• A key factor in the optimization of reservoir production
• High natural fractures – high production
• Helps to identify sweet spots
• Anisotropic properties: intensity and orientation
• Anisotropy analysis: amplitude and velocity
(Photos courtesy of Brian Cardott)
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AASPI
Outline
• Introduction
• Motivation
• Challenge
• Methodology
• VVAz
• AVAz
• Azimuthal crosscorrelation
• Application
• Geology background
• Azimuthal attributes
• Conclusion
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Velocity vs. Azimuth (VVAz)
• Advantages
• Easy to generate azimuthally-binned data
• Computation is fast and simple, providing a level of confidence
• Requires phase- but not amplitude-preservation
• Disadvantages
•
Suffers from vertical resolution problems associated with Dix’s equation
Amplitude vs. Azimuth (AVAz)
• Advantages
• Easy to generate azimuthally-binned data
• Computation is fast and simple, providing a level of confidence
• Computations are volumetric within the (properly registered) zone of interest
• Disadvantages
•
Requires amplitude-preserving processing and migration (AVAz)
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Velocity variation with angle and azimuth
(VVAZ)
Vint(φ)=V0+εcos[2(φ- φsym)]
If ε is zero, it becomes interval velocity.
N
φ
θ
N
φsym
sym
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Amplitude vs. Offset (AVO)
}sin2θ
R(θ,φ)=A+{Biso
Amplitude vs. Azimuth (AVAz)
R(θ,φ)=A+{Biso+Banisocos[2(φ- φsym)]}sin2θ
N
φ
θ
N
φsym
(Rueger, 1996)
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Workflows
Conventional VVAz:
•
•
•
•
Generate long-offset sectors or ’tiles’ at different azimuths φ (unmigrated)
At discrete picked horizons, compute VRMS as a function of azimuth, φ
Compute interval velocities Vint(φ) using Dix’s equation
Fit a sinusoidal curve to Vint(φ) to obtain the magnitude and azimuth of
anisotropy
AVAz:
• Generate long-offset sectors or ’tiles’ at different azimuths φ (migrated)
• Pick discrete upper and lower horizons and generate either flattened or stratal
slices throughout the volumetric zone of interest
• At every time or depth sample, fit a sinusoid to the amplitude as a function of
azimuth φ
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Residual “VVAz” Workflow
Shot
gathers
Prestack
Time
migration
Migrated
gathers
VVAz
Anisotropy
Dynamic
alignment
Azim 1
gathers
Azim 2 …… Azim 8
gathers
gathers
Structure
oriented filter
AVAz anisotropy
Baniso, ψaniso
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Dynamic alignment
• Correlate adjacent azimuths
• Find ∆τ and value of highest correlation coefficient
• Autocorrelate & crosscorrelate
Azim 1
Azim 2
Azim 8
Time
……
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Dynamic alignment
• Least-squares fit ∆τ to find εaniso and φsym
Vint(φ)=V0+εcos[2(φ- φsym)]
φsym
V0+εcos[2(φ- φsym)]
V0 + ε
V0
ε aniso
ε iso
V0 - ε
Azimuth, φ
(Modified from Roende et al., 2008)
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Dynamic alignment
• Stretch and squeeze data to provide flattened events
εaniso and φsym
Isotropic Layer 1
High anisotropy
Anisotropic Layer 2
Azimuthal data
Dynamic
alignment
Isotropic Layer 1
Anisotropic Layer 2
Aligned data
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AASPI
Outline
• Introduction
• Motivation
• Challenge
• Methodology
• VVAz
• AVAz
• Azimuthal crosscorrelation
• Application
• Geology background
• Azimuthal attributes
• Conclusion
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Stratigraphic Cross Section
Marble Falls
Unconformity
(Modified from Pollastro et al., 2009)
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Stacked azimuth sector gathers
Anisotropy indicators
CMP no.
0.5
Time (s)
0.6
CMP CMP CMP
398 399 400
0.7
0.8
Data aligned
Data Misaligned
(Roende et al., 2008)
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Amplitude as a function of azimuth (AVAz)
0.6
Time (s)
~8 ms
0.7
0.8
~3500 ft
(Roende et al., 2008)
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AVAz products
0.4
Time (s)
Top Marble Fall
1.0
Top Ellenburger
Intercept, A
Time (s)
0.4
1.0
Isotropic gradient, Biso
Time (s)
0.4
1.0
Anisotropic gradient, Baniso
Low
High 1 mile
(Modified from Roende et al., 2008)
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AASPI
Outline
• Introduction
• Motivation
• Challenge
• Methodology
• VVAz
• AVAz
• Azimuthal crosscorrelation
• Conclusion
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Anticipated Challenges
• Will there be a clear correlation between AVAz and
VVAz?
• Can the residuals be computed gather by gather, or will
layer-stripping become important?
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Acknowledgements
AASPI
• Marathon Oil Co. for a license to their survey
• Sponsors of the AASPI consortium for financial support and
technical encouragement
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