avo, ava e inversione sismica...1 ud5 petri ii mod. 2020-2021 laurea magistrale in geoscienze a.a....
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UD5
PETRI II MOD. 2020-2021
Laurea Magistrale in Geoscienze
A.A. 2020-2021
PETROFISICA INTEGRATA II MODULO
UD5 AVO, AVA e
INVERSIONE SISMICA Emanuele Forte
e-mail: [email protected]
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Discriminate between gas, oil & brine saturation;
Localize major rock lateral/vertical variations (sand and clay content, porosity changes, …)
Estimate main petrophysical parameters
QUANTITATIVE/PETROGRAPHYCAL ANALYSIS
REFLECTION SEISMIC: AMPLITUDE ANALYSIS
New analysis/interpretation techniques were implemented to obtain more affordable results to:
AMPLITUDE VERSUS OFFSET – AVO
AMPLITUDE VERSUS AZIMUTH - AVA
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Reflection Amplitudes RR , RT , are determined by the contrast in rock properties VP , VS , r and angle of
incidence q
θ
RI RR
RT
Incident wave
Reflected wave
Transmitted wave
θ
1PV
2PV
1SV
2SV2r
1r
Medium 1
M
edium 2
Velocities Density
REFLECTION SEISMIC: AMPLITUDE ANALYSIS
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PETRI II MOD. 2020-2021 CMP
Reflector
Sn S3 S2 S1 R1 R2 R3 Rn
Near offset
Far offset
REFLECTION SEISMIC: AMPLITUDE ANALYSIS
Information on the same point from several different OFFSETS
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Amplitude
Variation
Offset
with
Reflect
ion
Amplitud
e
Offset (m)
REFLECTION SEISMIC: AVO
Amplitude Versus Offset or Amplitude Variation with Offset – AVO is an ANALYSIS Technique to
understand any anomalous behaviour of AMPLITUDE
Typically, reflection amplitude decreases (falls) with offset due to geometrical spreading, attenuation and other factors.
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S -wave
Reflected
P-wave
incident
S-wave
Transmitted
P-wave
Reflected
P-wave
Transmitted
INTERFACE
MEDIUM 1 VP1 , VS1 , ρ1
MEDIUM 2 VP2 , VS2 , ρ2
1
2
2
1
2
2
1
1
2
2
1
1 sinsinsinsin
SSPP VVVVpparameterray
For a P-wave with vertical incidence the Rp can be calculated by:
1122
1122
rr
rr
PP
PPP
VV
VVR
12
12
PP
PPP
II
IIR
RP
SSSPSSSP
PSPPPSPP
SSSPSSSP
PSPPPSPP
Q 1
Aki & Richards (1980)
2222
2
221111
2
11
2
2
2222221
2
111111
2211
2211
2sin)sin21(2sin)sin21(
)sin21(cossin2)sin21(cossin2
sincossincos
cossincossin
SPSP
SSSS
VVVV
VVVVP
rrrr
rrrr
2222
2
221111
2
11
2
2
2222221
2
111111
2211
2211
2sin)sin21(2sin)sin21(
)sin21(cossin2)sin21(cossin2
sincossincos
cossincossin
SPSP
SSSS
VVVV
VVVVR
rrrr
rrrr
Reflected (and transmitted) AMPLITUDE f(Vp, Vs, r, angle)
REFLECTION COEFFICIENT Rp
i.e.
The general case (P and S waves) in much more complicated and in linear (matrix) form is:
Where:
Propagation/Transmission
Reflection
REFLECTION SEISMIC: AVO
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S
SS
P
PSR
V
VV
V
VVR
22
2
22sin 4)(cos2
1))(41(
2
1)( r
qr
rq q
q
Reflection Amplitude:
There are several simplified eq. for specific angle ranges:
Aki & Richards (1980)
r
2PV
r
SV
Lithology fluid types
fluid saturation Porosity
Anisotropy etc…
Velocities VP , VS are in turn dependent on elastic constants λ, µ,and ρ, and inherent rock properties
Moreover:
REFLECTION SEISMIC: AVO
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Lithology Fluid saturation
Observable parameters Seismic amplitude (Rc)
VP VS Density (ρ)
2D, 3-D seismic data Borehole log data
The seismic response & rock properties
Elastic Constants λ µ ρ κ
Can indicate the differences between fluids, porosity, saturation,…
REFLECTION SEISMIC: AVO
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REFLECTION SEISMIC: AVO
There are many possible applications depending from the final objectives, the available data
and the data quality
1) AVO ANALYSIS Study the Amplitude behaviour on multi-offset (multi-fold) 2D or 3D registered data
2) AVO MODELING 1D, 2D and 3D synthetic offset/angle gathers calculations (also from well log data). Modeling can be performed first, in order to determine what type of AVO anomaly may be anticipated.
3) AVO INVERSION extract lithologic and pore fluid information, removing the effects of the seismic wavelet and optimising thin-layer resolution
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REFLECTION SEISMIC: AVO ANALYSIS
1
2
1 sin BRR PPP
A simplified equation for the reflection amplitude is:
Shuey’s eq. linear relation between A(r) and incidence angle sin2Θ
201
PRABB = “AVO gradient”
B= AVO gradient f(=Poisson) Physical rock/fluid parameter liquid saturation
RP = A = “AVO Intercept parameter” f(porosity)
BA 9
4
BRR PS 2
1Rs=shear wave reflectivity Fluid saturation
S
S
P
S
P
P
V
V
V
Vc
V
VF
1f=fluid factor
- F0 water saturated rocks - F<0 possible top of oil reservoir - F>0 below a gas (or oil) reservoir
(Smith e Gidlow, 1987)
Many analysis techniques:
hydrocarbon indicator
Intercept (A) & Gradient (B)
Scaled Poisson’s Ratio
Shear wave reflectivity
Fluid factor
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AVO Crossplot AVO Crossplot A graph showing amplitudes value as a function of: AVO intercept attribute (A) AVO gradient attribute (B)
REFLECTION SEISMIC: AVO ANALYSIS
For brine-saturated (clastic) rocks in a particular site, there may be a well-defined relationship between the AVO intercept (A) and the AVO gradient (B). Thus, in a given time window, non hydrocarbon-bearing clastic rocks often exhibit a well-defined background trend; deviations from this background are indicative of hydrocarbons or unusual lithologies.
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The classification of AVO responses should be based on position of the reflection of interest on an A versus B crossplot. First, the background trend within a given time and space window must be defined. This can be done with well control if the seismic data are correctly amplitude calibrated, or with the seismic data itself if care is taken to exclude prospective hidden hydrocarbon-bearing zones. Top of gas sand reflections then should plot below the background trend and bottom of gas sand reflections should plot above the trend. We can classify the gas sand response according to position in the A-B plane of the top of gas sand reflections.
AVO Crossplot - CLASSIFICATION
REFLECTION SEISMIC: AVO ANALYSIS
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AVO Crossplot - CLASSIFICATION
REFLECTION SEISMIC: AVO ANALYSIS
REAL EXAMPLES
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N
In-line 280 Oil in Top K Fm.
REFLECTION SEISMIC: AVO ANALYSIS
REAL EXAMPLES
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Normal Stack
Near Offset Stack
Far Offset Stack
RC + RC -
REFLECTION SEISMIC: AVO ANALYSIS
REAL EXAMPLES
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Ampl. extraction - near stack Ampl. extraction - far stack
Amplitude extraction
far-near difference
RC + RC -
REFLECTION SEISMIC: AVO ANALYSIS
REAL EXAMPLES
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VS (m/s)
VP (
m/s
)
Sh
aly
san
d
REFLECTION SEISMIC: AVO ANALYSIS
REAL EXAMPLES
Crossplot of VP vs. VS from Seismic + log data
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horizon_25
horizon_25
GR indicates sand
Resistivity indicates
hydrocarbon
Reflection
Am
plit
ude
AVO indicates Class III
Offset
REFLECTION SEISMIC: AVO ANALYSIS
REAL EXAMPLES AVO Integration and Calibration with LOGs
C1
C2
C3
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REFLECTION SEISMIC: AVO MODELING
Classically, changes in reservoir fluid type and saturation, as well as changes in porosity, are modeled
LOG data Modeling of a “virtual” acquisition : AVO applicability AVO validation
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REFLECTION SEISMIC: AVO MODELING
Improved (quantitative) interpretation of AVO analyses
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REFLECTION SEISMIC: AVO INVERSION What is Inversion? Seismic (AVO) inversion is the process of determining what physical characteristics of rocks and fluids could have produced the seismic record you are viewing i.e. it’s a procedure trying to determine the INPUT by looking at the output of an experiment
• Elastic Impedance (EI) inversion (Connolly, 1999)
• Lambda-Mu-Rho (LMR) inversion (Goodway, 1997)
Reflection amplitude
RP
RS
Pre-stack CMP gathers
Log data
VP VS ρ
Recorded data
ZP
ZS
2
SZr
22 2 SP ZZ r
λρ
µρ
inversion
Acoustic impedance
Elastic constants
ROCK/FLUID DATA
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EI inversion: Generalization of acoustic impedance for variable angle of incidence
)sin41()sin8()tan1( 222
)( qqq rq KK
SP VVEI
S
P
VV
Kwhere
Connolly, 1999
REFLECTION SEISMIC: AVO INVERSION
2D interpreted seismic section
Elastic Impedance
LMR Analysis
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Connolly, 1999
Very Low values of Elastic Impedance Gas saturation
REFLECTION SEISMIC: AVO INVERSION
Savic et al, 2000
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λ & μ are elastic constants
λ = fluid incompressibility is in theory affected only by the type
of fluid in the pore space Low λ suggests gas saturation
μ = rigidity is in theory affected only by the type of matrix High μ suggests hard compact materials
REFLECTION SEISMIC: AVO INVERSION
Lambda-Mu-Rho (LMR) inversion
Low λ + High μ Gas Sand
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λρ fluid indicator
(LR)
µρ matrix indicator
(MR)
VSHALE log of 1A
1 km
(GP
a)*
(g/c
c)
Resistivity log of 1A
1 km
(GP
a)*
(g/c
c)
REFLECTION SEISMIC: AVO INVERSION
Tim
e
Tim
e
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λρ fluid indicator
(LR)
µρ matrix indicator
(MR)
REFLECTION SEISMIC: AVO INVERSION
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Sand
will have
high r
igidity
Gas will have low fluid incompressibility
REFLECTION SEISMIC: AVO INVERSION LMR crossplot
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REFLECTION SEISMIC: AVO INVERSION
Rock and Fluid probability
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Feeder-channel
Lobe
structure 1km
100ms
Turbidite system outline
3-D seismic Facies and Fluid probability
Relatively large negative
gradient G (yellow)
Relatively small
gradient (blue)
Oil sands
Shale
Brine sands
Interbedded
sand-shales
REFLECTION SEISMIC: AVO INVERSION
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Amplitude Vs. Azimuth In general AMPLITUDE VARIATIONS (anisotropy) + local velocity variations (heterogeneity) + Discontinuity trend + Density variations + Anisotropy of secondary porosity/permeability
Unhomogeneous fluid distribution/saturation
REFLECTION SEISMIC: AVA
ms
3.5 km/s
5.0 km/s
Similar techniques as for the AVO analysis, but MORE UNCERTAINTIES And 3D dense data are required
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AVO: PROS
Qualitative AND QUANTITATIVE data Volumetric info (extrapolation of log data) No needs for new data acquisition, reprocessing of old datasets Specific rock (matrix) and Fluid indicators Possible 4D analyses
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Tuning and thin-bed effects Noise and processing effects Lateral velocity trends in overburden Needs for validation (logs, wells, …) Site dependent results Representative statistics? Supervised seismic interpretation
AVO: LIMITS
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DOMANDE?