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


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PETRI II MOD. 2020-2021 CMP

Reflector

Sn S3 S2 S1 R1 R2 R3 Rn

Near offset

Far offset


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


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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:


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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,…


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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

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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)


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


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AVO Crossplot - CLASSIFICATION


REAL EXAMPLES

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N

In-line 280 Oil in Top K Fm.


REAL EXAMPLES

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Normal Stack

Near Offset Stack

Far Offset Stack

RC + RC -


REAL EXAMPLES

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Ampl. extraction - near stack Ampl. extraction - far stack

Amplitude extraction

far-near difference

RC + RC -


REAL EXAMPLES

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VS (m/s)

VP (

m/s

)

Sh

aly

san

d


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


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


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


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)


Tim

e

Tim

e

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λρ fluid indicator

(LR)

µρ matrix indicator

(MR)


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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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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


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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?

AVO, AVA e INVERSIONE SISMICA...1 UD5 PETRI II MOD. 2020-2021 Laurea Magistrale in Geoscienze A.A....

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Transcript of AVO, AVA e INVERSIONE SISMICA...1 UD5 PETRI II MOD. 2020-2021 Laurea Magistrale in Geoscienze A.A....