Calibration of anisotropic velocity models

using sonic and Walkaway VSP

measurements

Rafael Guerra and Erik Wielemaker

Schlumberger Wireline

Contact authors at:

JGuerra5@slb.com

EWielemaker@slb.com

London, 27-Sep-2018

London, 27-Sep-2018 Calibration of Anisotropic Models using Sonic & Walkaway VSP 2

Agenda

• Introduction

• Elastic Anisotropy principles

• Key applications in oil industry

• How do we measure elastic anisotropy in-situ?

• Case studies

• Conclusion

London, 27-Sep-2018 Calibration of Anisotropic Models using Sonic & Walkaway VSP 3

Elastic anisotropy – geophysics & geomechanics

Simple definition:- Elastic wave velocity depends on propagation direction (geophysics)- Rock mechanical properties depend on measurement direction (geomechanics)

incompressibility

k = Bulk Modulus

DV/V = Volumetric strain

F

F = k DV/V

F = m e

e = tan q = Shear strain

m = Shear Modulusrigidity

F = Shear stress

F = normal stress Vp

m

4 31 2

Z Vp p

Vs

m

1 2

Z Vs s

q

F

No volumechange

volumechange

rigidity

Simple connection between geophysics and geomechanics:

P-waves

S-waves

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Simple Example (rocks)

Fractured rock (HTI, FVTI,…) Laminated shale (VTI)

The arrows indicate direction of maximum rock compressibility & minimum rigidity

It coincides with the direction with the lowest velocities:• P-wave velocity (Vp) and S-wave* velocity (Vs)

*In anisotropic media there are two S-waves (Sv/Sh or S1/S2) with <> polarizations & speeds

London, 27-Sep-2018 Calibration of Anisotropic Models using Sonic & Walkaway VSP 5

Simple Example (sonic logs)

(Hornby et al., 2003)

Using anisotropy corrected Sonic

Synthetic from uncorrected sonic

• Compressional sonic logs in same field for different well deviations

40% Vp

variations

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Polar anisotropy (VTI) – what it is?

“The wave velocity varies with the propagation angle from vertical”

• Shales exhibit polar anisotropy

• This talk covers mainly polar anisotropy

(Adapted from Oilfield Review, 1994)

group velocities

and polarizations

Horizontal velocity (km/s)

Ve

rtic

al v

elo

city

(km

/s)

(Horne et al., 2012)

Haynesville Shale

P

Sh

Sv

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Polar Anisotropy (VTI) - Thomsen parameters e, d, g

Vp0 vertical P-wave velocity

Vs0 vertical S-wave velocity

e ~ “%” of P-wave anisotropy (horizontal vs vertical velocity)

g ~ “%” of SH-wave anisotropy (horizontal vs vertical velocity)

d anisotropy curve ‘Shape’ parameter (P- and Sv-waves)

e

g

d

Vp0

Vs0

Vertical

Velocity

Horizontal

Velocity

(Horne et al., 2010)

P

SvSh

Isotropic seismic analyses:

- Vp, Vs, density

Anisotropic seismic analyses:

- Vp0, Vs0, density- e, d- g (microseismic, multicomponent seismic)

- Tilt of symmetry axis TTI

London, 27-Sep-2018 Calibration of Anisotropic Models using Sonic & Walkaway VSP 8

Key types of anisotropy

The elastic tensor relates stress to strain (Hooke’s law)

The elastic tensor has <=21 independent parameters

• Isotropy (ISO): 2 parameters (Vp, Vs) ISO-2

• Tilted TI (TTI): 5 parameters (Vp0,Vs0,e,d,g) + tilt

• VTI: 5 parameters (Vertical axis / polar)

• HTI: 5 parameters (Horizontal axis / azimuthal)VTI-5 HTI-5

• Orthorhombic (ORT): 9 parameters (2 sym. planes)

• FVTI = Fractured VTI

8 parameters (1 sym. plane)

ORT-9

FVTI-8

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VTI-anisotropy impact on Geomechanics

Key applications: wellbore stability, well design and hydraulic fracturing

(Waters et al., 2011)

Barnett shale:

- Low stress in high-clay volumes usingisotropic model

- Higher stress predicted using anisotropic model

Minimum horizontal stress is computed from vertical stress, elasticity, closure stress data and a poroelastic model

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TTI-anisotropy impact on Surface Seismic

Key applications: seismic depth imaging, amplitude vs angle (AVA), AVA vs azimuth in fractured reservoirs (AVAz), Full-waveform

Inversion (FWI), etc.

(WG, data courtesy

of Wintershall)

Isotropic PSDM

(Gerritsen et al., 2016)

ST

ISO

(Jones et al., 2003)

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TTI-anisotropy impact on Surface Seismic

(Jones et al., 2003)

Anisotropic seismic imaging results in:

• Sharper images

• More accurate structures

• Improved well ties

• Improved amplitude analyses

(Jones et al., 2003)

ST

TTI(WG, data courtesy

of Wintershall)

Anisotropic PSDM

(Gerritsen et al., 2016)

London, 27-Sep-2018 Calibration of Anisotropic Models using Sonic & Walkaway VSP 12

How to measure TTI? Surface seismic

Limitations: opening angles and data

quality decrease with depth and

uncertainty in Vp0, e, d increases

More robust results if combined with

borehole anisotropy measurements

Upscaling borehole data and joint

tomography are key processes

(Woodward et al., 2008)

• Grid tomography workflow updates Vp0, e, d

• Borehole data constraints (usually markers & vertical velocities)

(Bakulin et al., 2010)

Model updates without and with steering filter by

joint tomography of seismic and checkshots

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How to measure TTI? Sonic logs

Vertical wells drilled through flat shales:• Advanced LWD and wireline sonic measure vertical

shear (C44=C55) and also horizontal shear (C66) from

Stoneley mode Thomsen g

NOTE: dipmeter required to know relative dips

(Valero et al., 2009)

Deviated wells drilled through shales:• Wireline dipole required to discriminate Sv & Sh shears

• Monopole compressional and Stoneley are used

Constraints from a priori anisotropy database, VSP or

multi-well sonic data Thomsen e, d, g

(Holstein et al., 2007)

~ similar VTI

effect on LWD

Quadrupole

Polar anisotropy signature of sonic recorded in vertical well flat layers

VTI

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How to measure TTI? Wireline Walkaway VSPs

Slowness

Polarization

Method*

* References:

- Parscau & Nicoletis, 1990

- Leaney & Esmersoy, 1989

- Horne & Leaney, 2000

- Leaney & Hornby, 2007

Delivers locally: Thomsen e, d and tilted axis

Each Walkaway shotpoint produces 4 points in plot above

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Walkaway & SonicTI-anisotropy – Middle East

(Waqas et al., 2017)

Thomsen parameters:e ~ 0.6 , d ~ 0.1 (Walkaway), g ~ 1 (Sonic)

Polarization Inclination (deg)

Slo

wn

ess

(ms/

km)

(Donald et al., 2018)

Geophysics

Geomechanics

London, 27-Sep-2018 Calibration of Anisotropic Models using Sonic & Walkaway VSP 16

Sonic VTI signature in shales

Isotropic Stoneley model

Isotropic Dipole

Flexural model

(Guerra et al., 2016)

Walkaway VTI signature

in shales and Thomsen

parameters computed

Walkaway & Sonic TI-anisotropy – West Africa (1/2)

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Walkaway & Sonic TI-anisotropy – West Africa (2/2)

Correlation between e and gseen in cores (above) was

estimated in-situ from

collocated Walkaway & Sonic

measurements and used to

extend the anisotropy logs (Guerra et al., 2016)

Borehole anisotropy

improved velocity

model and minimized

Walkaway travel time

residuals

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Walkaway & Sonic TI-anisotropy – North Sea (1/2)

Slowness-

Polarization

TI-anisotropy

Inversion

(Mogensen et al., 2018)

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Walkaway & Sonic TI-anisotropy – North Sea (2/2)

(Mogensen et al., 2018)

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Sonic & Walkaway FVTI – Middle East

(Leaney & Jocker, 2018)

Two ortho-walkaways give 6 FVTI parameters:Vp0, Vs0, ε, δ, δN, δV

g, δH

missing(Mizuno

et al, 2015)

The sonic method relies on core database and advanced wireline sonic measurement to deliver

in vertical wells 7 FVTI parameters:

Vp0, Vs0, ε, δ, g, δN, δV

Missing: δH

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Conclusions

• We have reviewed elastic anisotropy and its importance in:

• Seismic data analyses

• Geomechanical studies

• In-situ borehole measurements of anisotropy from sonic & VSP are crucial

to calibrate earth models and minimize uncertainties

• New advances still needed in the integration of the different measurement

scales, from core to field scale, and in handling lower anisotropy symmetries

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