4.2 Sonic Measurements

8/12/2019 4.2 Sonic Measurements

1/59

Advanced Formation Evaluation

October 2012

by Alain Brie

Sonic Measurements

Acquisition and Evaluation

4.2


2/59


3/59


4/59

3 Advanced Formation EvaluationAlain Brie 2012

Borehole Compensated Measurement

Making a measurement with two transmitters on top andbottom of the receivers achieves borehole compensation.

The BHC measurement is the average of the upper andlower transmitter measurements,

BHC corrects the effect of sonde tilt and boreholeenlargement,

The T-R spacings of the BHC tool are 3 ft- 5 ft

2

LTxUTxBHC

ttt

+=


5/59


Long Spacing Sonic

Short spacing measurements such as the 3 5 ft BHC limitthe depth of investigation and can cause adverse effects onthe measurement.

In very large boreholes the tool can read the mudtinsteadof the formation,

In case of alteration (shale swelling) the log can read thealtered zone instead of the virgin formation,

Increasing the TR Spacing increases the depth ofinvestigation and provides a reliable measurement of theformation,

The Long Spaced Sonic sonde with TR spacings of 8 10 ft

and 10 12 ft was designed for this purpose.

Sh

allowSection

IntermediateSection


6/59


Legacy Sonic Tools

Digital Cartridge

Array SonicBHC LSS

Legacy Sonic Tools can be operated with acommon digital cartridge that digitizes sonicwaveforms downhole.

The BHC sonde has 3 - 5 ft spacing

The Long Spacing Sonde has 8-10 ft and 10-12ft

Spacings, The Array Sonic has 3-5 ft, 8-10 ft spacings and an

array of 8 receivers every 6 in located at 8 ft fromthe upper transmitter.

All these measurements are monopole.

LTx

UTx

LTx

UTx

LTx

UTx


7/59


8/59


Slowness-Time Coherence Computation

The Slowness Time Coherence techniquescans the waveforms for all possible timesand move-out to find coherent arrivals inthe waveform.

Coherence is the ratio of coherent energyalong a move-out over the total energy,

Coherence of 1 is perfect correlation, Low coherence means no correlation.

The calculated coherence for each timeand slowness value is plotted on the STPlane.


9/59


ST Plane

TRline

Each point on the ST plane represents theresult of a coherence computation at a certaintime and move-out.

Contours a drawn around zones of equalcoherence,

Low coherences are shown in blue and high

coherences in red, High coherence peaks indicate a highly

correlated event propagating at this time andslowness,

The arrival time of events propagating alongthe borehole should be close to their slownesstimes the TR spacing.

0 Coherence 1

Time s

Slowness

s/ft


10/59


Field STC Processing Results

Field STC processing for monopole P&S (DSI SAM-4).

This display is used to control the quality of theSTC computation.

Track 3 shows the Slowness coherence projectionoverlaid with the resulting DT logs,

Low coherences are shown in blue and highcoherences in red,

Continuous red bands indicate good quality data(good coherence),

DT logs should follow track red bands,

Result logs are shown in Track 2,

Other information is shown in Track 1.


11/59


Monopole Borehole Propagation

Monopole sonic tools measure head waves,

Not body waves

The transmitter sends a pressure pulse thatpropagate as compressional and shear body wavesin the formation,

Body waves induce head waves in the formationwhen they are faster than the mud compressional,

Sonic receivers record the head waves,

The shear headwaveonly exists in fast formations

Other waves propagate in the borehole

? The pseudo-Rayleigh wave,

? The Stoneley wave,

? Normal and leaky modes (borehole arrivals).


12/59


Compressional and Shear Waves

Consider a pile of disks. If we excite a vibration by knocking

vertically on the top it propagates down as acompressional wave,

With a compressional wave particle motionis parallel to the propagation direction,

If we excite a vibration by knocking the pilelaterally it propagates down as a shearwave,

With a shear wave particle motion isperpendicular to the propagation direction,

Compressional

(extensional)

Shear

(flexural)


13/59


Monopole-Dipole Excitation

In monopole excitation one point source sends a

pressure pulse in all directions.

With dipole excitation two point sources side-by-side pulse in opposite phase creating a lateral

push-pull effect.

Quadrupole excitation uses four point sources;

one diagonal pulses in phase opposition with the

other diagonal.


14/59


Dipole Shear Sonic Imager

DSI Tool String

The DSI, dipole shear imager tool acquiresboth monopole and dipole measurements.

Its main features are:

Two dipole transmitters in perpendiculardirections,

One monopole transmitter with high and low

frequency drives, Array of 8 receivers stations with dipole and

monopole capability,

Long spacing for reading past altered zones,

Isolation joint to prevent direct wavetransmission through the tool body.


15/59


Electrodynamic Dipole Transmitter

There are various ways to generate dipole excitation in the borehole.

The electrodynamic transmitterworks as a loudspeaker, pushing

the mud laterally,

The push excites

the borehole in

flexion,

Flexural wave propagates verticallyalong the borehole, while particle

motion is transverse; it is therefore

close to a shear wave.

Propagation

DisplacementElectrodynamic Transmitter Flexural Wave


16/59


DSI Upper Dipole (SAM-2)

Dipole DSI Waveforms

Flexural


17/59


Flexural Wave Dispersion

Flexural Dispersion Curves in 8 in. Borehole

Inflection Point

The flexural wave is linked to the wellboreand varies with frequency. This is calledfrequency dispersion.

At low frequency the flexural slownessreaches the formation shear slowness,

At higher frequencies the flexural slowness

increases, The maximum amplitude of the flexural is

at the inflection point,

Dispersive STC processing accounts fordispersion and outputs formation sheardirectly.


18/59


Dipole Processing Quality Control Display

Quality control plot for dipole shearprocessing..

ST Projection with log tracking results ispresented in Track 3,

Coherence in Track 1,

Arrival frequency and filter band in Track 2,

Filtered waveform and reconstructed sheararrival time in Track 4.


19/59


Sonic Scanner Tool

TheSonic Scannertool is the latest development in Schlumberger sonic technology. The

SScan benefits from the experience acquired with the DSI-1 and DSI-2 tools and offers

superior dipole as well as monopole measurement capabilities.

The Sonic Scanner is modular, in the basic configuration it replaces all prior monopole

tools: BHC, LSS and AS.

In the full configuration it replaces the DSI and adds new capabilities for anisotropic and

inhomogeneous formation analysis.

The Sonic Scanner tool was designed by computer modeling, it has predictable acoustics

allowing full characterization of its response and frequency behavior for high fidelity answers.

S i S B i C fi i


20/59


Sonic Scanner Basic Configuration

Minimum Service Sonde

Basic Configuration

Monopole only tool to replace old technology sondes.

True BHC with upper and lower monopole transmitters.

Large 13 receivers array provide robust measurement and

multiple spacings from 1 to 7 ft.

Cement bond log (CBL) and variable density log (VDL)

measurement Improved behind casing monopole measurement with

CBL/VDL simultaneous acquisition

Measurements

Monopole P&S

Cement Evaluation Altered zone evaluation

S i S F ll C fi ti


21/59


Sonic Scanner Full Configuration

Full Service

Configuration

Basic configuration measurements plus:

Long-spacing 10.8 to 16.8 ft monopole with MF transmitter.

Low frequency monopole Stoneley measurement.

Wideband dipole measurements from X and Y transmitters.

All modes including BCR acquired all the time.

Improved behind casing dipole measurement with CBL/VDL

simultaneous acquisition.

Measurements

Dipole X&Y and anisotropy

Monopole P&S and Stoneley

Cement Evaluation

Stress Eval

Altered zone

S i S W f


22/59


Monopole Far (MF)

Sonic Scanner Waveforms

High quality, high consistency waveforms; Very wideband dipole waveforms for high quality answers, especially in cased hole and new

applications (formation alteration and stress evaluation).

Dipole (XD)


23/59

Alt ti E l ti F Di l Di i


24/59


Alteration Evaluation From Dipole Dispersion

Chemical or mechanical alteration of the formationnear the wellbore increases the dispersion of thedipole flexural wave.

Evaluation of dispersion provides:

More accurate formation shear,

Information on formation weaknesses and potentialfailure,

Formation stress information.

Dipole Anisotropy Radial Profiling


25/59


Dipole Anisotropy Radial Profiling

Systematic acquisition of XD and YDdipole waveforms provides shear slownessin the fast and slow directions in case ofanisotropy.

More accurate shear determination forformation evaluation,

Dispersion analysis further provides stress

information,

This is essential information for rockmechanics evaluation.

Elastic Waves Velocities


26/59


Elastic Waves Velocities

GKVp

+= 34

Compressional Shear

GVs=

GKV

t

p +

==3

4

8.3048.304

GV

t

s

s

8.3048.304==

Units: KandG in GPa in g/cc Vp ansVs in km/s t andts ins/ft

Sound waves are elastic waves that propagate in the ground as vibrations.

In an isotropic, homogeneous (HI) medium only two moduli and thedensity are necessaryto determine the velocity of the compressional and shear body waves.

K is the bulk modulus, G is the shear modulus and r is the density

ElasticWavesAnalogywithSpring Mass Systems


27/59


Elastic Waves Analogy with Spring-Mass Systems

Propagation of a vibration in asystem of spring and masses.

Displacement of masses.

Density

Moduli

Elastic MediumSpring-Mass System

Spring Stiffness

Mass

Velocity

Slowness

Elastic Moduli


28/59


Elastic moduli represent the resistance of amaterial to deformation.

Bulk modulus is the resistance to compression

Shear modulus is the resistance to distortion

Elastic Moduli

cK=

sG=

Compression

Shear

Elastic Moduli


29/59


Young Modulus and Poissons ratio are oftenused in rock lab and rock mechanics.

Young Modulus is the resistance to uniaxialcompression (as in a press)

Poissons ratio characterizes lateral expansion asthe sample is compressed

1

3

1

2

==

1

uE=

Uniaxial Compression

Elastic Moduli

PoissonsRatio is linked toVp/Vs


30/59


Poissons ratio is linked to the Vp/Vs ratio.

The physical limits of Poissons Ratio are:

Poissons Ratio and Vp/Vs are linked as:

From which the physical limits for Vp/Vs(isotropic material) are:

Poissons Ratio is linked to Vp/Vs

5.00

VsVp/2

1/

2/

2

12

2

=VsVp

VsVp

soft

Elastic Moduli Equivalence


31/59


Elastic Moduli Equivalence

-Poissons Ratio

-Lame Constant

-

-

E

G

K

-Youngs Modulus

Shear Modulus

Bulk Modulus

K, G E, ,

K

K

3

9

+

)3(2

23

+

K

K

++ )23(

)(2

+

3

23 K)21)(1(

+E

)21(3 E

)1(2 +E

32+

Two elastic constants are sufficient to describe elastic properties of a HI medium

(Homogeneous Isotropic),

Young modulus and Poissons ratio are used in rock mechanics,

The Lame constants and are used in theoretical physics.

DynamicElasticModuli


32/59


Dynamic Elastic Moduli

Factorsof InfluenceonSonicSlowness


33/59


Porosity

Lithology (mineralogy) including clay content

Pore fluid

Pore shapes

Micro structure Stress (pressure) and compaction

Factors of Influence on Sonic Slowness

Sizes of pores and grains have no influence (Sonic wavelength is much larger)

Equations that do not account for all effects are approximate and limited,

Most equations only account for Porosity and Lithology effects,

Elastic Moduli are needed to account for fluid effect (GassmannEquation).

WyllieSonicPorosity


34/59


Wyllie Sonic Porosity

In 1950 MrWyllie proposed a simple time-average response equationbased on a

correlation of laboratory measurements to linktand porosity:

mf

m

S tt

tt

=

mf

m

S tt

tt

Cp

=

1

In unconsolidated sands the Wyllie Porosity is larger than true porosity. A

compaction factor (multiplier) is added:

Cp: compaction factor is 1 in well consolidated sands

up to 2 in loose sands.

mf ttt += ).1(.

TheWyllie sonic porosity is this obtained as:

( ) mf

tCptCpt

+= ..1..

SonicPorosityEquations


35/59


Sonic Porosity Equations

Raymer-Hunt-Gardner Equation - RHG

mf ttt

+

=

2)1(1

)1(11

=

sttm

)(1

t

tt

s

m

=

s = 1.45 in sandstones1.60 in carbonates.

Velocity Equation - VelC

and

with

Sonic Porosity Chart Por-3


36/59


Sonic Porosity Chart Por 3

Porosity Evaluation from Sonic (Por-3)

V, ft/s t, s/ftWater 5300189

26000 38.5

Dolomite 23000 43.5

Limestone 21000 47.6

Hard Sands19500 51.3Soft Sands 18000 55.5

HardSa

ndston

es

SoftS

andsto

nes

Lim

esto

ne

Do

lomite

UncompactedSandstones

Mineral End Points


37/59


e a d o s

1.5378.255.52.65Soft sand

1.8492502.98Anhydr ite

1.73116.5672.16Salt1.878.543.52.87Dolomite

1.8688.547.52.71Limestone

1.598851.52.65Hard sand

Vp/VsmtsmtmmEnd points

End points for Wyllie Equation

These are slightly different from mineral values.


38/59

Sonic Porosity Equations In Sandstones


39/59


y q

Slowness in Unconsolidated Sandstones


40/59


Compressional slowness in shallow unconsolidated sands

Group 1 Group 2 Group 3

Sonic Porosity Equations In Carbonates


41/59


Crossplot compressional t-porosity in a water-bearing limestone

y q

SSPI =

62 s/ft

15 PU

10.5 PU

t= 62 s/ft, Sonic = 10.5 PUND = 15 PU

Secondary Porosity Index

SPI = 15 10.5 = 4.5 PU

Porosity Evaluation from Slowness


42/59


Porosity Evaluation Recommendations


43/59


Well compacted sands

Unconsolidated sands

Carbonates

Metamorphic and igneous rocks

Both Wyllie and VelCare adequate

Moderate unconsolidation: VelC

Substantial unconsolidation: calibrated Wyllie with Cp

VelCis a good average

Can use a dual porosity model : intergranular / isolated pores

Wyllie often gives good results for unfracturedblock

Vp/Vs Crossplot in Shaly Sands -Effect of Gas


44/59


Original Crossplot for Gas Sands.

Brie et al. -SPE 1995

Vp/Vs vst crossplot

Updated Vp/Vs Crossplot for Sands and Carbonates


45/59


p p p

Vp/Vs vst crossplot

UpdatedVp/Vs Crossplot

Includes:

- Effect of water salinity in sands,

- Water and gas trends in carbonates

Brie, SPWLA-2001


46/59

Live Oil Slowness


47/59


Effect of Live Oil on Vp/Vs


48/59


Live oil has an effect ont andVp/Vs.Although intuitively oil is liquid like water, it ismore compressible.

At high GOR the effect of live oil on sonictandVp/Vs is comparable to that of gas.

Sands with live oil, gravity 35 API, at 200Fand 5000 psi

For different GOR

Evaluation of Gas Effect on Sonic Slowness


49/59


Elastic physics govern sound propagation in materials and rocks.To understand the

effect of the pore fluid, especially that of gas, on sound velocity and slowness we haveto go back to the elastic properties of the rock. We have seen that sound velocity are

linked to elastic moduli and density with the relations:

23

42 sbpb VVK = 2

sbVG =

Where K is the bulk modulus,G the shear modulus andb is the rock density.

and

and

GKVp

+= 3

4

GVs=

Starting from the logs K and G can be obtained with the expressions:


50/59

Partially Saturated Rocks


51/59


In partially saturated rocks the effective fluid bulk modulus,

Kfe, must be entered in the Gassmannequation.Geophysicist useWoods Law , a compliance law to evaluate

Kfe.

However Woods Law is too abrupt at sonic frequency where

a more gradual change is observed.

A realistic approximation is provided by thepower mixing

law:

The exponente is usually around 5.Note that when saturation is less than 50% the fluid bulk

modulus is practically that of gas.Fluid Mixing Laws (low pressure)

Kfe

= Kmf

Kg( )Sxo e + Kg

0 100Liquid Saturation %

0

2.5

PoreFlu

idModulus

GPa

Woods

Law

Powe

rMix

ingLaw

g

xo

mf

xo

fe K

S

K

S

K

+= 11

Effect of Gas on Vp/Vs


52/59


( ) ( ) 3

422

+== G

K

mVs

Vp

dryVs

Vp dry

In shaly sands theVp/Vs of the dry rock is constant andequal to theVp/Vs of the minerals; 1.5 to 1.58(depending on additional minerals).

This expression provides a link between dry bulk

modulusKdry and the shear modulusG, obtained fromtshear.

Kdry can then be used to estimate the hydrocarbon

volume fromt, ort at different saturation conditions(fluid substitution).

Effect of Gas ont Slowness


53/59


Light hydrocarbon effect is large in

porous, unconsolidated formations.

The effect of gas on compressionalt issmall to negligible in low porosity, compact

formations.

Vp/Vs Crossplot in Tight Gas Sands -Algeria


54/59


Although the porosity is very low, the

effect of gas on Vp/Vs is visible in tight

sands with porosities of 10 PU or less.

Vp/Vs Crossplot in Low Porosity Oil Sand


55/59


Although the effect is small, the effect of live oil on

Vp/Vs is visible in tight sands with porosities of 6 to11 PU.

A quicklookmethod can be used to detect oil

bearing intervals in the field.

A polynomial correlation is adjusted to fit the Wet

Sand line on theVp/Vs crossplot

The wett can then be calculated fromtshear.

Live Oil properties:

41 deg API1301 cuft/bbl260F

4500 psi

Chardacet al. SPE-2003

432.... sssswet tetdtctbat ++++=

In the example shown: a = -0.8482

b = 0.7665

c = -1.450 10-3

d = 1.391 10-6

e = -5.364 10-10

t Overlay for HC Detection in Low Porosity Sands


56/59


The wett calculated fromtshear. with the polynomial

calculation is drawn with the measuredtlog

A separation betweentlog andtwet indicate the

presence of light hydrocarbon in the formation.

There is good agreement with the subsequent ELAN

evaluation.

Chardacet al. SPE-2003

Vp/Vs Crossplot in Limestone


57/59


Comparison of model curves with data

from a clean limestone with some gasintervals.

Wet Vp/Vs varies little and does not

depend on spherical porosity fraction,

Dry (gas bearing) Vp/Vs decreases with

porosity and with decreasing spherical

porosity fraction.

Generalized DryVp/Vs for Complex Lithology


58/59


In shaly sandsVp/Vsdry is constant around 1.53, but in carbonates it changes with porosity and

spherical pores fraction. A generalized equation forVp/Vsdry is given by the expression:

)9.01()53.1/(1.3/)/( spfVsVpVsVpVsVp mmdry =

Vp/Vsm :Vp/Vs of the minerals

(solids),

spf: Fraction of spherical

pores (molds or vugs) inthe porosity

Vp/Vsm The generalizedVp/Vsdry allows us to

evaluate the hydrocarbon effect and

do fluid substitution on the sonic logs

in all lithologies.

Sonic Applications


59/59


Seismic

Time Depth curveSynthetic seismogram

AVO/AVA calibration

Anisotropy

Cased Hole

Cement Bond Evaluation

Rock Mechanics

Pore pressure

Stress orientation and magnitude

Hydro-FracdesignWell bore stability evaluation

Sanding prediction

Petrophysics

Porosity-Lithology (especially in CH)Gas detection

Fractured reservoir evaluation

Permeability

Sonic Imaging

4.2 Sonic Measurements

Documents

Transcript of 4.2 Sonic Measurements