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Bächle et al. 2007, AAPG Long Beach

CSLUniversity of Miami

Effect of Spherical Pore Shapes on Acoustic Properties in Carbonates

Gregor Baechle1, Layaan Al-Kharusi1, Gregor Eberli1, Austin Boyd2 and Alan Byrnes3

1Comparative Sedimentology Laboratory 2Schlumberger Doll Research, Boston

3Kansas Geological Survey




Outline

• Porosity & Pore Type Effects– Dominant oomoldic pore type– Minor inter-crystalline pore type

• Fluid & Pressure Effects– 3-30 MPa differential pressure– 7 different pore fluids



Key Points – Part 1 • Large p-wave velocity scatter (2500 m/s) at given

porosity• Our results question two major assumptions in

respect to moldic, spherical pores:– Moldic pores are always associated with strong rock-

frame– Amount of spherical porosity is related to velocity

variations at a given porosity

Not dominant pore type, but minor inter-crystalline pore type in rock frame causes slow velocity at given porosity



1000

2000

3000

4000

5000

6000

7000

microporosityinterpart./crystalline por.densely cemented, low porousmoldic porosityintraframe porosity

0 10 20 30 40 50 60

predominant pore types:

porosity (%)

Vp (m

/s)

Conceptual Pore Type Model



oomoldic pore type ~ strong cemented frame spherical pores shapes ~ incompressibility

25002500

30003000

35003500

40004000

45004500

50005000

55005500

60006000

65006500

70007000

VELO

CIT

Y (

VELO

CIT

Y ( m

/sm

/s))

00 0.050.05 0.10.1 0.150.15 0.20.2 0.250.25 0.30.3 0.350.35 0.40.4 0.450.45 0.50.5PorosityPorosity

VPVP--PHIPHI

SalehSaleh and and CastagnaCastagna, 2004, Geophysics, , 2004, Geophysics, (drawing modified by Ralf Weger)Simplified Rock Model

Pore Space (1)Solid Frame (2)Contact Area (3)

Aspect Ratio:

High

Low

increase in spherical porosity



3000

3500

4000

4500

5000

5500

6000

6500

7000

10 20 30 40

Porosity (%)

Velo

city

(m/s

)

Oomoldic porosityplugs-dryWyllie Trendline Cacite

A

E

D

C

B

F

Large Scatter in Vp-Porosity Plot

Wyllie time average

30MPa; dry conditions



2.5% 3.6%

6.1%8.5%

5 mm1mm 1mm0.5 mm



Conceptual Model

1. Micropores between mosaic cement(“microcrack porosity”)

2. Micropores in fine grained cement

0.5 mm



3000

4000

5000

6000V

p

5 10 15 20 25 30Pressure

P-W

ave

Vel

ocity

(m/s

)

Differential Pressure (MPa)



1.6

1.65

1.7

1.75

1.8

1.85

1.9

1.95

Vp/

Vs

0 10 20 30Pressure

Vp/

Vs

ratio

Differential Pressure (MPa)

Increasing Vp/Vs with pressure



Vp/Vs

Vp/Vs > 1.8

Vp/Vs < 1.75



Key Points – Part 1 • Large p-wave velocity scatter (2500 m/s) at given

porosity• Our results question two major assumptions in

respect to moldic, spherical pores:– Moldic pores are always associated with strong rock-

frame– Amount of spherical porosity is related to velocity

variations at a given porosity

Not dominant pore type, but minor inter-crystalline pore type in rock frame causes slow velocity at given porosity



Key Points (Part 2) • P-wave velocity of rocks with oomoldic

porosity changes significantly with different pore fluids

• Bulk modulus shows linear relationship with changes in fluid modulus

• Fluid saturation effects on bulk modulus is function of porosity & pore types



Brine (0.2g/l)

Ethylene Glycol

80%Glycol 20%Ethanol



Ethanol Pentane

0

2

4

6

8

10

12

14

16

18

20

0 1 2 3 4

Kfluid (GPa)

Vis

cosi

ty (c

P)

Kfluid

Hea

vy o

il

Form

atio

n w

ater

Live

oil

Wet

gas

Dry

gas

(K=0

)



• Vpbrine ≠ Vp 57% Glycol at low pressure (but similar Kfluid)

• Density Effect: VpEthanol and VpPentane < VpDry

3000

3200

3400

3600

3800

4000

4200

4400

0 5 10 15 20 25 30Effective pressure (MPa)

Vel

ocity

(m/s

)

dry

brine

glycol

57%glycol

30%glycol

et hanol

pent ane

P-W

AVE

VEL

OC

ITY

(m/s

)Dry

Ethanol &Pentane

Glycol



Bulk modulus increases with Kfluid

Bulk modulus less sensitive to pressure changes than to saturation changes

15

20

25

30

8 13 18 23 28Effective pressure (MPa)

Mod

uli (

GP

a)

dry

br ine

glycol

57% glycol

30% glycol

et hanol

pent ane

Bul

k M

odul

us (G

Pa)

3.23 GPa

2.3 GPa

1.75GPa

1.1 GPa

0.7 GPa



Simplified Gassmann Equation*

Simplified Linear RelationSimplified Linear Relation

Kfl

µ=µo

Ksat

*Mavko et al., 1995; Han and Batzle, 2004; Rasolofosaon and Zinszner, 2004

Steeper slope ~ larger increase of sonic velocity Steeper slope ~ larger increase of sonic velocity with fluid substitutionwith fluid substitution

Kdry

Assumption:

Kgrain >> Kfluid



Slope Function of Pore Type & Porosity

5

10

15

20

25

30

0 1 2 3 4(K)fluid (MPa)

Mod

uli (

GPa

)

K_Pe=11MPa K_Pe=21MPaK_Pe=28MPa nu_P=11MPanu_P=21MPa nu_P=28MPa



Slope is the combined effect of porosity and pore type

1

1.5

2

2.5

3

3.5

4

4.5

10 15 20 25 30 35

Porosity (%)

Ksa

t-Kflu

id S

lope

Interparticle to vugy porosity

Oomoldic porosity

Microporosity



Key Points (Part 2) • P-wave velocity of rocks with oomoldic

porosity changes significantly with different pore fluids

• Bulk modulus shows linear relationship with changes in fluid modulus

• Fluid saturation effects on bulk modulus is function of porosity & pore types



Conclusions & Implications

– Large velocity scatter (2500 m/s) at given porosity observed in rocks dominated by oomoldic porosity

– Minor inter-crystalline microporosity causes slow velocity at given porosity

– The rock frame affects velocity at high frequencies and not the dominant pore shape or pore type!

– Ground truth of theoretical models is necessary to obtain meaningful acoustic characterization of carbonates rock/pore types



AcknowledgementIndustrial Associates Consortium For Research

and Training in Modern and Ancient Carbonates

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