Page 1
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
CSLUniversity of Miami
Page 2
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 3
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 4
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 6
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 7
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
2.5% 3.6%
6.1%8.5%
5 mm1mm 1mm0.5 mm
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
Conceptual Model
1. Micropores between mosaic cement(“microcrack porosity”)
2. Micropores in fine grained cement
0.5 mm
Page 9
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
3000
4000
5000
6000V
p
5 10 15 20 25 30Pressure
P-W
ave
Vel
ocity
(m/s
)
Differential Pressure (MPa)
Page 10
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
Vp/Vs
Vp/Vs > 1.8
Vp/Vs < 1.75
Page 12
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 13
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 14
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
Brine (0.2g/l)
Ethylene Glycol
80%Glycol 20%Ethanol
57%Glycol 43%Ethanol
30%Glycol 70%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
)
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
• 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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 19
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
Page 20
Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
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
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Bächle et al. 2007, AAPG Long Beach
CSLUniversity of Miami
AcknowledgementIndustrial Associates Consortium For Research
and Training in Modern and Ancient Carbonates