rock physics models for marine gas hydrates
DESCRIPTION
Rock Physics Models for Marine Gas Hydrates. Darrell A. Terry, Camelia C. Knapp, and James H. Knapp Earth and Ocean Sciences University of South Carolina. Long Range Research Goals. - PowerPoint PPT PresentationTRANSCRIPT
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Rock Physics Models for Marine Gas Hydrates
Darrell A. Terry, Camelia C. Knapp, and James H. Knapp
Earth and Ocean SciencesUniversity of South Carolina
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Long Range Research Goals
• Further develop statistical rock physics to associate seismic properties with lithology in marine gas hydrate reservoirs
• Investigate AVO and seismic attribute analysis in a marine gas hydrate reservoir
• Analyze anistropic seismic properties in a marine gas hydrate reservoir to delineate fracture structures and fluid flow pathways
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Outline
• What is Rock Physics?• Models Used by JIP• Brief Theoretical Background• Recent Updates Suggested for Models• Candidate Models to Use• Role of Well Log Data• Future Directions
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What is Rock Physics?
• Methodology to relate velocity and impedance to porosity and mineralogy
• Establish bounds on elastic moduli of rocks– Effective-medium models – Three key seismic parameters
• Investigate geometric variations of rocks– Cementing and sorting trends– Fluid substitution analysis
• Apply information theory– Quantitative interpretation for texture, lithology, and
compaction through statistical analysis
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Models Used by JIP
(from Dai et al, 2004)
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Models Used by JIP
(from Dai et al, 2004)
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Theoretical Background
Effective-medium models for unconsolidated sediments
• Mindlin, 1949 (Hertz-Mindlin Theory)• Digby, 1981; Walton, 1987• Dvorkin and Nur, 1996• Jenkins et al, 2005• Sava and Hardage, 2006, 2009• Dutta et al, 2009
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Theoretical Background
(from Walton, 1987)
(from Mindlin, 1949)
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Theoretical Background
Modifications for saturation conditions and presence of gas hydrates
• Dvorkin and Nur, 1996• Helgerud et al, 1999; Helgerud, 2001
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Why Use Jenkins’ Update?
• Hertz-Mindlin theory often under predicts Vp/Vs ratios in comparison with laboratory rocks and well log measurements (Dutta et al, 2009) for unconsolidated sediments.
• A similar problem is noted in Sava and Hardage (2006, 2009).
• Additional Degree-of-Freedom
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Comparisons with Jenkins’ Update
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11.5
2
2.5
3
3.5
4
4.5
5 Critical Porosity = 0.4, n = 8.5, Pressure = 2 MPa, Porosity = 0.4
Gas Hydrate Saturation (nondimensional)
Sat
-roc
k P
-wav
e V
eloc
ity (
km/s
)
Rough Sphere (Walton, 1987; Dvorkin & Nur, 1996)
Smooth Sphere (Walton, 1987) Alpha = 0.2 (Jenkins, 2005)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 11.5
2
2.5
3
3.5
4
4.5
5 Critical Porosity = 0.4, n = 8.5, Pressure = 2 MPa, Porosity = 0.4
Gas Hydrate Saturation (nondimensional)
Sat
-roc
k P
-wav
e V
eloc
ity (
km/s
)
Rough Sphere (Walton, 1987; Dvorkin & Nur, 1996)
Smooth Sphere (Walton, 1987) Alpha = 0.8 (Jenkins, 2005)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1
1.5
2
2.5
3 Critical Porosity = 0.4, n = 8.5, Pressure = 2 MPa, Porosity = 0.4
Gas Hydrate Saturation (nondimensional)
Sat
-roc
k S
-wav
e V
eloc
ity (
km/s
)
Rough Sphere (Walton, 1987; Dvorkin & Nur, 1996)
Smooth Sphere (Walton, 1987) Alpha = 0.8 (Jenkins, 2005)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.5
1
1.5
2
2.5
3 Critical Porosity = 0.4, n = 8.5, Pressure = 2 MPa, Porosity = 0.4
Gas Hydrate Saturation (nondimensional)
Sat
-roc
k S
-wav
e V
eloc
ity (
km/s
)
Rough Sphere (Walton, 1987; Dvorkin & Nur, 1996)
Smooth Sphere (Walton, 1987) Alpha = 0.2 (Jenkins, 2005)
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Baseline Model
• Hertz-Mindlin theory (Jenkins et al, 2005)
• Effective dry-rock moduli (Helgerud, 2001)
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Baseline Model
• Gassmann’s equations
• Velocity equations
• Poisson’s ratio
• Bulk density
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Model Configurations
• Gas Hydrate Models (for solid gas hydrate)– Rock Matrix (Supporting Matrix / Grain)– Pore-Fluid (Pore Filling) Rock Matrix Pore-Fluid
GH
GR
GR GR
GR
GR GR
GR
GH
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Model Configurations
• Pore-Fluid • Rock Matrix
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Well Log Data
• Mallik 2L-38• JIP Wells
– Keathley Canyon– Atwater Valley
(Data Digitized from Collett et al, 1999)
2 3 4 5
850
900
950
1000
1050
1100
1150
Track 19 (1)
Compressional Velocity (km/s)
Dep
th (
m)
0.5 1 1.5 2
850
900
950
1000
1050
1100
1150
Track 19 (2)
Shear Velocity (km/s)
Dep
th (
m)
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Well Log Data: Crossplot
• Mallik 2L-38• Other logs for crossplots
– Porosity– Resistivity– Gas Hydrate Saturation
• Crossplots with third attribute• Generate probability
distribution functions (PDFs)
0.5 1 1.5 21.5
2
2.5
3
3.5
4
4.5
5
5.5
6 Crossplot: P-Wave vs S-Wave
Shear Velocity (km/s)
Com
pres
sion
al V
eloc
ity (
km/s
)
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MC-118 Stacking Velocities
• WesternGeco: locations of stacking velocity profiles for 3D stack– 253 profiles– Spaced 40 CMPs apart, inline and crossline– Convert to interval velocities
-88.53 -88.52 -88.51 -88.5 -88.49 -88.48 -88.47 -88.46 -88.45
28.83
28.84
28.85
28.86
28.87
28.88
28.89
WesternGeco Stacking Velocity, Profiles with Velocity Reversals
Longitude, degrees W
Lat
itude
, de
gree
s N
-88.53 -88.52 -88.51 -88.5 -88.49 -88.48 -88.47 -88.46 -88.45
28.83
28.84
28.85
28.86
28.87
28.88
28.89
WesternGeco Stacking Velocity, Profile Chart
Longitude, degrees W
Lat
itude
, de
gree
s N
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MC-118 Stacking Velocities
1500 2000 2500 3000 3500 4000
0
2
4
6
8
10
12
WesternGeco Stacking Velocities, Profile 81, Lon -88.4937, Lat 28.8543
RMS, m/s
Two-w
ay Tra
vel Ti
me, s
1500 2000 2500 3000 3500 4000
0
2
4
6
8
10
12
WesternGeco Stacking Velocities, Profile 63, Lon -88.4936, Lat 28.8479
RMS, m/s
Two-w
ay Tra
vel Ti
me, s
1500 2000 2500 3000 3500 4000
0
2
4
6
8
10
12
WesternGeco Stacking Velocities, Profile 101, Lon -88.4938, Lat 28.8607
RMS, m/s
Two-w
ay Tra
vel Ti
me, s
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Future Directions: Synthetic Seismic Models
Velocity Model
X (m)
Y (
m)
100 200 300 400 500 600 700 800 900 1000
100
200
300
400
500
600
700
800
900
1000
Reflectivity Model
X (m)
Y (
m)
100 200 300 400 500 600 700 800 900 1000
100
200
300
400
500
600
700
800
900
1000
Synthetic CSG with Shot at 960 m
X (m)
Tim
e (s
)
100 200 300 400 500 600 700 800 900 1000
0.2
0.4
0.6
0.8
1
1.2
1.4
X (m)
Y (
m)
Stacked Image for 96 Shot Gathers
100 200 300 400 500 600 700 800 900 1000
100
200
300
400
500
600
700
800
900
1000
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Future Directions
• Create Rock Physics Templates• Amplitude Variation with Offset (AVO)• Seismic Inversion (WesternGeco data, Pre-Stack
Gathers)– Acoustic impedance– Elastic Impedance– Attribute analysis
• Assign Lithology and Estimate Gas Hydrate Probabilities Based on Information Theory
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ReferencesDai, J.; Xu, H.; Snyder, F.; Dutta, N.; 2004. Detection and estimation of gas hydrates using rock physics seismic inversion:
Examples from the northern deepwater Gulf of Mexico. The Leading Edge, January 2004, p. 60-66.Digby, P. J.; 1981. The effective elastic moduli of porous granular rocks. J. Appl. Mech., v. 48, p. 803-808.Dutta, T.; Mavko, G.; Mukerji, T.; 2009. Improved granular medium model for unconsolidated sands using coordination
number, porosity and pressure relations. Proc. SEG 2009 International Exposition and Annual Meeting, Houston, p. 1980-1984.
Dvorkin, J.; Nur, A.; 1996. Elasticity of high-porosity sandstones: Theory for two North Sea data sets. Geophysics, v. 61, p. 1363-1370.
Helgerud, M. B.; Dvorkin, J.; Nur, A.; Sakai, A.; Collett, T.; 1999. Elastic-wave velocity in marine sediments with gas hydrates: Effective medium modeling. Geophys. Res. Lett., v. 26, n. 13, p. 2021-2024.
Helgerud, M. B.; 2001. Wave Speeds in Gas Hydrate and Sediments Containing Gas Hydrate: A Laboratory and Modeling Study. Ph.D. Dissertation, Stanford University, April 2001.
Jenkins, J.; Johnson, D.; La Ragione, L.; Maske, H.; 2005. Fluctuations and the effective moduli of an isotropic, random aggregate of identical, frictionless spheres. J. Mech. Phys. Solids, v. 53, pp. 197-225.
Mindlin, R. D.; 1949. Compliance of elastic bodies in contact. J. Appl. Mech., v. 16, p. 259-268.Sava, D.; Hardage, B.; 2006. Rock physics models of gas hydrates from deepwater, unconsolidated sediments. Proc. SEG
2006 Annual Meeting, New Orleans, p. 1913-1917.Sava, D.; Hardage, B.; 2009. Rock-physics models for gas-hydrate systems associated with unconsolidated marine
sediments. In: Collett, T.; Johnson, A.; Knapp, C.; Boswell, R.; eds. Natural gas Hydrates – Energy Resource Potential and Associated Geologic Hazards. AAPG Memoir 89, p. 505-524.
Walton, K.; 1987. The effective elastic moduli of a random packing of spheres. J. Mech. Phys. Solids, v. 35, n. 2, pp. 213-226.
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Model Configurations
• Partial Gas Saturation Models (for free gas)– Homogeneous Gas Saturation– Patchy Gas Saturation