Rock Physics Models for Marine Gas Hydrates

1. 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

2. 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

3. 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

4. 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

5. Models Used by JIP (from Dai et al, 2004)

6. Models Used by JIP (from Dai et al, 2004)

7. 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

8. Theoretical Background (from Walton, 1987) (from Mindlin, 1949)

9. Theoretical Background Modifications for saturation conditions and presence of gas hydrates • Dvorkin and Nur, 1996 • Helgerud et al, 1999; Helgerud, 2001

10. 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

11. Comparisons with Jenkins’ Update

12. Baseline Model • Hertz-Mindlin theory (Jenkins et al, 2005) • Effective dry-rock moduli (Helgerud, 2001)

13. Baseline Model • Gassmann’s equations • Velocity equations • Poisson’s ratio • Bulk density

14. Model Configurations • Gas Hydrate Models (for solid gas hydrate) • Rock Matrix (Supporting Matrix / Grain) • Pore-Fluid (Pore Filling) Rock Matrix Pore-Fluid

15. Model Configurations • Pore-Fluid • Rock Matrix

16. Well Log Data • Mallik 2L-38 • JIP Wells • Keathley Canyon • Atwater Valley (Data Digitized from Collett et al, 1999)

17. 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)

18. 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

19. MC-118 Stacking Velocities

20. Future Directions: Synthetic Seismic Models

21. 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

22. References Dai, 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.

23. Model Configurations • Partial Gas Saturation Models (for free gas) • Homogeneous Gas Saturation • Patchy Gas Saturation