1. Quantifying methane hydrate saturation in different geologic settings Gaurav Bhatnagar 1, George J. Hirasaki 1, Walter G. Chapman 1 Brandon Dugan 2, Gerald R. Dickens 2 1. Dept. of Chemical and Biomolecular Engineering., Rice University 2. Dept. of Earth Science, Rice University AGU Fall Meeting December 13, 2006

2. Objectives Develop a general numerical model for simulating accumulation of gas hydrates in marine sediments over geological time scales Use dimensionless scalings to depict hydrate saturation dependence on the large parameter set using a few simple plots

3. Model schematic

4. Outline  Phase equilibrium  Component mass balances  Simulation Results  General hydrate distributions

5. Phase Equilibrium

6. Methane Solubility Profile Vertical depth normalized with the depth of the BHSZ Methane concentration normalized with triple point solubility

7. Outline  Phase equilibrium  Component mass balances  Simulation Results  General hydrate distributions

8. Component Mass Balances - Organic Assumptions –Sedimentation rate is constant with time –Densities of all components remain constant –Organic component advects with the sediment velocity –Organic decay occurs through a first order reaction Organic carbon in sediments Convective flux Reaction term Damkohler no. = Pe 1 Peclet no. =

9. Organic concentration profile

10. Component Mass Balances - Methane Assumptions –Hydrate and gas phases form as soon as local solubility is exceeded (no kinetic limitation) –Hydrate and gas phases advect with the same velocity as the sediments

11. Methane Balance (contd.) β : Normalized organic content at seafloor (quantifies net carbon input from top) Pe 2 : Peclet no. for external flow = = Ratio of (External Flux/Diffusion)

12. Outline  Phase equilibrium  Component mass balances  Simulation Results  General hydrate distributions

13. Hydrate accumulation with underlying free gas

14. Hydrate accumulation without free gas below

15. Outline  Phase equilibrium  Component mass balances  Simulation Results  General hydrate distributions

16. Parameter space for biogenic sources

17. Parameter space for biogenic sources with Da For each pair of curves: 1.Hydrate formation with free gas below 2.Hydrate formation without free gas 3.No hydrate formation

18. Scaling of variables Scale x-axis to represent net methane generated within the HSZ instead of just the input Methane generated within HSZ (from analytical solution to organic balance)

19. Scaled parameter space (biogenic source)

20. Compute average hydrate saturation and plot contour plots Average hydrate saturation also scales with the scaling shown before Hydrate saturation distribution (biogenic)

21. Hydrate saturation averaged over GHSZ (biogenic)

22. Parameter space for deeper sources

23. Scaled parameter space for deeper sources

24. Again compute average hydrate saturation as before Average hydrate saturation does not scale with the scaling shown before for this case (Pe 1 + Pe 2 ) The quantity that remains invariant in this case is the flux of hydrate, defined as Pe 1 Scales with the original choice of dimensionless groups and is plotted along contour lines Hydrate saturation distribution (deeper source)

25. Hydrate saturations from deeper sources Contours of Pe 1

26. Sensitivity to seafloor parameters

27. Conclusions Better physical understanding of this system can be obtained from our general dimensionless model compared to previous site-specific models Hydrate layer can extend down to BHSZ with free gas below or remain within HSZ with no free gas Dependence of hydrate saturation on various parameters can be depicted using simple contour maps. This helps in summarizing results from hundreds of simulations in just two plots. Hydrate saturation at any geological setting can be inferred from these plots without any new simulations

28. Financial Support: Shell Center for Sustainability & Kobayashi Graduate Fellowship