Consortium on Processes in Porous Media

1. Effects of Lithologic Heterogeneity and Focused Fluid Flow on Gas Hydrate Distribution in Marine Sediments Sayantan Chatterjee Walter G. Chapman, George J. HirasakiRice University, Houston, Texas April 26, 2011 Consortium on Processes in Porous Media Shared University Grid at Rice NSF Grant EIA-0216467 DE-FC26-06NT 42960

2. Gas hydrates: “Ice that burns” Samples from Cascadia margin, offshore Oregon Torres et al., Earth Planet. Sci. Lett., (2004) Courtesy: USGS

3. Motivation Potential energy resource Geohazard: Submarine slope failure Global climate change Needed – A fundamental understanding of the dynamics of gas hydrate systems

4. Model overview • Simulate gas hydrate and free gas accumulation in heterogeneous marine sediment over geologic time scales • Key features: • CH4 phase equilibrium and solubility curves • Sedimentation and compaction • Mass conservation: organic matter, sediment, CH4 and water • CH4 generation by in situ methanogenesis (biogenic) • CH4 advected up from deep external sources (thermogenic) • Water migration with dissolved gas (advection) and diffusion • Heterogeneity: High permeability conduits (e.g., vertical fracture systems, chimney structures, and sand layers)

5. Sediment flux Organic Carbon Seafloor Hydrate dissociation due to burial below BHSZ Lt BHSZ Methane solubility curve Free gas recycle back into HSZ Schematic of hydrate formation and burial Sediment flux Sediment flux Seafloor Fluid flux Sedimentation Hydrate layer extending downwards Depth BHSZ External fluid flux Subsidence Subsidence Subsidence Geological Timescale Bhatnagar et al., Am. J. Sci., (2007)

6. Sediment flux Organic Carbon Seafloor Hydrate layer extending downwards Hydrate dissociation due to burial below BHSZ Lt BHSZ Methane solubility curve Free gas recycle back into HSZ Schematic of hydrate formation and burial Sediment flux Sediment flux Seafloor Depth BHSZ Subsidence Subsidence Subsidence Geological Timescale Bhatnagar et al., Am. J. Sci., (2007)

7. Key dimensionless groups and scaled variables • Peclet numbers: Pe1: Ratio of advective fluid flux (due to sedimentation and compaction) to methane diffusion Pe2: Ratio of advective fluid flux (due to external sources) to methane diffusion • Damköhler number: Da: Ratio of methanogenesis reaction rate to methane diffusion • Beta: β: Normalized organic matter concentration deposited at the seafloor relative to 3-phase equilibrium CH4 concentration

8. 1-D model: Effect of upward fluid flux Pe2 = -5 Parameters Pe1 = 0.1 Da = 0 β = 0 Peak Sh = 6% Peak Sg = 5% Bhatnagar et al., Am. J. Sci., (2007)

9. 1-D model: Effect of upward fluid flux Pe2 = -5 Pe2 = -15 Parameters Pe1 = 0.1 Da = 0 β = 0 Peak Sh = 6% Peak Sh = 21% Peak Sg = 5% Peak Sg = 21% Bhatnagar et al., Am. J. Sci., (2007)

10. 2-D homogeneous model (validation with 1-D) Parameters Pe1 = 0.1 Pe2 = -15 Da = 0 β = 0 Nsc = 104 N’tϕ = 1.485 Peak Sh = 20% BHSZ Peak Sg = 17%

11. 2-D homogeneous model (validation with 1-D) Sh = 20% Peak Sh = 20% BHSZ Peak Sg = 17% Sg = 19%

12. Net fluid flux and steady state average hydrate saturation <Sh> Parameters Pe1 = 0.1 Net fluid flux Pe1 + Pe2 Average hydrate flux Pe1<Sh>  Hydrate saturation <Sh>

13. Effect of a vertical fracture system 2700 mbsl Seafloor kfrac = 100 kshale 2 Lt BHSZ 13 2 Lt

14. Vertical fracture system with in-situ methanogenesis Seafloor Peak Sh = 26% BHSZ Parameters Pe1 = 0.1 Pe2 = 0 Da = 10 β = 6 Peak Sg = 29%

15. Vertical fracture system with deep methane sources Seafloor Peak Sh = 48% BHSZ Parameters Pe1 = 0.1 Pe2 = -2 Da = 10 β = 6 Peak Sg = 42%

16. Effect of permeability anisotropy (kv < kh) Seafloor Peak Sh = 53% BHSZ Parameters Pe1 = 0.1 Pe2 = -2 Da = 10 β = 6 kv/kh = 10-2 Peak Sg = 40%

17. Effect of permeability anisotropy (kv < kh) Seafloor Peak Sh = 53% BHSZ Parameters Pe1 = 0.1 Pe2 = -2 Da = 10 β = 6 kv/kh = 10-2 Peak Sg = 40%

18. Local fluid flux and Pe1<Sh>

19. Result summary – Immobile gas Homogeneous Shand Sg in fracture Biogenic source only Parameters Pe1 = 0.1 Pe2 = 0 Da = 10 β = 6 11% 26% 29% 14%

20. Result summary – Immobile gas Homogeneous Shand Sg in fracture Biogenic source only Parameters Pe1 = 0.1 Pe2 = 0 Da = 10 β = 6 11% 26% 29% 14% Homogeneous Shand Sg in fracture Biogenic + external flux 48% 14% Parameters Pe1 = 0.1 Pe2 = -2 Da = 10 β = 6 17% 42%

21. Effect of free gas migration into the GHSZ Seafloor Time = 6.4 Myr Peak Sh = 59% BHSZ Parameters Pe1 = 0.1 Pe2 = -2 Da = 10 β = 6 Sgr = 5% Peak Sg = 33%

22. Effect of free gas migration into the GHSZ Seafloor Time = 19.2 Myr Peak Sh = 75% BHSZ BHSZ Parameters Pe1 = 0.1 Pe2 = -2 Da = 10 β = 6 Sgr = 5% Peak Sg = 62%

23. Effect of a dipping sand layer 2700 mbsl Seafloor High permeability sand layer deposited between two shale sediments ksand = 100 kshale q 2 Lt BHSZ 10 Lt 23

24. Preferential accumulation within high permeability dipping sand layers Seafloor Peak Sh = 59% BHSZ Peak Sg = 38%

25. Conclusions • Enhanced hydrate and free gas saturations occur within high permeability conduits (e.g., vertical fracture systems, chimney structures, and sand layers) • Enhanced hydrate and free gas saturation within high permeability conduits is related to increased, focused, localized, advective fluid flux (PeLocal) • PeLocal can be used to compute average hydrate saturation <Sh> similar to our 1-D correlation

26. Questions

27. Back up slides

28. Constitutive relationships Darcy flux for water Darcy flux for free gas Phase saturations Effective stress - porosity relationship

29. 2-D water mass balance Dimensionless water mass balance

30. 2-D sediment mass balance Dimensionless sediment mass balance

31. 2-D organic mass balance Dimensionless organic carbon mass balance

32. 2-D methane mass balance Dimensionless methane mass balance

33. Porosity and normalized organic carbon profile Porosity reduction (compaction) Biogenic source of methane Normalized Depth Normalized Depth Reduced porosity Normalized organic content α Organic matter leaving the GHSZ is dependent on the ratio Pe1/Da Bhatnagar et al., Am. J. Sci., (2007)

34. 1-D model: Dissolved CH4 concentration, gas hydrate and free gas saturation Seafloor Pe2 = -2 Seafloor Peak Sh = 2% Peak Sg = 1% 34 Adapted from Bhatnagar et al., Am. J. Sci. (2007)

35. Net fluid flux and steady state average hydrate saturation <Sh> Pe1<Sh> Cascadia Margin (Site 889) Fluid velocity ~ 1 mm/yr Pe1 = 0.061 <Sh> = 3% Increasing external flux Parameters Pe1 = 0.1, β = 6 Nsc = 104 (Hydrostatic) N’tϕ = 1.485, tfinal = 12.8 Myr Bhatnagar et al., Am. J. Sci., (2007) Net fluid flux Pe1 + Pe2 Average hydrate flux Pe1<Sh>  Hydrate saturation <Sh> 35

36. 1-D and 2-D model liquid flux comparison Pe2 = -20 1-D model 2-D model Pe2 = -40

37. Effect of a high permeability sand layer 2700 mbsl Seafloor High permeability sand layer deposited between two shale sediments ksand = 100 kshale 916 mbsf BHSZ kfrac = 100 kshale 37 4.58 km

38. Combined effect of vertical fracture system and dipping sand layer 2700 mbsl Seafloor High permeability sand layer deposited between two shale sediments ksand = 100 kshale 916 mbsf BHSZ 38 4.58 km kfrac = 100 kshale

39. Peak Sh = 11% BHSZ Parameters Pe1 = 0.1 Pe2 = 0 Da = 10 β = 6 Nsc = 104 N’tϕ = 1.485 Peak Sg = 13%