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

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

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

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Simulate gas hydrate and free gas accumulation in heterogeneous marine sediment over geologic time scales Key features: CH 4 phase equilibrium and solubility curves Sedimentation and compaction Mass conservation: organic matter, sediment, CH 4 and water CH 4 generation by in situ methanogenesis (biogenic) CH 4 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) Model overview 4

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

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

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Key dimensionless groups and scaled variables Peclet numbers: Pe 1 : Ratio of advective fluid flux (due to sedimentation and compaction) to methane diffusion Pe 2 : 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 CH 4 concentration 7

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1-D model: Effect of upward fluid flux Bhatnagar et al., Am. J. Sci., (2007) Parameters Pe 1 = 0.1 Da = 0 β = 0 Peak S h = 6% Peak S g = 5% Pe 2 = -5 8

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1-D model: Effect of upward fluid flux Bhatnagar et al., Am. J. Sci., (2007) Peak S h = 21% Peak S g = 21% Pe 2 = -15 Parameters Pe 1 = 0.1 Da = 0 β = 0 Peak S h = 6% Peak S g = 5% Pe 2 = -5

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2-D homogeneous model (validation with 1-D) BHSZ Peak S h = 20% Peak S g = 17% Parameters Pe 1 = 0.1 Pe 2 = -15 Da = 0 β = 0 N sc = 10 4 N t ϕ = 1.485

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2-D homogeneous model (validation with 1-D) BHSZ Peak S h = 20% Peak S g = 17% S g = 19% S h = 20%

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Net fluid flux Pe 1 + Pe 2 Average hydrate flux Pe 1 Hydrate saturation Net fluid flux and steady state average hydrate saturation Parameters Pe 1 = 0.1

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13 k frac = 100 k shale Effect of a vertical fracture system 13 Seafloor 2 L t 2700 mbsl BHSZ

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Peak S h = 26% Peak S g = 29% Vertical fracture system with in-situ methanogenesis Seafloor Parameters Pe 1 = 0.1 Pe 2 = 0 Da = 10 β = 6

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BHSZ Peak S h = 48% Peak S g = 42% Vertical fracture system with deep methane sources Seafloor Parameters Pe 1 = 0.1 Pe 2 = -2 Da = 10 β = 6

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BHSZ Peak S h = 53% Peak S g = 40% Effect of permeability anisotropy (k v < k h ) Seafloor Parameters Pe 1 = 0.1 Pe 2 = -2 Da = 10 β = 6 k v /k h = 10 -2

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BHSZ Peak S h = 53% Peak S g = 40% Effect of permeability anisotropy (k v < k h ) Seafloor Parameters Pe 1 = 0.1 Pe 2 = -2 Da = 10 β = 6 k v /k h = 10 -2

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Local fluid flux and Pe 1

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Result summary – Immobile gas 26% 29% Parameters Pe 1 = 0.1 Pe 2 = 0 Da = 10 β = 6 11% 14% Biogenic source only Homogeneous S h and S g in fracture

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Result summary – Immobile gas 26% 29% 42% 48% Parameters Pe 1 = 0.1 Pe 2 = 0 Da = 10 β = 6 Parameters Pe 1 = 0.1 Pe 2 = -2 Da = 10 β = 6 11% 14% 17% 14% Biogenic source only Biogenic + external flux Homogeneous S h and S g in fracture

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Effect of free gas migration into the GHSZ Parameters Pe 1 = 0.1 Pe 2 = -2 Da = 10 β = 6 S gr = 5% Seafloor BHSZ Time = 6.4 Myr Peak S g = 33% Peak S h = 59%

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Effect of free gas migration into the GHSZ Parameters Pe 1 = 0.1 Pe 2 = -2 Da = 10 β = 6 S gr = 5% Seafloor BHSZ Time = 19.2 Myr BHSZ Peak S g = 62% Peak S h = 75%

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23 k sand = 100 k shale Effect of a dipping sand layer 23 Seafloor 2 L t 10 L t 2700 mbsl High permeability sand layer deposited between two shale sediments BHSZ

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Seafloor Peak S g = 38% Peak S h = 59% BHSZ Preferential accumulation within high permeability dipping sand layers

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Conclusions 25 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 (Pe Local ) Pe Local can be used to compute average hydrate saturation similar to our 1-D correlation

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Questions 26

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Back up slides 27

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

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2-D water mass balance Dimensionless water mass balance 29

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2-D sediment mass balance Dimensionless sediment mass balance 30

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

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Dimensionless methane mass balance 2-D methane mass balance 32

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Porosity reduction (compaction)Biogenic source of methane Organic matter leaving the GHSZ is dependent on the ratio Pe 1 /Da Reduced porosity Normalized Depth Porosity and normalized organic carbon profile 33 Bhatnagar et al., Am. J. Sci., (2007) Normalized organic content α Normalized Depth

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34 1-D model: Dissolved CH 4 concentration, gas hydrate and free gas saturation 34 Seafloor Adapted from Bhatnagar et al., Am. J. Sci. (2007) Peak S h = 2% Peak S g = 1% Pe 2 = -2

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Net fluid flux Pe 1 + Pe 2 Average hydrate flux Pe 1 Hydrate saturation 35 Net fluid flux and steady state average hydrate saturation 35 Bhatnagar et al., Am. J. Sci., (2007) Pe 1 Cascadia Margin (Site 889) Fluid velocity ~ 1 mm/yr Pe 1 = = 3% Increasing external flux Parameters Pe 1 = 0.1, β = 6 N sc = 10 4 (Hydrostatic) N t ϕ = 1.485, t final = 12.8 Myr

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1-D and 2-D model liquid flux comparison 1-D model 2-D model Pe 2 = -40 Pe 2 =

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

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

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BHSZ Peak S h = 11% Peak S g = 13% Parameters Pe 1 = 0.1 Pe 2 = 0 Da = 10 β = 6 N sc = 10 4 N t ϕ = 1.485

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