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 Processesin Porous Media

DE-FC26-06NT 42960Shared University Grid at Rice

NSF Grant EIA-0216467

Samples from Cascadia margin, offshore OregonTorres et al., Earth Planet. Sci. Lett., (2004)

2

Gas hydrates: “Ice that burns”

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

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)

Model overview

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Hydrate dissociation due to burial below BHSZ

Organic Carbon Seafloor

Sediment flux

Lt

BHSZ

Free gas recycleback into HSZ

Methane solubility curve

Geological Timescale

Subsidence

Schematic of hydrate formation and burial

Bhatnagar et al., Am. J. Sci., (2007)5

Sediment flux Sediment flux

BHSZ

Subsidence Subsidence

Seafloor

External fluid flux

Fluid fluxSedimentation Hydrate layer

extending downwards

Hydrate dissociation due to burial below BHSZ

Organic Carbon Seafloor

Sediment flux

Lt

BHSZ

Free gas recycleback into HSZ

Methane solubility curve

Hydrate layer extending downwards

Geological Timescale

Subsidence

Schematic of hydrate formation and burial

Bhatnagar et al., Am. J. Sci., (2007)6

Sediment flux Sediment flux

BHSZ

Subsidence Subsidence

Seafloor,

1 f sed t

m

U LPe

D

,2 f ext t

m

U LPe

D

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

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1-D model: Effect of upward fluid flux

Bhatnagar et al., Am. J. Sci., (2007)

ParametersPe1 = 0.1Da = 0β = 0

Peak Sh = 6%

Peak Sg = 5%

Pe2 = -5

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1-D model: Effect of upward fluid flux

Bhatnagar et al., Am. J. Sci., (2007)

Peak Sh = 21%

Peak Sg = 21%

Pe2 = -15

ParametersPe1 = 0.1Da = 0β = 0

Peak Sh = 6%

Peak Sg = 5%

Pe2 = -5

2-D homogeneous model (validation with 1-D)

BHSZ Peak Sh = 20%

Peak Sg = 17%

Parameters

Pe1 = 0.1Pe2 = -15Da = 0β = 0Nsc = 104

N’tϕ = 1.485

2-D homogeneous model (validation with 1-D)

BHSZ Peak Sh = 20%

Peak Sg = 17% Sg = 19%

Sh = 20%

Net fluid flux Pe1 + Pe2 Average hydrate flux Pe1<Sh> Hydrate saturation <Sh>

Net fluid flux and steady state average hydrate saturation <Sh>

ParametersPe1 = 0.1

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kfrac = 100 kshale

Effect of a vertical fracture system

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Seafloor

2 Lt

2700 mbsl

BHSZ

BHSZPeak Sh = 26%

Peak Sg = 29%

Vertical fracture system with in-situ methanogenesis

Seafloor

Parameters

Pe1 = 0.1Pe2 = 0Da = 10β = 6

BHSZPeak Sh = 48%

Peak Sg = 42%

Vertical fracture system with deep methane sources

Seafloor

Parameters

Pe1 = 0.1Pe2 = -2Da = 10β = 6

BHSZPeak Sh = 53%

Peak Sg = 40%

Effect of permeability anisotropy (kv < kh)

Seafloor

Parameters

Pe1 = 0.1Pe2 = -2Da = 10β = 6kv/kh = 10-2

BHSZPeak Sh = 53%

Peak Sg = 40%

Effect of permeability anisotropy (kv < kh)

Seafloor

Parameters

Pe1 = 0.1Pe2 = -2Da = 10β = 6kv/kh = 10-2

Local fluid flux and Pe1<Sh>

Result summary – Immobile gas

26%

29%

ParametersPe1 = 0.1Pe2 = 0Da = 10β = 6

11%

14%

Biogenic source only

Homogeneous Sh and Sg in fracture

Result summary – Immobile gas

26%

29%

42%

48%

ParametersPe1 = 0.1Pe2 = 0Da = 10β = 6

ParametersPe1 = 0.1Pe2 = -2Da = 10β = 6

11%

14%

17%

14%

Biogenic source only

Biogenic + external flux

Homogeneous Sh and Sg in fracture

Homogeneous Sh and Sg in fracture

Effect of free gas migration into the GHSZ

Parameters

Pe1 = 0.1Pe2 = -2Da = 10β = 6Sgr = 5%

Seafloor

BHSZ

Time = 6.4 Myr

Peak Sg = 33%

Peak Sh = 59%

Effect of free gas migration into the GHSZ

Parameters

Pe1 = 0.1Pe2 = -2Da = 10β = 6Sgr = 5%

Seafloor

BHSZ

Time = 19.2 Myr

Peak Sg = 62%

Peak Sh = 75%

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ksand = 100 kshale

Effect of a dipping sand layer

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Seafloor

10 Lt

2700 mbslHigh permeability sand layer deposited between two shale sediments

BHSZ

Seafloor

Peak Sg = 38%

Peak Sh = 59%

BHSZ

Preferential accumulation within high permeability dipping sand layers

Conclusions

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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

Questions

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Back up slides

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Constitutive relationshipsDarcy flux for water

Darcy flux for free gas

Phase saturations

Effective stress - porosity relationship

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2-D water mass balance

Dimensionless water mass balance

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2-D sediment mass balance

Dimensionless sediment mass balance

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2-D organic mass balance

Dimensionless organic carbon mass balance

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Dimensionless methane mass balance

2-D methane mass balance

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Porosity reduction (compaction) Biogenic source of methane

Organic matter leaving the GHSZ is dependent on the ratio Pe1/Da

,1 Sedimentation

Reactionf sed

t

UPe

Da L

Reduced porosity

Porosity and normalized organic carbon profile

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1

0.7

0.1

t

o

N

Bhatnagar et al., Am. J. Sci., (2007)

Normalized organic content α

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1-D model: Dissolved CH4 concentration, gas hydrate and free gas saturation

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Seafloor

Seafloor

Adapted from Bhatnagar et al., Am. J. Sci. (2007)

Peak Sh = 2%

Peak Sg = 1%

Pe2 = -2

Net fluid flux Pe1 + Pe2 Average hydrate flux Pe1<Sh> Hydrate saturation <Sh>35

Net fluid flux and steady state average hydrate saturation <Sh>

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Bhatnagar et al., Am. J. Sci., (2007)

Pe1<Sh>Cascadia Margin (Site 889)Fluid velocity ~ 1 mm/yrPe1 = 0.061<Sh> = 3%

Increasing external flux

ParametersPe1 = 0.1, β = 6Nsc = 104 (Hydrostatic)N’tϕ = 1.485, tfinal = 12.8 Myr

1-D and 2-D model liquid flux comparison

0

1 1

lm

l lw m

c

c c

0

1

lm

l lw m

c

c c

1-D model 2-D model

Pe2 = -40

Pe2 = -20

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37

ksand = 100 kshale

Effect of a high permeability sand layer

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Seafloor

916 mbsf

4.58 km

2700 mbsl

High permeability sand layer deposited between two shale sediments

kfrac = 100 kshale

BHSZ

38

ksand = 100 kshale

Combined effect of vertical fracture system and dipping sand layer

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Seafloor

916 mbsf

4.58 km

2700 mbsl

BHSZ

kfrac = 100 kshale

High permeability sand layer deposited between two shale sediments

BHSZ Peak Sh = 11%

Peak Sg = 13%

Parameters

Pe1 = 0.1Pe2 = 0Da = 10β = 6Nsc = 104

N’tϕ = 1.485