Quantifying%20methane%20hydrate%20saturation%20in%20different%20geologic%20settings

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Quantifying methane hydrate saturation in
different geologic settings
Gaurav Bhatnagar1, George J. Hirasaki1, Walter G.
Chapman1 Brandon Dugan2, Gerald R. Dickens2 1.
Dept. of Chemical and Biomolecular Engineering.,
Rice University 2. Dept. of Earth Science, Rice
University AGU Fall Meeting December 13, 2006
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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

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Model schematic
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Outline

• Phase equilibrium
• Component mass balances
• Simulation Results
• General hydrate distributions

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Phase Equilibrium
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Methane Solubility Profile

• Vertical depth normalized with the depth of the
  BHSZ
• Methane concentration normalized with triple
  point solubility

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Outline

• Phase equilibrium
• Component mass balances
• Simulation Results
• General hydrate distributions

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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

Reaction term
Convective flux
Organic carbon in sediments
Pe1 Peclet no.
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Organic concentration profile
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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

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Methane Balance (contd.)
ß Normalized organic content at seafloor
(quantifies net carbon input from top) Pe2
Peclet no. for external flow Ratio of
(External Flux/Diffusion)
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Outline

• Phase equilibrium
• Component mass balances
• Simulation Results
• General hydrate distributions

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Hydrate accumulation with underlying free
gas
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Hydrate accumulation without free gas
below
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Outline

• Phase equilibrium
• Component mass balances
• Simulation Results
• General hydrate distributions

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Parameter space for biogenic sources
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Parameter space for biogenic sources with
Da

• For each pair
• of curves
• Hydrate formation with free gas below
• Hydrate formation without free gas
• 3. No hydrate formation

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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)

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Scaled parameter space (biogenic source)
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Hydrate saturation distribution (biogenic)

• Compute average hydrate saturation ltShgt and plot
  contour plots
• Average hydrate saturation also scales with the
  scaling shown before

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Hydrate saturation averaged over GHSZ
(biogenic)
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Parameter space for deeper sources
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Scaled parameter space for deeper sources
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Hydrate saturation distribution (deeper
source)

• Again compute average hydrate saturation ltShgt as
  before
• Average hydrate saturation does not scale with
  the scaling shown before for this case (Pe1
  Pe2)
• The quantity that remains invariant in this case
  is the flux of hydrate, defined as Pe1ltShgt
• Scales with the original choice of dimensionless
  groups and is plotted along contour lines

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Hydrate saturations from deeper sources
Contours of Pe1ltShgt
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Sensitivity to seafloor parameters
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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

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Financial Support Shell Center for
Sustainability Kobayashi Graduate Fellowship