Quantifying methane hydrate saturation in different geologic settings
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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

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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


Objectives
Objectives settings

  • 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


Model schematic

Sediment flux U settingss

Sedimentation

Sedimentation

TOC α0

Seafloor

Seafloor

Hydrate layer

extending downwards

Hydrate dissociation due to burial below BHSZ

Fluid flux

Uf,sed

Lt

BHSZ

BHSZ

Methane

solubility

curve

Free gas might recycle

back into HSZ

External flux

Uf,ext

Subsidence

Subsidence

Subsidence

Time

Model schematic


Outline
Outline settings

  • Phase equilibrium

  • Component mass balances

  • Simulation Results

  • General hydrate distributions



Methane solubility profile
Methane Solubility Profile settings

  • Vertical depth normalized with the depth of the BHSZ

  • Methane concentration normalized with triple point solubility


Outline1
Outline settings

  • Phase equilibrium

  • Component mass balances

  • Simulation Results

  • General hydrate distributions


Component mass balances organic

Damkohler no. = settings

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. =



Component mass balances methane
Component Mass Balances - Methane settings

  • 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


Methane balance contd
Methane Balance (contd.) settings

β : Normalized organic content at seafloor (quantifies net carbon input from top)

Pe2 : Peclet no. for external flow = = Ratio of (External Flux/Diffusion)


Outline2
Outline settings

  • Phase equilibrium

  • Component mass balances

  • Simulation Results

  • General hydrate distributions




Outline3
Outline settings

  • Phase equilibrium

  • Component mass balances

  • Simulation Results

  • General hydrate distributions



Parameter space for biogenic sources with da
Parameter space for biogenic sources with settingsDa

  • For each pair

  • of curves:

  • Hydrate formation with free gas below

  • Hydrate formation without free gas

  • 3. No hydrate formation


Scaling of variables
Scaling of variables settings

  • 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)



Hydrate saturation distribution biogenic
Hydrate saturation distribution (biogenic) settings

  • Compute average hydrate saturation <Sh> and plot contour plots

  • Average hydrate saturation also scales with the scaling shown before





Hydrate saturation distribution deeper source
Hydrate saturation distribution (deeper source) settings

  • Again compute average hydrate saturation <Sh> 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 Pe1<Sh>

  • Scales with the original choice of dimensionless groups and is plotted along contour lines


Hydrate saturations from deeper sources
Hydrate saturations from deeper sources settings

Contours of Pe1<Sh>



Conclusions
Conclusions settings

  • 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


Financial Support: settings

Shell Center for Sustainability

&

Kobayashi Graduate Fellowship


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