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 University2. Dept. of Earth Science, Rice University

AGU Fall MeetingDecember 13, 2006

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

Model schematic

Seafloor Seafloor

Sediment flux Us

Subsidence

Sedimentation

Subsidence

Sedimentation

Subsidence

Lt

BHSZ BHSZ

Hydrate layer extending downwards

Hydrate dissociation due to burial below BHSZ

Free gas might recycleback into HSZ

Fluid flux Uf,sed

External fluxUf,ext

TOC α0

Methane solubility curve

Outline

Phase equilibrium

Component mass balances

Simulation Results

General hydrate distributions

Phase Equilibrium

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

Outline

Phase equilibrium

Component mass balances

Simulation Results

General hydrate distributions

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

Organic carbon in sediments

Convective fluxReaction term

Damkohler no. = Pe1 Peclet no. =m

tsedf

DLU ,

11(1 ) (1 )sPe U Da

t z

Organic concentration profile

SedimentationReaction

,1 f sed

t

UPeDa L

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

Methane Balance (contd.)

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

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

textf

DLU ,

4

1 1

1 1 1(1 )

1 (1 ) (1 )(1 ) (1 )

1 (1 )

l h gh g m h m h g m g

l h gf m s h m h s g m g

lCH sm

h gorg f

S S c S c S ct

U c PeU S c PeU S cz

McS S Daz z M

(1 ) ,

Outline

Phase equilibrium

Component mass balances

Simulation Results

General hydrate distributions

Hydrate accumulation with underlying free gas

Hydrate accumulation without free gas below

Outline

Phase equilibrium

Component mass balances

Simulation Results

General hydrate distributions

Parameter space for biogenic sources

Parameter space for biogenic sources with Da

For each pairof curves:

1. Hydrate formation with free gas below

2. Hydrate formation without free gas

3. No hydrate formation

Scaling of variables

• Scale x-axis to represent net methane generated within the HSZ instead of just the input

Methane generatedwithin HSZ (from analytical solution to organic balance)

11z

Scaled parameter space (biogenic source)

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

• Average hydrate saturation also scales with the scaling shown before

Hydrate saturation distribution (biogenic)

tL

ht

h zdzSL

S0

~)~(1

Hydrate saturation averaged over GHSZ (biogenic)

Parameter space for deeper sources

Scaled parameter space for deeper sources

• 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 saturation distribution (deeper source)

tL

ht

h zdzSL

S0

~)~(1

Hydrate saturations from deeper sourcesContours of Pe1<Sh>

Sensitivity to seafloor parameters

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

Financial Support:

Shell Center for Sustainability

&

Kobayashi Graduate Fellowship