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 AGU Fall Meeting December 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

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 flux Reaction term Damkohler no. = Pe 1 Peclet no. =

Organic concentration profile

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) Pe 2 : Peclet no. for external flow = = Ratio of (External Flux/Diffusion)

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 pair of 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 generated within HSZ (from analytical solution to organic balance)

Scaled parameter space (biogenic source)

Compute average hydrate saturation and plot contour plots Average hydrate saturation also scales with the scaling shown before Hydrate saturation distribution (biogenic)

Hydrate saturation averaged over GHSZ (biogenic)

Parameter space for deeper sources

Scaled parameter space for deeper sources

Again compute average hydrate saturation as before Average hydrate saturation does not scale with the scaling shown before for this case (Pe 1 + Pe 2 ) The quantity that remains invariant in this case is the flux of hydrate, defined as Pe 1 Scales with the original choice of dimensionless groups and is plotted along contour lines Hydrate saturation distribution (deeper source)

Hydrate saturations from deeper sources Contours of Pe 1

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