Overpressure development and effect of heterogeneities on gas hydrate distribution

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Overpressure development and effect of
heterogeneities on gas hydrate distribution
By Gaurav Bhatnagar Consortium for Processes
in Porous Media March 26, 2008
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Gas Hydrates

• Gas hydrates are crystalline compounds formed by
  the inclusion of molecules (e.g. methane, ethane)
  in water cages
• Stable at high pressures and low temperatures,
  typical of deep water marine sediments and
  permafrost environments
• Natural gas hydrates often found with an
  underlying free gas layer

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Outline

• Previous work
• Effect of overpressure
• 2-D model with heterogeneities
• Conclusions

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Previous Work

• Developed a general 1-D model for simulating gas
  hydrate accumulation in marine sediments due to
  biogenic/deeper methane sources
• Used appropriate scaling schemes to generate
  simple contour plots for average hydrate
  saturation
• Developed new model and theory to predict gas
  hydrate saturation using depth of sulfate-methane
  transition as a proxy

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Model Schematic
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Key Dimensionless Groups
Pe1 Peclet no.
Damkohler no.
ß Normalized organic content at seafloor
(quantifies net carbon input from top) Pe2
Peclet no. for external flow Ratio of
(External Flux/Diffusion) Cm,ext Methane
concentration in external flux
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Average Hydrate Saturation (Biogenic
Sites)
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Average Hydrate Saturation (Deeper Source)
Contours of Pe1ltShgt
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Outline

• Previous work
• Effect of overpressure
• 2-D model with heterogeneities
• Conclusions

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Motivation

• Overpressure is defined as pore water pressure in
  excess of the hydrostatic pressure
• Overpressure can develop as a result of
  compaction at sites characterized by low
  permeability and/or fast sedimentation rates
• Development of overpressure not only changes
  fluid flow, but also affects hydrate/free gas
  saturation

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Sedimentation-Compaction Group

• Define a new dimensionless group that denotes the
  ratio of sediment permeability to sedimentation
  rate
• Higher values of Nsc imply high permeability
  and/or low sedimentation rate, resulting in
  hydrostatic pore pressure. Conversely, relatively
  low values of Nsc cause development of
  overpressure

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Effect of Nsc on Pore Pressure
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Effect on Hydrate/Free Gas Saturation
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Effect on Hydrate/Free Gas Sediment Volume
Fraction
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Outline

• Previous work
• Effect of overpressure
• 2-D model with heterogeneities
• Conclusions

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Objectives

• Model free gas migration and study effect of
  overpressure on gas column thickness
• Extend previous work to 2-D modeling with
  heterogeneities
• Study effect of vertical fractures, dipping sand
  layers and combinations of both on gas hydrate
  saturation

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Free Gas Migration

• Previous results assumed free gas phase to be
  immobile within the sediment matrix. However,
  free gas becomes mobile when its saturation
  exceeds the critical saturation
• Free gas can then rise buoyantly upwards and
  enter the GHSZ, where it is converted to gas
  hydrate
• Gas hydrate precipitation at the base of the GHSZ
  increases capillary entry pressure for gas
  causing a free gas column to form beneath the
  hydrate layer

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Results

• Results are presented for two cases
• Case 1 High permeability with Nsc 1000 (i.e.,
  zero overpressure due to compaction)
• Case 2 Low permeability with Nsc 10 (i.e.,
  significant overpressure due to compaction)
• The state at which gas pressure becomes equal to
  the lithostatic stress is termed critical-state
  e.g., Flemings et al., 2003, Hornbach et al.,
  2004

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Case 1 Pressure Profiles (Nsc1000)
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Case 1 Pressure Profiles (Nsc1000)
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Case 1 Pressure Profiles (Nsc1000)
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Case 2 Pressure Profiles (Nsc10)
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2-D Heterogeneous Gas Hydrate Systems
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Gas Hydrate Systems with Fractures
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Gas Hydrate Systems with Fractures
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Gas Hydrate Systems with Fractures
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Gas Hydrate Systems with Fractures
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Gas Hydrate Systems with Sand Layers
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Effect of Dipping Sand Layers ( 0.25)
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Effect of Dipping Sand Layers ( 0.75)
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Fracture Dipping Sand Layer ( 0.25)
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Fracture Dipping Sand Layer ( 0.75)
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Outline

• Previous work
• Effect of overpressure
• 2-D model with heterogeneities
• Conclusions

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Conclusions

• Overpressure Development
• Overpressure can develop in gas hydrate systems
  due to low permeability and/or high sedimentation
  rates
• Can be studied using a single dimensionless group
• Reduces steady-state gas hydrate and free gas
  saturations
• Has small effect on thickness of the GHSZ
• Reduces maximum thickness of free gas column
  beneath the GHSZ before fracturing occurs

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Conclusions

• 2-D Model
• Heterogeneous systems, such as fractures and/or
  dipping sand layers, can be studied using the 2-D
  model
• Fluid focusing through these high permeability
  conduits results in relatively high hydrate
  saturation
• These effects are dynamic in nature i.e., gas
  hydrate saturation becomes laterally uniform as
  the high permeability feature moves out of the
  GHSZ

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• Financial Support
• Shell Center for Sustainability
• Kobayashi Graduate Fellowship
• Hin Wei Wong Fellowship
• Department of Energy (DE-FC26- 06NT42960)