GAS HYDRATE STABILITY: DISSOLUTION VS.

DISSOCIATION

Rachel Marie Wilson ,Laura L. Lapham, Jeff Chanton,

Dissociation: occurs when the hydrate is exposed to P/T regimes not in the stability zone

Dissolution: the hydrate is stable at the P/T regime, but surrounding gas concentration is under-saturated

4 Primary Factors Controlling Hydrate Stability

1) Pressure

2) Temperature

3) Salinity

4) Guest concentration in the surrounding environment

1. Pressure2. Temperature3. Guest Concentration

Lapham , et al. (2010) Earth and Planetary Science Letters

Synthetic Hydrate taken down into the water column within the hydrate stability zone:

Hester et al. (2009) 110 cm/yr Rehder et al. (2004) 167 cm/yr

Hydrate Dissolution Rates

Pressure and Temperature OK

Methane concentration below stability

2004 2006

Barkley Canyon, Cascadia Margin

Hydrate stability: seafloor observations

Photos Ross Chapman

MacDonald et al. (2005)

Bush Hill, Gulf of Mexico

Based on CH4 concentrations measured at these sites:

Diffusion controlled dissolution = 30 cm/yr

10 cm 17 cm

2004

24 cm Observed rate = 3.5 cm/yr

20032002

Lapham et al. (2010) Earth and Planetary Science Letters

Lapham, et al. (2010) Earth and Planetary Science Letters

Exposed hydrate mounds present on the seafloor in under-saturated conditions should be rapidly dissolving

Evidence does not support dissolution at the rate we would expect

Observed rates of exposed hydrate dissolution appear to be an order of magnitude lower than we would expect

Something is acting to slow the hydrate dissolution

Hydrate may be re-supplied from below

Recap

Bigalke et al. (2009) have demonstrated that hydrate dissolution rates are diffusion controlled (i.e. kd = D/z)

We want to ask: What are the influences on z (boundary layer) that could be enhancing hydrate stability in the natural environment?

Hydrate Composition Oil/biofilms Sediment

Figure from Bigalke, N Rehder, G and Gust, G (2009) Marine Chemistry 115: 226–234

Could the presence of other guest molecules (ethane, propane) be acting to

slow the dissolution rate?

Gas inlet Gas and water inlet

Original drawing in Google SketchUp by LLL

ExperimentalSetup (How)

Procedure ~300mL SDS solution

introduced to chamber

“source” gas (methane) introduced to Pressurize (700-800psi)

Stir slowly to stimulate hydrate formation

Hydrate evolution monitored by P/T

Once P/T stabilizes, hydrate formation is considered complete

Headspace is flushed w/ N2 to replace CH4 at pressure

Gas inlet Gas and water inlet

Original drawing in Google SketchUp by LLL

*Hydrate forming

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Date

Tem

p (d

egC

)

In deeper water

Just below window

Fridge*

*

Could methane be dissolving into the oil

layer?

StudyDissolution

rateRehder et al. (2009) synthetic hydrate in water column

167 cm/yr

Hester et al. (2004) synthetic hydrate in water column

110 cm/yr

Bigalke et al. (2009) synthetic hydrate stirred lab study

~100 cm/yr

In situ natural hydrate in water column at Barkley Canyon site

3.5 cm/yr

Synthetic methane hydrate lab experiment, no stirring

30 cm/yr

Synthetic mixed-gas hydrate lab experiment, no stirring

27 cm/yr

Synthetic methane hydrate lab experiment, no stirring, with oil

~100 cm/yr

Summary and Future Work

Results indicate that mixed gas hydrates have similar dissolution rates to pure methane hydrate formations

The addition of mineral oil significantly increased dissolution rates, contrary to expectations. Oil was methane-free, in nature oil would be saturated with methane Incorporating oil into hydrate structure? Incorporating methane into oil? More complex oil mixtures?

Hydrate dissolution rates may be slowed by biofilm armoring or coatings

Salinity is potentially an important factor to consider

• Biofilms

• sediment studies

“filling-type” hydrate

Water-wet sand layer

Hydrate

gas headspace

Filter on port tip

Proposed Work

Dissolution of hydrate lens in sands

Dissolution of “filling-type” hydrate in sands

We expect hydrate dissolution to be diffusion

controlled thus the two experiments should yield similar rates. However if surface interaction effects

do influence hydrate dissolution, the dispersed

hydrate will be more affected (greater interaction area)

Saturated water layer

methane

N2

O2

Ar

Bubbly Gulch, gas-active, buried hydrates

*Note Scale

Differences

Biogenic methane

Acknowledgments Collaborators

Brian Anderson, West Virginia University Nagasree Garapati, WVU

Funding Agencies NETL Hydrate Research via the Department of

Energy Gulf of Mexico Hydrate Research Consortium Mississippi Mineral Resources Institute