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. Pressure 2. Temperature 3. 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

Barkley Canyon, Cascadia Margin Hydrate stability: seafloor observations Photos Ross Chapman

MacDonald et al. (2005) Bush Hill, Gulf of Mexico

Based on CH 4 concentrations measured at these sites: Diffusion controlled dissolution = 30 cm/yr 10 cm 17 cm cm Observed rate = 3.5 cm/yr 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

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 inletGas and water inlet Original drawing in Google SketchUp by LLL Experimental Setup (How)

 ~300mL SDS solution introduced to chamber  “source” gas (methane) introduced to Pressurize ( psi)  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/ N 2 to replace CH 4 at pressure Gas inletGas and water inlet Original drawing in Google SketchUp by LLL * Hydrate forming * *

Could methane be dissolving into the oil layer?

StudyDissolution rate Rehder 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 N2N2 O2O2 Ar

Bubbly Gulch, gas-active, buried hydrates

*Note Scale Differences Biogenic methane

 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