The overall goals of this research are (1) to determine the physical fate of single and multiple methane bubbles emitted to water columns by dissociating gas hydrates at seep sites deep within the hydrate stability zone or at the updip limit of gas hydrate stability, and (2) to quantitatively link theoretical and laboratory findings on methane transport to an analysis of real-world field-scale methane plume data placed within context of degrading methane hydrate province on the U.S. Atlantic margin.  

The project is designed to advance on three interrelated fronts (numerical modeling, laboratory experiments, and analysis of field-based plume data) simultaneously. The fundamental objectives of each of these components are the following:

• Numerical modeling: Constraining the conditions under which rising bubbles become armored with hydrate, the impact of hydrate armoring on the eventual fate of a bubble’s methane, and the role of multiple bubble interactions in survival of methane plumes to very shallow depths in the water column.
• Laboratory experiments: Exploring the parameter space (e.g., bubble size, gas saturation in the liquid phase, and “proximity” to the stability boundary) for formation of a hydrate shell around a free bubble in water, the rise rate of such bubbles, and the bubble’s acoustic characteristics using field-scale frequencies.
• Field component: Extending the results of numerical modeling and laboratory experiments to the field-scale using new and existing public domain, state-of-the-art data on U.S. Atlantic margin methane seeps without acquiring new field data in the course of this particular project. This component has been developed to quantitatively analyze data on Atlantic margin methane plumes and place them and their corresponding seeps within the context of gas hydrate degradation processes on this margin.

Massachusetts Institute of Technology, Cambridge, MA 02139

Collaborators
University of New Hampshire, Durham, NH 03824
United States Geological Survey, Woods Hole, MA 02543

Numerous studies have considered the perturbations in pressure, temperature, pore water salinity, and other conditions that may cause breakdown of natural gas hydrates in marine sediments (e.g., Kennett et al., 2003; Mienert et al., 2005; Reagan and Moridis, 2008; Biastoch et al., 2011; Ruppel, 2011; Phrampus and Hornbach, 2012; Ferre et al., 2012). In contrast, only a few studies—most notably the work of McGinnis and co-workers [McGinnis et al., 2006; Greinert and McGinnis, 2009; Greinert et al., 2010)—have focused on the fate of non-catastrophically-released individual methane bubbles once they enter the water column that act as the critical buffer between the sediment and the atmosphere. In recent years, it has become widely accepted that the water column may itself be an important chemical sink for methane [Reeburgh, 2007; Mau et al., 2007; Kessler et al., 2011; Ruppel and Kessler, 2017]. Less widely understood are the physics of bubble rise; the controls on and timescales of the formation of bubble-encasing hydrate shells; the role that hydrate formation around rising bubbles may play in mitigating their dissolution and allowing methane to reach shallow parts (e.g., upper mixed layer) of the water column; and how modern shipboard imagery of methane plumes can be used to estimate bubble size, height of final bubble rise, and methane flux from the seafloor on both a local and regional basis.

This research project will in part constrain the conditions under which this relatively direct injection path from the seafloor to the atmosphere might be expected to operate. Currently, this direct injection path appears to be limited to shallow water settings [McGinnis et al., 2006] and catastrophic methane release events [Leifer et al., 2006]. The numerical modeling and the laboratory experiments to be undertaken in this research program will permit the project team to assess whether other conditions (including pre-existing methane supersaturation of the water column or formation of a hydrate shell around a rising bubble) also permit relatively direct injection of methane into the upper mixed layer of the ocean, where methane can more easily access the atmosphere.

The new microscale numerical model will be the first to adopt the phase field modeling approach to the consideration of systems other than simple liquid-solid. This will be an important contribution to hydrates research since the framework will provide a more natural way to numerically model the three-phase (hydrate/liquid/free gas) system as two phases (hydrate-liquid/gas).

The laboratory experiments will, for the first time, constrain the pressure conditions for the onset of hydrate shell formation and precisely measure the rise rate of hydrate encrusted bubbles for more realistic bubble sizes. The research will also be the first to examine the role of dissolved gas saturation in the water column in promoting hydrate shell formation around bubbles and the first to use high-frequency acoustics similar to those used by research ships to calibrate the acoustic characteristics of individual bubbles.

The “field" component of this proposal relies primarily on the quantitative analysis of plume characteristics in existing, recently-acquired, public-domain multibeam and scientific echosounder data collected by NOAA Ocean Exploration and made available through various online portals. In addition, the USGS is providing new quantitative echosounder data from other cruises on the Atlantic margin. The field component also involves the synthesis of disparate geophysical, physical oceanography, and geological data sets to develop a margin-wide interpretation for seep sourcing, gas hydrate dynamics, and past and future states of the gas hydrate system on the Atlantic margin. This will be the first study to integrate laboratory acoustics calibration on a single hydrate-encased bubble with the analysis of Atlantic margin gas plume multibeam data collected at a similar frequency.

A 2D Hele-Shaw model was developed to examine the migration of methane in a water-filled Hele-Shaw cell. The model, grounded on the theory of nonequilibrium thermodynamics, is able to explain fundamental observations of hydrate growth on liquid-gas interfaces, including the pressure-composition phase stability diagram, and the direction and rate of hydrate growth across the interface.

The flow-loop bubble capture chamber was designed and constructed. The system is now functional and capable of operating at pressures high enough to form xenon hydrate. A protocol has been developed that allows the capture of single gas bubbles. Imagery tracking the formation and evolution of hydrate shells on bubbles suggests bubbles need not be in water supersaturated with hydrate-forming gas in order to form hydrate shells. Hydrate shell formation appears to require the surrounding water to only contain enough dissolved hydrate former that the rate of gas lost to dissolution from the bubble drops below the rate at which gas from the bubble is consumed to form a hydrate shell. Over time, the bubble shrinks as gas from the bubble is consumed to form a hydrate shell which itself is being dissolved in order to push the concentration of gas dissolved in the surrounding water toward the solubility limit.

Raw split-beam echosounder and multibeam echosounder acoustic data collected by the NOAA Ship Okeanos Explorer over several hundred seeps of free gas on the U.S. Atlantic margin has been reviewed for quality control and processed to extract metrics that are relevant to the evolution and fate of the gas bubbles as they rise. An algorithmic routine was established to more accurately generate these profiles.

A global map of the depth below the sea surface of the top of gas hydrate stability was developed. In addition, decades’ worth of conductivity-temperature-depth (CTD) data from near the seafloor were used to determine the empirical updip limit (based on actual observed pressure and temperature) of gas hydrate stability on U.S. margins. A compilation of published and newly-discovered seep locations was also developed for the continental U.S. during a period of performance that included the recognition by NOAA of hundreds of previously-unknown seeps on the Northwest Pacific U.S. margin and findings of dozens more seeps by the USGS on the U.S. Atlantic margin.

Phase 1 – $256,072
Phase 2 – $381,441
Phase 3 – $293,327
Planned Total Funding: DOE Contribution: $830,840

Phase 1 – $74,968
Phase 2 – $72,799
Phase 3 – $76,192
Planned Total Funding: Performer Contribution: $223,959

NETL – Joe Renk (Joseph.Renk@netl.doe.gov or 412-386-6406)
Massachusetts Institute of Technology – Ruben Juanes (juanes@mit.edu or 613-253-7191) 
University of New Hampshire – Thomas Weber (weber@ccom.unh.edu or 603-862-1659) 
United States Geological Survey-Woods Hole – Carolyn Ruppel (cruppel@usgs.gov or 508-457-2339) and William Waite (wwaite@usgs.gov or 508-457-2346)