|Fate of Methane Emitted From Dissociating Marine Hydrates: Modeling, Laboratory, and Field Constraints
||Last Reviewed 11/15/2013
The overall goals of this research are (1) to determine the physical fate of single and multiple methane bubbles emitted to the water column 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 the analysis of real-world field-scale methane plume data placed within the context of the 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, “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
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]. Less widely understood is 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 us 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 not on the acquisition of new field data, but rather 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. 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.
New project awarded on October 1, 2013
Current Status (November 2013)
Phase-field models of static gas bubbles and buoyant hydrate-coated bubbles will be developed. These models will then be extended to a large collection of rising bubbles within the hydrate stability zone, accounting for the collective buoyant behavior of the bubbles. Researchers will use this model to reproduce controlled bench-scale experiments with multiple bubbles and also conduct laboratory experiments to systematically measure bubble rise rates and bubble evolution as functions of pressure (proxy for water depth) and the saturation of the dissolved hydrate-forming gas in the water column.
Project Start: October 1, 2013
Project End: September 30, 2016
Project Cost Information:
Phase 1 – DOE Contribution: $299,874, Performer Contribution: $74,968
Phase 2 – DOE Contribution: $291,199, Performer Contribution: $72,799
Phase 3 – DOE Contribution: $304,767, Performer Contribution: $76,192
Planned Total Funding–DOE Contribution: $895,840; Performer Contribution: $223,959
NETL – Skip Pratt (firstname.lastname@example.org or 304-285-4396)
Massachusetts Institute of Technology – Ruben Juanes (email@example.com or 613-253-7191)
University of New Hampshire – Thomas Weber (firstname.lastname@example.org or 603-862-1659)
United States Geological Survey-Woods Hole – Carolyn Ruppel (email@example.com or 508-457-2339)
Quarterly Research Performance Progress Report [PDF-418KB] October - December, 2013