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NETL’S METHANE HYDRATE RESEARCH

Charles E. Taylor, Jonathan Lekse, and Niall English
U.S. Department of Energy, National Energy Technology Laboratory
P.O. Box 10940, Pittsburgh, PA 15236-0940, USA
(p) 412-386-6058, (f) 412-386-5920, <[email protected]>

Methane hydrates are clathrates (crystalline solids whose building blocks consist of a gas molecule surrounded by a cage of water molecules similar to water ice, except that the crystalline structure is stabilized by the guest gas molecule within the cage of water molecules) where methane is the guest molecule and water is the host molecule. Methane hydrates are stable and occur naturally the ocean depths and permafrost areas. At standard temperature and pressure (STP) one volume of saturated methane hydrates contains approximately 180 volumes of methane. Current estimates suggest that there is at least twice as much organic carbon contained in methane hydrates than all other forms of fossil fuels combined. The methane hydrate deposits along the coast and in permafrost areas of the United States are estimated to contain 320,000 Tcf of methane. In order to tap into this vast resource, research is needed to understand the fundamental physical properties of hydrates.

The NETL Methane Hydrate Research Group conducts research in four key areas: Modeling, Computational, Thermal Physical Properties, and Kinetic Properties. The modeling effort focuses on being the Beta-tester of the Tough2 model. Computational research models the formation and dissociation of hydrates. Thermal Physical Properties research focuses on the measurement of thermodynamic properties of hydrates (both synthetic and naturally occurring). Kinetic Properties research measures the kinetic properties of methane hydrates (both synthetic and naturally occurring) including the physical properties of hydrates synthesized in one of the many view cells at NETL that range in volume from 1 mL to 15 L.

Our most recent research has focused on the formation and dissociation kinetics of methane hydrates formed in water and in a sand/water matrix. In our 1 L cell, we have compared the formation of methane hydrate in water and two different pourosity sands with the formation of methane hydrate without sand. A 225 ppm solution of sodium dodecylsulfate (SDS) in distilled water was used in each of the experiments. Two different grain sizes of sand were used in these experiments, sea sand with an average grain size of 350 µm and U.S. Silica’s F-110 with an average grain size of 110 µm to synthesize the hydrates (Figure 1). The molar uptake of methane on a mL of water present basis is shown in Figure 2. Of interest is that all three samples absorbed roughly the same amount of methane on a molar water basis. Also of interest is that the two different sand/hydrate samples exhibited the same initiation of hydrate formation time. The Raman spectrum of the methane hydrate formed in the F-110 sand is shown in Figure 3. This spectrum correlates well with other Raman spectrum obtained in our laboratory of methane hydrates.

Molecular dynamics (MD) studies on the dissociation of methane hydrate are currently under investigation at NETL. The objective of this MD study is to investigate the microscopic details of methane hydrate (type I) break-up at 276.65 K and 69 bar, in an effort to complement the experimental work in macroscopic dissociation of methane hydrates at this temperature and pressure which has been carried out at in our laboratory on the dissociation of methane hydrates that we have synthesized. The kinetic mechanisms of clathrate hydrate crystallisation and dissociation, on both a macroscopic and a molecular level, are understood rather poorly. Any investigations thereof tend to be at least an order of magnitude inferior in accuracy vis-à-vis experimental measurements or molecular simulation of equilibrium thermophysical properties.

It was found that the smaller nanocrystals and those of lower methane occupation tended to dissociate more rapidly than larger, fully-occupied crystallites. The crystal-liquid systems prepared by melting around the constrained crystallite core were found to exhibit an initial period of reorientational relaxation of the molecules in the interfacial liquid layer, leading to a delay in the onset of cluster dissociation. Such a delay in break-up was not as evident for the implanted systems, although the subsequent dissociation rates were similar for each mode of system preparation, for a given nanocrystal size and system composition.

Figure 1. Photographs of methane hydrate made in the 1 L cell. A: pure methane hydrate; B: methane hydrate in sea sand; C: methane hydrate in F-110 sand.

Figure 2. Methane uptake by the hydrates as a function of time.

Figure 3. Raman spectrum of mehtane hydrate in F-110 sand.