Ocean Floor Methane Gas Hydrate Exploration

R.B. Coffin,1 R. Lamontagne,1 S. Rose-Pehrsson,1 K.S. Grabowski,2 D.L. Knies,2 S.B. Qadri,2 J. P. Yesinowski,1 J.W. Pohlman,3 M. Yousuf,4 and J.A. Linton5
1Chemistry Division
2Materials Science and Technology Division
3Geocenters, Inc.
4George Washington University
5Argonne National Laboratory

Introduction: Over the last decade, large deposits of methane hydrates have been identified along the world continental margins. Frozen mixtures of hydrocarbon gas (mostly methane) and water occur over large areas of the ocean floor and vastly exceed other carbon-energy reservoirs. With a maximum content of 164 m3 of methane and 0.8 m3 of water at standard temperature and pressure per cubic meter of hydrate and an estimated range of 26 to 139 X 1015 m3 globally, this is a significant new energy source. The content of methane in hydrates is variable and is controlled by geothermal gradients and biological methane production. International research has begun, with a primary goal of obtaining the methane in these hydrates as an energy source.

This requires a broad range of scientific efforts to address the methane hydrate presence, develop mining strategies, and predict the impact on the environment and platform stability. The Naval Research Laboratory (NRL) has developed strong research topics regarding methane hydrates over the last 30 years. NRL has unique field and laboratory expertise that couples physical, chemical, and biological parameters to address methane hydrate distribution, formation, and stability. Recent, current, and planned field work is active on the Texas-Louisiana Shelf in the Gulf of Mexico, Nankai Trough off the eastern coast of Japan, Blake Ridge in the northwestern Atlantic Ocean, the Cascadia Margin in the northeastern Pacific Ocean, and the Haakon-Mosby Mud Volcano (MV) in the Norwegian-Greenland Sea (Fig. 7).

Fig7 Image
World methane hydrate distribution in the ocean, Arctic region. NRL regions of interest are highlighted around the U.S., Canada, Norway, and Japan.

Research Approach: Key program efforts at NRL includes integration of: geoacoustical surveys to predict hydrate locations; methane sensors to trace hydrate rich regions in the ocean floor; field and laboratory analysis of hydrate structure and content; and stable and radio carbon isotope analysis to assist in the interpretation of methane sources and understanding of the hydrate content and stability. The following sections describe Code 6000's efforts in this project.

Testing and Development of a Methane Sensor: Methane sensing is applied to identify potential hydrate-rich regions in the sediments and to study the flow of methane from these regions into the water column. A methane sensor, METS, from ASD Sensortechnik Gmgh (Germany) became commercially available at the start of the Methane Hydrate Advanced Research Initiative (ARI). The METS sensor specifications list an operational depth range from 0 to 2000 m, temperature range of 0 to 40 C, and a methane concentration range of 50 Ámol/l to 10 Ámol/l. The methane sensor is a semiconductor (metal oxide) that works on the principle of hydrocarbon adsorption. The data collected during the summer 2000 cruise to the Gulf of Mexico was obtained from one sensor, D21. The METS sensor was placed on the forward platform of the submersible in view of the operator. This allowed the operator and observer to properly annotate the sampling events since multiple experiments were ongoing on each dive. Figure 8 shows data collected while working with hydrate mounds or with pieces of hydrate. Methane concentrations rise from a background level of ~0.1 Ámols/l to a high of ~8.8 Ámols/l. The first peak (~2.1 Ámols/l) at 5,788 seconds was obtained when working around a loose piece of hydrate. While working a hydrate mound at 6,634 seconds, a concentration of ~2.9 Ámols/l was obtained. The highest concentration (~9 Ámols/l) occurred while cores were being taken in a hydrate mound. The peak located at 10,523 seconds occurred while working in a mussel bed located to the left of the hydrate mound.

Fig8 Image
This plot shows that high methane concentrations can be observed with the methane sensor in the vicinity of hydrate outcroppings. Work was conducted on submarine dives in the Gulf of Mexico.

Analysis of Hydrate Methane Sources, Structure, and Content: The formation, stability and required dissociation energy in hydrates varies as a function of physical, biological, and chemical factors. The chemical and biological parameters are a major effort in methane hydrate research conducted by Code 6000 scientists. For analysis of the hydrate gas sources, scientists at NRL are applying carbon isotope analysis to differentiate between thermogenic and biogenic gas sources. We find a broad range of gases in the hydrates (Table 1), and the methane source in the hydrates varies at different sites between biogenic and thermogenic origin (Fig. 9). In Fig. 9 gases with more negative values (below -50 0/00) are more influenced by biogenic cycles; above this value, more thermogenic methane is present. The carbon isotope data for the higher molecular weight gases do not show large variation. These have isotope signatures indicating a thermogenic origin.

Fig9 Image

Stable carbon isotope data for carbon gases in hydrates taken on submarine dives on the Texas- Louisiana Shelf in the Gulf of Mexico (GOM) and the Haakon-Mosby Mud Volcano (HMMV) in the Norwegian Greenland Sea. Carbon isotope data include compounds ranging from methane to butane and carbon dioxide.

We couple the carbon isotope analysis with a survey of the hydrate structure. Currently two approaches for structural analysis are being applied: proton nuclear magnetic resonance (NMR) and x-ray diffraction (in conjunction with Argonne National Laboratory). High-pressure and low-temperature xray diffraction at the Advanced Photon Source at Argonne National Laboratory has examined the possible presence of structure H hydrate in natural hydrate material. Figure 10 shows peaks that may originate from structure H (red labels), but additional peaks appear to arise from structure I or II hydrate, or water ice Ih (blue, cyan, and green labels, respectively). Further analysis is needed to verify the presence of structure H hydrate. Such a result would confirm the incorporation of higher mass hydrocarbons into the hydrate structure (Table 1). More detailed structure analysis and thermal expansion and bulk modulus measurements are underway that will document this information for the first time for natural hydrate material.

Fig10 Chart FIGURE 10
X-ray diffraction of natural hydrate collected from the Gulf of Mexico. Measurement performed at 500 psi and 150 K using 33 keV (0.3757 Ä) photons from the Advanced Photon Source. Diffraction peaks labeled from hydrate structure H (red), structure I (blue), and structure II (cyan), and water ice Ih (green).

This research contributes to understanding methane hydrate formation, content, and stability. NRL collaborations with scientists at industry, government, and university facilities throughout the United States, Canada, Norway, Japan, Korea, and Russia address topics in future energy, ocean floor fuel cell development, coastal stability, ocean carbon cycling, and global warming.

Acknowledgments: The NRL research program is an Advance Research Initiative, "Alteration of Sediment Properties Through Dissociation of Gas Hydrates." Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science.

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