Natural gas hydrates are solids that form from a combination of water and one or more hydrocarbon or non-hydrocarbon gases. In physical appearance, gas hydrates resemble packed snow or ice. In a gas hydrate, the gas molecules are "caged" within a crystal structure composed of water molecules. Sometimes gas hydrates are called "gas clathrates". Clathrates are substances in which molecules of one compound are completely "caged" within the crystal structure of another. Therefore, gas hydrates are one type of clathrate.
Gas Hydrate Importance to the Energy Industry and Society:
- Are a potential energy resource
- May play a role in past and future climate changes
- Potential to cause production (flow assurance) problems
Physics and Chemistry of Hydrates
Gas hydrates are stable only under specific pressure-temperature conditions. Under the appropriate pressure, they can exist at temperatures significantly above the freezing point of water. The maximum temperature at which gas hydrate can exist depends on pressure and gas composition. For example, methane plus water at 600 psia forms hydrate at 41º F, while at the same pressure, methane + 1% propane forms a gas hydrate at 49º F. Hydrate stability can also be influence by other factors, such as salinity (Edmonds et al., 1996).
Per unit volume, gas hydrates contain a tremendous amount of gas. For example, 1 m3 of hydrate disassociates at atmospheric temperature and pressure to form 164 m3 of natural gas + 0.8 m3 of water (Kvenvolden, 1993).
The natural gas component of gas hydrates is typically dominated by methane, but other natural gas components (e.g., ethane, propane, CO2) can also be incorporated into a hydrate. The origin of the methane in a hydrate can be either thermogenic or biogenic gas. Microbial gas formed during early digenesis of organic matter can become part of a gas hydrate in continental shelf sediment. Similarly, thermogenic gas leaking to the surface from a deep thermogenic gas accumulation can form a gas hydrate in the same continental shelf sediment.
Geological Settings Where Gas Hydrates Occur
Gas hydrates can be detected seismically (e.g., Hornbach et al., 2003), as well as using well logs (Goldberg and Saito, 1998). Gas hydrates occur in two discrete geological situations:
- Marine shelf sediments (distributed worldwide, see, for example, Kvenvolden 1993; Kvenvolden and Lorenson, 2000).
- Onshore polar regions beneath permafrost
Hydrates occur in these settings because the pressure-temperature conditions are within the hydrate stability field (see, for example, Lerche and Bagirov, 1998).
Gas Hydrate Importance to the Energy Industry and Society
Gas hydrates are of interest primarily for 3 reasons:
(1) Gas hydrates are a potential energy resource: Considering the planet as a whole, the quantity of natural gas in sedimentary gas hydrates greatly exceeds the conventional natural gas resources (e.g., Kvenvolden, 1993). As a result, numerous studies have discussed the energy resource potential of gas hydrates (see, for example, Collett, 1993, 1997, 2002; Iseux, 1992; Kvenvolden, 1993; Milkov and Sassen, 2003).
However, utilization of gas hydrates as an energy resource has been largely inhibited by the lack of economical methods for production for most hydrate accumulations, especially marine shelf hydrates. A variety of mechanisms have been proposed for economically developing gas hydrates as an unconventional gas source (e.g., see discussions in Goel et al., 2001, Sawyer et al., 2000).
Thus far, the only method that has been successful used to economically produce gas from gas hydrates is the "depressurization method". This method is applicable only to hydrates that exist in polar regions beneath permafrost. This method is applicable when a free gas phase exists beneath the hydrate accumulation. Under such circumstances, production of the free gas leg using conventional gas development techniques produces a pressure drop. This pressure drop causes the overlying hydrate to become unstable and to progressively disassociate into free gas + water, a process that adds gas to the underlying free gas accumulation.
(2) Potential role of gas hydrates in past and future climate changes: Gas hydrates are also of interest because of their potential role in climate change. Gas hydrates in continental shelf sediments can become unstable either as a result of warming bottom water, or as a result of a pressure drop due to a reduction in sea level (such as during an ice age). If these marine gas hydrates begin to rapidly disassociate into gas + water, then the methane trapped in the gas hydrates can be released to the atmosphere.
Methane is a greenhouse gas. In fact, methane is many times more effective as a greenhouse gas than is CO2. Therefore, if the flux of methane to the atmosphere from dissociating hydrates is sufficient in quantity, this methane can cause global warming. This process is believed to have influenced past climate changes (see, for example, Henriet, 1998; Haq, 1998; Hesselbo et al., 2000; Kvenvolden, 1991), and may enhance the current global warming episode by way of a "positive feedback" loop. Specifically, as the earth warms, increasing bottom water temperatures could cause gas hydrate disassociation in many marine shelf locations. This gas hydrate disassociation would cause further warming due to the greenhouse effects of the gas which is released.
(3) Production (flow assurance) problems: Gas hydrates can spontaneously form in petroleum production equipment and pipelines associated with deep-water petroleum production and arctic on-shore petroleum production. These unwanted hydrates can clog equipment, preventing optimum production of hydrocarbons.
Various methods are used to prevent hydrate formation in petroleum production and transportation equipment (see, for example, Paez et al., 2001; Reyma and Stewart, 2001; Yousif and Dunayevsky, 1997; Behar et al., 1994).
Research on Gas Hydrates
A variety of gas hydrate research programs are currently underway in different parts of the world, and a listing of all of them is beyond the scope of this article. However, a few key research programs are particularly worthy of note:
The US Department of Energy Gulf of Mexico Joint Industry Project (JIP) is an aggressive multimillion dollar gas hydrate research program. JIP participants include the US Department of Energy and a group of petroleum industry companies, including ConocoPhillips, Halliburton, Japan National Oil company, MMS, Reliance Industries, Schlumberger and TotalFinaElf. In 2004, the JIP program cored multiple gas hydrate accumulations in the GOM. This multiyear program is summarized by Shirley (2004).
Another program of note was undertaken by a Japanese government-sponsored gas hydrate research organization: the Research Consortium for Methane Hydrate Resources in Japan (also known as the MH21 Research Consortium). That program drilled and cored numerous wells in the Nankai Trough offshore of eastern Japan.
However, MH21 Research Consortium gas hydrate research extends beyond coastal Asia. In 2002, production testing of gas hydrates in the Mackenzie Delta (Canada) was conducted by an international consortium that included the Japan National Oil Company and the Geological Survey of Canada. Detailed results of that project were presented at a conference in Chiba, Japan in December 2003.
Additional Information on Gas Hydrates
At the bottom of this article, we have listed over 50 published articles that provide information on a variety of different aspects of gas hydrates. Three excellent gas hydrate review articles are also available on line at the US Energy Information Administration web site:
- “The Future Supply Potential of Natural Gas Hydrates”
- “Natural Gas Hydrates Update 1998-2000” by David F. Morehouse
- “Natural Gas Hydrates Update 2000-2002” by David F. Morehouse
For more information on the techniques described here, or to discuss a specific project, e-mail us at firstname.lastname@example.org, or
call us at U.S. (214) 584-9169.
Bakker, J., 1998, Improvements in clathrate modeling II: the H20-CO2-CH4-N2-C2H6 fluid system, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates - Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 75-105.
Behar, E., A. Delion, J. Herri, A. Sugier, and M. Thomas, 1994, Hydrates Problem Within the Framework of Multiphase Production and Transport of Crude Oils and Natural Gases - part1 - Physical Chemistry of Hydrates Formation and Dissociation,: Rev. IFP, v. 49, p. 265.
Booth, J. S., W. J. Winters, W. P. Dillon, M. P. Clennell, and M. M. Rowe, 1998, Major occurrences and reservoir concepts of marine clathrate hydrates: implications of field evidence, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates - Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 113-128.
Collett, T. S., 1993, Natural gas hydrates of the Prudhoe Bay and Kuparuk River area, North Slope, Alaska: AAPG Bulletin, v. 77, p. 793-812.
Collett, T. S., 1993, Natural gas production from Arctic gas hydrates, in D. G. Howell, ed., The Future of Energy Gases - U.S. Geological Survey Professional Paper 1570: Washington, United States Government Printing Office, p. 299-311.
Collett, T. S., 1997, Gas Hydrate Resources of Northern Alaska: Bull. Canadian Petrol. Geol., v. 45, p. 317-338.
Collett, T. S., 2002, Energy resource potential of natural gas hydrates: AAPG Bulletin, v. 86, p. 1971-1992.
Collett, T. S., K. J. Bird, K. A. Kvenvolden, and L. B. Magoon, 1989, Gas hydrates of Arctic Alaska: AAPG Bulletin, v. 73, p. 345-346.
Collett, T. S., K. J. Bird, K. A. Kvenvolden, and L. B. Magoon, 1989, The origin of natural gas hydrates on the North Slope of Alaska, in J. H. Dover, and J. P. Galloway, eds., Geologic Studies in Alaska by the U.S. Geological Survey, 1988: U.S. Geological Survey Bull. 1903, p. 3-9.
Edmonds, B., R. Moorwood, and R. Szczepanski, 1996, A Practical Model for the Effect of Salinity on Gas Hydrate Formation, SPE Paper 35569: SPE.
Ginsburg, G., V.Borisov, A.Novozhilov, and A.Milkov, 1996, Helium concentrations in natural gases indicative of hydrate formation and dissociation (with the reference to the Messoyakha field), Proceedings of 2nd International Conference on Natural Gas Hydrates, Toulouse, France, p. 563-568.
Ginsburg, G. D., 1998, Gas hydrate accumulation in deep-water marine sediments, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates - Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 51-62.
Goel, N., M. Wiggins, and S. Shah, 2001, Analytical modeling of gas recovery from in situ hydrates dissociation: Journal of Petroleum Science and Engineering, v. 29 (2), p. 115-127.
Goldberg, D., and S. Saito, 1998, Detection of gas hydrates using downhole logs, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates - Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 129-132.
Grauls, D., 2001, Gas hydrates: importance and applications in petroleum exploration: Marine and Petroleum Geology, v. 18, p. 519-523.
Haq, B. U., 1998, Natural gas hydrates: searching for the long-term climatic and slope-stability records, in J. P. Henriet, and J. Mienert, eds., Gas Hydrates: Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 303-318.
Henriet, J.-P., 1998, Gas Hydrates: Relevance to World Margin Stability and Climate Change, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates: Relevance to World Margin Stability and Climate Change: London, The Geological Society, Special Publications.
Hesselbo, S. P., D. R. Gröcke, H. C. Jenkyns, C. J. Bjerrum, P. Farrimond, H. S. Morgans Bell, and O. R. Green, 2000, Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event: Nature, v. 406, p. 392-395.
Hornbach, M. J., W. S. Holbrook, A. R. Gorman, K. L. Hackwith, D. Lizarralde, and I. Pecher, 2003, Direct seismic detection of methane hydrate on the Blake Ridge: Geophysics, v. 68, p. 92-100.
Iseux, J. C., 1992, Gas hydrates: a new source of natural gas, in R. Vially, ed., Bacterial Gas: Paris, Editions Technip, p. 205-222.
Johnson, A., and T. Collett, 2003, Gas hydrate research advances: AAPG Explorer, v. 24 (November 2003), p. 34.
Kvenvolden, K. A., 1991, A review of Arctic gas hydrates as a source of methane in global change, in G. Weller, C. L. Wilson, and B. A. B. Severin, eds., International Conference on the Role of the Polar Regions in Global Change: Proceedings of a conference held June 11-15, 1990 at the University of Alaska Fairbanks, Geophysical Institute and Center for Global Change and Arctic System Research, University of Alaska Fairbanks, p. 696-701.
Kvenvolden, K. A., 1993, Gas hydrates as a potential energy resource - a review of their methane content, in D. G. Howell, ed., The Future of Energy Gases - U.S. Geological Survey Professional Paper 1570: Washington, United States Government Printing Office, p. 555-561.
Kvenvolden, K. A., 1993, A primer on gas hydrates, in D. G. Howell, ed., The Future of Energy Gases - U.S. Geological Survey Professional Paper 1570: Washington, United States Government Printing Office, p. 279-291.
Kvenvolden, K. A., 1994, Natural gas hydrate occurrence and issues, in E. D. Sloan, J. Happel, and M. A. Hnatow, eds., International Conference on Natural Gas Hydrates, Annals of the New York Academy of Sciences, p. 232-246.
Kvenvolden, K. A., 1995, A Review of the Geochemistry of Methane in Natural Gas Hydrate: Organic Geochemistry, v. 23, p. 997-1008.
Kvenvolden, K. A., 1998, A primer on the geological occurrence of gas hydrates, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates - Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 9-30.
Kvenvolden, K. A., 1999, Biogenic methane and gas hydrate, in C. P. Marshall, and R. W. Fairbridge, eds., Encyclopedia of Geochemistry: Dordrecht, The Netherlands, Kluwer Academic Publishers, p. 30.
Kvenvolden, K. A., 1999, Potential effects of gas hydrate on human welfare: Proceedings National Academy of Sciences USA, v. 96, p. 3420-3426.
Kvenvolden, K. A., G. D. Ginsburg, and V. A. Soloviev, 1993, Worldwide distribution of subaquatic gas hydrates: Geo-Marine Letters, v. 13, p. 32-40.
Kvenvolden, K. A., and A. Grantz, 1990, Gas hydrates of the Arctic Ocean region, in A. Grantz, L. Johnson, and J. F. Sweeney, eds., The Arctic Ocean region: Boulder, Colorado, Geological Society of America, The Geology of North America, p. 539-549.
Kvenvolden, K. A., and T. D. Lorenson, 2000, The global occurrence of natural gas hydrate, in C. K. Paull, and W. P. Dillon, eds., Natural Gas Hydrates: Occurrence, Distribution, and Dynamics, AGU Monograph, p. 55.
Kvenvolden, K. A., and E. Suess, 1991, Gas hydrates and Ocean Drilling: JOIDES J., v. 17, p. 29-31.
Lee, M. W., D. R. Hutchinson, W. P. Dillon, J. J. Miller, W. F. Agena, and B. A. Swift, 1993, Method of estimating the amount of in situ gas hydrates in deep marine sediments: Marine and Petroleum Geology, v. 10, p. 493-506.
Lerche, I., and E. Bagirov, 1998, Guide to gas hydrate stability in various geological settings: Marine and Petroleum Geology, v. 15, p. 427-438.
Lorenson, T. D., and K. A. Kvenvolden, 2001, A Worldwide Assessment of Coincidental Gas Hydrate and Petroleum Gas Occurrences, 2001 AAPG Annual Convention Program with Abstracts, v. 10: Tulsa, AAPG, p. A120.
Makogon, Y. F., and S. A. Holditch, 2001, Gas hydrates - 1: Lab work clarifies gas hydrate formation, dissociation: Oil & Gas Journal, Feb. 5 issue, v. 99, p. 47-52.
Milkov, A. V., 2000, Gas hydrate stability in the Gulf of Mexico: Significance to resource estimation, geohazards, and global change: AAPG Bulletin, v. 84, p. 1869.
Milkov, A. V., 2002, Global distribution and significance of natural gas hydrate [Abstract], Second Tsunami Symposium, Program and Abstracts May 28-30 Honolulu, Hawaii, p. 23.
Milkov, A. V., and R. Sassen, 2001, Role of gas hydrate in global change appears overestimated: results of modeling in the northwestern Gulf of Mexico [Abstract], Eos Trans. AGU, 82 (47), Fall Meet. Suppl., p. PP21B-0470.
Milkov, A. V., and R. Sassen, 2002, Economic geology of offshore gas hydrate accumulations and provinces: Marine and Petroleum Geology, v. 19, p. 1-11.
Milkov, A. V., and R. Sassen, 2002, Economic geology of offshore gas hydrates: where to explore for reserves? [Abstract], AAPG Annual Convention, Official Program, v. 11, p. A122.
Milkov, A. V., and R. Sassen, 2003, Assessing the economic potential of individual gas hydrate accumulations in the Gulf of Mexico continental slope, AAPG Annual Meeting, Salt Lake City, Utah, May 11-13 (Abstracts), v. 12: Tulsa, AAPG, p. A120.
Milkov, A. V., and R. Sassen, 2003, Preliminary assessment of resources and economic potential of individual gas hydrate accumulations in the Gulf of Mexico continental slope: Marine and Petroleum Geology, v. 20, p. 111-128.
Paez, J. E., R. Blok, H. Vaziri, and M. R. Islam, 2001, Problems in Gas Hydrates: Practical Guidelines for Field Remediation, SPE Paper # 69424: Society of Petroleum Engineers.
Rempel, A. W., and B. A. Buffett, 1998, Mathematical models of gas hydrate formation, in J.-P. Henriet, and J. Mienert, eds., Gas Hydrates - Relevance to World Margin Stability and Climate Change: London, The Geological Society, p. 63-74.
Reyma, E., and S. Stewart, 2001, Case History of the Removal of a Hydrate Plug Formed During Deep Water Well Testing, SPE Paper 67746: SPE.
Sassen, R., J.S.Watkins, C.L.Decker, D.A.Defreitas, and A.V.Milkov, 1999, Structural setting and geochemistry of gas hydrates, Gulf of Mexico continental slope, AAPG Annual Convention, Official Program, v. 8, p. A122.
Sassen, R., S.T.Sweet, D.A.DeFreitas, A.V.Milkov, G.Salata, and E.W.McDade, 1999, Geology and geochemistry of gas hydrates, central Gulf of Mexico continental slope: AAPG Bulletin, v. 83, p. 1363.
Sassen, R., S. T. Sweet, D. A. Defreitas, A. V. Milkov, G. Salata, and E. W. McDade, 1999, Geology and Geochemistry of Gas Hydrates, Central Gulf of Mexico Continental Slope: Gulf Coast Assoc. Geol. Societies Transactions, v. XLIX, p. 462-468.
Sassen, R., S. T. Sweet, A. V. Milkov, D. A. DeFreitas, and M. C. K. II, 2001, Thermogenic vent gas and gas hydrate in the Gulf of Mexico slope: Is gas hydrate decomposition significant?: Geology, v. 29, p. 107-110.
Sassen, R., S. T. Sweet, A. V. Milkov, D. A. DeFreitas, M. C. K. II, and H. H. Roberts, 2001, Stability of thermogenic gas hydrate in the Gulf of Mexico: Constraints on models of climate change, in C. K. Paull, and W. P. Dillon, eds., Natural gas hydrates: Occurrence, distribution, and dynamics, v. 24, AGU, p. 131-143.
Sawyer, W., C. Boyer, J. Franz, and A. Yost, 2000, Comparative Assessment of Natural Gas Hydrate Production Models, SPE Paper 62513: SPE.
Shirley, K., 2003, GOM gas hydrate opportunities explored - Love 'em or hate 'em- They're there: AAPG Explorer (January 2004), v. 25, p. 22-23.
Sloan, E. D., 1990, Natural gas hydrate phase equilibria and kinetics; understanding the state-of-the-art: Revue de l'Institut Francais du Petrole, v. 45, p. 245-266.
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