Charting the Future of Methane Hydrate Research in the United States (2004) / Chapter Skim
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2 Why Study Gas Hydrate?
2 Why Study Gas Hydrate?
Pages 23-42
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From page 23... ...
. Although the total amount of methane trapped in gas hydrate and the geological processes that lead to concentrated gas hydrate deposits are poorly understood, existing knowledge suggests that gas hydrate represents a potential fossil fuel resource for the future (e.g., Kvenvolden, 1993a; Kvenvolden and Lorenson, 2001; Milkov and Sassen, 2002)
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. Natural gas hydrate recovered from the seafloor or from the subsurface decomposes rapidly when it is sampled, leading to the dramatic phenomenon of burning ice (Figure 2.2)
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. These estimates range over several orders of magnitude and are generally based on an estimate of the volume of continental margins and Arctic permafrost basins that fall within the gas hydrate stability zone (GHSZ)
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figure reprinted with permission from Dr. Stephen Masutani, University of Hawaii, Hawaii Natural Energy Institute, Ocean Resources Applications Laboratory.
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A shortfall in natural gas supply from conventional and unconventional sources is expected to occur in about 2020 (Energy Information Administration, 2002)
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To do this, a better understanding of the geologic factors that lead to highly concentrated hydrate deposits and their and their geophysical dynamics is needed. Gas hydrate equilibrates with gas, and a better knowledge of the gas-gas hydrate system is required to understand resources on the seafloor as well as the potential effects of hydrate on global climate and on seafloor stability.
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From page 29... ...
has proposed that gas hydrate may act as a sort of capacitor in the global carbon and climate cycle, storing large amounts of methane until nature triggers a change in the system that results in destabilization, releasing it to the ocean and atmosphere. Several investtigators have postulated both negative and positive feedback effects from gas hydrate destabilization in response to global warming and/or sea level change (Paull et al., 1991; Kennett et al., 2003)
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From page 30... ...
. To quantify the role of gas hydrate in global warming, research is needed in the following areas: · quantification of the distribution of gas hydrate in marine sedi ments and polar land areas; · determination of the geologic time scales over which gas hydrate forms; · determination of methane flux, including oxidation effects, into the ocean from vents associated with focused methane hydrate deposits; and · knowledge of the historical latitudinal distributions and geometric configurations of land masses and ocean basins.
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31 rvey. systems.
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Any of these processes can result in hydrate dissociation and a dramatic change in the geotechnical properties of the sediment, leading to borehole instability, release of gas, and potential structural and safety concerns. Although there is anecdotal evidence of structural collapse due to gas hydrate dissociation, industry generally avoids areas where hydrate deposits are suspected to be, thereby limiting accessibility to hydrocarbon resources.
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As the geothermal gradient increases, the thickness of the stability zone decreases. Subsurface hydrology can locally perturb the geothermal gradient by transporting warm fluids from greater depth into the gas hydrate stability field.
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. Understanding this complexity in nature requires interdisciplinary studies that include laboratory experiments to monitor gas hydrate formation under controlled conditions, numerical modeling experiments to extend laboratory results, and field observations.
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· How much methane is venting through the water column and in sediment porewaters? · How much methane is coming from an underlying gas hydrate deposit versus an underlying gas reservoir versus in situ sediment (biogenic)
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From page 36... ...
The vigorous expulsion of gas bubbles in the water column above seafloor hydrate mounds, even when the mound is well within the nominal gas hydrate stability field, testifies to the dynamic nature of these seafloor deposits and the large amounts of gas often associated with them. The time scales over which these mounds form, evolve, and are destroyed (either by dissociation or by mechanical removal from the seafloor due to a spontaneous buoyancy instability and/or earthquakes)
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From page 37... ...
indicates that it results from a decrease in velocity with depth. It is thought that this decrease in velocity with depth occurs because gas hydrate-bearing sediments with relatively high seismic velocity overlie free gas containing sediments with lower velocity.
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Comprehensive studies of boreholes in gas hydratebearing regions include Ocean Drilling Program (ODP) Leg 164 to the Blake Ridge gas hydrate province of the Atlantic continental margin, ODP Legs 146 and 204 to the gas hydrate province of the Cascadia accretionary complex off the U.S.
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From page 39... ...
The targeted projects funded by the DOE Methane Hydrate R&D Program and discussed in Chapter 3 all follow this model to some degree. Below are some of the insights and milestones determined from laboratory studies, since Humphrey Davy first discovered hydrate in 1811, almost two centuries ago:
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From page 40... ...
Accurate hydrate thermodynamic models, tested against this field and laboratory data, have provided an extremely cost-effective alternative to field experiments for the assessment of gas hydrate production techniques (Ballard and Sloan, 2002)
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FEASIBILITY OF PRODUCING METHANE FROM GAS HYDRATE Although the total amount of gas hydrate in the marine environment is estimated to be much greater than the amount of hydrate in the permafrost, permafrost hydrate is more accessible. Therefore, the first major effort to evaluate the feasibility of producing methane from gas hydrate was conducted by an international consortium in the northern Canadian permafrost.
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