Marine and Petroleum Geology, Marine Gas Hydrates, Marine Geology, environmental protection, methane gas, methane gas hydrates, resources development, energy density, gas hydrate, Earth and Planetary Science Letters, gas hydrates, global warming, International Materials Science Society, Gornitz V. Potential, Lopez M. Architecture, International Conference, College of Mining Engineering, greenhouse gas, Alexei V. Milkov, natural gas, Future Energy, temperature, Northern Hemisphere, Global estimates, marine sediments, energy resources, National Key Basic Research Program, American Association of Petroleum Geologists, methane hydrates, sustainable development, Koh C. Clathrate Hydrates, methane oxidation, methane hydrate, National Natural Science Foundation of China, Lerche I. Estimates, energy resource, development, Stephen J. Regional, global carbon cycle, future energy source, Ananthaswamy A. Crocodiles
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Energy Procedia www.elsevier.com/locate/procedia
2012 International Conference
on Future Energy, Environment, and Materials Marine Gas Hydrates: Future Energy or Environmental Killer? Zhen-guo Zhang a,b ,Yu Wang a,b, Lian-feng Gao a,b, Ying Zhang a,b, Chang-shui Liu a,b, * a College of Mining Engineering, Hebei United University, Tangshan 063009, China. b Key Laboratory Mineral Development and Security Technology of Hebei Province, Tangshan 063009 China
Abstract Marine gas hydrates are regard as a new energy in the 21st century with the characteristics of high energy density, huge amount of resources and cleaning. It has important significances for resources development, environmental protection and global climate changing. This large marine gas hydrate reservoir is further suggested as an important component of the global carbon cycle and as a future energy source. But its decomposition and release can lead to decrease the stability of seabed, causing submarine landslide. And its overflowed can intensify the greenhouse effect, interaction between which can lead to more serious ecological and environmental disasters. Due to the limitations of the occurrence mode and the technical level
of marine gas hydrates, at present, the development and utilization of the resources are still tentative. How to use these future energy is a challenge, it must be considered the only way to protect biodiversity and vulnerable marine ecosystems effectively. O©©p2e20n01a11c1cPeuPssbuluibsnlhdieesdrhCebydC EbBlyYse-EvNilCesr-eNBvDi.Velr.icSLeentldseec..tSioenleacntdi/oonr paenedr-/roevripeweeurn-rdeevrireewspounnsdibeilritryeospf Ionntesrinbaitliiotynaol fM[antaermiaelsoSrcgieannciezeSro]ciety. Keywords: Marine gas hydrates; future energy; resource characteristics; environmental effects 1. Introduction Gas hydrates are also known as natural gas hydrates, methane gas hydrates, etc., which are a solid icelike substance formed by hydrocarbon gas molecules methanebased and water molecules. Gas hydrates are mainly distributed in the slope of the seabed sediments with the depth of 400~1000m, temperature below 10 and the pressure is greater than 3.5 Mpa, and the Arctic tundra in Russia, Canada and other countries. Ocean is the main place for gas hydrates to formation and preservation. Gas hydrates, with the characteristics of high energy density, huge amount of resources and cleaning, are considered as an ideal * Corresponding author. Tel.: 8615032921800; fax: 86-0315-2592148 E-mail address: [email protected]
1876-6102 © 2011 Published by Elsevier B.V. Selection and/or peer-review under responsibility of International Materials Science Society. Open access under CC BY-NC-ND license. doi:10.1016/j.egypro.2012.01.149
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alternative energy in the 21st century. It has important significances for resources development, environmental protection and global climate changing.
2. Marine Gas Hydrates: Resource Characteristics
2.1. Resource Characteristics of Marine Gas Hydrates
Methane is the dominant hydrocarbon in gas hydrates, but other higher hydrocarbon gases (ethane, propane through isopentane) may also be significant [2, 3]. The global volume of hydrate-bound gas is uncertain . It appears that the global estimates of hydrate-bound gas decreased by at least one order of magnitude from 1970s~early 1980s (abourt 1017~1018 m3) to late 1980s~early 1990s (1016 m3) to late 1990s~present (1014~1015 m3). The decrease of estimates is a result of growing knowledge of the distribution and concentration of gas hydrates in marine sediments and ongoing efforts to better constrain the volume of hydrate-bearing sediments and their gas yield. Gornitz et al. suggests that the volume of methane is 1.8 Ч1016 ~ 3.4Ч1017 m3 in the global submarine gas hydrates reservoir . However, estimates as high as 7600Ч1015 m3  and as low as 0.2Ч1015 m3  are also given in the literature. It is generally believed that the estimated volume is 1013 and 20Ч1015 m3 (standard conditions) of methane gas in onshore and deep offshore areas , which is more credible. This number could meet mankind energy needs for almost 1,000 years. Only methane in gas hydrates in the Black Ridge in the southeastern United States can meet energy consumption
in the United States (now the consumption level) for 105 years. Current estimates of the carbon content in global fossil-fuel minerals (oil, natural gas and coal) is 5Ч1012t, yet the carbon content in methane is twice of fossil-fuel minerals, up to 10Ч1012t . Second, the gas hydrates are a kind of clean energy, which contain high purity methane and less harmful gases. environmental pollution
is in low degree compared with coal, oil, natural gas. Furthermore, gas hydrates have high energy value. 1m3 of gas hydrates is equivalent to 164m3 of methane in the normal conditions . Energy density (methane in sediments per unit volume under standard conditions) is 10 times of the other non-conventional energy sources
and is 2 to 5 times of conventional natural gas.
2.2. Distributing of Marine Gas Hydrates
For the above reasons, gas hydrates are believed as a new alternative energy of contemporary fossil fuels. It has enormous potential to develop and utilize. And it has been paid great attention by governments, scientific research
units, enterprises and other related departments in the developed countries
, such as the United States, Germany, Japan, France and Russia. Geological, geophysical, and geochemical evidence or samples of gas hydrates collected from DSDP (Deep Sea Drilling Project), ODP (Ocean Drilling Program) and IODP (Integrated Ocean Drilling Program). They are reported more than 40 sites which find localities in the oceanic area. However, intact gas hydrates samples have been recovered at only 30 of these localities, and mainly geophysical evidences are BSR (Bottom Simulating Reflectors) and BZ (Blank Zone). Because unambiguous evidences of gas hydrates from cores and Drill Hole
s are often lacking, the number of sites where gas hydrates are suggested to occur may be overestimated. Global DSDPODP-IODP geochemical data identify many additional deep-water marine sites with large sulfate gradients that lack BSRs, perhaps suggesting the occurrence of previously unrecognized gas hydrate localities. But, about 90% of global oceans have not found any effective evidence. The obvious evidences show the gas hydrates distributing in regular where distribute in the accretion prism of initiative continental margin, the active area of isobathic ocean current in passive continental margin
, the mud volcanoes and a few gas-oil seepage area
(Shown in Table 1) .The factors of marine geology, including sedimentary velocity, content of the TOC, granularity of sediment, hydro-dynamic condition and ocean productivity, are illustrated (Shown in Figure 1). Table 1 Distributing of the marine gas hydrates
Location Hydrate Ridge Cascadia Margin Mexico Margin Costa Rica
Margin Guatemala Margin Peru Margin Nankai Trough Okushiri Ridge Blake Ridge Gulf of Mexico
Amazon Fan Norwegian Sea Svalbard Margin Low Congo Basin Milano-Napoli Mud Volcano
Leg/Site ODP204/1244,1249 ODP146/888,892;ODP204/1248,1249;IODP311/1326-1329 DSDP66/490,492 DSDP84/565;ODP170/1041;ODP205/1253 DSDP67/497,498;DSDP84/568,570;ODP206/1256 ODP112/ 685,688;ODP201/1228 ODP131/808;ODP190/1173 ODP127/796 DSDP76/ 533;ODP164/994~ 997;ODP172/1061 DSDP618 ODP 155/935 ODP 104/642 ODP162/986 ODP175/1077 ODP160/970,971
Sediment Type Clay, Silty clay, clayey silt. Silt,Sand-silt-clay Clay, Silty clay, Volcanic ash Nannofossil clay, Volcanic ash, Sand-sized glauconite Pebbles of basalt, Silt, Nannofossil clay, Silt,Volcanic ash Pelagic clay;Silty clay Pelagic clay,Volcanic ash SiltSand-silt-clay SiltSand, Pelagic clay Silt, Sand-silt-clay SiltSand-silt-clay Pelagic clay, Silt, Sand Pelagic clay, Silt, Sand Clay, Silt, Sand-silt-clay Clay, Silt
3. Marine Gas Hydrates: Environmental Effects
Marine gas hydrates are an important carbon reservoir on lithosphere surface, and a key link in global carbon cycle. But gas hydrates are unstable in nature. Their formation need special conditions of pressure and temperature which is controlled by seawater depth, seafloor temperature and geothermal gradient. The gas hydrate stability zone (GHSZ) are only relativity concept, and they determined by finding the intersection of the local P-T conditions with experimentally determined conditions for three-phase equilibrium among water, hydrate and free gas. Any change in temperature and pressure will cause it to decompose or produce. The decomposition and the release of submarine gas hydrates can lead to decrease the stability of seabed strata, causing submarine landslide. And methane gas overflowed can intensify the greenhouse effect, interaction between which can lead to more serious ecological and environmental disasters.
3.1. Release of Marine Gas Hydrates and Global Warming
Although the volume concentration of methane in the atmosphere is only 1/200 of the concentration of CO2, methane is an important greenhouse gas, whose global warming potential index (GWP) is 3.7 times of CO2 by mole number and 20 times of CO2 by weight. Absorption or release of methane gas hydrates on global climate has a significant impact. Especially the rapid release of methane gas in gas hydrates is likely the culprit to lead to short-scale global climate change
. The existing research shows in Paleocene- Eocene global temperature became warm suddenly. It is believed to have occurred suddenly about 55Ma or so (55.6 Ma) and called "Latest Paleocene Thermal Maximum" (LPTM) [12~15]. The temperature in Northern Hemisphere increased 6-12during the period of 1Ma. Temperature changed so quickly, which
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closely related to the changes in global sea level caused by the decomposition and the release of the large number of gas hydrates [16~18].
3.2. Release of Marine Gas Hydrates and Seabed Geological Disasters
Different from the accumulation model of coal, oil, gas and other energy minerals, marine gas hydrates lack of consolidation cap and prone to phase transformation. Regardless of temperature-pressure balance on any changes, the stability zone will be reduced due to the pressure on the surface or temperature increasing, and induced hydrate dissociation. The sensitivity of oceanic gas hydrates and submarine slope stability to the combined forcing of sea level changes and bottom water perturbation is a critical issue for risk assessment
in the seabed geological disasters.Methane gas released will break the shackles of gas hydrate stability zone (GHSZ), upward from its weaknesses. And the submarine sediments will move downward under the action of gravity, leading to submarine landslide (Shown in Fig.2). Sea-level changes in the geological history caused undersea landslides continuously. Usually the sediments in the lower continental slope
formed a thick layer of several superimposed landslide, such as Cae Fear on continental rise offshore southeastern North America
, Beaufort Sea continental slope in the north of Alaska, Amazon deep sea fan, Norwegian continental margin [19~22]. Seismic reflection profiles show that landslide area caused by the decomposition of gas hydrates in the slope of Beaufort Sea in Alaska almost covers the area of occurrence of gas hydrates .
Fig 1. Distribution of marine gas hydrates (after initiall report of DSDP,ODP and IODP)
Fig 2. Seafloor slide caused by marine gas hydrate discompose and release
3.3. The Intrinsic Link of Release of Marine Gas Hydrate and Ecological Disaster
The release of methane in marine gas hydrates can not only cause the greenhouse effect rapidly, but also induce seabed geological disasters. These changes will ultimately affect the biological species on the Earth. The release of gas hydrates caused global warming, which had produced an important role on the evolution of land mammals. Great thermal event appeared in about 55Ma (Paleocene-Eocene). The fundamental reason was that the decomposition of a large number of gas hydrates led to a sudden global warming. Northern Hemisphere temperatures increased 6-12 in the short term. This change made crocodiles appear in the Arctic Circle . Now the direct relationship
between submarine extinction and gas hydrates is concerned very much by scientists. Based on the records of core high-resolution carbon
isotope from sites 892 in Ocean Drilling Program (ODP Leg146) and sites 995 (ODP Leg164) [25~27], Dickens believes large quantities release of methane in gas hydrates in the short period was the direct reason that 1/2 to 2/3 of benthic animals became extinct in the turn of the Paleocene and Eocene (about 55Ma) .
Resources and environment are the conditions for human to survive, and they are the basic of Sustainable Development
for society and economy. The development and utilization of new energy sources must be based on the premise of safety development and environment-friendly. Marine gas hydrates show one important role in the global carbon cycle, but they are regarded as energy resources firstly. Gas hydrate may be considered a future energy source not because the global volume of hydratebound gas is large, but because some individual gas hydrate accumulations may contain significant and concentrated resources that may be profitably recovered in the future. The decomposition and the release of gas hydrates in large-scale in geological history caused the greenhouse effect and extinction of marine life. These events show, in the process of the development and use of marine gas hydrates, the safety of production technology must be solved at first. Due to the limitations of the occurrence mode and the technical level of marine gas hydrates, at present, the development and utilization of the resources are still tentative. How to use these future energy must consider the only way to protect biodiversity and vulnerable marine ecosystems effectively. And it has great significance for the sustainable development of human.
This research was supported by National Natural Science
Foundation of China (40972079 and 41030853); National Key Basic Research Program (2007CB411703).
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Z Zhang, Y Wang, L Gao, Y Zhang, C Liu