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Natural Gas Hydrate

Natural gas hydrate is white or light gray ice with cage-like structure composed of water molecules and small molecules. Snow-like crystalline compounds, because the gas molecules are mainly methane (CH4) (>90%) it looks like ice and can burn when exposed to fire, it is also called "flammable ice". With high resource density, wide global distribution, and extremely high resource value, it has become a long-term research hotspot in the oil and gas industry. Since the 1960s, some countries have formulated research plans for natural gas hydrate exploration and development. So far, people have discovered more than 230 hydrate deposits in offshore waters and permafrost regions, and a large number of gas hydrate hot research areas have emerged.Because the ocean methane hydrates comprise a large pool of potentially releasable carbon, they have the potential to have a strong, long-term impact on Earth's climate.

different types

So far, three different structural types of hydrates have been discovered: type I, type II and type H. The main difference between them lies in the structure of single crystals and the ratio of gas to water molecules. We usually say that hydrate is Type I can contain methane molecules. The density of hydrates in nature is generally between 0.8-1.0g/cm3. In addition to thermal expansion In addition to expansibility and thermal conductivity, its spectral properties, mechanical properties and transfer properties are similar to ice, but hydrates can Efficient storage of gas, 1 unit volume of hydrate decomposition can release 164-172 unit volume under normal pressure Methane, which determines its huge prospects as a new energy source.

Where it is

The vast areas of the oceans and polar regions meet the conditions for gas hydrate formation. Approximately 27% of the land is a potential area where natural gas hydrate can be formed, and there are approximately 90% of the area is also such a potential area However, little is known about the distribution of natural gas hydrates in deep ocean basins, which account for most of the ocean. The reason for this is that the area currently engaged in gas hydrate investigation has not yet set foot in the ocean basin.

nature

After the combustion of natural gas hydrate, there is almost no residue, and the pollution is much smaller than that of coal, oil and natural gas. One cubic meter of combustible ice can be converted into 164 cubic meters of natural gas and 0.8 cubic meters of water. During mining, only the solid "natural gas hydrate" needs to be heated and decompressed to release a large amount of methane gas.

1 cubic meter of combustible ice can release 164 cubic meters of natural gas and 0.8 cubic meters of fresh water under normal temperature and pressure, so solid natural gas hydrates are often distributed in seabed sediments or cold permafrost with a water depth of more than 300 meters in. Submarine gas hydrates rely on the pressure of a huge layer of water to maintain their solid state, and their distribution can range from the seabed to within 1,000 meters below the seabed, and further deep, the solid state is destroyed due to the increase in ground temperature. exist.

Composition structure

"Natural gas hydrate" is the "ice block" that natural gas crystallizes and transforms under the action of 0°C and 30 atmospheres. The methane in the "ice cube" accounts for 80% to 99.9%, which can be directly ignited. It can be represented by mCH4·nH2O. The gas represents CH4, C2H6, C3H8, C4H10 and other series and CO2, N2, H2S, etc., which can form single or multiple natural gas hydrates.

The main gas that forms natural gas hydrates is methane. Natural gas hydrates with a content of more than 99% of methane molecules are usually called methane hydrates. There are two dodecahedrons (20 end points so there are 20 water molecules) and six tetradecahedrons (24 water molecules) in a water cage structure in each unit cell. The hydration value of 20 can be obtained by MAS NMR. The spectrum of methane gas hydrate clathrates was recorded at 275 K and 3.1 MPa, showing that each cage reflects a peak, and gaseous methane also has individual peaks.

conditions to create

Low temperature: generally less than 10°C

High pressure: generally less than 200bar

Sufficient air and water sources

difficuties

They are very sensitive to pressure and temperature, and humans cannot simply mine them out and transport them to land. Combustible ice is often buried several hundred meters below the seabed at a water depth of about 500 meters. The pressure there is much higher than sea level and the temperature is close to 0 degrees Celsius. After leaving the high-pressure and low-temperature environment, methane hydrate will decompose before the methane is extracted.

Mining method

Thermally stimulated mining of hydrate

The heat-shock method is a way to directly supply the reservoir heat from the external environment outside the hydrate reservoir by injecting hot water under the condition of little change in pressure to increase the temperature of the hydrate reservoir. A mining plan that decomposes hydrates to recover gas

Decompression mining of hydrate

The depressurization method is currently one of the main natural gas hydrate extraction methods. It is the process of reducing the pressure of the gas hydrate reservoir by pumping to make it lower than the phase equilibrium pressure of the hydrate under the temperature conditions of the region, so that the hydrate is decomposed from the solid phase to the process of producing methane gas.

Inhibitor mining of hydrate

By injecting chemical inhibitors (including brine, methanol, ethanol, ethylene glycol, glycerol, etc.), the phase equilibrium conditions for hydrate formation can be changed, the hydrate stability temperature can be reduced, and the steady-state conditions of the natural gas hydrate stability zone can be changed. Lead to the decomposition of some natural gas hydrates.

Gas replacement mining of hydrate

This technology introduces another kind of guest molecule CO2 into the natural gas hydrate, reduces the partial pressure of the CH4 molecule in the hydrate phase and replaces the CH4 molecule from the hydrate, so as to achieve the purpose of mining CH4.

Existing area and reserves

Weather hydrates are most commonly found in permafrost and shallow sediments of the deep bottom. The ecology below the permafrost zone and the ocean's depths, on the other hand, are vastly different.Methane is widely recognised as the most significant hydrocarbon gas in the ocean environment, whereas ethane and other heavy hydrocarbons make up the majority of the permafrost environment. This noticeable shift in gas composition suggests that microbial activity isn't the main factor in the formation of these hydrocarbons. These vapours, on the other hand, might be escaping from the depths.Thermal breakdown of organic materials buried in deep strata produces light hydrocarbons.

Natural gas hydrates may occur on around a quarter of the world's land and a third of the world's seafloor, and their total calorific value is roughly double that of other fossil fuels. According to current estimations, the reserves identified in the South China Sea + tundra can last China 200 years and Japan 100 years along its coast.

Identification method-BSR

BSR stands for "seabed simulated reflector," which is a geophysical detection method. Gas hydrate deposits have a higher longitudinal wave velocity than other sediments on continental borders. As a result, earthquake velocity and other elastic characteristics are extremely valuable for understanding gas hydrate dispersion.Furthermore, the P-wave velocity and Poisson's ratio of free gas-containing deposits that may occur below the gas hydrate stability zone are lower. As a result, by comparing existing data with the petrophysical model technique, the resultant velocity and elastic parameter structure may be utilised to estimate the saturation of natural gas hydrate semi-quantitatively.Then, using parameters such as thickness and area, compute the gas hydrate reserves. This strategy, however, is not without flaws. First and foremost, some ground feature data is extremely ambiguous, making it impossible to fully assess BSR's existence and continuity. Second, existing sampling data suggests that locations with gas hydrates may not be detected by BSR.

environmental problems

The most worrying thing in the research and exploitation of natural gas hydrate is the environmental problems it may cause. The exploitation of natural gas hydrate reservoirs will change the temperature and pressure conditions on which natural gas hydrates occur, and cause the decomposition of natural gas hydrates. During the exploitation of natural gas hydrate reservoirs, if the temperature and pressure conditions cannot be effectively controlled, a series of environmental problems may occur. The above several exploitation methods have their own shortcomings, and these shortcomings mainly cause environmental problems. . Climate change: Methane is a powerful greenhouse gas. Its greenhouse effect is 20 times that of carbon dioxide. The total amount of methane in the global seabed combustible ice is about 3000 times the total amount of methane in the earth's atmosphere. If a large amount of methane enters the atmosphere in a short time due to careless development, it will cause terrible climate change and geological disasters. Ecological impact: If the development is not careful, it may cause natural gas to leak into the ocean, increasing oxidation and lack of oxygen in the ocean, causing a devastating blow to marine life. Submarine geological disasters: such as submarine landslides, submarine earthquakes, etc., can also cause seawater vaporization and tsunamis, and even cause seawater turbulence and airflow negative pressure entrainment.

reference list

https://www.zhihu.com/topic/20082243/top-answers zhihu

http://www.giec.cas.cn/kxcb/zthd/6th/6thshow/201707/P020170706392817122567.pdf Jinan Guan

http://china-isa.jm.china-embassy.org/chn/gjhd/hdzy/t218971.htm

https://www.bbc.com/ukchina/simp/vert-fut-46451105 Martha Henriques(2018-12-10)

https://www.nature.com/articles/nature02135 Nature volume 426, pages353–359(2003)

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018RG000638 P.B. Flemings A. Malinverno T.S. Collett K. Darnell(22 August 2019)

https://www.pnas.org/content/106/49/20596David Archer, Bruce Buffett, and Victor Brovkin(December 8, 2009)

Zou Cain. Unconventional oil and gas geology. Beijing: Geological Publishing House, 2013: 330