Natural gas hydrate has dual effects on global carbon cycle and climate change: first, methane gas in hydrate is directly or indirectly released into the atmosphere in the form of CO2 through chemistry and biochemistry; Second, low-carbon methane can replace multi-carbon fossil fuels and reduce man-made greenhouse gas emissions. Natural gas hydrate is extremely unstable in nature, and small changes in temperature and pressure conditions will cause its decomposition or formation. The natural gas hydrate hills and hills were photographed in the water depth of 500 meters off the coast of Louisiana. By comparing the videos of 1992 and 1993, the disappearance of one hill and the rebirth of another hill were identified. The air flow around this area continuously releases less than 69.6% CH4, 6.3% C2H6, 1.7% C3H8, 1.4% N2, 8% CO2 and trace butane, pentane and oxygen. In sediment, organic matter and CO2 can generate a large amount of methane under the action of bacteria, and deep-sea strata can also convert organic matter buried in geological history into natural gas, which can form natural gas hydrate under suitable temperature and pressure conditions. On the other hand, when the temperature increases or the pressure decreases, natural gas hydrate will decompose and release methane into the atmosphere. Due to the huge reserves of natural gas hydrate, its methane throughput is also very large; Therefore, natural gas hydrate is an unstable carbon pool in the shallow layer of the geosphere, an important link in the global carbon cycle, and plays an important role in carbon exchange among the geosphere, hydrosphere and hydrosphere.
Methane is a highly active greenhouse gas, and its impact on global warming is 20 times greater than that of a considerable amount of carbon dioxide. During the Pleistocene, global climate change (regression) led to the release of a large amount of methane from natural gas hydrate in land and marine environment, which in turn caused global climate change. Global warming, melting glaciers and ice sheets, causing sea level rise; The rise of sea level causes the increase of underwater hydrostatic pressure and the stability of natural gas hydrate, while the increase of water temperature has the opposite effect. For most submarine natural gas hydrates on the continental margin, the water depth is more than 300 ~ 500 m, and the fluctuation of sea level and the change of submarine water temperature have an impact on natural gas hydrates. The above changes are also due to the different latitudes of natural gas hydrate occurrence areas, and the relationship between stability and instability of natural gas hydrate is also different. The measurement of the British continental shelf area of about 600,000 km2 shows that the amount of methane escaping into the atmosphere every year is 6.5438+200,000 tons ~ 3.5 million tons, accounting for 2% ~ 4% of the total methane discharged into Britain. Therefore, this kind of emission is more prominent in the sea area where natural gas hydrate is widely distributed, and it has become an important subject that needs to be monitored and studied in advance for the development and utilization of natural gas hydrate.
The increase of seawater temperature may lead to hydrate decomposition and release methane gas. The released methane gas has increased the global carbon storage. It can bubble or diffuse into the water column, conduct horizontal convection through the flow of seawater, and carry out chemical and biochemical reactions in the water column. If the release rate of methane gas exceeds the oxidation rate, it will eventually enter the atmosphere through bubbling. Because this release will lead to cascade effect, it has great potential for climate impact. Including the expansion of the atmosphere, the rise of ocean temperature and the acceleration of existing hydrate decomposition. The recent deep-sea survey found pockmarks and other structures, indicating that a large amount of fluid was released from the seabed in the past. The decomposition and release of hydrate is one of the possible reasons. The numerical study of joint storage and ocean carbon cycle model shows that it has obvious influence on climate change on a hundred-year scale. The recent simulation of hydrate decomposition caused by ocean temperature change shows that storing shallow hydrate on a scale of ten years can release a large amount of methane gas, and only need to increase the heat of 1℃ in the sediments in the hydrate storage area. On the contrary, the simulation study of hydrate behavior in cold deep sea area does not show large-scale instability or methane gas release.
(2) engineering geological response of natural gas hydrate
Sultan et al. [2004] made a new study on the influence of natural gas hydrate decomposition on the stability of submarine slope. They studied the thermodynamic and chemical equilibrium of hydrate in soil by considering factors such as temperature, pressure, chemical properties of pore water and average pore size distribution. The model uses enthalpy formula based on the law of conservation of energy. The improved model shows that hydrate will be dispersed at the top of hydrate occurrence zone due to the increase of temperature and pressure to ensure chemical balance with surrounding water, which is consistent with the experimental results. The model is used to analyze the Storegga landslide on the Norwegian continental margin, and the influence of sea level change and seawater temperature change on hydrostatic pressure is considered in the calculation. The simulation results show that natural gas hydrate decomposes at the top of the landslide, which breaks the previous understanding that hydrate is only dispersed at the bottom of the occurrence zone.
Submarine geological disasters are an important content of natural gas hydrate resources development research. The relationship between natural gas hydrate and submarine landslide was recognized as early as 1970s. There are nearly 200 landslides in the continental margin of the Atlantic Ocean in the United States, which are considered to be caused by the decrease of sea level and isobaric pressure, and the decomposed natural gas hydrate escapes methane gas, resulting in slope instability.
At the same time, most landslides in this sea area are distributed in or near the gas hydrate distribution area, which also shows this point. The collapse of sea platforms in other sea areas is also related to natural gas hydrate, such as slopes and platforms in southwest Africa, Norwegian continental margin, beaufort continental margin, Caspian Sea, North Panama continental shelf and Newfoundland. Once the landslide begins, the free gas under the hydrate layer will rise along the fracture, and the metastable hydrate of the raw rice will also release methane gas. The research shows that most large landslides are related to the instability of natural gas hydrate, or to the "sliding" of collapsed materials on hydrate. The interaction between submarine landslide and hydrate, on the one hand, submarine landslide provides rich material conditions for the formation of natural gas hydrate, which is beneficial to the formation of hydrate; The formation of hydrate plays a role in fixing loose sediments accumulated by slump; On the other hand, the gas and water released in the process of gas hydrate decomposition increase the pore pressure, which leads to sediment sliding and new submarine landslide. Therefore, when developing and utilizing submarine gas hydrate, we should fully consider and study submarine geological disasters and design feasible technical schemes.
In marine sediments, when natural gas hydrate is formed, it can produce cementation in pores, which makes the continental slope zone in an obvious stable state. When natural gas hydrate is released due to the change of pressure and temperature conditions, it will first lead to instability in many parts of the continental slope, and huge slump will slide into the deep sea, and the ecological environment of the deep sea will suffer disastrous consequences (Figure 8.20).
Fig. 8.20 Comprehensive schematic diagram of environmental and engineering geological effects of marine natural gas hydrate.
According to the results of previous seabed exploration, scientists explained that 8,000 years ago, the sediments with a total amount of about 5600m3 on the Norwegian continental margin slipped 800km from the upper edge of the human slope to the Norwegian basin, and the tsunami caused by the huge amount of soil pushing away seawater caused devastating consequences, and terrible waves suddenly swallowed up the coastline. Scientists speculate that Storega, an extremely famous submarine landslide, is probably one of the world-famous largest sliding bodies formed by the release of natural gas hydrate.
Natural gas hydrate, as a possible cover of closed deposits, is beneficial to the accumulation of hydrocarbon compounds that migrate upward. However, if such a gas reservoir is formed near natural gas hydrate during drilling, explosive pressure release may occur, which is called "blowout". Scientists realize that the vulnerability of natural gas hydrate has an important influence on well location selection, drilling and casing running scheme. Naturally, the instability of natural gas hydrate will also pose a threat to submarine pipelines, cables and other engineering facilities and construction, and even cause terrible consequences.
Geological, geophysical and geochemical indicators of a large number of natural gas hydrates found on the northern slope of the South China Sea. Among them, the typical hydrate occurrence area with clear characteristics and multiple evidences is consistent with the distribution range of submarine landslide, which shows that hydrate has a very close relationship with submarine landslide.
The submarine landslide developed on the northern slope of the South China Sea has the characteristics of loose structure, high mud content, high water content, high void ratio, high organic carbon and high hydrocarbon gas content, which provides rich material sources and storage space for the formation of natural gas hydrate, and the formation of hydrate can consolidate the loose sediments accumulated by landslide. However, the decomposition of natural gas hydrate will cause new submarine landslides.
According to the stable temperature and pressure conditions of natural gas hydrate, it existed at least at the end of Eocene, when the ocean cold water circle (water temperature < 10℃) was formed. Prior to this, the bottom seawater temperature in the Late Cretaceous and Paleocene was estimated to be 7- 10℃, and a thin natural hydrate formation layer may also be formed in deeper water. Natural gas hydrate formed under suitable conditions is filled in the gaps of sediments, which plays a role in hindering the consolidation of sediments and mineral cementation. When the pressure decreases or the temperature increases, the stable depth of natural gas hydrate decreases, and the bottom of hydrate layer becomes unstable, releasing methane much larger than the volume of hydrate, forming an aerated layer, reducing the strength of sediments and leading to large-scale landslides. Before Oligocene, there was no large ice sheet. In the case of long-term low water surface, the instability of natural gas hydrate may become the first-order driving force for submarine landslides and shallow structural changes. At the end of Early Eocene (49.5Ma) and the middle Oligocene (30Ma), there were two sea level drops, both of which were accompanied by man-made landslides. The analysis of the seismic profile of the continental margin of New Jersey shows that there were four large landslides in the Early Tertiary, all of which corresponded to the main low water level period. During the Pleistocene Ice Age, the sea level dropped by about1000 m, and the hydrostatic pressure on the continental shelf and slope dropped by about 1000kPa, which reduced the stable depth of natural gas hydrate by about 20 m ... This may be the reason for the large-scale landslide on the continental margin of the world at that time. The possible connection between natural gas hydrate and submarine landslide has been reported all over the world. Re-studying the seismic profile and stratigraphic data of continental margin and analyzing the shallow structural phenomena within the stable depth of natural gas hydrate are likely to find more evidence of the existence of natural gas hydrate in geological history.
The negative effects of hydrate decomposition have aroused great concern of relevant experts. Ogisako and others believe that in order to analyze the deformation mechanism of submarine hydrate-bearing sediments, the deformation mechanism of submarine surface and hydrate-bearing layer should be studied separately. The decomposition of hydrate in hydrate-bearing layer can be described by tunnel model. Ogisako and others further divided the sediments in the South China Sea Trough into two types: cohesive soil and sandy soil. These two kinds of soils have axisymmetric characteristics and conform to the viscoelastic model. On the basis of the above assumptions, Ogisako et al. used finite element method to study the deformation mechanism of hydrate-bearing sediments in the South China Sea Trough of Japan during hydrate decomposition.
Masayuki, another Japanese geologist, also studied the mechanical properties of hydrate-bearing sediments. His experimental method is to design a drainage system, then put the hydrate and sand synthesized in the laboratory into the system, and then observe the deformation mechanism of hydrate-containing sediments during hydrate decomposition. The drainage system adopts two-dimensional pressure technology, which can simulate the pressure system in deep water environment. In addition, the drainage system designed by Masayuki and others can also control the speed of water injection and drainage.
The decomposition of submarine hydrate not only leads to the decrease of submarine stability and the occurrence of submarine landslide, but also leads to the occurrence of tsunami. Take the United States as an example Tsunamis along its east coast, Gulf of Mexico, west coast, Alaska and Hawaii have increased in intensity and frequency. Some studies show that the damage caused by the tsunami is much more serious than people think. In view of the existing problems, the United States carried out tsunami investigation and study to further understand the frequency, amplitude, potential hazard assessment, formation mechanism and the relationship with hydrate decomposition.
Based on the study of modern geological environment and disasters, geoscientists began to pay more attention to the environment and disasters in geological history. Paleontologists have put forward various hypotheses about several major biological extinctions in geological history. For example, the extinction of animals in the late Devonian, the widespread distribution of black shale made many geologists think that the global hypoxia event led to the extinction of this species. The latest carbon isotope data show that δ 13C on standard profiles in Iran and parts of southern China decreased by -5‰ and-1.5‰ respectively. The anomaly of carbon isotope is related to the disturbance of global carbon cycle, and correspondingly, the test results of oxygen isotope also show similar anomalies. The above phenomenon shows a global warming event. Methane released by natural gas hydrate leads to global warming, which eventually leads to extinction.
There are natural gas hydrate outcrops at the Bush Mountain hydrate leakage station in the northern Gulf of Mexico. A newly designed fluid flux measuring instrument/chemical sampler named MOSQUITO has been deployed in Bush Mountain for 430 days to determine how the underground dynamic flux affects the stability of natural gas hydrate and quantify the related methane flux into the sea. Among them, three fluid flowmeters are placed at the outcrop near the gas hydrate mound, and the fourth one is used to monitor the background conditions. The measurement results of the flux meter reveal that the literature characteristics of surface water near the mound dike are complex and changeable. In the range of-16 1 ~ 273 cm/yr, there are frequent activities of downward flow and upward flow, as well as temporary changes of the horizontal component of flux. The continuous chemical record of flux shows that natural gas hydrate is actively formed in sediments. Solomon et al. proposed that the long period (up to 4 months) of seawater flowing downward near the natural gas outlet was driven by partial pressure. The high-frequency variation of velocity (day-week) may be due to the transient changes of sediment permeability and three-dimensional fluid flux field, which is the result of active natural gas hydrate, authigenic carbonate precipitation and free gas. The formation of natural gas hydrate is attributed to the long-term methane gas precipitation from the centralized natural gas outlet, followed by the more dispersed intergranular methane flux. The methane flux from the concentrated natural gas outlet through Bushan cold spring is estimated to be 5× 106mol/ year. This remarkable flux confirms that hydrocarbon leakage in the Bush Mountains and similar northwest Gulf of Mexico may be an important natural source of methane entering the ocean or atmosphere.
The "methane eruption hypothesis" holds that natural gas hydrate and seabed leakage are the main geological factors controlling the Quaternary atmospheric and climate changes. However, methane has a wider geological source and played an important role in the past climate change. In addition to offshore seepage, related methane geological emission (GEM) also comes from land seepage, including mud volcanic activity, micro-seepage and geothermal flux. All gems are the second important natural source of methane in the atmosphere. It seems that the amount of methane that GEM enters the atmosphere on land is more than that that seeps out at sea. The transport of methane (rich in 13C) with obviously heavy isotope value from land source to atmosphere is mainly controlled by endogenous geological processes (geodynamics), which leads to the change of gas flow on the geological age scale and the human scale on the Millennium scale, and only a small part of it is controlled by exogenous (surface) geological processes, so it is not affected by negative feedback. The ultimate impact of methane enrichment in the atmosphere does not necessarily require catastrophe or sudden release, as stated in the "gas-water composite explosion hypothesis". The enhanced exhaust process from Mi Yuan contributed to the methane trend observed in ice core records, which can explain the peak of methane concentration increase in the late Quaternary and the accompanying enrichment of heavy isotope methane. This hypothesis should be verified by various interdisciplinary studies based on atmospheric, biological and geological indicators.