Environmental effects of natural gas hydrate exploration and development

1. Natural gas hydrate and global carbon cycle and greenhouse effect

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. Natural gas hydrate hills and hills were photographed below 500 meters in Louisiana. By comparing the videos of 1992 and 1993, the disappearance of one mountain and the rebirth of another mountain are determined. The continuously released airflow around the hillside contains 69.6% CH4, 6.3% C2H6, 1.7% C3H8, 1 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 contrary, 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 the carbon cycle among the lithosphere, hydrosphere and atmosphere.

Methane is an important greenhouse gas, so the release or absorption of methane by natural gas hydrate will have a great impact on the global climate. At present, although the volume concentration of methane in the atmosphere is only 1/200 of that of CO2, its global warming potential index (GWP) is 3.7 times that of CO2 by mole and 10 times that of CO2 by mass. During the period of 1980 ~ 1990, the contribution of methane to the greenhouse effect accounted for 12%, while the total contribution of methane and other trace greenhouse gases accounted for 43%, only slightly lower than that of CO2 (57%). From before the industrial revolution to now, the concentration of CO2 in the atmosphere has increased by 25% (from 280× 10-6 to 350× 10-6), while the concentration of methane has doubled (from 0.8× 10-6 to1.7). This shows that methane concentration increases faster, so its relative contribution to the greenhouse effect will increase in the future.

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 rise of water temperature plays the opposite role. 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 stable and unstable changes of natural gas hydrate is different. It is estimated that in the past 654.38+00000 years, the surface temperature of the polar shelf has increased by more than 654.38+00℃, and the influence of temperature increase has exceeded that of sea level rise, resulting in a large amount of methane release, with an average annual release of 5.6× 654.38+009 m3, equivalent to 654.38+0% of all atmospheric methane sources. For another example, the measurement of the British continental shelf area of about 60× 104km2 shows that the amount of methane escaping into the atmosphere every year reaches12×104t ~ 3.5x106t, 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 on the seabed, 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.

Second, the relationship between natural gas hydrate and global climate change

Natural gas hydrate is not only a huge carrier of future energy, but also an important factor of climate change. The existing research results show that in the past 200,000 years, the methane content in the atmosphere has a close coupling relationship with the temperature of the earth (figure 1- 10), but the reason and mechanism have not been clarified. It is speculated that the global temperature rise may be the result of the instability of a large number of natural gas hydrates. In fact, only melting 1m3 hydrate can release as much as 160 ~ 200m3 methane, and some of it will definitely enter the atmosphere. On the contrary, the warming of the lower atmosphere will heat the ocean and trigger more vicious cycles of hydrate decomposition and atmospheric warming.

Figure 1- 10 Records of temperature, carbon dioxide and methane changes in the Vostok ice core in Antarctica for 200,000 years.

The Sea of Okhotsk on the east coast of Asia is roughly equivalent to the total area of the North Sea and the Baltic Sea. The Sea of Okhotsk is usually covered with ice for seven months in a year, and methane constantly escapes from the gas hydrate deposits on the seabed, forming plumes. 199 1 year, the Russian scientific research team detected that the methane concentration in the water under the ice was 65 ml/L. When the ice melted in the following summer, the index was only 0. 13mL/L, and the rest obviously escaped into the atmosphere. This measurement clearly shows that methane hydrate under the ocean is an important source of atmospheric methane.

Evolution trajectory of hydrate under the condition of sea level fluctuation

When the water depth changes, the hydrostatic pressure will change with the change of sea level. The stability of hydrate depends on the following two factors: ① the extent of sea level rise or fall; (2) How quickly the change of seabed temperature affects the change of seabed sediment temperature. An initial depth generally has a lower temperature base value, so high overlying water pressure and low water temperature are the conditions for producing a large number of hydrates. In shallow water, when the water depth and pressure decrease, the temperature base value will also increase, so the stability of hydrate will be lower than the minimum stability condition. Therefore, in the case of shallow initial depth and high water temperature base value, hydrate may not be generated.

With the increase of water depth and the decrease of water temperature, the stability conditions of hydrate are shown in figure1-1(a) and figure1-1(b) respectively. Whether the phase path ends at point B, B' or B "depends on the degree of sea level decline. When the sea level drops slightly, the pressure on the hydrate stability curve remains unchanged, so the phase path will end at point B. However, when the sea level drops sharply, the phase path will pass through the hydrate stability curve to reach point B', and as a mixture of natural gas and water, the hydrate will end at point B. " On the other hand, as shown in figure1-1(b), if the sea level rises slightly and stays at the original point A, the (gas/water mixture) phase diagram will end at point B (as shown in figure1-kloc-0//kloc-0) When the sea level rises on a large scale, the phase diagram crosses B' (as shown in figure1-1(b)), and the hydrate formation conditions can be reached at point B, ending the evolution history "(point B represents hydrate, not gas/water mixture).

The rising and falling cycles of sea level can produce three phase cycles (as shown in figure1-1(c), marked with a, b and c respectively). Due to the heat conduction to the sediment, there is a time delay in the adjustment process of the sediment temperature. Any cyclic phase path rotates counterclockwise from position 1, which means that as the temperature response slows down, there will be hysteresis. In ring A (figure1-1(c)), the sea level rises and falls beyond the hydrate stability curve, so it is a process from gas/water mixture-hydrate to gas/water mixture. This process occurs when the sea level rises for the first time (from 1 position to position 2), then the sediment temperature drops (from position 2 to position 3), then the sea level drops (from position 2 to position 4), and finally the sediment temperature rises (from position 4 to 1 position). In rings B and C, hydrate is preserved (ring B) and gas/water mixture always exists (ring C).

Many scholars have discussed the feedback of natural gas hydrate to global climate change, which is different in polar regions and middle and low latitudes. During the interglacial period, global warming, melting of glaciers and ice sheets, and unstable natural gas hydrate in permafrost strata released methane due to rising temperature and falling pressure, which produced a greenhouse effect and positive feedback to global warming. At the same time, in the marginal sea at the middle and low latitudes, on the one hand, the rise of seawater temperature will make natural gas hydrate unstable, on the other hand, due to the rise of sea level, the hydrostatic pressure on the seabed will increase, which will increase the stability of natural gas hydrate. Because of the large heat capacity of seawater, the temperature rise of bottom seawater will not be significant, and the influence of hydrostatic pressure may be dominant, so the overall effect may be to improve the stability of natural gas hydrate and produce negative feedback on global warming. During the ice age, all the above processes can be reversed. Kvervolden( 1988) thinks that the positive feedback of polar gas hydrate plays a leading role in the process of modern global warming, and the methane released in this process is estimated to be 3×10/2g every year, which is 1% of the methane increment in the global atmosphere. It is generally believed that the ice age was caused by Milankovic orbital force, but this mechanism can explain the extensive and slow changes of the ice age cycle, but it cannot explain the sudden termination of the ice age. Paull et al. (1996) explained the termination of the ice age with the negative feedback of natural gas hydrate in the continental margin sea, but this could not explain the suddenness of the termination. It is considered that as long as the temperature warms slightly, the positive feedback of polar gas hydrate can accelerate this process and make the ice age suddenly end. However, this effect will lead to the uncontrollable release of methane from natural gas hydrate and the uncontrollable global warming that follows; Actually, this phenomenon has not been observed. Therefore, the sudden end of the ice age remains a mystery.

Figure 1- 1 1 Schematic diagram of hydrate evolution trajectory under sea level fluctuation.

Many researchers believe that a large area of explosive methane release will make the climate change dramatically in a short time. James P.Kennett, a marine geographer at the University of California, Santa Barbara, put forward a hypothesis that in the last ice age of about 1.5× 104a, catastrophic methane release may cause a significant increase in temperature in just a few decades.

The researchers also found an older sign that methane released by natural gas hydrate has affected global climate change. Fossil evidence that influenced the global climate at the end of Paleocene about 5500× 104 years ago shows that the temperature of the ocean and land rose sharply during this period, resulting in a worldwide temperature anomaly (LPTM = the peak of the late Paleocene), and many single-celled organic species living on the seabed sediments became extinct. The carbon isotope of microorganisms has become the key to explain the rapid rise of temperature. This famous global temperature anomaly, as shown by the global carbon isotope changes during this period, is accompanied by extremely strong methane release from marine natural gas hydrate sediments.

For the relationship between natural gas hydrate and global change, it is urgent to deeply and quantitatively study its role in global carbon cycle and its feedback to global warming, cooling and corresponding sea level changes. As mentioned above, the direction and intensity of this feedback may change with latitude or climate process; Revealing its laws will be of great significance to understanding global changes, especially the reasons for the alternation of glacial and interglacial periods. In order to study the total contribution of natural gas hydrate to carbon cycle and greenhouse effect, it is necessary to study the feedback mechanism on the basis of experiments and simulations, accurately estimate the amount of methane released or absorbed by natural gas hydrate in different environments and conditions, and the amount of methane released after passing through the water layer without being dissolved or oxidized and reaching the atmosphere, so as to quantitatively estimate the total amount of methane released or absorbed by natural gas hydrate in the world under a given climate change, that is, the sum of two opposite effects in middle and low latitudes and positive feedback effects in polar regions.

Three. Geological hazard factors of natural gas hydrate

The scientific community generally believes that natural gas hydrate will eventually become a clean energy source with great potential for human beings in the future. At the same time, the study shows that when the surrounding environmental conditions of natural gas hydrate in sediments change for various reasons, the temperature and pressure balance will be destroyed, leading to the disintegration and escape of natural gas hydrate, which may lead to geological disasters or have an impact on global climate change. The occurrence of instability has a complex interaction with the change of environmental conditions of natural gas hydrate occurrence. The stability of natural gas hydrate is determined by pressure, temperature and gas. In the typical water temperature change process, the stable threshold temperature of pure methane hydrate starts at about 5℃ and 50Pa pressure (equivalent to about 500m water depth). In the case of mixing other gases, especially hydrogen sulfide, the stability range will be significantly expanded. At the same temperature, adding about 20% hydrogen sulfide to the mixed hydrate of methane and carbon dioxide will reduce the pressure by about 65438±00Pa, or increase it by about 2℃ at the same pressure. Natural gas hydrate with different components will be formed in different temperature and pressure ranges. In addition, the composition and availability of pore water, gas saturation, possible catalytic characteristics of host rocks, porosity and continuous stability are also of great significance to the stable range of sediments.

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. Nearly 200 landslides have been mapped on the Atlantic continental margin of the United States, which are considered to be caused by the decline of sea level, the decrease of confining pressure and the release of methane gas by decomposing natural gas hydrate. 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 plateaus in other sea areas is also related to natural gas hydrate (such as continental slopes and plateaus in southwest Africa, Norwegian continental margin, beaufort continental margin, Caspian Sea, North Panamanian continental shelf and Newfoundland). During the late Pleistocene regression, the sea level dropped by about1000 m, resulting in a decrease in seabed pressure of 1000 kPa. The decrease of total pressure causes the decomposition of the bottom of natural gas hydrate, releasing excessive methane and water, resulting in slope instability and disastrous consequences. The research shows that there are two different mechanisms to trigger the Amazon submarine landslide: ① the rapid decline of sea level makes the natural gas hydrate unstable and the overlying sediments slide; (2) The glaciers in the Andes receded and then Amazon sediments poured into the continental slope, which led to submarine landslides caused by overloading. According to the change of atmospheric methane content observed in the ice core, the former explanation seems more reasonable. The fuse of submarine landslide may be a small earthquake, proluvial brought by a batch of rivers, or even a big storm surge. Once the landslide begins, the free gas under the hydrate layer will rise along the fracture, and the metastable hydrate will also release methane gas. Research shows that most large landslides are related to the instability of natural gas hydrate or the "sliding" of collapsed materials on hydrate (Figure 1- 12). 1929, the submarine landslide in Newfoundland, Canada caused 27 deaths and huge economic losses; 1979, the tsunami caused by the submarine landslide on the French coast caused 1 1 deaths. Therefore, when developing and utilizing submarine gas hydrate, we should fully consider and study submarine geological disasters and design feasible technical schemes.

Figure 1- 12 Comprehensive schematic diagram of marine hydrate environmental impact.

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 the change of pressure and temperature conditions leads to the release of natural gas hydrate, it will first lead to instability in many parts of the continental slope belt, form huge slump blocks and slide into the deep sea, and the deep sea ecological environment will suffer disastrous consequences.

According to the previous detection results of the seabed, scientists explained that about 5600m3 of sediment located in front of 0.8× 104a on the Norwegian continental margin slipped 800km from the upper edge of the continental slope to the Norwegian basin, and the tsunami caused by the huge amount of soil pushing 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 biggest landslides formed by the release of world-famous natural gas hydrate.

1in the summer of 998, Russian researchers from Shirshov Institute of Oceanography discovered an unstable hydrate deposit on the west coast of Norway. They believe that the natural decomposition of columns and hydrates produced by submarine faults can slowly release methane into the atmosphere, but this process is sometimes more explosive. An international team led by You Jurgen Mienert of Tromso University in Norway recently discovered that there are many huge craters at the bottom of Barents Sea (just at the northeast end of Norway), and the largest crater is 700 meters wide and 300 meters deep. These craters of different sizes are densely distributed near methane hydrate deposits, which clearly shows that a catastrophic methane explosion has occurred. Faults and other structural evidence indicate that they may have occurred at the end of the last ice age. This eruption may follow the theory used to explain the reason of the landslide in Storega: the warming ocean makes the hydrate unstable, and when it reaches a certain critical point, it will erupt like a volcano.

Because hydrate contains more gas than autogenous volume 100 times, if it encounters structural action such as fracture, it will decompose like an instantaneous explosion, forming a gas/water mixture with a density of 0. 1kg/m, forming a powerful hydrodynamic flow, vortex and cyclone on the sea surface. In this environment, ships, planes and offshore drilling facilities will soon sink into the seabed. Scientists realize that the fragility of natural gas hydrate has an important influence on well location selection, drilling and casing running scheme, and 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.

According to the analysis of stable temperature and pressure conditions of natural gas hydrate, it existed at least at the end of Eocene, that is, 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 gas hydrate layer may be formed in deeper water. Natural gas hydrate formed under suitable conditions fills in the gaps of sedimentary layers, 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 Middle Oligocene (30Ma), there were two sea level drops, both accompanied by large-scale landslides. The seismic profile analysis of the continental margin of New Jersey shows that there were four large landslides in the Early Tertiary, all corresponding 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.