Know "flammable ice"

Natural Gas Hydrate (hereinafter referred to as Gas Hydrate) because of its appearance as ice and can be burned in the event of fire, it is also known as "flammable ice" or "solid gas" and "gas ice." It is ice-like, non-stoichiometric, composed of water and natural gas mixed under conditions (suitable temperature, pressure, gas saturation, salinity of water, Hp, etc.) under medium and high pressure and low temperature conditions. Caged crystalline compounds. It can be represented by CH4nH2O, where M represents the gas molecule in the hydrate and n is the hydration index (ie, the number of water molecules). Components that make up natural gas, such as CH4, C2H6, C3H8, C4H10 equivalents, and CO2, N2, H2S, etc., can form single or multiple natural gas hydrates. The main gas that forms natural gas hydrates is methane, and natural gas hydrates with a methane molecular content of more than 99% are often referred to as methane hydrates (Methane Hydrate).

Natural gas hydrates are widely distributed in nature on the slopes of the mainland, islands, uplifts of active and passive continental margins, polar continental shelves, and deep water environments of the oceans and some inland lakes. Under standard conditions, the decomposition of a unit volume of gas hydrate can produce up to 164 units of methane gas, which is an important potential future resource.

Gas hydrates are a new type of mineral resource discovered during the scientific investigation in the 20th century. It is a kind of solid substance produced when water and natural gas are mixed under high pressure and low temperature conditions. It looks like ice, snow or solid alcohol and can be ignited to burn. It is called "flammable water", "air ice" and "solid gas". As a strategic resource with commercial development prospects in the 21st century, natural gas hydrates are a new type of high-efficiency energy. Their composition is similar to the natural gas components that people usually use, but they are more pure and they only need to integrate solid natural gas. "Hydrate" heating and decompression can release a large amount of methane gas.

Natural gas hydrates are easy to use, have high combustion values, and are clean and pollution-free. It is understood that the global reserves of natural gas hydrates are twice that of existing natural gas and oil reserves, and have broad prospects for development. The United States, Japan, and other countries have all discovered and produced natural gas hydrates in their respective sea areas. According to estimates, natural gas in the South China Sea The amount of hydrate resources is 70 billion tons of oil equivalent, which is equivalent to about one-half of China's current total land-based oil and natural gas resources.

China's first production of natural gas hydrate (flammable ice) samples

China successfully drilled a physical flammable gas sample “flammable ice” in the northern part of the South China Sea, becoming the fourth country after the United States, Japan, and India to collect physical samples of hydrates through the national R&D plan.

In the early morning of May 1, 2007, China's first sampling in the northern part of the South China Sea was successful, confirming that there are abundant natural gas hydrate resources in the northern part of the South China Sea, indicating that China's natural gas hydrate survey and research level has entered the world's advanced ranks.

The scientific name of "flammable ice" is "natural gas hydrate", which is the "ice cube" of natural gas crystallized at 0°C and 30 atmosphere pressure. Methane in "ice" accounts for 80% to 99.9%, can be directly ignited, and almost no residue is produced after combustion. The pollution is much smaller than that of coal, oil, and natural gas. 1 cubic meter of combustible ice can be converted into 164 cubic meters of natural gas and 0.8 cubic meters of water. At present, the conventional oil and natural gas resources owned by the world will gradually deplete after 40 or 50 years. Scientists estimate that the distribution of combustible ice on the seabed is about 40 million square kilometers, which accounts for 10% of the total area of ​​the sea. The reserves of combustible ice on the sea floor are enough for human use for 1,000 years, and thus the scientists are hailed as “future energy” and “21st century energy”. .

It is reported that so far, at least 30 countries and regions around the world are conducting research and investigations on the flammable ice.

Combustible ice is mainly stored in the permafrost zone on the sea floor or in cold areas, making it hard to find and explore. The newly developed instrument with extremely high sensitivity can instantly measure the precise content of various ultra-micro amounts of methane, ethane, propane and hydrogen in the seabed soil and rocks, and determine the presence or absence of flammable ice resources and the amount of resources. And other indicators.

Traditional mining methods

(1) Thermally Excited Mining The thermal excitation mining method directly heats the natural gas hydrate layer, so that the temperature of the natural gas hydrate layer exceeds its equilibrium temperature, thereby prompting the decomposition of natural gas hydrate into a method of mining water and natural gas. This method has experienced the development of direct injection of hot fluid heating, fire flooding, downhole electromagnetic heating, and microwave heating into the natural gas hydrate formation. The thermal excitation method can realize the cyclic heat injection, and the action method is faster. The continuous improvement of heating methods has promoted the development of thermally stimulated mining methods. However, this method has not yet solved the problem of low heat utilization efficiency and can only perform local heating. Therefore, this method still needs further improvement.

(2) Decompression mining method The decompression mining method is a method that promotes natural

Gas hydrate decomposition mining method. There are two main ways of pressure reduction: 1 Use low-density mud drilling to achieve the purpose of decompression; 2 When there is free gas or other fluid below the natural gas hydrate layer, lower the free gas or other fluid under the gas hydrate layer to lower The pressure of the gas hydrate layer. The decompression mining method does not require continuous excitation and has a low cost and is suitable for large-area mining. It is especially suitable for the exploitation of natural gas hydrate reservoirs in the presence of underlying free gas layers. It is the most promising technology for traditional gas hydrate extraction methods. . However, it has special requirements for the nature of natural gas hydrate reservoirs. Only when the natural gas hydrate reservoir is located near the temperature-pressure equilibrium boundary, the decompression mining method is economically viable.

(3) Chemical reagent injection and recovery method Chemical reagent injection and mining method injects certain chemical reagents such as salt water, methanol, ethanol, ethylene glycol and glycerin into the gas hydrate layer to destroy the phase balance of the gas hydrate reservoir. Conditions promote the decomposition of natural gas hydrates. Although this method can reduce the initial energy input, the defect is obvious. It requires expensive chemical reagents, has a slow effect on the natural gas hydrate layer, and it also brings some environmental problems. Therefore, this method is currently used. Investment in research is relatively small.

New mining method

(1) CO2 displacement mining method. This method was first proposed by Japanese researchers, and the method is still based on the pressure conditions of the gas hydrate stabilization zone. Under certain temperature conditions, the pressure required to maintain the stability of natural gas hydrates is higher than that of CO2 hydrates. Therefore, within a certain pressure range, natural gas hydrates will decompose, and CO2 hydrates are easy to form and remain stable. If CO2 gas is injected into the natural gas hydrate reservoir at this time, the CO2 gas may generate CO2 hydrate with the water decomposed from the natural gas hydrate. This effect

The heat released allows the decomposition reaction of natural gas hydrates to continue.

(2) Solid mining method. The solids extraction method initially collects solid hydrates from the seabed and pulls the gas hydrates into shallow water for controlled decomposition. This method has evolved into a hybrid mining method or mining mud mining method. The specific steps of this method are to first promote natural gas hydrates to decompose into gas-liquid mixed phase in situ, collect mixed mud mixed with gas, liquid and solid hydrate, and then introduce the mixed mud into a surface operation ship or a production platform for processing. , to promote the complete decomposition of natural gas hydrates and to obtain natural gas

Typical Mining Research Examples Mining of Gas Hydrates in the Maesoha Gas Field

The Mesojaha gas field was discovered in the late 1960s and is the first and so far

The only gas field that has been commercially mined for gas hydrate reservoirs. The gas field is located in the northwestern part of the former Soviet Union in West Siberia. The annual permafrost thickness in the gas field is more than 500 m, which is favorable for the occurrence of natural gas hydrates. The Maesoja gas field is a conventional gas field. Natural gas in the gas field migrates through the cap rock. Under favorable environmental conditions, a gas hydrate layer is formed above the gas field. The gas hydrate reservoir in the gas field was first inadvertently mined via a decompression route. By exploiting the conventional natural gas below the gas hydrate reservoir, the pressure of the gas hydrate layer is reduced, and the gas hydrate decomposes. Later, in order to promote the further decomposition of natural gas hydrates and maintain gas production, chemical inhibitors such as methanol and Calcium chloride were deliberately injected into gas hydrate reservoirs.

Gas Hydrate Test Collection in Mackenzie Delta

The Mackenzie Delta region is located in the northwestern part of Canada and is located in an arctic cold environment with favorable conditions for the formation and preservation of natural gas hydrates. The study of natural gas hydrates in this area has a long history. The evidence of the presence of natural gas hydrates was found occasionally in the 800-1100 m well below the permafrost as it was drilled in the Mallik L238 well in the conventional exploration wells between 1971 and 1972; in 1998, it was drilled for natural gas hydrate exploration. In the Well Mallik 2L238, gas hydrates were discovered in the wells from 897 to 952 m, and natural gas hydrate cores were produced. In 2002, a global gas hydrate pilot mining study was carried out in the Mackenzie Delta. The project was jointly invested by nine institutions including five countries including the Canadian Geological Survey, the Japan Petroleum Corporation, the German Institute of Earth Sciences, the U.S. Geological Survey, the U.S. Department of Energy, the Indian Gas Supply Company, and the Indian Oil and Gas Company. This is the first natural gas hydrate extraction experiment in the area. It is also the first time in the world that this kind of large-scale international quarrying of natural gas hydrates has been conducted.

Test of Gas Hydrate Recovery in the Slope Region of Northern Alaska

The Prudhoe Bay-Kupalak River region in northern Alaska is located on the slopes of northern Alaska. In 1972, Arco Oil and Exxon Petroleum produced natural gas hydrate cores at 664-667 m intervals while drilling conventional wells in the Prudhoe Bay oil field. Later, a large number of gas hydrate researches were conducted in the northern slope of Alaska. On this basis, a remarkable research project on gas hydrate test mining was implemented in the area in 2003. The project was jointly initiated by Anadarko Petroleum, Noble Corporation, Mau2rer Technologies, and the U.S. Department of Energy's Methane Hydrate Research and Development Program. The goal was to drill natural gas hydrate research and test wells, the Hot Bing 1 well. This is the first exploratory well drilled for natural gas hydrate research and test production in the northern slopes of Alaska.

Environmental issues in the exploitation of natural gas hydrates

The exploitation of natural gas hydrate reservoirs will change the temperature and pressure conditions on which natural gas hydrates will occur, causing the decomposition of natural gas hydrates. If the control of temperature and pressure conditions cannot be effectively achieved during the exploitation of natural gas hydrate reservoirs, a series of environmental problems may occur, such as the intensification of greenhouse effect, changes in marine ecology, and seafloor slump events.

(1) Methane is a strong greenhouse gas and its contribution to atmospheric radiation balance is second only to carbon dioxide. On the one hand, the amount of methane contained in global gas hydrates is approximately 3,000 times the amount of methane in the atmosphere; on the other hand, the amount of methane produced by the decomposition of natural gas hydrates into the atmosphere is only 0.5% of the total amount of atmospheric methane. It will also significantly accelerate the progress of global warming. Therefore, if the methane gas cannot be well controlled during the gas hydrate extraction process, it will inevitably increase the global greenhouse effect. In addition to the greenhouse effect, the exploitation of natural gas hydrates in the marine environment will bring more problems. 1 The methane entering the sea will affect the marine ecology. After methane enters seawater, rapid microbial oxidation will occur, affecting the chemical properties of seawater. If methane gas is discharged into seawater in large quantities, its oxidation will consume a large amount of oxygen in seawater and cause the ocean to form an oxygen-deficient environment, thus causing damage to the growth and development of marine microorganisms. 2 If the amount of methane that enters the seawater is particularly large, it may also cause vaporization of seawater and tsunami. It may even cause turbulence in seawater and negative pressure entrainment of air currents, which will seriously endanger the sea surface operations and even the maritime aviation operations.

(2) The decomposition of natural gas hydrates during the mining process will also generate large amounts of water, releasing the pore space of the rock formation and deteriorating the consolidation of the gas hydrate storage area and causing geological disasters. Decomposition of marine gas hydrates may lead to seafloor slumps]. Recent studies have found that the decline in the stability of the continental slope due to the decomposition of natural gas hydrates in the seabed is an important cause of the occurrence of seafloor slump. If a large number of gas hydrates are decomposed during the drilling process, it may also lead to deformation of the drilling and increase the risk of offshore drilling platforms.

(3) How to treat the water produced by the decomposition of natural gas hydrate during the exploitation of natural gas hydrates is also a problem that should be taken seriously.