natural gas

[tiān rán qì]

Mixture of hydrocarbon and non hydrocarbon gas contained in the formation

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Natural gas refers to all gases naturally existing in nature, including Atmosphere 、 Hydrosphere , and lithosphere Gases (including Oilfield gas 、 Gas field gas 、 Mud volcanic gas 、 CBM and Biogenic gas Etc.).

The definition of "natural gas", which has been widely used for a long time, is based on energy narrow sense Definition refers to the natural Hydrocarbons And non hydrocarbon gases. stay Petroleum geology In, it usually refers to oil field gas and gas field gas. It is mainly composed of hydrocarbons and contains non hydrocarbon gases.

On May 6, 2020, PetroChina Southwest Oil and Gas Field Company It is disclosed that the company has found a new zone rich in natural gas. It is estimated that Potential resources Over one trillion cubic meters^[1] 。

On February 28, 2023, the National Bureau of Statistics issued the Statistical Bulletin of the National Economic and Social Development of the People's Republic of China in 2022. According to the preliminary accounting, the consumption of natural gas decreased by 1.2%, and the consumption of clean energy such as natural gas, hydropower, nuclear power, wind power, and solar power accounted for 25.9% of the total energy consumption, up 0.4 percentage points.^[7]

In 2023, the natural gas output will be 229.71 billion cubic meters, up 5.8% year on year, with an annual increase of 10 billion cubic meters for seven consecutive years On.^[8]

In 2024, the natural gas output will be 232.43 billion cubic meters, with a year-on-year growth of 5.6% and consumption growth of 7.2%.^[9]

Chinese name: natural gas

Foreign name: Natural gas
Main components: Hydrocarbon and non hydrocarbon gases

catalog

▪ improve our life
five Causes
▪ Biogenesis
▪ Organic genesis
▪ Inorganic origin
six Ranking of reserves in various countries
seven toxicity hazard

source

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Natural gas is stored in underground porous rock formations, including oil field gas, gas field gas, coal bed gas, mud volcanic gas and biogenic gas, and a small amount is also produced in coal seams. It is a high-quality fuel and chemical raw material.

Natural gas is mainly used as fuel to produce carbon black, chemicals and Liquefied petroleum gas , produced by natural gas propane 、 butane It is an important raw material for modern industry. Natural gas mainly consists of gas Low molecular hydrocarbon It is mixed with non hydrocarbon gas.

It is mainly composed of methane (85%) and a small amount of ethane (9%), propane (3%), nitrogen (2%) and butane (1%). It is mainly used as fuel, as well as raw materials for manufacturing acetaldehyde, acetylene, ammonia, carbon black, ethanol, formaldehyde, hydrocarbon fuel, hydrogenated oil, methanol, nitric acid, synthetic gas, vinyl chloride and other chemicals. Natural gas is compressed into liquid for storage and transportation. Coal miners, nitric acid manufacturers, power plant workers, organic chemical synthesizers, gas users, petroleum refiners, etc. have the opportunity to contact this product. It mainly enters the human body through the respiratory tract. genus Simple suffocating gas 。 When the concentration is high, it causes hypoxia due to replacement of air, resulting in shortness of breath and loss of consciousness; In serious cases, people may die of asphyxia due to low blood oxygen. High pressure natural gas can cause frostbite. Incomplete combustion can produce carbon monoxide 。^[2]

Physical and chemical properties

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Natural gas is a general term for the mixed gas existing in underground rock reservoirs with hydrocarbon as the main body. Its specific gravity is about 0.65, lighter than air, and it is colorless, tasteless, and non-toxic.

Main components of natural gas alkane , where methane Accounting for the vast majority, and a few others ethane 、 propane and butane In addition, there are hydrogen sulfide 、 carbon dioxide , nitrogen and water vapor and a small amount carbon monoxide And trace Noble gas , such as helium and argon Etc. Before the natural gas is sent to the end user, in order to facilitate leak detection mercaptan 、 Tetrahydrothiophene To add odor to natural gas.

Natural gas is insoluble in water, with a density of 0.7174kg/m three The relative density (water) is 0.45 (liquefaction), the ignition point (℃) is 650, and the explosion limit (V%) is 5-15. Under standard conditions, methane to butane exists as a gas, Pentane The above is liquid. Methane is the shortest and lightest hydrocarbon molecule.

Organic sulfide and hydrogen sulfide （H two S) It is a common impurity that must be removed in advance in most cases when natural gas is used. Natural gas with many sulfur impurities is described as "sour" in English.

The combustion heat value of natural gas per cubic meter ranges from 8000 kcal to 8500 kcal. The combustion heat value of each kilogram of liquefied gas is 11000 kcal. The specific gravity of gaseous liquefied gas is 0.55. The combustion heat value of each cubic meter of liquefied gas is 25200 kcal. Each bottle of liquefied gas weighs 14.5kg, and the total combustion heat value is 159500 kcal, which is equivalent to the combustion heat value of 20 cubic meters of natural gas.

Methane combustion equation

Complete combustion: CH four +2O two = CO two +2H two O (reaction condition is ignition)

Methane+oxygen → carbon dioxide+water vapor

Incomplete combustion: 2CH four +3O two =2CO+4H two O

Methane+oxygen → carbon monoxide+water vapor

Unit of measurement

Kilowatt hour (kw · h) or joule (J)

Gas station sales unit: CNG yuan/cubic meter (yuan/m three ）LNG yuan/kg

Composition classification

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1. Natural gas exists underground Phase state Can be divided into Free state 、 Dissolved state 、 Adsorptive state And solid hydrates. Only free natural gas can be developed and utilized after forming natural gas reservoir through accumulation.

2. Natural gas can be divided into Associated gas And non associated gas.

Associated gas: oilfield gas produced with crude oil in symbiosis. The associated gas is usually the volatile part of crude oil, which exists in the form of gas on the oil bearing layer. It is found in all the formations with crude oil, but the proportion of oil and gas is different. Even in the same oil field, the sources of oil and gas are not necessarily the same. They come together in the same rock reservoir by different ways and processes.

Non associated gas: including natural gas from pure gas field and natural gas from condensate gas field, both of which exist as gas in the formation. After flowing out of the wellhead from the formation, the natural gas in the condensate field is separated into gas-liquid two phases as the pressure drops and the temperature rises. The gas phase is the natural gas in the condensate field, and the liquid phase is the condensate, called condensate. If it is non associated gas, it has nothing to do with liquid accumulation and may be produced from plant substances. Gas field gas and oil field gas are the main natural gas production in the world. The exploitation of coalbed methane has been paid more and more attention.

3. According to the state of natural gas reserves, it can be divided into structural natural gas, water-soluble natural gas and coal mine natural gas. Structural natural gas can be divided into wet natural gas produced with crude oil and dry natural gas without liquid composition.

4. Natural gas can be divided into biogenic gas Oil type gas and Coal formed gas 。 Inorganic gas, especially non hydrocarbon gas, is highly valued.

5. According to the occurrence of natural gas underground, it can be divided into oilfield gas, gas field gas, condensate gas, water-soluble gas, coal-bed gas, and solid gas hydrate.^[3]

Basic characteristics

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Natural gas is one of the safer fuels. It does not contain carbon monoxide and is lighter than air. Once leaked, it will immediately spread upward, which is not easy to accumulate and form explosive gas. Its safety is relatively higher than other fuels.

Using natural gas as energy can reduce the consumption of coal and oil, thus greatly improving environmental pollution; As a clean energy, natural gas can reduce sulfur dioxide And dust emission nearly 100%, reducing carbon dioxide 60% of emissions and Nitrogen oxides 50% of the emissions, and help reduce the formation of acid rain Global greenhouse effect And fundamentally improve the environmental quality.

As a vehicle fuel, natural gas has the advantages of high unit heat value, low exhaust pollution, reliable supply and low price, and has become the development direction of clean fuel for vehicles in the world, while natural gas vehicles have become the fastest growing and most used new energy vehicles.

However, for the greenhouse effect, natural gas, like coal and oil, will produce carbon dioxide. Therefore, natural gas cannot be regarded as a new energy source. Its advantages include:

Green and environmental protection

Natural gas is a kind of clean and environment-friendly high-quality energy, almost free of sulfur, dust and other harmful substances. When burning, it produces less carbon dioxide than other fossil fuels, resulting in low greenhouse effect, which can fundamentally improve environmental quality.

economical and practical

Natural gas and Artificial gas In comparison, the price of year-on-year calorific value is similar, and the natural gas is clean, which can extend the service life of the cooker and help users reduce maintenance costs. Natural gas is a clean gas with stable supply and can improve air quality, so it can provide new impetus for the economic development of the region, drive economic prosperity and improve the environment.

Safe and reliable

Natural gas is non-toxic, easy to emit, lighter than air, and not easy to accumulate into explosive gas. It is a relatively safe gas.

improve our life

As families use safe and reliable natural gas, it will greatly improve the home environment and improve the quality of life.

Calculation of natural gas oxygen consumption: about 2.0 cubic meters of oxygen and about 10 cubic meters of air are required for the complete combustion of 1 cubic meter of natural gas (100% purity).

Causes

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The origin of natural gas is diverse. The formation of natural gas runs through diagenesis, plutonic, epigenetic and metamorphic processes. All types of organic matter can form natural gas, sapropelic organic matter can generate oil and gas, and humic organic matter mainly generates gaseous hydrocarbons.

Biogenesis

In the early diagenesis (stage), in the shallow biochemical action zone, the natural gas formed by the mass fermentation and synthesis of sedimentary organic matter by microorganisms is called biogenic gas. Sometimes it is mixed with gas formed by early low-temperature degradation. Biogenic gas occurs in the rock stratum with shallow burial, new age and low degree of evolution, mainly containing methane gas. The prerequisite for the formation of biogenic gas is more abundant organic matter and a strong reducing environment.

The organic parent material most conducive to gas generation is herbaceous humic sapropelic humic type, which is mostly distributed in the delta and swamp lakeshore zone with abundant supply of terrigenous materials, and the sand shale series containing terrigenous organic matter is usually the most favorable. The reason why it is difficult to form a large amount of biogenic gas in the sulfuric acid layer is that sulfuric acid has obvious resistance to methanogens two Preferentially reduce sulfate to form metal sulfide or hydrogen sulfide for sulfur ions, so carbon dioxide cannot be converted into methane by hydrogen.

Methanogen The growth of E h <- 300mV (i.e. oxygen and SO in formation water four 2- After all are restored in turn, they will reproduce in large numbers); Secondly, the pH value should be close to neutral, generally 6.0~8.0, and the optimal value is 7.2~7.6; Moreover, the growth temperature of methanogens is 0~75 ℃, and the optimal value is 37~42 ℃. Without these external conditions, methanogens cannot reproduce in large numbers and form a large amount of methane gas.

Organic genesis

Oil type gas

The natural gas formed by sedimentary organic matter, especially sapropel organic matter, together with oil in the process of thermal degradation into oil, or the natural gas formed by thermal cracking of organic matter and early formed liquid oil in the post formation stage is called oil type gas, including wet gas (oil associated gas), condensate gas and pyrolysis gas.

Like the gradual formation of oil through pyrolysis of organic matter, the formation of natural gas also has obvious vertical zoning. At the top of the profile (diagenetic stage) is biogenic gas, and at the later stage of the plutonic stage is low molecular weight gaseous hydrocarbon (C two ～C four ）It refers to wet gas and condensate gas formed by reverse evaporation of light liquid hydrocarbon due to high temperature and pressure. At the lower part of the profile, due to temperature rise, the generated oil is cracked into small molecule light hydrocarbon until methane, and the organic matter also further generates gas, mainly methane Petroleum cracking gas It is the final product of gas generation sequence, and this stage is usually called dry gas zone.

From oil associated gas → condensate gas → dry gas, the methane content gradually increases, so the drying coefficient increases.

Coal formed gas

The natural gas generated by thermal evolution of coal measure organic matter (including coal seams and dispersed organic matter in coal measure strata) is called coal type gas.

During coal field mining, a large amount of gas is often emitted, such as a gas outburst in a well in Hechuan District, Chongqing, with a gas emission of 1.4 million cubic meters, which shows that coal measures can indeed generate natural gas.

Coal formed gas is a multi-component mixed gas, in which the hydrocarbon gas is mainly methane, and the content of heavy hydrocarbon gas is small, generally dry gas, but there may also be wet gas, even condensate gas. Sometimes it can contain more Hg steam and N two Etc.

Coal type gas can also form super large gas fields. Since the 1960s, super large coal type gas fields have been found in K2 in northern West Siberia, P in the eastern Netherlands basin and southern North Sea basin. The proven reserves of these three gas areas are 22 trillion cubic meters, accounting for 1/3 of the total proven natural gas reserves in the world. According to statistics (M.T. Halbuddi, 1970), 16 of the 26 atmospheric fields discovered in the world are coal type gas fields, accounting for 60% of the total, with reserves accounting for 72.2%. It can be seen that coal type gas plays an important role in the composition of combustible natural gas resources in the world.

Coal forming process and formation of coal formed gas: coal forming process can be divided into two stages: mud carbonization and coalification. In the previous stage, plant remains and fragments accumulated in swamp, lake or shallow sea environment formed peat, the predecessor of coal, through biochemical action; With the subsidence of the basin, the burial depth and temperature pressure increase, the peat enters the coalification stage from the peat carbonization stage. During the coalification process, the peat becomes lignite through microbial enzymolysis, compaction, dehydration and other actions; When the burial is gradually deepened, the formed lignite is transformed in the sequence of long flame coal → gas coal → fat coal → coking coal → lean coal → lean coal → anthracite under the action of temperature, pressure and time.

The actual measurement shows that the volatile matter of coal decreases significantly with the increase of coalification. From lignite to bituminous coal to anthracite, the volatile matter decreases from 50% to 5%. These volatiles are mainly composed of CH four 、CO two 、H two O、N two 、NH three The escape of gaseous products is the basis for the formation of coal formed gas and the main volatile products released during coalification.

1. Total volume port of volatile products during coalification;

2．CO two

3．H two O

4．CH four

5．NH three

6．H two S

From the perspective of forming coal formed gas, attention should be paid to the four relatively obvious changes of coal forming substances during coalification (called coalification jump in coal petrology):

The first jump occurred at the beginning of long flame coal, with carbon content Cr=75-80%, volatile matter Vr=43%, Ro=0.6%;

The second jump occurred in the fat coal stage, Cr=87%，Vr=29%，Ro=1.3%；

The third jump occurs in the phase of bituminous coal → anthracite, Cr=91%，Vr=8%，Ro=2.5%；

The fourth jump occurred in the stage of anthracite → metamorphic anthracite, Cr=93.5%，Vr=4%，Ro=3.7%， The condensation degree of aromatic fused rings is greatly improved.

Among the four jumps, the first and second jumps are the most obvious ones that lead to the change of coal quality. Coalification jump It is not only the qualitative change of coal, but also a gas (methane) peak for each jump.

The formation and yield of coal formed gas are not only related to coal rank, but also related to the coal petrographic composition of coal. Humic coal can be divided into three macerals: vitrinite, lipidoid and inertinite. Among humic coal in most coal fields in China, vitrinite has the highest content of each component, accounting for 50%~80%, inertinite accounts for 10%~20% (the highest is 30%~50%), and lipidoid has the lowest content, Generally not more than 5%.

In coal forming process, the contribution of each maceral to gas forming is different. On the basis of successfully separating and purifying the organic macerals of coal, Changqing Oilfield and Institute of Geochemistry, Chinese Academy of Sciences (1984) carried out a simulation experiment on the thermal evolution of organic macerals of low rank coal, and discussed the hydrocarbon generation mechanism of different macerals. It is found that the final hydrocarbon generation efficiency ratio of the three macerals is about lipid group: vitrinite: inert group=3:1:0.71, and the gas production capacity ratio is about 3.3:1:0.8, indicating that the inert group also has a certain gas generation capacity.

Inorganic origin

All elements on the earth have gone through a process similar to the nuclear fusion on the sun without exception. During a certain period after the carbon element is formed by nuclear fusion of some lighter elements, it reacts with the hydrogen element in the original atmosphere to generate methane.

The combustible gases distributed in the deep magmatic activity, metamorphic rocks and space of the universe, as well as the gases generated from the decomposition of inorganic salts in rocks, are all inorganic or abiogenic gases. It is dry gas, mainly methane, sometimes containing CO two 、N two , He and H two S. Hg steam, or even one of them, forms non hydrocarbon gas reservoirs with industrial significance.

Noble gases He, Ar, etc ， Because of their special geochemical behavior, scientists often use them as tracers of geochemical processes. Isotopic ratio of He and Ar three He/ four He、 forty Ar/ thirty-six Ar is an extremely important means to find out the origin of natural gas. As the two are increasing along the atmosphere → crust source → crust and mantle source mixing → mantle source, the former is 1.39 × 10 -6 →>10 -5 The latter is 295.6 →>2000. In addition, according to the radioactive origin of Ar isotopes in wall rocks and gas reservoirs, the formation age of gas can also be calculated.

methane

Inorganic synthesis: CO two +H two →CH four +H two O condition: high temperature (250 ℃), iron group elements

Methane in the earth's original atmosphere: absorbed in the mantle, discharged along deep faults, volcanic activities, etc

Methane in plate subduction zone: H, C, CO/CO produced by dehydration and decomposition of oceanic plate subduction under high temperature and pressure two →CH four

carbon dioxide

High CO content in natural gas two As high hydrocarbon bearing gas, it is also of important economic significance for CO two For gas reservoir, the one with economic value is CO two Natural gas with content>80% (volume concentration) can be widely used in industry, agriculture, meteorology, medical treatment, catering industry, environmental protection and other fields. CO in natural gas from Well Shuishen 9, Shatouwei, Sanshui Basin, Guangdong Province, China two The content is up to 99.55%, and the daily gas production is 5 million cubic meters, making it a gas reservoir with high economic value.

CO discovered in the world two Gas reservoirs are mainly distributed in Mesozoic Cenozoic volcanic area, fault activity area, oil and gas enrichment area and coal field area. From the perspective of causes, there are the following:

Inorganic origin:

① CO rich magma in the upper mantle two When the magma rises along the weak zone of the crust and the pressure decreases, CO two Escape.

② Carbonate rocks can form a large amount of CO by high-temperature baking or deep metamorphism two When groundwater is involved or contains Al, Mg and Fe impurities, considerable CO can also be generated at 98~200 ℃ two , this origin CO two Feature: CO two Content>35%, δ 13CCO2>- 8 ‰.

③ The interaction of carbonate minerals with other minerals can also generate CO two , such as dolomite and kaolinite.

In addition, organic origin includes biochemical action, thermochemical action and oxidation of oxidized coal in oil field

nitrogen

N two It is the main component in the atmosphere. According to research, the maximum concentration and fugacity of molecular nitrogen appear in the nitrogen-containing strata at the edge of the ancient platform, especially within the boundary of the distribution area of evaporite beds. Nitrogen is transferred from water layer to gas reservoir and reduced from nitrate. Its precursor is NH four + 。

N two If the content is more than 15%, it is a nitrogen rich reservoir two The main types of genesis are:

① N produced by decomposition of organic matter two : peak at 100-130 ℃, generated N two The amount accounts for 2.0% of the total gas amount, and the content is low; (Organic)

② Thermal decomposition and degassing of crustal rocks: such as the gas yield of diabase thermal analysis, N two Up to 52%, such N two Enrichment;

③ Denitrification of underground brine (nitrate): nitrate generates N through biochemical action two O+N two ；

④ N of mantle source two : For example, iron meteorite contains tens to hundreds of ppm nitrogen;

⑤ N of atmospheric source two : N in atmosphere two The hot spring gas is mainly mixed with the deep water circulation.

Seen from the isotopic characteristics, generally the heaviest nitrogen is concentrated in nitrate rock, the heavier nitrogen is concentrated in aromatic compounds, and the lighter nitrogen is concentrated in ammonium salt and amino acids.

hydrogen sulfide

Almost all discovered gas reservoirs in the world have H two S gas, H two Gas reservoirs with S content>1% are rich in H two S gas reservoir shall be more than 5% if it is of commercial significance.

According to research (Zhabrew et al., 1988), H two The S enrichment area is mainly a large oil and gas bearing sedimentary basin, and the sedimentary section of these basins contains thick carbonate evaporite rock series.

H in nature two There are two main types of S generation:

① Biogenesis (organic): including biodegradation and biochemical action;

② Thermochemical origin (inorganic): thermal degradation, thermochemical reduction, high-temperature synthesis, etc. According to thermodynamic calculation, gypsum (CaSO) in natural environment four ）Reduced to H by hydrocarbons two The demand temperature of S is up to 150 ℃, so the high content of H found in nature two S gas reservoir occurs in deep carbonate evaporite series, and the carbonate reservoir is good.

Ranking of reserves in various countries

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In 2005, the world's total proven natural gas reserves were 179.53 trillion cubic meters.

Major natural gas producing countries in the world at the beginning of the 21st century (unit: 100 million cubic meters)
country	reserves	Annual output
Russia	481 thousand and 400	six thousand three hundred and twenty-eight
Iran	275 thousand	About 896.63
Qatar	257 thousand and 800	three hundred and seventy-eight
Saudi Arabia	67 thousand and 500	six hundred and fifty-six point eight
The United Arab Emirates	60 thousand and 600	four hundred and fifty-eight
U.S.A	56 thousand	five thousand five hundred and thirty-two point seven seven
Algeria	45 thousand and 200	eight hundred and twenty
Norway	44 thousand and 600	eight hundred and eighty
Venezuela	41 thousand and 900	two hundred and eighty-four
Nigeria	34 thousand	two hundred and twenty-three point eight eight

toxicity hazard

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When the content of natural gas in the air reaches a certain level, people will suffocate. Natural gas is not as toxic as carbon monoxide. It is harmless to human body in essence. However, if the natural gas is in a high concentration and the oxygen in the air is not enough to sustain life, it will still cause death. After all, natural gas cannot be used for human respiration. As a fuel, natural gas will also cause casualties due to explosion.

Although natural gas is lighter than air and easy to disperse, when it gathers in a closed environment such as a house or tent and reaches a certain proportion, it will trigger a powerful explosion. The explosion may flatten the whole house, or even affect adjacent buildings. The lower explosion limit of methane in air is 5% and the upper explosion limit is 15%.

The explosion of compressed natural gas to be used in the engine of natural gas vehicle is basically unavailable under spontaneous conditions due to the volatile nature of the gas, so external force is required to maintain the concentration of natural gas between 5% and 15% to trigger the explosion.

the way to distinguish

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Natural gas is widely distributed in nature, with various genetic types, different thermal evolution degrees, and various geochemical characteristics. Therefore, it is difficult to identify natural gas with unified indicators. The practice shows that the comprehensive discrimination with multiple indicators is more reliable than that with a single indicator. From the perspective of the items involved in the identification of the origin of natural gas, there are mainly four items: isotope, gas component, light hydrocarbon and biomarker compound. Some of the items have absolute significance because of their clear identification criteria, while some of the items overlap on the three types of genetic gas and have only certain relative significance.

Derivative products

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Natural gas is an important energy, widely used as urban gas and industrial fuel; However, the commonly called natural gas only refers to a combustible gas rich in hydrocarbons stored in the deeper stratum, while the natural gas associated with oil is often called oilfield associated gas.

Natural gas fuel is the first widely used alternative fuel, which is divided into compressed natural gas (CNG) and liquified natural gas (LNG). Industrial natural gas can be burned with external mixing burner.

Liquefied gas

liquified natural gas

When natural gas is cooled to about - 162 ℃ under normal pressure, it will change from gaseous state to liquid state, which is called liquefied natural gas (LNG). The main component of LNG is methane, with a small amount of ethane, propane and nitrogen. Natural gas is further purified during liquefaction. Methane is more pure, almost free of carbon dioxide and sulfide, and colorless, tasteless and non-toxic.

Liquefied natural gas (LNG) has become a new industry in China and is developing rapidly. In addition to solving transportation and storage problems, liquefied natural gas (LNG) technology is also widely used in peak shaving devices when natural gas is used. Since the origin of natural gas is often not in industrial or densely populated areas, transportation and storage problems must be solved. The main component of natural gas is methane, and its critical temperature is 190.58K, so it cannot be liquefied by pressure alone at normal temperature. The liquefaction and storage technology of natural gas has gradually become a major advanced technology.

Advantages of LNG

Compared with natural gas, LNG has the following advantages:

① Easy to store and transport

The density of LNG is 625 times that of methane under standard conditions. That is to say, 1m ³ of LNG can be gasified into 625m ³ of natural gas, which shows the convenience of storage and transportation.

② Good safety

The main mode of natural gas storage and transportation is compression (CNG). Due to the high pressure of compressed natural gas, there are many potential safety hazards.

③ Low indirect investment

The volume energy density of compressed natural gas (CNG) is about 26% of that of gasoline, while the volume energy density of liquefied natural gas (LNG) is about 72% of that of gasoline, more than twice that of compressed natural gas (CNG). Therefore, the vehicles using LNG have a long journey, which can greatly reduce the number of vehicle filling stations.

④ Peaking effect

As the fuel of civil gas or power plant, natural gas will inevitably have fluctuations in demand, which requires peak shaving in supply.

⑤ Environmental protection

Natural gas must be subject to strict pre purification before liquefaction, so the impurity content in LNG is far lower than CNG, creating conditions for automobile exhaust or emissions when used as fuel to meet more stringent standards (such as "Euro II" or even "Euro III").

Liquefied petroleum gas

Liquefied petroleum gas is one of petroleum products. Its English name is liquefied petroleum gas (LPG for short). It is a colorless and volatile gas obtained by pressurization, cooling and liquefaction of refinery gas or natural gas (including oilfield associated gas).

Liquefied petroleum gas (LPG for short) is a kind of petroleum tail gas left in the process of refining gasoline, kerosene, diesel oil, heavy oil and other oil products from petroleum. Through certain procedures, the petroleum tail gas is recycled, pressurized measures are taken to make it become liquid, and loaded in pressurized containers. The name of LPG comes from this. Its main components are ethylene, ethane, propylene, propane and butane, and it also contains a small amount of pentane, pentene and trace sulfur compound impurities. The composition of liquefied gas obtained from natural gas is basically free of olefins. It is liquid in the gas cylinder. Once it flows out, it will vaporize into a combustible gas about 250 times the original volume, and it is very easy to spread. It will burn or explode when encountering an open fire. Therefore, special attention should be paid to the use of liquefied gas.

Liquefied coalbed methane

China is a major coal producer in the world, and the corresponding reserves of coal-bed methane are also large. The reserves are basically the same as that of natural gas. Its basic component is methane. In addition to being a cheap chemical raw material, it is mainly used as a fuel. It is not only used as a living fuel for residents, but also used as a fuel for vehicles, ships, planes and other means of transportation. Because of its high calorific value and less environmental pollution caused by combustion products, CBM is considered as a high-quality clean fuel.

There are several main benefits to use coal-bed methane after liquefaction:

① Economy

Low investment cost and quick recovery.

② Security

"Gas mining before coal mining" has become the basic way of energy utilization in developed countries. "Gas mining before coal mining" greatly improves the safety of coal mining.

③ Policy

This method can save energy and make full use of energy, which is in line with relevant national policies. It is conducive to obtaining government support.

The coal-bed gas liquefaction equipment is basically the same as the natural gas liquefaction equipment, but because the content of oxygen and nitrogen in most coal-bed gas is slightly higher than that in natural gas, a rectification system needs to be added.

Compressed gas

Compressed Natural Gas (CNG) is natural gas pressurized and stored in a container in gaseous state. In addition to using natural gas from oil fields and natural gas fields, compressed natural gas can also be used to produce biogas (mainly methane) manually.

The composition of compressed natural gas is the same as that of pipeline natural gas, and the main component is methane (CH four ）。 CNG can be used as vehicle fuel. CNG can be used to produce LNG (Liquefied Natural Gas), which is called NGV (Natural Gas Vehicle). Liquefied Petroleum Gas (LPG for short) is often confused with CNG. In fact, there are obvious differences between them.

Fire hazard of CNG compressed natural gas

1. Combustion explosiveness - combustible gas can burn or explode when encountering ignition source when it is within the explosive concentration range.

2. Diffusivity - the gas diffusivity is affected by the density of the gas itself. The lighter the density is than air, the greater the diffusivity.

3. Expansibility - The compressed gas expands due to heat, which increases the pressure of the cylinder and may cause the cylinder to break or explode.

The combustion gas sources in people's lives can be roughly divided into three categories: liquefied petroleum gas (Y), artificial gas (R) and natural gas (T).

Artificial gas

Coal gas is made from solid raw materials such as coal or coke through dry distillation or gasification. Its main components are carbon monoxide, methane and hydrogen. Therefore, gas is toxic and easy to form explosive mixture with air, so high attention should be paid when using it.

purpose

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Industrial fuel

Natural gas is used instead of coal for plant heating, production boilers and gas turbine boilers in thermal power plants. Natural gas power generation is an effective way to alleviate energy shortage, reduce the proportion of coal-fired power generation, and reduce environmental pollution. In terms of economic benefits, natural gas power generation requires less investment per unit installed capacity, short construction period, and low on grid price, so it has strong competitiveness.

For natural gas power generation, natural gas generator sets are installed after natural gas treatment to provide power.

Process production

Such as paint baking production line, tobacco drying, asphalt heating and insulation.

Natural gas chemical industry

Natural gas is the best raw material for manufacturing nitrogen fertilizer, which has the characteristics of less investment, low cost and less pollution. The proportion of natural gas in raw materials for nitrogen fertilizer production is about 80% in the world on average.

Urban gas industry

In particular, domestic fuels include conventional natural gas and unconventional natural gas, such as coalbed methane and shale gas. It is mainly incorporated into the pipeline after production for daily use of natural gas. With the improvement of people's living standards and the enhancement of environmental awareness, the demand for natural gas in most cities has increased significantly. The economic benefit of natural gas as a civil fuel is also greater than that of industrial fuel.

CNG vehicle

Using natural gas instead of automobile oil has the advantages of low price, less pollution and safety. According to the statistics of the International Natural Gas Vehicle Organization, the average annual growth rate of natural gas vehicles is 20.8%. There are about 12.7 million vehicles using natural gas in the world, and the total number will reach 70 million in 2020, most of which are compressed natural gas vehicles.

Natural gas is a high-quality and efficient clean energy. The emissions of carbon dioxide and nitrogen oxides are only about half and one-fifth of that of coal, and the emissions of sulfur dioxide are almost zero. As a clean and efficient fossil energy, the development and utilization of natural gas has attracted more and more attention all over the world. Globally, the amount of natural gas resources is far greater than oil, and the development of natural gas has sufficient resource guarantee.

Efficiency enhancing natural gas

It is a new type of energy saving and environmental friendly industrial gas based on natural gas, which is formed after the gas agent intelligent mixing equipment and natural gas synergist are mixed. The combustion temperature can be raised to 3300 ℃. It can be used for industrial cutting, welding, and breaking. It can completely replace acetylene gas and propane gas. It can be widely used in steel mills, steel structures, and shipbuilding industries, and can be safely used in cabins, The products on the market are Sharp Gas and Sharp Natural Gas Synergist.

People's awareness of environmental protection has increased, and the world's demand for clean energy has soared. Governments of all countries have also communicated this trend through legislative procedures. Natural gas was once regarded as one of the cleanest energy sources. In addition, the Persian Gulf crisis in the Middle East in 1990 deepened the determination of the United States and major oil consuming countries to research and develop alternative energy. Therefore, before the real alternative energy was found, The demand for natural gas will naturally increase.

Distribution area

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China has a wide distribution area of sedimentary rocks and many continental basins, forming superior geological conditions for various natural gas reserves. According to the forecast of national natural gas resources in 1993, China's total natural gas resources reached 38 trillion m three The onshore natural gas is mainly distributed in the central and western regions, accounting for 43.2% and 39.0% of the onshore resources respectively.

Stratigraphic distribution of natural gas resources in China Cenozoic The Tertiary and Paleozoic strata are dominant, and the Cenozoic accounts for 37.3% of the total resources, Mesozoic 11.1%， Upper Paleozoic 25.5%， Lower Paleozoic 26.1%。 The genetic type of natural gas resources is that highly mature cracked gas and coal-bed gas occupy the dominant position, accounting for 28.3% and 20.6% of the total resources respectively, the associated gas of oil fields accounts for 18.8%, coal-bed adsorbed gas accounts for 27.6%, and biological gas accounts for 4.7%.

China's proven natural gas reserves are concentrated in 10 large basins, which are Bohai Bay, Sichuan, Songliao, Junggar, Yinggehai Qiongdongnan, Qaidam, Tuha, Tarim, Bohai and Ordos in turn. China's gas fields are mainly small and medium-sized. Most of the gas fields have complex geological structures and are difficult to explore and develop. During 1991-1995, China's natural gas production increased from 16.073 billion meters three Increase to 17.947 billion m three The average annual growth rate is 2.33%.

China's natural gas resources are mainly distributed in the central and western basins of China. At the same time, China also has unconventional CBM prospective resources mainly enriched in North China. Under China's 9.6 million square kilometers of land and more than 3 million square kilometers of sea area under its jurisdiction, there are very rich natural gas resources. Experts predict that the total amount of resources can reach more than 40-60 trillion cubic meters, making it a large country with natural gas resources.

China is rich in coal resources. According to statistics, it has 600 billion tons, ranking the third in the world. Coal accumulating basins are developed. Coal type gas has been found in North China, Ordos, Sichuan, Taiwan East China Sea, Yinggehai Qiongdongnan, Tuha and other basins. According to the research, most of the gas in the central atmospheric area of the Ordos Basin comes from the C-P coal measure strata of the Upper Paleozoic (Upper Paleozoic: Lower Paleozoic gas source=7 ∶ 3 or 6 ∶ 4), which shows that the coal measure strata have great potential to generate natural gas.

According to the study of gas fields in Sichuan Basin, China (Baozi, 1988), the Paleozoic gas fields in the basin were formed during the high temperature methane generation period. From Triassic to Sinian, the drying coefficient increased from small to large (T: 35.5 → P: 73.1 → Z: 387.1), and the heavy hydrocarbons increased to less. In South Sichuan gas field, natural gas and thermal asphalt coexist, which indicates that natural gas is formed by thermal metamorphism of oil.

East is the East China Sea Basin. There has been the dawn of natural gas;

South is Yinggehai Qiongdongnan and Yungui. There has also shown the majesty of the atmospheric area;

The west is the Tarim Basin, Tuha Basin, Junggar Basin in Xinjiang and Qaidam Basin in Qinghai. At the western end of the ancient Silk Road, the drums of the oil and gas battle are beating louder and louder. They will not only become an important area for China's oil strategy to take over, but also the fire of natural gas has been raging, and the trend of starting a prairie fire is unstoppable;

North is the vast area of Northeast and North China. There are many large and old oil fields there. Driven by the future high-tech, they will not only maintain stable oil and gas production, but also possibly climb new peaks;

Middle, Ordos Basin and Sichuan Basin. The natural gas exploration field in the Ordos Basin is expanding and the proven reserves are increasing year by year, and the development project is being carried out. Sichuan Basin is the main area of natural gas production in China. With new discoveries and breakthroughs, the development of natural gas will enter a new stage and reach a new level.

Mining method

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Like crude oil, natural gas is also buried in underground closed geological structures, some of which are stored in the same layer as crude oil, and some exist alone. Natural gas stored in the same layer with crude oil will be extracted together with crude oil.

For the single phase gas reservoir, we call it gas reservoir. Its exploitation method is very similar to that of crude oil, but also has its special features. Due to the low density of natural gas, it is 0.75~0.8kg/m three The pressure of wellbore gas column on the bottom hole is small; The viscosity of natural gas is small, and the flow resistance in the formation and pipeline is also small; And because of the large expansion coefficient, its elastic energy is also large.

Therefore, the natural gas is generally produced by self injection. This is basically the same as the automatic injection oil recovery method. However, because the pressure of gas wells is generally high and natural gas is flammable and explosive, the pressure bearing capacity and sealing performance of gas production wellhead devices are much higher than those of oil production wellhead devices.

Natural gas exploitation also has its own characteristics. First of all, natural gas, like crude oil, and bottom or edge water are often a storage system. With the process of natural gas exploitation, the elastic energy of water will drive water to enter gas reservoirs along high permeability zones. In this case, due to the hydrophilicity of the rock itself and the role of capillary pressure, the invasion of water does not effectively displace the gas, but seals the gas not discharged from the seams, holes or pores, forming a dead gas zone.

The amount of high-pressure gas trapped in the water invasion zone can be as high as 30%~50% of the rock pore volume, thus greatly reducing the ultimate recovery factor of the gas reservoir. Secondly, after the gas well produces water, the flow resistance of the gas flow into the bottom of the well will increase, and the total energy consumption of the upward pipe flow of the gas and liquid phases along the oil well will increase significantly. With the increasing impact of water invasion, the gas production rate of the gas reservoir decreases, the flowing capacity of the gas well weakens, and the production of single well decreases rapidly until the production stops due to serious water accumulation at the bottom of the well. The treatment of gas reservoir flood mainly starts from two aspects: drainage and water plugging. Water plugging is to separate the gas producing layer from the water producing layer by mechanical plugging, chemical plugging and other methods, or to establish a water barrier in the reservoir.

Compressed natural gas pressure reducing station

There are many methods, the main principle is to remove the water in the shaft, and the technical term is drainage gas production method. The small tubing drainage gas production method is based on the principle that under a certain gas production rate, the smaller the tubing diameter, the greater the gas flow velocity and the stronger the liquid carrying capacity. If the tubing diameter is selected reasonably, there will be no bottom hole ponding. This method is suitable for gas wells with high formation pressure and low water production at the initial stage of water production.

Foam drainage gas production method is to add foaming agent into the well through tubing or casing, and the foaming agent dissolves into the water at the bottom of the well to form bubbles, which can not only reduce the relative density of liquid accumulation, but also bring the water produced in the formation out of the ground with the gas flow. This method is suitable for gas wells with high formation pressure and relatively low water production.

Plunger gas lift drainage gas production method is to put a plunger into the tubing. When the plunger is lowered, the flow channel in the plunger is open, and the plunger moves downward under its own weight. When reaching the bottom of the oil pipe, the flow passage in the plunger automatically closes. Since the pressure acting on the bottom of the plunger is greater than the pressure acting on its top, the plunger starts to move upward and drain the water above the plunger to the ground. When it reaches the top of the oil pipe, the flow passage in the plunger is automatically opened, and then turns to downward movement. Through the reciprocating movement of the plunger, the accumulated liquid can be discharged continuously. This method is applicable to gas wells with sufficient formation pressure and large water production.

The deep well pump drainage gas production method is to use the deep well pump, sucker rod and surface pumping unit in the well to control the bottom hole pressure by pumping through tubing and gas production through casing. This method is suitable for gas wells with low formation pressure, especially for water producing gas wells in the middle and later stages of production, but the operation cost is relatively high.

exploration and development

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On September 10, 2018, the State Council issued Several Opinions on Promoting the Coordinated and Stable Development of Natural Gas (hereinafter referred to as the Opinions). It is proposed to increase domestic exploration and development efforts. Strive to achieve a domestic natural gas output of more than 200 billion cubic meters by the end of 2020.

Data shows that in 2017, China's natural gas production increased by more than 10 billion cubic meters to 148.03 billion cubic meters, up 8.2% year on year. From January to July, China's apparent consumption of natural gas totaled 157.56 billion cubic meters, up 16.47% year on year; China's natural gas output was 90.49 billion cubic meters, up 5.5% year on year, 3.3 percentage points lower than the growth rate from January to June 2017.

The Opinions proposed that the construction of production, supply, storage and marketing system should be strengthened to promote the dynamic balance between supply and demand of natural gas. First, increase domestic exploration and development efforts. Strictly implement the exit mechanism of oil and gas exploration blocks, fully implement the competitive transfer of blocks, encourage the market-oriented transfer of mining rights, accelerate the use of proved and unused reserves, and strengthen the assessment of energy security of state-owned oil and gas enterprises. Second, improve the diversified overseas supply system of natural gas. Accelerate the diversification of import countries (regions), transportation modes, import channels, contract modes and participants. Third, build a multi-level reserve system. Promote the construction of gas storage capacity of local governments and urban gas enterprises as a whole, realize the intensive and large-scale operation of gas storage facilities, avoid "blooming everywhere", and encourage various investment entities to jointly build gas storage facilities. Fourth, strengthen natural gas infrastructure construction and interconnection. We will promptly introduce a reform plan for the oil and gas pipeline network system, and promote the fair opening of infrastructure to third-party market players; Deepen the reform of "decentralization, management and service", and shorten the cycle of project construction procedures and approval.

The Opinions stressed the need to deepen the reform in the natural gas field and establish and improve the system and mechanism for coordinated and stable development. First, establish a natural gas supply and demand forecasting and early warning mechanism. The second is to establish a comprehensive coordination mechanism for natural gas development. "Coal to gas" adheres to the principle of "gas based reform" and gradually highlights the guarantee of gas demand in Beijing Tianjin Hebei and surrounding areas and key areas such as the Fenwei Plain. Third, establish and improve the demand side management and peak shaving mechanism of natural gas. Fourth, establish and improve the emergency system of natural gas supply guarantee. Implement the main responsibility of the local people's governments at all levels for ensuring the supply of gas for people's livelihood, and do a good job in the hierarchical supply guarantee plan and user peak shaving plan. Fifth, rationalize the natural gas price mechanism. Ensure the smooth implementation of the price reform plan for residential valve stations, accelerate the establishment of an upstream and downstream natural gas price linkage mechanism, and implement differentiated price policies such as seasonal price differences and interruptible gas prices. Sixth, strengthen the safe operation mechanism of the whole natural gas industry chain.^[4]

technical parameter

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The upper limit of sulfur standard for civil natural gas is: Class I gas is less than or equal to 100mg/m3, Class II gas is less than or equal to 200mg/m3, and Class III gas is less than or equal to 460mg/m3. Therefore, the maximum amount of sulfur dioxide released after the combustion of 1 cubic meter of natural gas is (460mg/32) * 64=920mg.

national standard	Standard No
Measurement of natural gas flow with standard orifice flowmeter	GB/T 21446-2008
Industrial automation systems and integration - Life cycle data integration of process plants (including oil and gas production facilities) - Part 2: Data model	GB/T 18975.2-2008
Natural gas Determination of sulfur compounds Part 2: Determination of hydrogen sulfide content by methylene blue method	GB/T 11060.2-2008
Conversion between water content and water dew point of natural gas	GB/T 16781.1-2008
Standard reference conditions of natural gas	GB/T 19205-2008
Natural gas Determination of mercury content Part 1: Iodine chemisorption sampling method	GB/T 22634-2008
Design of LNG equipment and installation onshore device	GB/T 22724-2008
Determination of natural gas energy	GB/T 22723-2008
Technical requirements for retrofitting of gas vehicles Part 1: Compressed natural gas vehicles	GB/T 18437.1-2009
Specification for type approval test of natural gas vehicles	GB/T 23335-2009
Periodic inspection and evaluation of CNG metal liner filament wound gas cylinders for automobiles	GB 24162-2009
Compressed natural gas cylinder valve for vehicles	GB 17926-2009
Periodic inspection and evaluation of station compressed natural gas cylinders	GB 24163-2009
Circumferentially wound compressed natural gas cylinder with steel liner for vehicles	GB 24160-2009
Frozen Light Hydrocarbon Fluid LNG Shipboard Trade Handover Procedure	GB/T 24964-2010
LNG equipment and installation ship shore interface	GB/T 24963-2010
Natural gas Determination of sulfur compounds Part 4: Determination of total sulfur content by oxidation microcoulometry	GB/T 11060.4-2010
Natural gas Determination of sulfur compounds Part 5: Determination of total sulfur content by hydrogenolysis ratemeter colorimetry	GB/T 11060.5-2010
Natural gas Determination of sulfur compounds Part 1: Determination of hydrogen sulfide content by iodometry	GB/T 11060.1-2010
Natural gas Determination of sulfur compounds Part 3: Determination of hydrogen sulfide content by lead acetate reaction rate dual optical path detection method	GB/T 11060.3-2010
Natural gas Determination of mercury content Part 2: Gold platinum alloy mercury homogenization sampling method	GB/T 16781.2-2010
Gas turbine procurement Part 5: Application in petroleum and natural gas industry	GB/T 14099.5-2010
Reciprocating piston natural gas compressor for automobile filling station	GB/T 25360-2010
LNG filling device for automobiles	GB/T 25986-2010
Crash safety requirements for fuel system of compressed natural gas vehicles	GB/T 26780-2011
Design and construction of on-site assembled vertical cylindrical flat bottom steel LNG storage tanks Part 1: General	GB/T 26978.1-2011
Design and construction of on-site assembled vertical cylindrical flat bottom steel LNG tanks Part 2: Metal components	GB/T 26978.2-2011

Production

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The production of natural gas has kept growing while the import has declined. From January to February 2022, China will produce 37.2 billion cubic meters of natural gas, a year-on-year increase of 6.7%, 4.4 percentage points faster than that in December of last year, with an average daily output of 630 million cubic meters. The import of natural gas was 19.6 million tons, down 3.8% year on year, and up 4.6% in December of last year.^[5]

In 2023, the natural gas output will be 229.71 billion cubic meters, up 5.8% year on year, with an annual increase of 10 billion cubic meters for seven consecutive years On.

In 2024, the natural gas output will be 232.43 billion cubic meters, with a year-on-year growth of 5.6% and consumption growth of 7.2%.^[9]

Engineering technical problems

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On June 27, 2022, at the closing ceremony of the 24th annual meeting of the China Association for Science and Technology, the China Association for Science and Technology solemnly released 10 engineering technical problems that play a key role in engineering technology innovation, including "How to extract helium from low-grade helium containing natural gas?" 。^[6]

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