CN110972485B

CN110972485B - Method for increasing production of coal bed gas by injecting high-temperature air

Info

Publication number: CN110972485B
Application number: CN201780088693.6A
Authority: CN
Inventors: 陈信平
Original assignee: Beijing Jiemaodi China Energy Technology Co ltd
Current assignee: Inner Mongolia Zhongkuang Clean Coal Technology Co ltd
Priority date: 2017-03-23
Filing date: 2017-03-23
Publication date: 2022-08-12
Anticipated expiration: 2037-03-23
Also published as: WO2018170830A1; CN110972485A

Abstract

A method for injecting high-temperature air into a coal seam to increase the yield of coal seam gas. The method comprises the steps of injecting high-temperature air into a coal bed from one or more gas injection wells, stripping and expelling adsorbed coal bed gas on the surfaces of coal micropores and fractures by nitrogen, promoting the desorption of the coal bed gas and transporting the coal bed gas to a production well, enabling oxygen in the coal bed gas to be retained in the coal bed or consumed due to physical adsorption, chemical adsorption and oxidation reaction under the action of low-temperature oxidation, collecting the coal bed gas or mixed gas of the coal bed gas and the nitrogen from the multiple production wells, and improving the yield of the coal bed gas well and the recovery ratio of the coal bed gas. The same or better yield increasing effect can be obtained by injecting high-temperature air.

Description

Method for increasing production of coal bed gas by injecting high-temperature air

Technical Field

The invention relates to a method for increasing coal bed methane by injecting high-temperature air into a coal bed methane reservoir. In particular, the invention relates to a method for injecting high-temperature air into a coal seam from one or more wells (hereinafter referred to as "gas injection wells") communicated with the coal seam, wherein nitrogen in the coal seam "strips" and "drives off adsorbed coal seam gas on the surfaces of coal micropores and cracks, desorption of the coal seam gas is promoted, the coal seam gas is transported to a production well, oxygen in the coal seam gas is retained in the coal seam or consumed by low-temperature oxidation due to physical adsorption, chemical adsorption and oxidation reaction, and produced coal seam gas and mixed gas of the coal seam gas and the nitrogen are collected from the one or more wells (hereinafter referred to as" production wells ") communicated with the coal seam, so that the yield of the coal seam gas well and the recovery rate of the coal seam gas are improved.

Background

Coal bed gas is natural gas in coal beds, also called gas, and the main component of the coal bed gas is methane. The coal bed gas is not only a clean energy source of natural gas, but also a cause of coal mine gas outburst and explosion.

World coal bed gas reserves are huge, and production is very low because the drilling-fracturing-draining-depressurizing-gas production technical mode from the United states used in the current coal bed gas development can only develop the coal bed gas reservoir with the best quality, and is not suitable for most coal bed gas reservoirs. This consistent failure of current mainstream coal bed gas development technology has resulted in coal bed gas blocks that recapture investment and profit or have this prospect being a low probability lucky event, both world and china. The world coal bed gas industry has not declined, with only a few exceptions to success. Only by developing a new coal bed gas development technology suitable for the country according to the respective national conditions, all countries in the world can develop coal bed gas resources of the country with economic benefits.

In 1998, Puri and Stein proposed a method for increasing the production of coal bed gas by injecting inert gas (e.g., nitrogen) or gas that does not chemically react with coal (e.g., CO) from a gas injection well into the coal bed ₂ ) Obtaining coal bed gas from one or more production wells can increase the production of coal bed gas (see Rajen Puri and Michael h. steel. 1988. Method of coalbed methane production. U.S. patent No. 4883122). The Puri and Stein patent methods are only suitable for high-quality coal bed gas reservoirs with high permeability, are inapplicable to most coal bed gas reservoirs and cannot be implemented. In 2015, chenyiping proposes a pressure control method for increasing and producing coal bed methane by nitrogen injection, which is suitable for increasing and producing coal bed methane by nitrogen injection and decomposition of any coal bed (see chenyiping, in 2015, a pressure control method for increasing and producing coal bed methane by nitrogen injection, Chinese patent number ZL 201510467978.3). The Chen Xinping patent method takes parameters such as wellhead pressure, gas production rate, and volume percentage content change of nitrogen of a production well as the reaction of a coal bed gas reservoir to nitrogen injection pressure and nitrogen injection rate of a nitrogen injection well, feeds the parameters back to the nitrogen injection well, and adjusts the nitrogen injection pressure and the nitrogen injection rate, so that nitrogen can be injected into a low-permeability coal bed without fracturing surrounding rocks of a coal bed roof and a bottom plate.

CO injection ₂ The coal bed gas production increase is high in cost, and the gas coal bed is converted into a carbon dioxide coal bed. For coal mining, CO ₂ Is as harmful as methane, even CO ₂ The harm is greater because of CO ₂ And more difficult to pump. Australia has a large amount of high CO ₂ Coal bed due to CO ₂ Difficult to be controlled and cannot be exploited. Therefore, CO injection ₂ Coal bed gas enhancement is not widely usedCoal bed gas development technology.

Note N ₂ Increasing the coal seam gas does not substantially harm future coal mining operations, however, note N ₂ The high cost of the coal bed gas production is influenced by N injection ₂ The popularization and application of the coal bed gas increasing technology. Note N ₂ The cost of the coal bed gas increasing technology is about 0.50 yuan/m ³ Left and right. The current price of the coal bed gas wellhead is 1.20-1.50 yuan/m ³ In the meantime. Note N ₂ The cost accounts for 42-33% of the sale benefit of the coal bed gas, and the proportion is too heavy, so that the N injection is carried out ₂ Increasing production of coal bed gas is not economically efficient in most cases. Note N ₂ The cost of the coal bed gas production increasing technology is composed of two parts: (1) separating nitrogen from air by using a nitrogen making machine or a cryogenic method, mainly separating oxygen from nitrogen; and (2) injecting nitrogen into the coal seam using a high pressure booster. The two parts are substantially equal in cost. Thus, if the coal seam can be injected with air instead of nitrogen, N is obtained and injected ₂ The yield increase effect of coal bed gas is the same or better, and the cost is only equivalent to the N injection ₂ Half of the cost of increasing the coal bed gas, therefore, the method for increasing the coal bed gas by injecting air has great use value and great contribution to the development of the coal bed gas in the world.

Disclosure of Invention

The invention relates to a method for increasing coal bed methane by injecting high-temperature air into a coal bed methane reservoir. In particular, the invention relates to a method for injecting high-temperature air into a coal seam from one or more wells (hereinafter referred to as "gas injection wells") communicated with the coal seam, wherein nitrogen in the coal seam "strips" and "drives off adsorbed coal seam gas on the surfaces of coal micropores and cracks, desorption of the coal seam gas is promoted, the coal seam gas is transported to a production well, oxygen in the coal seam gas is retained in the coal seam or consumed by low-temperature oxidation due to physical adsorption, chemical adsorption and oxidation reaction, and produced coal seam gas and mixed gas of the coal seam gas and the nitrogen are collected from the one or more wells (hereinafter referred to as" production wells "), so that the yield of the coal seam gas well and the recovery rate of the coal seam gas are improved.

The invention is realized by adopting the following technical means:

the invention relates to a method for increasing coal bed methane by injecting high-temperature air into a coal bed methane reservoir. In particular, the invention relates to a method for injecting high-temperature air into a coal seam from one or more wells communicated with the coal seam, wherein nitrogen in the high-temperature air "strips" and "drives" coal-bed methane in an adsorption state on the micropore and fissure surfaces, the desorption of the coal-bed methane is promoted, the coal-bed methane is transported to a production well, oxygen in the coal-bed methane is retained in the coal seam or consumed due to physical adsorption, chemical adsorption and oxidation reaction due to low-temperature oxidation, and the produced coal-bed methane and mixed gas of the coal-bed methane and the nitrogen are collected from the one or more wells communicated with the coal seam, so that the yield of a coal-bed gas well and the recovery rate of the coal-bed methane are improved.

The specific implementation steps and characteristics of the method for increasing the yield of the coal bed methane by injecting high-temperature air into the coal bed are as follows:

(a) determining the initial temperature, the heating rate, the normal operation temperature and the accelerated oxygen consumption temperature of air injected into the coal bed according to the characteristics of the coal bed gas reservoir;

(b) starting gas injection operation according to the initial temperature and the heating rate at the gas injection starting stage until the gas injection temperature reaches the normal operation temperature, and entering the normal gas injection yield increasing operation stage;

(c) monitoring the bottom pressure and temperature of the gas injection well, and monitoring the gas composition, the bottom pressure and temperature, the top pressure and temperature of the production well;

(d) when the moisture content in the gas of the production well is obviously reduced, a water injection high-pressure switch in a gas injection well mouth or a gas injection pipeline is opened, and the water injection high-pressure switch of the gas injection well mouth is closed until the moisture content in the gas of the production well is recovered to be normal;

(e) when the oxygen content in the gas of the production well is abnormally increased or the oxygen content in the gas of the production well exceeds the standard specified by the coal bed gas production specification, raising the gas injection temperature to the accelerated oxygen consumption temperature, and reducing the gas injection temperature to the normal operation temperature until the oxygen content in the gas of the production well is recovered to be normal;

(f) determining the highest allowable gas temperature of the production well according to the characteristics of the coal bed gas reservoir and the performance of the coal bed gas post-processing equipment;

(g) when the gas temperature of the production well is increased to be close to the highest allowable temperature, stopping gas injection operation until the bottom hole pressure of the production well is reduced to be close to the hydrostatic pressure of a coal bed gas reservoir or until the gas yield of the production well is reduced to be close to an economic threshold value, starting gas injection operation according to the initial temperature and the temperature rise rate according to the operation program of the gas injection starting stage until the gas injection temperature reaches the normal operation temperature, and entering the normal gas injection yield-increasing operation stage again;

(h) when the gas temperature of the production well is increased to be close to the maximum allowable temperature and the temperature reduction effect of the step (g) is poor or the step (g) cannot be implemented due to the poor temperature reduction effect, injecting normal-temperature air or normal-temperature nitrogen again until the gas temperature of the production well is sufficiently reduced, starting gas injection operation according to the initial temperature and the temperature increase rate according to the operation program of the gas injection starting stage until the gas injection temperature reaches the normal operation temperature, and entering the normal gas injection yield-increasing operation stage again;

(i) steps (d), (e), (g) and (h) may be repeated as many times as necessary.

Compared with the prior art, the method has the following obvious beneficial effects and advantages:

firstly, the method injects air into the coal bed, nitrogen in the air is stripped and the adsorbed coal bed gas on the surface of the coal micropore, the desorption of the coal bed gas is promoted and the coal bed gas is transported to a production well, and the N injection is realized ₂ Increasing production of nitrogen in coal bed gas, increasing production effect and injecting N ₂ The yield increasing effect of increasing the yield of coal bed gas is similar.

Secondly, the method of the invention does not need to separate nitrogen and oxygen, the low-temperature oxidation of the coal replaces a nitrogen making machine, and the cost is only equivalent to the N injection ₂ The cost of producing the coal bed gas is increased by half, and the cost of intensively developing the coal bed gas is effectively reduced.

Thirdly, the high-temperature air injected into the coal seam by the method directly increases the temperature of the coal seam, and the physical adsorption, chemical adsorption and chemical reaction in the low-temperature oxidation of the coal are all exothermic, so that the temperature of the coal seam is also increased. According to the research, the adsorption quantity of the coal to the methane is reduced by about 0.18m on average from 30 ℃ to 40 ℃ and every 1 ℃ rise of the temperature ³ Perton (see Fuxuehai, Qin Yong, Wei Doau 2007. coal bed gas geology[M]Section four of chapter iv. Beijing: china university of mining publishers). Therefore, the method provided by the invention has the advantages that the temperature of the coal bed is increased, the desorption of the coal bed gas is promoted, the recovery ratio of the coal bed gas can be increased, and the residual gas content of the coal bed is reduced to a lower level.

The coal bed gas reserves in the world are huge, and the yield is low because the currently used coal bed gas development technology can only develop the coal bed gas reservoirs with the best quality and is not suitable for most coal bed gas reservoirs. The technology provided by the invention can develop most coal bed gas reservoirs, so that huge coal bed gas resources benefit mankind.

Drawings

FIG. 1 is a graph showing the trend of oxygen absorption for different coal types at different adsorption ambient temperatures.

FIG. 2 is a graph of oxygen consumption by low temperature oxidation of coal as a function of temperature.

FIG. 3 shows the oxygen consumption of coal by oxidation of coal at different temperatures for 1 hour.

FIG. 4 shows the results of oxygen consumption measurements for the same coal at the same temperature and different oxidation times.

FIG. 5 is a schematic diagram showing the temperature of the coal bed around the gas injection well and the low-temperature oxidation of coal 4 years after injecting high-temperature air.

Detailed Description

The present invention will be further described with reference to the following embodiments.

Theoretical main points of low-temperature oxidation of coal

In the scientific literature, "low-temperature coal oxidation" refers to the physical, physico-chemical, and chemical actions that occur between coal and oxygen in an oxidation environment below the ignition point of coal. The coal ignition point rises along with the rise of the coal rank from the ignition point of lignite (270-310 ℃) to the ignition point of coking coal (350-370 ℃). The low-temperature oxidation of coal refers to the physical, physical-chemical and chemical actions between coal and oxygen elements in an oxidation environment far below the ignition point of coal in a gas-injected coal bed, or in other words, the low-temperature oxidation of coal is limited to a stable oxidation reaction process in which the generated heat is equal to the dissipated heat, and does not include a stage in which the oxidation reaction is automatically accelerated to achieve uncontrollable spontaneous combustion.

The coal industry has comprehensively and fully researched the problem of low-temperature oxidation of coal for a long time, because the low-temperature oxidation stage is the initial stage of spontaneous combustion of coal and plays a decisive role in the development of spontaneous combustion of coal, and the research on the coal oxidation characteristic in the stage is of great significance for disclosing the mechanism of spontaneous combustion of coal and making a countermeasure from the source to prevent and control coal mine fires. The literature of the coal industry accumulated in the aspect of low-temperature coal oxidation is rich, and research results are obtained in the aspects of theoretical development, test equipment development, laboratory simulation test, numerical simulation, production application and the like. The basic theory of low temperature oxidation of coal has proven to be correct and reliable in practice, although there are still controversies between different perspectives and many points that are not completely clear, due to the extremely complex and varied structure of coal. The following summary states the theoretical points of low-temperature oxidation of coal closely related to the present invention and generally recognized by the coal industry (see: Li Wen, Li Baoqing, 1995. low-temperature oxidation and spontaneous combustion of coal, coal conversion, 18(1): 10-17; Daguanlong, 2008. research on low-temperature oxidation and oxygen absorption tests of coal, academic journal of Liaoning engineering technology (Nature science edition), 27(2): 172-175; Xuejing color, Schwann, Wenhu, Li, 2001. analysis of the influence factors of the composite thermal effect of coal and oxygen [ J ]. Chinese safety science, academic journal of China, 11(2): 31-36; Xiaoyun, Wangming, Lishuai, 2011. research on the characteristics of gas adsorption and analysis processes at low-temperature oxidation stage of coal, 2011 (2011) (102-) -104; Wu Pinna, Wang Fei Dunfeng, Wangfeng, Wenming, 3. coal low-temperature oxidation process of different indexes [ J ], 2013(2) 109-114; sixthly, Wuyang Yang, Mao dynasty, Hujiawei, Chenyili, 2014, coal low-temperature oxidation marker gas change rule, mineral engineering research, 29(3) 52-57; 2007, research on the change rule of gas products in the low-temperature coal oxidation process, coal mine safety, and 2007, 01: 1-4; the relationship between the change rule of the low-temperature oxidation structure of the coal and the spontaneous combustion process of the coal is 2007 the report of coal science 32(9) 939-; structure and reactivity of coal [ M ] Beijing, scientific Press, 2002):

● all coals, including anthracite coals, all coal and rock compositions, including inert components, are susceptible to oxidation, although the degree of oxidation sensitivity varies widely from coal to coal and from coal to coal composition.

● active structures capable of reacting with oxygen are present in coal at almost any temperature condition. The coal has a plurality of functional groups in the structure, and the element composition and the structure of the functional groups are greatly changed, so that the activity of the coal is greatly changed, and the coal has different oxidizing capacities; the functional groups and the coal structure main body can influence each other, so that the activity of the coal structure main body is changed, and the oxidizing capability of the functional groups is changed; this allows for the interaction of coal oxygen in the coal mass at almost any temperature condition.

● the number of functional groups such as aromatic ketone, aldehyde carbonyl, phenol, alcohol, ether, ester oxygen bond, etc. increases or increases from nothing to nothing with the increase of the oxidation temperature. Such functional groups are primarily oxygen-containing functional groups because during oxidation of coal, oxygen atoms are bonded to the more reducing functional groups. Thus, such functional groups increase in number with increasing temperature during low temperature oxidation.

● the majority of the functional groups decrease in number with increasing oxidation temperature, even after a certain temperature at which a functional group is no longer present. The functional groups generally belong to aliphatic structures, are mostly positioned in branched chain parts in the coal molecular structure and show stronger reducibility.

● the temperature at which the same functional group oxidizes in coal samples of different coal grades is different, and the temperature at which the same functional group oxidizes in different coal samples of the same coal grade may also be different due to the different main structures in which the functional groups are located. The host structure affects the oxidizability of the functional group.

● the main structure of the coal structure is not changed basically in the low-temperature oxidation process of the coal. Because the main structure of the coal structure needs higher energy to be activated to generate oxidation reaction, the main structure of the coal structure is not obviously changed under the low-temperature oxidation condition below 200 ℃.

● the coal will be physically adsorbed and chemically adsorbed on contact with oxygen to release heat, so that the coal is most easily activated, i.e. the structure with lower activation energy is activated by the heat released by physical adsorption and chemical adsorption, and then reacts with oxygen to release reaction products including index gas while releasing heat; the energy of the system is further increased along with the rise of the temperature of the coal body caused by the heat released by the initial oxidation reaction, so that other functional groups requiring larger activation energy (higher temperature) are activated to perform chemical reaction, more heat is released, the temperature of the coal body is further increased, the energy of the system is further increased, and the structure and the functional groups requiring larger activation energy are activated to perform oxidation reaction, so that the temperature of the coal body is continuously increased. However, this auto-acceleration of the oxidation reaction will slow down and stop due to insufficient oxygen supply.

● influence the intrinsic factors of low temperature oxidation of coal:

■ rank (or degree of coalification, degree of coal deterioration): it is generally considered that the activity of low-temperature oxidation of coal decreases as the degree of coalification increases. One reason for this is that low rank coal has a high oxygen content and an enhanced chemical binding capacity for moisture, making the coal more hydrophilic and more reactive to oxidation.

■ Sulfur content: it is believed that the oxidation of pyrite can produce an exothermic effect, which facilitates the continuous oxygen consumption between coal and oxygen, increasing the oxygen consumption rate. Inorganic sulfur and organic sulfur in coal behave differently in oxidation reactions. Oxidation of pyrite is significant at low temperatures (e.g., 25 ℃) and is more pronounced at high temperatures (e.g., 80 ℃). The mechanism of sulfur oxidation is: pyrite (FeS) ₂ ) By-product of oxidation H ₂ SO ₄ Not only improves the solubility and leaching capability of minerals in the coal, but also plays a catalytic role in the reaction of oxidizing C-H in the micro-components into oxygen-containing functional groups, thereby being rich in FeS ₂ The coal is easily oxidized. In addition, the particle size distribution of pyrite, the ability of carbonates in the coal to neutralize acids, etc., can also affect the rate of oxidation.

■ porosity and pore structure: the large porosity, good pore connectivity, and large internal surface area all contribute to the low temperature oxidation of coal, because these factors determine whether air can easily enter the coal, contact the effective surface area, and enter reactive vacancies. From laboratory measurements, researchers have found that the oxidation rate is proportional to the cubic root of the internal surface area, and pores with pore diameters >100nm play an important role in coal oxidation, respectively.

■ other influencing factors: the microscopic components affect the low-temperature oxidation of coal, vitrinite, alga and sporophyte are easy to be oxidized, and silk, keratin and cork are relatively difficult to be oxidized. Calcareous component (CaCO) ₃ And its conversion product CaSO ₄ ) It has catalytic action to low-temp oxidation of coal.

● influence the external factors of low temperature oxidation of coal:

■ temperature: when other conditions are unchanged, the temperature is different, the chemical reaction speed is different, the oxygen consumption rate of coal is different, and the chemical reaction constant and the surface reaction heat are functions of the temperature. As the temperature of the coal rises, the oxygen consumption rate and the oxidation heat release intensity of the coal sample both rise, and the higher the temperature is, the larger the amplitude of the oxygen consumption rate and the heat release intensity along with the temperature rise is. The amount of the oxidizing gas generated increases with an increase in the oxidizing temperature, and the amount of the most generated gas increases exponentially with an increase in the temperature.

■ time: under the limiting conditions of temperatures well below the coal ignition point, while the coal oxygen adsorption process is fast, the oxidation process is slow. The oxidation reaction process under such a limited condition generates heat equal to the amount of heat dissipated, and the oxidation reaction does not enter into the auto-acceleration process, and thus, it is a stable oxidation reaction process. For this oxidation process, time is an important factor; the change caused by the oxidation reaction process can be shown only if the time is long enough; the variation is often large as long as the time is long enough. For example, it has been observed that anthracite coal deposited in the open air has increased surface pores and fissures along the pores as the oxidation time increases; these are all effects of long term low temperature oxidation.

■ oxygen concentration: oxygen is the material basis for the coal oxygen reaction. The oxidation reaction of the surface active structure of the coal body is directly influenced by the oxygen concentration of the environment where the coal body is located. The oxygen concentration is different, the oxygen consumption rate of coal is different, the acceleration rate of oxidation reaction is different, and the acceleration limit is different.

■ moisture: moisture can be considered as an intrinsic factor, since moisture is one of the important components of coal; moisture can also be considered an external factor, as external intervention can greatly alter the moisture content in coal. Moisture affects both the physical adsorption and chemical reactions between coal and oxygen. A proper amount of water is beneficial to the low-temperature oxidation of the coal, and too much or too little water is extremely not beneficial to the low-temperature oxidation of the coal. If the coal body contains more moisture, a layer of water film is formed on the surface of the coal body by the excessive moisture to isolate oxygen, even if a small amount of oxygen is adsorbed by the coal body, most of heat released by adsorption can be absorbed and consumed by moisture evaporation, so that the heat around the coal body is difficult to accumulate, and the self-heating temperature rise of the coal is delayed. However, when the moisture in the coal is reduced to a certain degree, the ability of the coal to physically adsorb oxygen is reduced, and the dried coal does not substantially adsorb oxygen. The physical adsorption between coal and oxygen is the precursor of the chemical reaction between coal and oxygen, and the subsequent oxidation reaction is difficult without the precursor. In addition, moisture can promote the decomposition of the intermediate product peroxide of the coal-oxygen reaction, and has catalytic effect on the coal-oxygen reaction. Therefore, maintaining a suitable amount of water in the coal seam is one of the conditions under which low temperature oxidation continues.

● products of coal low-temperature oxidation process

■ gaseous products: various gaseous products are generated in the low-temperature oxidation process of coal, and CO are common ₂ 、CH ₄ 、C ₂ H ₆ 、C ₃ H ₆ 、C ₂ H ₄ 、H ₂ O, etc., see also H ₂ 、C ₃ H ₈ 、C ₂ H ₂ 、C ₄ H ₁₀ 、iC ₄ H ₁₀ And the like.

Removing CO ₂ And H ₂ Besides O, gaseous products generated in the low-temperature oxidation process of coal are all combustible gases.

Different coal types generate the same gas at different temperatures and different gas production rates: for example, lignite can be oxidized at normal temperature to produce CO, whereas anthracite can produce CO at temperatures in excess of 80 ℃. Generally, the temperature at which the same gas is produced increases with increasing rank and the rate of gas production decreases with increasing rank.

The initial temperature and the ending temperature of different gases generated by the same coal are different, and each gas is generated in a specific temperature range, although the temperature ranges for generating different gases may overlap or partially overlap: for example, researchers have found that the gas change rule generated by lignite oxidation is as follows: oxidized to generate CO gas just after contacting with air, and oxidized to generate C at 4 DEG C ₂ H ₆ Gas, oxidation at 90 ℃ to C ₂ H ₄ Gas, oxidation at 120 ℃ to C ₃ H ₈ Gas, oxidation at 140 ℃ to H ₂ The gas is oxidized at 160 ℃ to generate C along with the oxidation deepening ₂ H ₂ A gas.

The types and the sequences of the gases generated by different coal types are different, and even the types and the sequences of the gases generated by coal samples in different regions of the same coal type are also different: for example, researchers found that under the same temperature rise experimental conditions, the types and the sequence of the gases generated by lignite oxidation are CO and C ₂ H ₆ 、C ₂ H ₄ 、C ₃ H ₈ 、H ₂ 、C ₂ H ₂ (ii) a The gas produced by gas coal oxidation is CO and C in the order ₂ H ₄ 、H ₂ (C ₂ H ₆ Already present at the beginning of the experiment); the category and the sequence of the gas generated by the oxidation of the gas fat coal are CO and C ₂ H ₄ 、H ₂ (C ₂ H ₆ 、C ₃ H ₈ Already present at the beginning of the experiment); the category and the sequence of the gas generated by the oxidation of the anthracite are CO and H ₂ Although the temperature is raised from 20 ℃ to 210 ℃, C is not generated by oxidation ₂ H ₆ 、C ₂ H ₄ 、C ₃ H ₈ 。

Index gas (or marker gas): gas that is not originally present in the coal seam and is produced entirely by coal oxidation is referred to as "indicator gas". CO is the most prominent indicator gas because CO is a gas that can be produced by any coal at lower temperatures. C ₂ H ₄ Is also a commonly used indicator gas.

■ non-gaseous products: indeed, it has not been known to date with certainty which non-gaseous products and the amount thereof are produced during low temperature oxidation of coal, largely because it is difficult to distinguish between the original constituents of the coal and the non-gaseous products. Furthermore, the present invention does not find that the non-gaseous products are significantly related to the success of injecting high temperature air to increase the production of coal bed gas. Thus, the present invention is somewhat independent of non-gaseous products.

● mechanism of low-temperature oxidation of coal: due to the differences in rank (type of coal), grade (quality) of coal, micro-components and content thereof, mineral components and content thereof, structure, porosity and pore structure of coal, oxidation temperature, oxygen concentration, etc., due to the complexity of the low-temperature oxidation reaction itself, and the differences in research means (reaction apparatus, type of coal, amount of coal, and analysis techniques) employed by many researchers, there are many consensus and opposite opinions on the research of the low-temperature oxidation mechanism of coal. The present invention sets forth a consensus among many researchers for the primary purpose of clarifying the process of the present invention without pursuing all-inclusive. The low-temperature oxidation process of coal can be divided into three stages

■ physical adsorption stage: adsorption is an interfacial phenomenon, which is called adsorption, in which when two phases are brought into contact, a region having a composition and properties different from those of the interior of either phase exists at the interface. Coal-oxygen adsorption belongs to the category of solid-gas adsorption in physical adsorption. It is known from the general mechanism of physical adsorption that adsorption is caused by excess surface free energy. The surface free energy is the increment of the system free energy when the unit area of the adsorbent is increased under constant temperature and constant pressure, and is also called surface excess free energy or Gibbs free energy. The interface has the tendency of spontaneously reducing the interface energy, the solid interface is difficult to shrink because surface molecules or atoms cannot move freely, and the surface free energy is reduced only by adsorbing other molecules; this is the fundamental driving force for adsorption on solid surfaces. The greater the surface free energy of the solid, the greater the power to reduce energy by the adsorbed gas and the corresponding increase in the amount of adsorbed gas per unit area. The mechanism of physical adsorption of coal is of particular interest. Coal has a structure similar to that of a high polymer molecule, and a certain number of chemical bonds exist in a high polymer chain to form a three-dimensional network structure of the high polymer as cross-links. The lower the degree of deterioration of coal, the more developed its three-dimensional network structure. Gas molecules enter the lattice of macromolecular structures and enter the interstices enclosed by these macromolecules, creating an adsorbed artefact, which is in fact a "dissolution phenomenon" in which gas molecules are "absorbed" into the solid, sometimes referred to as a "solid solution phenomenon". Thus, the ability of coal to adsorb gases is much higher than other substances. The coal adsorbs oxygen and has specificity, namely dynamic property, different from that of coal adsorbing other gases. The oxygen molecules are attached to the surface of the coal, and the oxygen molecules are directed to two ways, wherein one way is to convert the oxygen molecules into chemical adsorption and prepare for chemical reaction; the other is to detach from the surface of the coal and change into gas phase again. When a certain oxygen molecule leaves its physisorption site, it is replenished by another oxygen molecule. Therefore, physical adsorption during low-temperature oxidation of coal becomes a dynamic adsorption process due to the presence of oxidation reaction. The dynamic process is generally characterized by the fact that the oxygen molecules entering the coal are larger than the oxygen molecules exiting the coal, and the difference between the two is the amount of oxygen consumed by the oxidation reaction of the coal. The physical oxygen uptake of coal is related to the degree of deterioration of coal, and the degree of deterioration increases and then decreases. The coal with low metamorphic grade has loose structure and large porosity, thus having strong physical adsorption capacity. The bituminous coal with higher deterioration degree has the advantages that the porosity of the coal is reduced due to the action of the ground pressure in the long deterioration process, the coal quality tends to be compact, and the oxygen adsorption capacity is greatly reduced. With the further deterioration of coal, under high temperature and high pressure, many micropores are generated in the coal body due to the carbonization action, the surface area is gradually increased, and the oxygen adsorption capacity of the anthracite is increased. The micro process of low-temperature oxidation of the coal is physical adsorption, chemical adsorption and oxidation reaction in sequence, and one oxygen molecule also acts with the coal structure according to the sequence. Thus, one of the most important roles of the process of coal physisorption of oxygen is to transport oxygen for the oxidation reaction and give off the heat of physisorption. The physical adsorption heat slightly raises the temperature of the coal body, and is the prelude that the structure which is very easy to be activated in the coal body is activated to absorb oxygen to generate chemical adsorption and chemical reaction, so that the low-temperature oxidation process of the coal is developed. Coal-oxygen physisorption is a prerequisite for the occurrence of an oxidation reaction, which is the result of adsorption of oxygen by coal.

■ chemisorption and partial chemical reaction stage: chemisorption is the transition process between the physical adsorption of coal to a chemical reaction, or transition state. Chemical reaction between the chemisorbed oxygen and the coal surface structure occurs essentially, and electron transfer occurs between oxygen atoms and atoms in the coal structure and the oxygen atoms are combined with surface bonding force similar to chemical bonds. Chemisorption itself is a chemical reaction and the heat of chemisorption is similar to the heat of chemical reaction. The coal oxygen chemical adsorption is a process that electrons of a coal active structure enter a track of unpaired electrons of oxygen molecules to form a stable system and release heat at the same time. The force that produces chemisorption is the chemical bond force. In the chemical adsorption process, processes such as transfer of electrons, rearrangement of atoms, destruction and formation of chemical bonds, and the like can occur. At this stage, chemisorption of oxygen on the coal surface and in the pores forms an unstable solid intermediate coal oxygen complex which further decomposes into gaseous products and stable solid compounds. In the stage, part of the coal and the oxygen react rapidly to generate oxidation products to release heat, part of the coal and the oxygen generate chemical adsorption to generate unstable complexes, and the unstable complexes are decomposed into gaseous products and solid compounds and release heat at the same time. The accumulation of these two portions of heat results in an increase in temperature, which promotes the oxidation of the coal into the next stage. At this stage, the gas products produced are mainly CO, CO ₂ 、H ₂ O。

■ chemical reaction stage: this stage can be divided into two successive sub-stages:

autothermal oxidation stage: as the temperature of coal rises, the oxidation speed is accelerated, the fat side chain is oxidized to generate gaseous hydrocarbon C ₂ H ₄ ，C ₂ H ₆ And C ₃ H ₈ 。

Accelerating the oxidation stage: deep oxidation, oxidizing partial bridge bonds on the coal molecule structural unit, accelerating the reaction speed and generating H ₂ And C ₂ H ₂ 。

Chemical reactions at this stage are not limited to surface chemical reactions, but may penetrate deep inside the coal structure. However, the chemical reactions at this stage are still limited to the side chains, bridges, functional groups of the coal structure and do not involve the main structure of the coal.

Technical key points and difficult points of method for increasing production of coal bed gas by injecting high-temperature air

On the basis of the theory of low-temperature oxidation of coal, the invention states the technical key points and the detailed difficult points of the method for increasing the yield of coal bed gas by injecting high-temperature air, and mainly how to consume the oxygen in the air injected into the coal bed? How does oxygen not become consumed remain in the coal seam and not be removed from the production well along with the coal seam gas and nitrogen?

● composition of air: the normal air composition is calculated by volume fraction: nitrogen (N) ₂ ) About 78.07% oxygen (O) ₂ ) About 20.94%, rare gas about 0.93% (helium He, neon Ne, argon Ar, krypton Kr, xenon Xe, radon Rn), carbon dioxide (CO) ₂ ) About 0.031%, and other gases and impurities about 0.03% (e.g., ozone (O) ₃ ) Nitrogen monoxide (NO), nitrogen dioxide (NO) ₂ ) Water vapor (H) ₂ O), etc.).

● the components of air injected into coal seam and the components of gas generated during low-temperature oxidation of coal

■ Nitrogen (including helium, neon, argon, krypton, xenon, radon): the rare gases helium, neon, argon, krypton, xenon, radon are inert gases having the same properties as nitrogen, and their functions and destinations are the same, so nitrogen is typically used for discussion. The nitrogen is stripped and the adsorbed coal bed gas on the surfaces of coal micropores and cracks is expelled, so that the desorption of the coal bed gas is promoted and the coal bed gas is transported to a production well. The yield increasing effect of injecting high-temperature air to increase the yield of coal bed gas is mainly borne by nitrogen. For the principle of increasing coal bed gas by nitrogen, see us patent No. 4883122 and chinese patent No. 201510467978.3, which are not repeated herein. A small part of nitrogen is adsorbed by the coal bed and stays in the coal bed, and most of nitrogen is transported to a production well along with the coal bed gas which is driven and discharged, enters a coal bed gas gathering and transportation system and finally returns to the atmosphere. In the initial stage of injecting high-temperature air, the percentage content of nitrogen in the gas produced by the production well is zero or close to zero; the percentage of nitrogen increases as the insufflation time increases.

■ carbon dioxide (including carbon dioxide produced by low temperature oxidation of coal): coal bed has extremely strong CO adsorption ₂ Due to CO ₂ Higher gram molecular weight and coal to CO ₂ Resulting in CO ₂ Has a large Langmuir volume and a low Langmuir pressure. Thus, trace amounts of CO in the high temperature air injected into the coal seam ₂ And carbon dioxide generated by low-temperature oxidation of coal, most of which is adsorbed by the coal bed and is retained in the coal bed, and only trace amount of CO is possible ₂ To the production well. This is a theory that has been demonstrated by the flow of multicomponent multiphase fluids in adsorption desorption porous media (see Zhu, J.2003.multicomponent multiphase flow in pore media with temperature variation or adsorption, Ph.D. separation, Standard University, Online athttp://pangea.stanford.edu/ ERE/pdf/pereports/PhD/Zhu03.pdf) Also confirmed by a number of laboratory experiments (see Parakh, S.,2007, Experimental Investigation of Enhanced cobalt Bed Methane Recovery, Ph.D. discovery. Stanford University). CO 2 ₂ The coal bed gas in the adsorption state is replaced while being adsorbed by the coal bed, and the coal bed gas is also increased (see Yejianping, Von Sanli, Fanzhi, Wang Guo Qiang.2007. Mini pilot test research on improving the recovery ratio of the coal bed gas by injecting carbon dioxide in south of Qin Water basin. Petroleum institute, 28(4): 77-80).

■ gas generated during low temperature oxidation of coal (except CO) ₂ External): h ₂ O originally is one of the components of the coal bed gas, enters the gathering and transportation system as one of the gas components of the production well, and is dehydrated and removed by the post-treatment equipment. CO removal ₂ And H ₂ Besides O, gaseous products generated in the low-temperature coal oxidation process are combustible gases, and the calorific values of the gaseous products are much higher than that of the main component methane of the coal bed gas and are high-calorific-value natural gas components. The additive is used as a coal bed gas component and is discharged into a coal bed gas gathering and transportation system from a production well, and the additive is beneficial and harmless to the production of the coal bed gas.

■ oxygen: the action and the homing of the oxygen are the most complex and the most critical to the success of injecting high-temperature air to increase the coal bed gas, and the detailed discussion is needed as follows:

oxygen retained in the coal seam: free oxygen remains in the coal seam in three forms as follows.

Dissolved oxygen: oxygen is not readily soluble in water, and only about 30mL of oxygen can be dissolved in 1L of water. When high-temperature air is injected to increase the yield of coal bed gas, most of water in the cracks, the large pores, the medium pores and the small pores of the coal bed is driven and discharged out of a coal bed gas well region by high-pressure gas, only water in the micropores is temporarily reserved, and the water in the micropores is finally driven and discharged into the cracks and then driven and discharged out of the coal bed gas well region. The coal bed only contains trace moisture, and the requirement of low-temperature oxidation of coal on moisture is met. Therefore, dissolved oxygen in the coal seam is negligible.

Free oxygen: free oxygen exists in the fissures, large, medium and small pores of the coal bed and enters the micropores as a single molecule. Thus, coal bed porosity is a major factor affecting free oxygen content. When high-temperature air is injected to increase the yield of coal bed gas, high-pressure free oxygen and nitrogen are filled in the cracks, large, medium and small pores of the coal bed together and enter the micropores as single molecules. Due to the dynamic property of coal oxygen adsorption in the low-temperature oxidation of coal, the content of free oxygen in the coal bed is dynamically changed; when the oxygen supply is sufficient, the content of free oxygen is large; when the oxygen supply is insufficient or the oxygen supply is stopped, the free oxygen content is consumed because the free oxygen is converted into a physical adsorption state, then into a chemical adsorption state, and then undergoes a chemical reaction. Therefore, the method for increasing the coal bed gas by injecting high-temperature air is not concerned with the dynamically changing free-state oxygen content in the coal bed, but is concerned with the oxygen content percentage of the gas in the production well, and the parameter of the accelerated oxygen consumption temperature is designed in the implementation step, when the oxygen content percentage in the gas in the production well is abnormally increased or the oxygen content in the gas in the production well exceeds the standard specified by the coal bed gas production specification, the method of the invention will increaseThe gas injection temperature is brought to the accelerated oxygen consumption temperature until the oxygen content in the gas in the production well returns to normal.

Adsorption state oxygen: FIG. 1 is a graph showing the trend of the amount of adsorbed oxygen in different coals at different adsorption ambient temperatures (see Yao chiffon, Wangdming, Zhongxian, Gunje, Hu nationality (not detailed in years). Experimental study on the process of coal oxygen recombination at low temperature stage. As can be seen from fig. 1, as the temperature of the coal sample increases, the oxygen uptake of the coal as a whole tends to decrease. Coal to O ₂ The adsorption of the oxygen-absorbing material mainly comprises two forms of physical adsorption and chemical adsorption, and the main oxygen absorption mode at the low-temperature stage is physical oxygen absorption. As the temperature of the environment in which the coal sample is placed increases, O ₂ The kinetic energy of the molecule becomes larger and the coal surface becomes O ₂ Is less affected by temperature changes, so that O ₂ More easily desorbed from the surface of the coal, and causes the reduction of the physical oxygen absorption of the coal. The chemisorption of coal increases with the increase in the temperature of the adsorption environment, but the absolute increase of the chemisorption is slow until the temperature of the coal reaches a certain temperature, and the absolute increase is insufficient to offset the decrease of the physical adsorption of coal, so that the total amount of the oxygen absorbed by the coal is reduced. In addition, the oxygen uptake of coal at the low-temperature stage is related to the degree of deterioration of coal, but the oxygen uptake does not simply decrease as the degree of deterioration of coal increases, but roughly shows the rule that the oxygen uptake of lignite is greater than that of anthracite and that of bituminous coal. As the temperature increases, the degree of deterioration decreases in influence on the amount of oxygen absorbed. At the adsorption ambient temperature of 100 ℃, the oxygen absorption of each coal is basically about 0.5ml/g, which shows that the oxygen absorption at the temperature is not greatly influenced by the deterioration degree of the coal. It should be noted that fig. 1 is only indicative and the data cannot be used to infer the oxygen uptake of an underground coal seam because (1) fig. 1 is plotted using data measured at normal pressure (one atmosphere, i.e., 0.1MPa), whereas the depth of a coal seam gas reservoir is typically between 500 and 1000m and the normal reservoir pressure is typically between 5 and 10 MPa; and the adsorption amount of coal to any gas is sensitively and rapidly increased along with the increase of the pressure, so that the coal cannot be adsorbed according to the conventional methodEstimating the oxygen absorption amount of the coal in the underground in-situ state by the oxygen absorption amount of the coal under pressure. (2) In laboratory measurement, the oxygen uptake of coal and the oxygen consumption of coal oxidation reaction are difficult to distinguish, and the measurement result of the oxygen uptake of coal in literature is little, and the measurement result of the oxygen uptake of coal under the condition of coal bed gas reservoir pressure is not available. Therefore, the invention estimates the coal oxygen uptake based on the coal nitrogen uptake. The basis for this estimation is as follows: (1) under the condition of similar molecular affinity, the adsorption quantity of coal to a gas is closely and positively correlated with the molecular weight of the gas. The molecular weight of oxygen (32) is greater than but close to the molecular weight of nitrogen (28). Assuming that coal molecules have similar affinities for nitrogen and oxygen and the environmental conditions are the same, the amount of oxygen absorbed by coal should be greater than the amount of nitrogen absorbed by coal. (2) Nitrogen is an inert gas and oxygen is a chemically very reactive gas, so it is certain that the oxygen uptake by coal should be greater than the nitrogen uptake by coal under the same conditions. (3) Under the conditions of 5-10 MPa pressure and 30 ℃ underground in-situ temperature of a coal bed, the nitrogen absorption amount of coal is 5-7.5 m ³ Between/ton (see yaoyang bin 2009 selective adsorption and desorption characteristics of coal on methane, carbon dioxide and nitrogen study 2009 asian pacific international coalbed methane conference and 2009 chinese coalbed methane academic seminar). The invention estimates the coal oxygen uptake under the same condition to be 4.0-7.0 m ³ Between/ton; this is a fairly conservative estimate. Thus, coal seams are a large reservoir of oxygen. Coal bed gas development practices prove that in most cases, the coal bed gas contains only trace oxygen or no oxygen at all, and therefore, the underground in-situ coal bed also contains only trace oxygen or no oxygen at all. Even though coal formation has historically contained oxygen in the coal seam, this oxygen has been depleted in a long geological history due to low temperature oxidation. Today's coal bed methane reservoirs are large empty as well as oxygen reservoirs. The oxygen storage can not be filled up even if the high-temperature air is continuously injected to increase the coal bed gas production for years and more than ten years. Since it is a leaky pool-physisorption-chemisorption-oxidation-the process will eventually consume the adsorbed oxygen. Therefore, the correct operation of injecting high-temperature air to increase the coal bed gas can ensure that the oxygen storage reservoir is never filled.

Consumed oxygen: FIG. 2 is a graph of oxygen consumption as a function of temperature for low temperature oxidation of a lean coal and an anthracite (see YIXIANGDAN, Wangdoming, Zhongxiaxing. 2010. study on activation energy of coal low temperature oxidation reaction based on oxygen consumption. coal mine safety, 2010(07): 12-15). In the graph, the vertical axis represents the difference between the oxygen concentration of the inlet gas and the oxygen concentration of the outlet gas of the experimental apparatus, and a larger difference indicates a larger oxygen consumption by the low-temperature oxidation. As can be seen from fig. 2, the oxygen consumption generally shows a tendency of decreasing first and then increasing. The physical adsorption is firstly generated when the coal and the oxygen are contacted, the adsorption process is very quick, the physical adsorption can reach about 80 percent of the saturated oxygen absorption amount within several seconds, then the adsorption rate is sharply reduced, and finally the adsorption balance is achieved. With the increase of the temperature, the physical adsorption is gradually transited to the chemical adsorption, the chemical reaction starts to occur between the coal and the oxygen, and the oxygen consumption is increased. When the temperature rises to a certain specific temperature range, the oxygen consumption is increased sharply, namely the oxygen consumption rate is obviously increased; for northern meager coal, the specific temperature range is about 65 ℃, and for frontier anthracite coal, the specific temperature range is between 80 ℃ and 90 ℃. Table 1 shows the oxygen consumption of coal by oxidation at 70 ℃ for 1 hour for five different coal types (see Juwei, Hokking, Zhongxing, Wangdinging. 2006. study on spontaneous combustion step-by-step self-activation reaction process of coal, proceedings of university of mining, China 36 (1): 111-. The test conditions and the data recording and sorting method are as follows: the particle size of the coal sample is 80-120 meshes (0.20-0.125 mm), the amount of the coal sample is 40g, the coal sample is placed in a distillation flask, the volume of the flask is 1000ml, namely the air amount is 1000ml, after the coal sample is oxidized for 1 hour at the constant temperature of 70 ℃ under the environment temperature, the oxygen concentration in the flask is tested, and the oxygen consumption per gram of coal is obtained according to the change of the oxygen concentration. As can be seen from Table 1, at this ambient temperature, oxygen consumption decreased as the rank increased. Table 1 merely shows that the oxygen consumption varies from coal to coal at the same ambient temperature. Higher rank coals consume less oxygen, perhaps because the ambient temperature has not reached a particular temperature range where the oxygen consumption rate is significantly increased. Table 2 and FIG. 3 show the oxygen consumption of coal (Zhaowei, supra) for 1 hour of oxidation at different temperatures for the same coal type (Zhaoli gas fat coal). The test conditions and data recording and finishing methods were the same as those in table 1, but the ambient temperature was different for each test, and fresh coal samples were used for each test. The percentage next to each data point in figure 3 is the oxygen concentration of the gas in the distillation flask at the end of the test. As can be seen from Table 2 and FIG. 3, the oxygen consumption for the oxidation of coal of the same coal type increased significantly with increasing ambient temperature at the same oxidation time, and increased more dramatically with increasing temperature before the oxygen concentration of the gas in the distillation flask was greater than 13% at the end of the test. After an oxygen concentration of less than 13%, the oxygen consumption increases more slowly with increasing temperature due to the decrease in oxygen concentration at higher temperatures. This is essentially the same as the conclusion obtained from fig. 1. Table 3 and FIG. 4 show the results of oxygen consumption measurements for the same coal type (i.e., the firewood gas fat coal) at the same temperature (70 ℃ C.) and different oxidation times (see Saiwei, supra). The test conditions and the data recording and finishing methods were the same as in tables 1 and 2 and fig. 3, and the test temperature was 70 ℃. As can be seen from Table 3 and FIG. 4, the oxygen consumption of the coal gradually increased with the increase of the oxidation time, i.e., the degree of oxidation of the coal gradually increased, but the increase of the oxygen consumption was not significant when the oxidation time reached 40 minutes (for the present coal type), indicating that the coal had been oxidized relatively sufficiently in such an environment.

TABLE 1 oxygen consumption of coal oxidation at 70 deg.C for 1 hour for different coal types

TABLE 2 oxygen consumption of coal oxidation at different temperatures for 1 hour for the same coal type

TABLE 3 oxygen consumption test results for the same coal type at the same temperature and different oxidation times

● the majority of oxygen will be retained in or consumed by the coal seam: the foregoing theoretical points of low temperature oxidation of coal and the discussion of the role and fate of the constituents of the air injected into the coal bed and the constituents of the gas produced during low temperature oxidation of coal have ensured that a substantial portion of the oxygen is retained or consumed in the coal bed. To more clearly illustrate how oxygen in the air is retained in the coal seam or consumed in the coal seam, fig. 5 schematically shows the temperature conditions of the coal seam around the gas injection well after 4 years of injecting high temperature air, the coal seam being assumed to be lignite. In the drawing of fig. 5, it is assumed that the characteristics of the coal bed gas reservoir are isotropic, the heat conduction of the high-temperature gas to the coal bed is the same in all directions, and the heat conduction between different portions of the coal bed itself is the same in all directions, so that the temperature rise of the coal bed due to the injection of the high-temperature gas is the same in all directions. Fig. 5 is a plan view, i.e. a bird's eye view, of the temperature of the coal seam around the nitrogen injection well, which is increased by injecting high-temperature gas under the assumed condition. In FIG. 5, the central black circle is the gas injection well with a gas injection temperature of 150 ℃; on the right side horizontal line of cross, outside-in has marked the radius of 1 st, 4 th, 6, 9 concentric circles in proper order: 300m, 254m, 200m, 100 m; on the lower part vertical line of cross, from bottom to top, marked the temperature of 1 st, 4 th, 6, near 9 concentric circles position coal seams in proper order: 40 ℃, 43 ℃, 50 ℃ and 70 ℃. In this figure, the central set of thick black concentric circles indicate the range of coal bed temperatures greater than 70 ℃; the middle set of thin black concentric circles indicates the temperature of the coal bed in the range of 70-43 ℃, and the radius of the last thin black concentric circle is about 245 m; four gray filled black circles on the thin black concentric circle represent four production wells closest to the gas injection well, which is equivalent to a first circle of production wells of the 250 m-250 m coal bed gas well pattern surrounding the gas injection well; the outer concentric circles of black dashed lines indicate the range of coal seam temperatures less than 43 c, which is not limited to the concentric circles of black dashed lines in the figure, but expands outward until abutting the influence range of the adjacent gas injection well. The influence range of one gas injection well is determined by the characteristics of the coal bed gas reservoir, gas injection operation parameters, gas injection time length and other factors, and ranges from hundreds of meters to thousands of meters. The coal bed is assumed to be lignite, and the low-temperature oxidation of the coal is mainly carried out in a temperature range indicated by a thick black concentric ring, so that the coal bed is called a chemical reaction zone; in the temperature range indicated by the thin black concentric circles, the low-temperature oxidation of coal is mainly based on chemical adsorption and has local chemical reaction, so the coal is called as a chemical adsorption and local chemical reaction zone; the temperature range indicated by the concentric circles with black dashed lines, in which the low temperature oxidation of coal predominates, is therefore referred to as the "physisorption zone". Each zone has a specific function of consuming or retaining oxygen, as described in detail below:

■ in the chemical reaction area, the physical adsorption between coal and oxygen is quickly converted into chemical adsorption, which is quickly converted into chemical reaction, and the heat of chemical reaction and the heat brought by high-temperature air are combined to maintain the limited automatic acceleration of chemical reaction in the area. The limited automatic acceleration is because the quantity of high-temperature air injected into the coal bed is limited, the carried oxygen is limited, the coal oxidation reaction cannot be automatically accelerated to an uncontrollable and burning stage, but the automatic acceleration rate of the coal oxidation reaction is quickly reduced to zero along with the increase of the distance from the gas injection well to the periphery, and the coal oxidation reaction enters a stable oxidation reaction process, namely the heat generated in the oxidation reaction process is equal to the dissipated heat. Because the underground coal seam is a closed system, although the gas injection well and the production well create several singularities on the closed system, the closure of the closed system of the underground coal seam is not basically changed, so the dissipated heat can effectively expand the range of the chemical reaction zone along with the high-temperature gas injection time. In the initial stage of injecting high temperature air, the chemical reaction area has limited range and limited oxygen consumption. Along with the prolonging of the gas injection time, the range of the chemical reaction zone is expanded, the oxygen consumption is gradually increased, and the chemical reaction zone becomes an important oxygen consumption space after 1-2 years.

■ in the chemisorption and local chemical reaction zones, not only is the conversion of coal-oxygen physisorption to chemisorption extended, but the conversion of chemisorption to chemical reaction is slow and limited to those reactive structures in the coal bed that require lower temperatures to react. This local chemical reaction results from the strong selectivity of chemisorption itself. Because chemisorption requires a certain activation energy, chemisorption always begins with the selection of active structures with higher activation energy. The chemical adsorption has the selective characteristic, and the complexity and the difference of the chemical structure of the coal organic matters are more remarkable. Another important feature of chemisorption in low temperature oxidation of coal is the formation of an unstable solid intermediate coal oxygen complex which further decomposes into gaseous products and stable solid compounds. Chemisorption is essentially a chemical reaction, called chemisorption rather than chemical reaction, because of the electron transfer between the adsorbate and the adsorbent, mainly because chemisorption is reversible, while chemical reactions, including local chemical reactions, are generally irreversible. The chemical reactions that take place in the chemisorption and local chemical reaction zones, which are therefore called "local chemical reactions", are surface chemical reactions, on the one hand because the subsequent chemical reactions that result from the selectivity of chemisorption take place only locally or preferentially locally, and on the other hand because the chemical reactions in this region take place only at the surfaces of the coal micropores, fissures. The temperature rise can increase the activation energy of the active structure and is beneficial to chemical adsorption, so that the chemical adsorption rate and the adsorption quantity can be increased by raising the temperature, and the range, the rate and the strength of local chemical reaction are improved. The chemical adsorption heat release and the local chemical reaction heat, together with the heat from the high-temperature air flow and the conduction heat from the chemical reaction area, raise the temperature of the chemical adsorption and local chemical reaction area, on one hand, the chemical adsorption and local chemical reaction area adjacent to the chemical reaction area is converted into the chemical reaction area, on the other hand, the chemical adsorption and local chemical reaction area is enlarged. Since the extent of the chemisorption and localized chemical reaction zone is much greater than the chemical reaction zone surrounding the gas injection well, the amount of oxygen consumed and retained in this zone may be greater than the oxygen consumption of the chemical reaction zone.

■ in the physical adsorption zone: the amount of oxygen consumed and retained in the physical adsorption zone is much greater than the sum of oxygen consumption in the chemical reaction zone and the chemical adsorption and local chemical reaction zones, mainly because of the following three reasons: (1) physics of physicsThe space capacity of the adsorption area is large. The coal bed can become a chemical reaction zone, a chemical adsorption zone and a local chemical reaction zone only when the coal bed needs to reach a certain temperature. All this is preceded by a physisorption zone. Although the chemical reaction area, the chemical adsorption area and the local chemical reaction area are rapidly enlarged along with the prolonging of the gas injection time, the amount of air injected into the coal bed is small, the heat capacity of the gas is small, and the coal bed is slowly and gradually heated by high-temperature air; under the condition of injecting high-temperature air to increase the yield of coal bed gas, the contribution of chemical reaction heat to the increase of the coal bed temperature is also a slow and gradual process due to the limitation of oxygen supply. For economic reasons, the gas injection well spacing should typically be greater than 1000 m. Therefore, even after several years of gas injection, the physical adsorption zone spatial capacity will be much greater than the sum of the spatial capacities of the chemical reaction zone and the chemisorption and local chemical reaction zones. (2) The oxygen uptake of coal is enormous. Assuming that the well spacing of the gas injection well is 1000m, the thickness of the coal bed is 6m, and the density of the coal is 1.4 tons/m ³ Then, the mass of coal in the cylindrical space affected by one gas injection well is equal to π 500 ² 6 x 1.4 t 6594000 tons; assuming the capacity of the gas injection supercharger to be 1000m ³ Hour, then, the amount of oxygen injected into the coal bed is equal to about 5040m per day ³ (ii) a It is continuously assumed that the saturated oxygen uptake of coal is 5.0m ³ And/ton, then 17.9 years is needed to saturate the amount of absorbed coal in the cylindrical space with a radius of 500m centered in the gas injection well, without considering the oxygen consumption of chemisorption and chemical reaction at all. (3) As discussed above, the physisorption zone is a leaky oxygen reservoir, since physisorption-chemisorption-oxidation-process eventually consumes physisorbed oxygen. As long as the time is long enough, both physical adsorption and chemical adsorption are the preparation and the prelude of the oxidation reaction, and the oxygen adsorbed on the surface of the coal micropores and fissures is finally consumed by the oxidation reaction. In other words, by correctly using the method for increasing the coal bed gas by injecting the high-temperature air provided by the invention, the oxygen absorption of the coal bed gas reservoir can never reach saturation. Therefore, the physical adsorption area is the last and most reliable guarantee for injecting high-temperature air to increase the yield of coal bed gas, and ensures that no excessive oxygen reaches a production well.

● method for obtaining high temperature air by heating air: first, the booster with the adjustable final cooler can be used to adjust the final cooler of the booster to achieve the desired air temperature. The method uses the booster to compress the air, which causes the air to heat up, without additional energy consumption to heat the air. Secondly, the high-pressure air output by the supercharger can be heated by using a pipeline gas heater with adjustable temperature to obtain the required air temperature. Third, the final cooler adjustable booster and the temperature adjustable duct gas heater can be used together to achieve the desired air temperature, or any other means of heating the high pressure air output from the booster to achieve the desired air temperature.

● for the sake of safe operation, the initial temperature is selected from the range of normal temperature to 70 deg.C, the heating rate is selected from the range of 20 deg.C/day to 10 deg.C/month, the normal operation temperature is selected from the range of triple point of the coal burning point of the gas-injected coal bed, and the accelerated oxygen consumption temperature is selected from the range of triple point of the coal burning point of the gas-injected coal bed.

● according to the rule that the oxygen consumption of coal seam sample is changed with the temperature, the initial temperature, the heating rate, the normal operation temperature and the accelerated oxygen consumption temperature are selected and determined.

● the method comprises selecting and determining initial temperature, heating rate, normal operation temperature, and accelerated oxygen consumption temperature, and adjusting the initial temperature, heating rate, normal operation temperature, and accelerated oxygen consumption temperature with the change of the characteristics of the coalbed methane reservoir along with the extension of high temperature air injection time.

After the theoretical basis, the technical points and the difficulties of increasing the coal bed methane by injecting the high-temperature air are discussed in detail, the method for increasing the coal bed methane by injecting the high-temperature air provided by the invention is summarized as follows:

the invention is realized by adopting the following technical means:

The specific implementation steps and characteristics of the method for increasing the coal bed gas by injecting the high-temperature air are as follows:

(c) monitoring the bottom pressure and temperature of a gas injection well, and monitoring the gas composition, the bottom pressure and temperature, the wellhead pressure and temperature of a production well;

(i) steps (d), (e), (g) and (h) may be repeated as many times as necessary.

Finally, it should be noted that although the present invention has been described in detail herein, it will be understood by those skilled in the art that the present invention may be modified and equivalents may be substituted; all such modifications and variations that do not depart from the spirit and scope of the invention are intended to be included within the scope of the appended claims.

Claims

1. A method for increasing the yield of coal bed gas by injecting high-temperature air comprises the steps of injecting high-temperature air into a coal bed from one or more gas injection wells communicated with a coal bed gas reservoir, stripping and expelling adsorbed coal bed gas on the surfaces of coal microporosities and cracks of nitrogen, promoting the desorption of the coal bed gas and transporting the coal bed gas to a production well, enabling oxygen in the coal bed to be retained in the coal bed or consumed due to physical adsorption, chemical adsorption and oxidation reaction by virtue of low-temperature oxidation, and collecting the produced coal bed gas and mixed gas of the coal bed gas and the nitrogen from the production wells communicated with the coal bed, so that the yield of a coal bed gas well and the recovery ratio of the coal bed gas are improved, and is characterized in that:

(i) steps (d), (e), (g) and (h) may be repeated as many times as necessary.

2. The method for increasing the yield of the coal bed gas by injecting the high-temperature air according to claim 1, wherein the temperature of the air injected into the coal bed is higher than the normal temperature.

3. The method for increasing the yield of the coal bed gas by injecting the high-temperature air as claimed in claim 1, wherein moisture is added into the gas injection pipeline to catalyze the reaction between coal and oxygen.

4. The method of claim 1, wherein when the oxygen content in the gas from the production well is higher than expected, the injection temperature is increased to enhance the chemical reaction between the coal and oxygen and increase the oxygen consumption.

5. The method for increasing the gas yield of the coal seam by injecting the high-temperature air according to claim 1, wherein when the gas temperature of the production well rises to exceed the expected temperature, the gas injection is stopped, or the normal-temperature air or the normal-temperature nitrogen is injected, so that the temperature of the coal seam is directly reduced, the chemical reaction between coal and oxygen is weakened, and the heating effect of the chemical reaction heat on the coal seam is reduced.

6. The method of claim 1, wherein the hot air is heated to a desired air temperature by using a booster with an adjustable final cooler, adjusting the final cooler of the booster, or by using a temperature-adjustable duct gas heater to heat the high-pressure air output from the booster to a desired air temperature, or by using both the booster with an adjustable final cooler and the temperature-adjustable duct gas heater to obtain a desired air temperature, or by using any other means to heat the high-pressure air output from the booster to a desired air temperature.

7. The method of claim 1, wherein the initial temperature is selected from the range of normal temperature to 70 ℃, wherein the heating rate is selected from the range of 20 ℃/day to 10 ℃/month, wherein the normal operating temperature is selected from the range of three minutes of the coal ignition temperature of the coal bed to be injected, and wherein the accelerated oxygen depletion temperature is selected from the range of four minutes of the coal ignition temperature of the coal bed to be injected.

8. The method for increasing production of coal bed methane by injecting high temperature air according to claim 1, wherein the initial temperature, the temperature rise rate, the normal operation temperature and the accelerated oxygen consumption temperature are determined according to the laboratory-determined rule that the oxygen consumption of the coal bed sample under the low temperature oxidation effect changes with the temperature.

9. The method for increasing production of coal bed methane by injecting high temperature air according to claim 1, wherein the initial temperature, the heating rate, the normal operation temperature and the accelerated oxygen consumption temperature are determined by taking various characteristics of the coal bed methane reservoir into consideration, and the initial temperature, the heating rate, the normal operation temperature and the accelerated oxygen consumption temperature are selected and determined, and the initial temperature, the heating rate, the normal operation temperature and the accelerated oxygen consumption temperature are adjusted along with the characteristic change of the coal bed methane reservoir along with the increase of the high temperature air injection time.