Light interference type methane concentration determination method based on environmental gas components and volume fraction

Abstract

The invention relates to a light interference type methane concentration determination method based on environmental gas components and volume fraction, and belongs to the technical field of coal mine underground gas detection. The method comprises the following steps: simplifying gas components in an underground environment according to the gas components in the underground environment and the influence degree of corresponding gas on methane concentration measurement; determining environment temperature and atmospheric pressure reference values in the methane concentration calculation process, and obtaining gas refractive indexes under the corresponding reference values; the method comprises the following steps: acquiring filtered water vapor and carbon dioxide and underground atmosphere which only filters out water vapor, sequentially filling a gas chamber to obtain two groups of mixed gas refractive index and interference fringe displacement data, constructing an equation set related to methane and carbon dioxide concentration based on the two groups of data, and solving the equation set; and correcting the concentration of methane and carbon dioxide according to a temperature and pressure correction coefficient calculated according to the actually measured temperature and pressure of the point to be measured. According to the method, various factors influencing the methane concentration measurement accuracy are comprehensively considered, and the measurement accuracy of the methane concentration is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas detection in coal mines, and relates to a method for measuring methane concentration by optical interference based on environmental gas components and volume fractions. Background Art

[0002] Effective prevention and control of gas disasters remain a difficult problem and challenge constantly faced by coal mining enterprises. Accurately detecting gas concentration has important practical guiding significance. About 90% of the components in gas are methane. Therefore, accurately measuring methane concentration is the cornerstone and guarantee for effective prevention and control of gas disasters. At present, optical interference type methane concentration detectors using optical principles have been widely used in coal mining enterprises due to their simple structure, strong stability and reliability.

[0003] In the traditional optical interference type methane concentration measurement model, in addition to the temperature and pressure correction coefficients, only the influence of moisture and CO 2 on the measurement result is considered, and it is filtered out before measurement, but the influence of the components and volume fractions of the mixed gas in the coal mine underground environment on the measurement result is not fully considered. In the traditional model, the refractive index of the mixed gas entering the gas chamber (measurement chamber) of the optical interference type methane concentration detector is expressed as Xn m +(1 - X)n a , where X represents the methane volume fraction, n m represents the refractive index of methane, and n a represents the refractive index of air. It can be seen from this formula that in the traditional model, the refractive indices of the gases other than methane in the underground mixed gas are all simplified as fixed values according to the refractive index of air, that is, regardless of whether the gas components and volume fractions at the measurement point are the same, the refractive index of the mixed gas other than methane is regarded as a fixed value and remains unchanged. However, research results show that in addition to the large influence of moisture and CO 2 on the measurement result, the concentrations of O 2 and N 2 in the underground environmental gas will also have a great influence on the measurement result of methane concentration, while the concentrations of CO, H 2 S, SO 2 and other gases have little influence on the measurement result due to their very low concentrations. This leads to large fluctuations or distorted measurement data in the detection results of optical interference type methane concentration detectors in some low-oxygen environments such as the return air corner of the underground working face, the measurement range is limited, the human input is wasted, and the effective judgment of the true value of the gas concentration is affected. Summary of the Invention

[0004] In view of this, the object of the present invention is to provide a method for measuring methane concentration by optical interference based on environmental gas components and volume fractions, improve the measurement accuracy and applicable range of the optical interference type methane concentration detector, solve the problem of unstable measurement results in the low-oxygen environment underground in coal mines, promote the development of precise methane concentration measurement technology, reduce the labor input cost at the same time, and enhance the mine safety guarantee.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for measuring methane concentration by optical interference based on environmental gas components and volume fractions, the method includes:

[0007] S1. Simplify the gas components in the underground environment according to the underground environmental gas components and the influence degree of the corresponding gases on the methane concentration measurement.

[0008] CO, H with tiny concentrations in the underground environmental gas and having no influence on the methane concentration measurement result 2 S, SO 2 and nitrogen oxide gases can all be regarded as N 2 , thus regarding the underground environmental gas as being composed of O 2 , N 2 , CH 4 and CO 2 to achieve the simplification of the gas components in the underground environment.

[0009] S2. Establish the reference values of environmental temperature and atmospheric pressure in the methane concentration calculation process, and obtain the gas refractive index under the corresponding reference values of environmental temperature and atmospheric pressure.

[0010] S3. At the underground methane concentration measurement point, collect the underground atmosphere, filter water vapor and carbon dioxide, then fill it into the gas chamber of the optical interference type methane concentration detector, calculate the refractive index of the first mixed gas in the gas chamber, and record the first displacement amount of the interference fringes in the optical interference type methane concentration detector;

[0011] S4. Clean the gas in the gas chamber of the optical interference type methane concentration detector, then fill the underground atmosphere filtered only of water vapor into the gas chamber, calculate the refractive index of the second mixed gas in the gas chamber, and record the second displacement amount of the interference fringes in the optical interference type methane concentration detector;

[0012] S5. According to the refractive index of the first mixed gas, the refractive index of the second mixed gas, the first displacement amount and the second displacement amount, construct a system of equations about methane concentration and carbon dioxide concentration and solve it;

[0013] S6. Measure and obtain the ambient temperature and atmospheric pressure values at the underground methane concentration measurement point to calculate the temperature and pressure correction coefficient, and correct the solved methane and carbon dioxide concentration data according to the temperature and pressure correction coefficient to obtain the true methane concentration and carbon dioxide concentration at the underground methane concentration measurement point.

[0014] Further, in step S2, the reference values of the ambient temperature and atmospheric pressure are respectively: the ambient temperature is 20 °C, and the atmospheric pressure is 1013.25 hPa.

[0015] Further, in step S3, the refractive index of the first mixed gas is calculated by the following formula:

[0016] n t =c 1 n 1 / (1 - c 2 ) + c 3 n 3 / (1 - c 2 )+(1 - c 1 - c 2 - c 3 )n 4 / (1 - c 2 )

[0017] In the formula, n t represents the refractive index of the first mixed gas, c 1 , c 2 , c 3 respectively represent the methane concentration, carbon dioxide concentration, and oxygen concentration in the underground gas, n 1 , n 3 respectively represent the refractive indices of methane and oxygen under the reference values of ambient temperature and atmospheric pressure; n 4 represents the refractive index of the remaining gas in the atmospheric environment at the underground methane concentration measurement point under the reference values of ambient temperature and atmospheric pressure, excluding oxygen, methane, and carbon dioxide. Since the concentrations of gases such as CO, H 2 S, SO 2 and nitrogen oxides in the remaining gas are very small and have no influence on the methane concentration measurement result, the refractive index n 4 of this remaining gas takes the refractive index value of nitrogen under the reference values of ambient temperature and atmospheric pressure.

[0018] In step S4, the refractive index of the second mixed gas is calculated by the following formula:

[0019] n = c 1 n 1 + c 2 n 2 + c 3 n 3 +(1 - c1 -c 2 -c 3 )n 4

[0020] In the formula, n represents the refractive index of the second mixed gas, and n 2 represents the refractive index of carbon dioxide under the reference values of ambient temperature and atmospheric pressure.

[0021] Furthermore, in step S5, the equations system constructed for methane concentration and carbon dioxide concentration is expressed as:

[0022]

[0023] n t =c 1 n 1 / (1 - c 2 ) + c 3 n 3 / (1 - c 2 ) + (1 - c 1 -c 2 -c 3 )n 4 / (1 - c 2 )

[0024] n = c 1 n 1 +c 2 n 2 +c 3 n 3 +(1 - c 1 -c 2 -c 3 )n 4

[0025] In the formula, n a represents the refractive index of the gas in the reference gas chamber of the methane concentration detector under the reference values of ambient temperature and atmospheric pressure; N t 、N b respectively represent the first displacement and the second displacement; L represents the length of the gas chamber for gas containing methane in the methane concentration detector, and λ represents the wavelength of light. Among them, the oxygen concentration c 3 can be obtained through a sensor, and c 1 and c 2 are solved based on the above equations system.

[0026] Furthermore, step S6 includes first calculating the temperature and pressure correction coefficient Q:

[0027] Q = [(273.15 + T) / 293.15]×[760 / (0.75P)]

[0028] Then, the methane concentration and carbon dioxide concentration obtained by solution are corrected by the temperature and pressure correction coefficient:

[0029]

[0030] In the formula, T represents the ambient temperature at the underground methane concentration measurement point, P represents the atmospheric pressure at the underground methane concentration measurement point, c 1ture , c 2ture respectively represent the true methane concentration and carbon dioxide concentration at the underground methane concentration measurement point.

[0031] The beneficial effects of the present invention are as follows:

[0032] (1) The present invention optimizes the method in the traditional optical interference type methane concentration measurement principle of regarding the refractive index of gases other than methane underground as a fixed value. Based on the actual components and volume fractions of underground environmental gases, a methane concentration measurement method with a variable refractive index for gases other than methane is proposed, which is more applicable to the variable underground coal mine atmospheric environment.

[0033] (2) The present invention comprehensively considers various factors affecting the measurement accuracy of methane concentration, including environmental factors such as the ambient temperature and atmospheric pressure at the measurement location, and at the same time considers the influence of gas concentrations such as oxygen and nitrogen underground on the methane concentration measurement, effectively improving the measurement accuracy of methane concentration and promoting the safe production of coal mines.

[0034] (3) The present invention can simultaneously measure the carbon dioxide gas concentration in the underground environment, and the operation process is the same as that of the traditional methane concentration detector. Therefore, for coal mine enterprises, only the calculation method proposed by the present invention needs to be referred to for measuring the carbon dioxide gas concentration, and there is no need to replace a large number of measuring instruments, thus effectively saving the production input cost while ensuring the measurement accuracy.

[0035] Other advantages, objectives and features of the present invention will be described to some extent in the subsequent specification, and to some extent, will be obvious to those skilled in the art based on the study of the following text, or can be taught from the practice of the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the following specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail preferably with reference to the accompanying drawings, wherein:

[0037] Figure 1 is a schematic flow chart of the optical interference type methane concentration measurement method based on environmental gas components and volume fractions according to the present invention;

[0038] Figure 2It is a schematic diagram of the optical structure of an optical interference type methane concentration detector. Specific embodiments

[0039] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the diagrams provided in the following embodiments only illustrate the basic concept of the present invention in a schematic manner. Without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0040] Among them, the drawings are only for illustrative purposes, showing only schematic diagrams, not physical diagrams, and cannot be understood as a limitation to the present invention; in order to better illustrate the embodiments of the present invention, some components in the drawings will be omitted, enlarged or reduced, which do not represent the dimensions of actual products; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0041] In the drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms such as "upper", "lower", "left", "right", "front", "rear", etc. indicating the orientation or positional relationship, it is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the positional relationship in the drawings are only for illustrative purposes and cannot be understood as a limitation to the present invention. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0042] In order to improve the measurement accuracy and applicable range of the optical interference type methane concentration detector and solve the problem of unstable measurement results in the low-oxygen environment in coal mines, based on the traditional optical interference type methane concentration measurement model, the present invention proposes an optical interference type methane concentration measurement method based on environmental gas components and volume fractions, aiming to improve the accuracy of methane concentration measurement, promote coal mine safety production, and at the same time save production input costs.

[0043] As Figure 1 shown, it is an optical interference type methane concentration measurement method based on environmental gas components and volume fractions provided by an embodiment of the present invention, including the following steps:

[0044] S1: According to the underground environmental gas components and the influence degree of the corresponding gases on the methane concentration measurement, simplify the gas components in the underground environment, and set that the gases in the underground environment in the methane concentration measurement method are O2 , N 2 , CH 4 , CO 2 consists of, where CH 4 , CO 2 , O 2 and other three kinds of gases, the volume concentrations of these three gases are respectively denoted as c 1 , c 2 , c 3 ; the remaining gases are regarded as N 2 for processing, and its volume concentration is denoted as c 4 , c 4 = 1 - (c 1 + c 2 + c 3 ).

[0045] Among them, the remaining gases include N 2 , CO, H 2 S, SO 2 and nitrogen oxides, etc. The concentrations of CO, H 2 S, SO 2 and nitrogen oxides and other gases are very low and have no influence on the measurement result of methane concentration. Therefore, a simplified treatment can be carried out, and all the remaining gases are regarded as N 2 for processing, that is, the concentration and refractive index value of the remaining gases both take the concentration and refractive index value of N 2 .

[0046] Among them, the value of the gas volume concentration c 3 is obtained through the sensor, and c 1 , c 2 are unknowns to be calculated.

[0047] S2: Unify the standard of environmental temperature and atmospheric pressure values in the methane concentration calculation process. Since the gas refractive index is affected by environmental temperature and atmospheric pressure, and the gas refractive index is different under different environmental temperatures or atmospheric pressures, it is necessary to first establish the reference values of environmental temperature and atmospheric pressure, and then obtain the specific values of the gas refractive index under the corresponding environmental temperature and atmospheric pressure conditions by looking up the table.

[0048] Generally, normal temperature and pressure (temperature T 0 = 20°C, pressure P 0 = 1013.25 hPa) is used as the reference benchmark to determine the exact refractive index values of four kinds of gases, namely CH 4 , CO 2 , O 2 , N 2 , which are respectively denoted as n 1 , n 2 , n 3 , n 4 .

[0049] In addition, under the conditions of the above unified standard ambient temperature and atmospheric pressure, the refractive index value of fresh air gas is n a . The refractive index is a known parameter and can be obtained by looking up a table.

[0050] S3: In the fresh air flow on the ground, before testing the optical interference type methane concentration detector, clean the air chamber, check the performance of the gas absorbent, check the airtightness, and zero the scale; after passing the inspection, fill the reference air chamber of the detector with fresh air and seal it.

[0051] Among them, the gas absorbents are soda lime and silica gel filled in the drying tube group respectively, where soda lime is used to absorb carbon dioxide and silica gel is used to absorb moisture.

[0052] S4: At the underground location where the methane concentration is to be measured, suck the underground atmosphere filtered of water vapor and carbon dioxide into the gas chamber of the optical interference type methane concentration detector, determine the refractive index n t of the mixed gas in the gas chamber, and read the moving distance N t of the interference fringes in the methane concentration detector.

[0053] Among them, the refractive index n t of the mixed gas in the gas chamber is calculated as follows:

[0054] n t = c 1 n 1 / (1 - c 2 ) + c 3 n 3 / (1 - c 2 ) + (1 - c 1 - c 2 - c 3 )n 4 / (1 - c 2 )

[0055] S5: Clean the gas in the gas chamber, then suck the underground atmosphere filtered only of water vapor into the gas chamber of the optical interference type methane concentration detector, determine the refractive index n of the mixed gas in the gas chamber again, and read the moving distance N b of the interference fringes in the methane concentration detector.

[0056] Among them, the calculation formula of the refractive index n is expressed as:

[0057] n = c 1 n 1 + c 2 n 2 + c 3 n 3 + (1 - c 1-c 2 -c 3 )n 4

[0058] S6: Based on the linear relationship between the displacement of the interference fringes and the methane concentration, and combined with the data read after inhaling the underground air twice, establish a system of equations containing the unknowns c 1 、c 2 and solve it.

[0059] The established system of equations is as follows:

[0060]

[0061] Among them, L is the length of the gas chamber for gas in the detector, and λ is the wavelength of light.

[0062] S7: Measure and obtain the environmental temperature and atmospheric pressure values at the methane concentration measurement location underground. According to the temperature and pressure correction coefficient Q, correct the methane and carbon dioxide concentration data obtained by solving the system of equations, and finally determine the true methane concentration c 1ture and carbon dioxide concentration c 2ture at the measurement point.

[0063] Among them, the method for determining the temperature and pressure correction coefficient Q is:

[0064] Q = [(273.15 + T) / 293.15] × [760 / (0.75P)]

[0065] In the above formula, T is the environmental temperature at the measurement location, in °C; P is the atmospheric pressure at the measurement location, in hPa.

[0066] Then c 1ture and c 2ture are corrected respectively by the following formulas:

[0067]

[0068] In this embodiment, the method of regarding the refractive index of the gases other than methane underground as a fixed value in the traditional optical interference type methane concentration measurement principle is optimized. Based on the actual components and volume fractions of the underground environmental gases, a methane concentration measurement method with the refractive index of the gases other than methane as a variable is proposed, which is more suitable for the variable atmospheric environment underground in coal mines. The method provided in this embodiment comprehensively considers various factors affecting the measurement accuracy of methane concentration, including environmental factors such as the environmental temperature and atmospheric pressure at the measurement location, and at the same time considers the influence of the gas concentrations such as oxygen and nitrogen underground on the methane concentration measurement, improving the measurement accuracy of methane concentration. In addition, the method provided in this embodiment can simultaneously measure the carbon dioxide gas concentration in the underground environment, and the operation process is the same as that of the traditional methane concentration detector.

[0069] Such asFigure 2 As shown, it is a schematic diagram of the optical structure of the optical interference type methane concentration detector, which has two reference gas chambers and one methane gas chamber. The two reference gas chambers are located on both sides of the methane gas chamber, and there is no communication between the reference gas chamber and the methane gas chamber.

[0070] Based on Figure 2 The working principle of the optical interference type methane concentration detector based on the above optical structure is that a laser is generated by a laser light source. The laser is incident on a parallel flat mirror through lens A and prism A in sequence and is divided into two beams. The first beam of light is reflected by the first surface of the parallel flat mirror and enters the reference gas chamber on one side. The second beam of light first passes through the first surface of the parallel flat mirror and is refracted, then is reflected by the second surface and enters the methane gas chamber. After the first beam of light exits the reference gas chamber, it enters the reference gas chamber on the other side after two reflections by prism C, and then exits to the parallel flat mirror, passes through the first surface of the parallel flat mirror and is refracted and reflected by the second surface to reach prism B, and then enters the eyepiece through lens B and lens C. After the second beam of light exits the methane gas chamber, it is reflected by prism C and enters the methane gas chamber again. After exiting the methane gas chamber again, it reaches the parallel flat mirror, passes through the first surface of the parallel flat mirror and is refracted and reflected by the second surface to reach prism B, and then enters the eyepiece through lens B and lens C. Since the first beam of light and the second beam of light come from the same coherent light source, light interference is generated. The displacement of the scale of the interference fringe relative to the zero scale of the interference fringe is observed and recorded through the eyepiece.

[0071] According to the above principle, two light absorption experiments are carried out respectively. For the first time, the underground gas filtered out of water vapor and carbon dioxide is filled into the methane gas chamber. After the laser is absorbed by the gas, interference is generated, and the first interference fringe scale displacement is recorded. For the second time, the underground gas filtered out of only carbon dioxide is filled into the methane gas chamber. After the laser is absorbed by the gas, interference is generated, and the second interference fringe scale displacement is recorded.

[0072] In summary, the present invention proposes an optical interference type methane concentration detector based on the environmental gas components and volume fractions, which optimizes the method of regarding the refractive index of the gas other than methane in the traditional optical interference type methane concentration measurement principle as a fixed value. Based on the actual components and volume fractions of the underground environmental gas, a methane concentration measurement method with the refractive index of the gas other than methane as a variable is proposed, which improves the measurement accuracy of methane concentration and is more suitable for the changeable underground coal mine atmosphere.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the purpose and scope of the present technical solution, and they should all be covered by the scope of the claims of the present invention.

Claims

1. A method for measuring methane concentration based on optical interference of ambient gas composition and volume fraction, characterized in that: The method includes: S1. Simplify the gas components in the downhole environment according to the gas components in the downhole environment and the influence of the corresponding gas on the methane concentration measurement; S2. Establish reference values ​​of ambient temperature and atmospheric pressure during the calculation of methane concentration, and obtain the gas refractive index under the corresponding reference values ​​of ambient temperature and atmospheric pressure; S3, at the underground methane concentration to be measured point, collect underground atmosphere, filter water vapor and carbon dioxide, and then fill it into the gas chamber of the optical interference type methane concentration measuring device, calculate the refractive index of the first mixed gas in the gas chamber, and record the first displacement of the interference fringes in the optical interference type methane concentration measuring device; S4, cleaning the gas in the gas chamber of the optical interference type methane concentration measuring device, then filling the gas chamber with underground atmosphere from which only water vapor has been filtered out, calculating the refractive index of the second mixed gas in the gas chamber, and recording the second displacement of the interference fringes in the optical interference type methane concentration measuring device; S5, constructing and solving a set of equations about methane concentration and carbon dioxide concentration according to the refractive index of the first mixed gas, the refractive index of the second mixed gas, the first displacement, and the second displacement; S6. Measure and obtain the ambient temperature and atmospheric pressure of the underground methane concentration test point to calculate the temperature and pressure correction coefficient, and correct the solved methane and carbon dioxide concentration data according to the temperature and pressure correction coefficient to obtain the actual methane concentration and carbon dioxide concentration of the underground methane concentration test point.

2. The method according to claim 1, characterized in that: In step S3, the refractive index of the first mixed gas is calculated by the following formula: n t =c1n1 / (1-c2)+c3n3 / (1-c2)+(1-c1-c2-c3)n4 / (1-c2) Where n t represents the refractive index of the first mixed gas, c1, c2, and c3 represent the methane concentration, carbon dioxide concentration, and oxygen concentration in the downhole gas, respectively, n1 and n3 represent the refractive indices of methane and oxygen, respectively, at the reference values ​​of ambient temperature and atmospheric pressure; n4 represents the refractive index of the remaining gases other than oxygen, methane, and carbon dioxide in the atmospheric environment of the downhole methane concentration test point at the reference values ​​of ambient temperature and atmospheric pressure, and the value of n4 is the refractive index value of nitrogen at the reference values ​​of ambient temperature and atmospheric pressure.

3. The method according to claim 1, characterized in that In step S4, the refractive index of the second mixed gas is calculated by the following formula: n=c1n1+c2n2+c3n3+(1-c1-c2-c3)n4 In the formula, n represents the refractive index of the second mixed gas, c1, c2, and c3 represent the methane concentration, carbon dioxide concentration, and oxygen concentration in the downhole gas, respectively, and n1, n2, and n3 represent the refractive indices of methane, carbon dioxide, and oxygen at the reference values ​​of ambient temperature and atmospheric pressure, respectively; n4 represents the refractive index of the remaining gases other than oxygen, methane, and carbon dioxide in the atmospheric environment of the downhole methane concentration test point at the reference values ​​of ambient temperature and atmospheric pressure, and the value of n4 is the refractive index value of nitrogen at the reference values ​​of ambient temperature and atmospheric pressure.

4. The method according to claim 1, characterized in that In step S5, the constructed equation group about methane concentration and carbon dioxide concentration is expressed as: n t =c1n1 / (1-c2)+c3n3 / (1-c2)+(1-c1-c2-c3)n4 / (1-c2) n=c1n1+c2n2+c3n3+(1-c1-c2-c3)n4 Where n t represents the refractive index of the first mixed gas, and n represents the refractive index of the second mixed gas; c1, c2, and c3 represent the methane concentration, carbon dioxide concentration, and oxygen concentration in the downhole gas, respectively, and c1 and c2 are solved by the equation group; n1, n2, and n3 represent the refractive indices of methane, carbon dioxide, and oxygen at the reference values ​​of ambient temperature and atmospheric pressure, respectively; n4 represents the refractive index of the remaining gases other than oxygen, methane, and carbon dioxide in the atmospheric environment of the downhole methane concentration test point at the reference values ​​of ambient temperature and atmospheric pressure, and the value of n4 is the refractive index value of nitrogen at the reference values ​​of ambient temperature and atmospheric pressure; n a Represents the refractive index of the gas in the reference gas chamber of the methane concentration measuring device at the reference value of ambient temperature and atmospheric pressure; N t 、N b They represent the first displacement and the second displacement respectively; L represents the length of the gas chamber in the methane concentration measuring device, and λ represents the wavelength of light.

5. The method according to any one of claims 2 to 4, characterized in that: The reference values ​​of the ambient temperature and the atmospheric pressure are specifically: the ambient temperature is 20° C., and the atmospheric pressure is 1013.25 hPa.

6. The method according to claim 1, characterized in that Step S6 includes first calculating the temperature and pressure correction coefficient Q: Q=[(273.15+T) / 293.15]×[760 / (0.75P)] Then the obtained methane concentration and carbon dioxide concentration are corrected by the temperature and pressure correction coefficient: In the formula, T represents the ambient temperature of the underground methane concentration test point, P represents the atmospheric pressure of the underground methane concentration test point, c1 and c2 represent the methane concentration and carbon dioxide concentration in the underground gas obtained by solving the constructed equations, respectively, and c 1ture 、c 2ture They respectively represent the actual methane concentration and carbon dioxide concentration at the underground methane concentration test point.

7. The method according to claim 1, characterized in that Step S1 includes considering CO, H2S, SO2 and nitrogen oxide gases in the downhole ambient gas with very small concentrations and no effect on the methane concentration measurement result as N2, thereby considering the downhole ambient gas to be composed of O2, N2, CH4 and CO2.

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