A method for predicting gas content in a coal seam

Abstract

The present application provides a kind of coal seam gas content prediction method, it is related to coalbed gas exploration and development technical field.The present application includes the following steps: first, select coal sample, by vitrinite random reflectance automatic determination image analysis method, obtain the maximum vitrinite reflectance R o,max Then, isothermal gas adsorption experiment is carried out to each coal sample, and the Langmuir volume V L And Langmuir pressure P L According to the maximum vitrinite reflectance R o,max Sample is divided into anthracite sample group, bituminous sample group and sub-bituminous sample group according to coal rank;Finally, the calculation formula of coal seam gas content change with depth under different coal rank is established, and the actual gas emission of coal mine is used to test, realize the calculation of coal seam gas content change with depth, the present application is simple and convenient, low in cost, can obtain the coal seam gas content of different coal mines, has good application effect and wide applicability.

Description

Technical Field

[0001] This invention belongs to the field of coalbed methane exploration and development technology, and specifically relates to a method for predicting the gas content of coal seams. Background Technology

[0002] Coalbed methane (CBM) is primarily composed of methane, which is mainly adsorbed on the surface of the coal matrix and buried within the coal-forming geology. CBM is both a potent greenhouse gas and a clean energy source. However, China's research on methane inventories is currently relatively insufficient in both depth and breadth, leading to significant resource waste. Over the past decades, the annual release of usable CBM into the atmosphere has caused a dramatic increase in atmospheric CH4 concentration.

[0003] Due to factors such as coal rank and mining depth, the patterns and characteristics of methane emission from coal mines vary across different regions. These factors pose significant challenges to methane emission calculations and coalbed methane extraction plans. Therefore, a convenient and accurate method for calculating the gas content of coal seams in different coal mines is urgently needed.

[0004] Currently, methods for calculating the gas content of coal seams are mainly divided into two categories. One category is the direct method and the improved direct method, which involves directly measuring the gas content according to the relevant standards of the U.S. Bureau of Mines. This method yields accurate results but is very labor-intensive and resource-intensive. The second category is the indirect method, one of which is the isothermal adsorption method. The isothermal adsorption method is well-developed, but it currently has two shortcomings: (1) it requires relatively complicated isothermal adsorption experiments, which are costly; (2) the results obtained are only applicable to a specific coal mine and cannot be extrapolated. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention aims to provide an accurate and rapid method for calculating the gas content of coal seams for three coal ranks: lignite, bituminous coal, and anthracite. This method is more convenient, lower in cost, and applicable to various coal mines with complex and varied geological conditions.

[0006] The technical solution of this invention is as follows:

[0007] A method for predicting the gas content of coal seams specifically includes the following steps:

[0008] Step 1: Select a coal sample and obtain the maximum vitrinite reflectance R using the automatic vitrinite random reflectance measurement image analysis method. o,max .

[0009] The automatic image analysis method for determining the random reflectance of vitrinite involves sequentially acquiring microscopic images of a coal sample using a microscope camera and an automatic stage under oil immersion objective and reflected light conditions with perpendicular incident light. Grayscale measurements are then performed on all measurement points in the microscopic images. The effective vitrinite measurement points are identified using the automatic random reflectance determination system, and the random reflectance values ​​of each effective vitrinite measurement point are statistically analyzed to obtain the maximum vitrinite reflectance of the sample.

[0010] Step 2: Isothermal gas adsorption experiments were conducted on each coal sample selected in Step 1 to obtain the isothermal adsorption test results. These results include the Langmuir volume V. L Langmuir pressure P L V L and P L It was obtained by fitting the results of isothermal adsorption tests.

[0011] The isothermal gas adsorption experiment is as follows: First, the sample is degassed by placing the coal sample in a sample tube. Then, it is heated under vacuum. The sample tube is then placed in the analysis station of a physical adsorption instrument. At the isothermal adsorption temperature, the adsorbate gas is gradually introduced into the evacuated sample tube. The adsorbate molecules entering the sample tube reach the surface of each pore on the coal sample. The adsorbate interacts with the active sites on the sample surface, generating intermolecular forces. The phenomenon of adsorbate molecules adhering to the solid surface is called adsorption. By measuring the adsorption amount corresponding to a series of pressures, a set of data on the adsorption amount corresponding to the pressure is obtained, resulting in an adsorption isotherm. After the adsorption process is completed, the Langmuir volume constant V of the corresponding coal sample is obtained by fitting according to the Langmuir monolayer adsorption theory. L Langmuir pressure constant P L .

[0012] The Langmuir volume V L and Langmuir pressure P L Isothermal gas adsorption experiments were conducted under equilibrium water conditions, and the experimental data were fitted according to the Langmuir monolayer adsorption theory. Equilibrium water conditions refer to the process of equilibrium water treatment of coal samples before conducting isothermal adsorption experiments, so that the moisture content of the coal samples is close to that of the original coal.

[0013] Langmuir volume V L and Langmuir pressure P L The relationship is represented by the following relation:

[0014]

[0015] Among them, V L P is the Langmuir volume constant; LLet P be the Langmuir pressure constant, when the adsorption pressure P equals the Langmuir pressure P. L At that time, the amount of coal adsorbed is equal to half of the Langmuir volume, i.e., P = P L At that time, V = V L / 2;V L and P L The magnitude of the value depends on the properties of the coal and is obtained by fitting the results of isothermal adsorption experiments.

[0016] Step 3: Based on the maximum vitrinite reflectance R obtained in Step 1 o,max The samples were divided into three groups according to coal rank: anthracite sample group, bituminous coal sample group, and sub-bituminous coal sample group.

[0017] Among them, the vitrinite reflectance range of lignite is R. o,max <0.5, the vitrinite reflectance range of bituminous coal is 0.5≤R o,max <2, the vitrinite reflectance range of anthracite is 2≤R o,max <8;

[0018] Step 4: Establish a calculation formula for the change of gas content in coal seams with mining depth under different coal ranks, and verify it with the actual gas emission of the coal mine.

[0019] The calculation formulas for the variation of gas content in coal seams of different coal ranks with mining depth are as follows:

[0020]

[0021] Where d is the raw coal mining depth in meters; R is the Langmuir constant; V gas This refers to the gas content of the coal seam, expressed in cubic meters (m³). 3 / t; A and B are undetermined coefficients; for the calculation formula of gas content in coal seams of different coal ranks, the values ​​of coefficients A and B are different. Among them, the coefficients of anthracite are named A1 and B1; the coefficients of bituminous coal are named A2 and B2; and the coefficients of lignite are named A3 and B3.

[0022] The Langmuir volume V measured for the anthracite, bituminous coal, and lignite sample groups taken in step 3. L Langmuir pressure P L The data were simulated using the Monte Carlo method. First, it was assumed that the data of the anthracite, bituminous coal, and lignite sample groups conformed to a log-normal distribution. Undetermined coefficients A1, B1, A2, B2, A3, and B3 were obtained through fitting. The Anderson-Darling test was used to measure the goodness of fit of the probability distribution function. If the goodness of fit was above 99%, the values ​​of the undetermined coefficients were considered reliable. If the goodness of fit was less than 99%, the probability distribution conformed to by the corresponding sample groups needed to be re-assumed until the goodness of fit was above 99%.

[0023] Substituting the undetermined coefficients A1, B1, A2, B2, A3, and B3 obtained from the above fitting into the formulas, we obtain the calculation formulas for the variation of gas content in coal seams with mining depth for the three coal ranks as follows:

[0024]

[0025]

[0026]

[0027] The subscripts 1, 2, and 3 represent the anthracite sample group, the bituminous coal sample group, and the sub-bituminous coal sample group, respectively.

[0028] Compared with the prior art, the technical solution proposed in this invention has the following beneficial effects:

[0029] First, the method of this invention provides a calculation formula for the change of coal seam gas content with mining depth under different coal ranks. The coal seam gas content can be obtained based solely on the mining depth, eliminating the need for cumbersome isothermal adsorption experiments, making the calculation fast and convenient. Second, since coalbed methane is a associated mineral resource of coal, the coal seam gas content is closely related to the coal rank, and the maximum vitrinite reflectance R... o,max It can reflect changes in coal rank. Based on the isothermal adsorption test results of 213 coal samples, this invention analyzes the samples according to R... o,max The sample groups were divided into three different coal ranks. A probability distribution function was established using the Monte Carlo simulation method. The undetermined coefficients were obtained by fitting the samples. This method can be used to calculate the gas content of coal seams in different coal mines with complex and variable geological conditions, and has wide applicability. Finally, the method of this invention is based on adsorption theory and Monte Carlo simulation, which can effectively avoid equipment wear and damage caused by experiments and can effectively reduce costs.

[0030] pass Figures 3 to 5 The comparison chart of coal seam gas content calculated using this invention and experimentally measured coal seam gas content shows that the coal seam gas content calculated using this invention can achieve the same calculation and testing results as those performed according to the standard procedure of "Method for Determination of Coal Seam Gas Content (GB / T19559-2004)". The calculation is simple, the cost is low, and it can obtain the coal seam gas content of different coal mines, showing good application effect and wide applicability. Attached Figure Description

[0031] To more clearly illustrate the specific methods in the embodiments of the present invention, the relevant drawings involved in the embodiments will be briefly described below. The drawings below are only preferred embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative changes.

[0032] Figure 1This is a flowchart illustrating the method for predicting coal seam gas content in this embodiment.

[0033] Figure 2 This is a graph showing the relationship between the gas content of coal seams of different coal ranks and the mining depth in this embodiment.

[0034] Figure 3 This is a comparison chart of the gas content of lignite calculated using the gas content prediction method of this embodiment and its actual measured value.

[0035] Figure 4 This is a comparison chart of the coal seam gas content calculated using the coal seam gas content prediction method of this embodiment and the actual measured value.

[0036] Figure 5 This is a comparison chart of the gas content of the coal seam calculated using the gas content prediction method of this embodiment and the actual measured value. Detailed Implementation

[0037] To facilitate understanding of this application, a more complete description will be provided below with reference to the accompanying drawings. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0038] Combining multiple prediction methods or developing new prediction methods is a major direction for the future development of coalbed methane resource evaluation. Coal adsorption of methane is a physical adsorption process on a solid surface, conforming to the Langmuir adsorption equation. Gas content is obtained through isothermal adsorption experiments, and further fitting yields Langmuir pressure and volume. Although the isothermal adsorption experiment process is relatively cumbersome, existing research results on coal sample methane adsorption can be integrated into a database. Combining the advantages of statistical methods and isothermal adsorption methods, this database can be used to calculate the Langmuir parameters of three coal ranks to predict coalbed methane content, thus achieving the prediction of coalbed methane content. Figure 1 As shown, a method for predicting the gas content of coal seams specifically includes the following steps:

[0039] Step 1: Select coal samples representative of the characteristics of coal-producing provinces in China, and obtain the maximum vitrinite reflectance R using the automatic determination and image analysis method of vitrinite random reflectance. o,max .

[0040] The automatic image analysis method for determining the random reflectance of vitrinite involves sequentially acquiring microscopic images of coal samples using a microscope camera and an automatic stage under conditions of oil immersion objective lens and reflected light (λ = 546 nm) of perpendicular incident light. Grayscale measurements are performed on all measurement points in the microscopic images. The effective measurement points of vitrinite are identified through the automatic random reflectance determination system, and the random reflectance values ​​of each effective measurement point are statistically analyzed to obtain the maximum vitrinite reflectance of the sample.

[0041] In this embodiment, a total of 213 coal samples from coal-producing provinces in China were selected. The maximum vitrinite reflectance R of each sample was obtained according to the standard "Automatic Measurement and Image Analysis Method for Random Vitrin Reflectance of Coal (GB / T 40485-2021)". o,max .

[0042] Step 2: Obtain the isothermal adsorption test results by conducting isothermal gas adsorption experiments on each coal sample selected in Step 1;

[0043] The isothermal gas adsorption experiment involves first degassing the sample to remove contaminants such as water and oil from the solid surface. In this embodiment, a 0.1g coal sample is placed in a sample tube and then heated under vacuum. The sample tube is then placed in the analysis station of the physical adsorption instrument. At the isothermal adsorption temperature, the adsorbate gas is gradually introduced into the evacuated sample tube. The adsorbate molecules quickly reach the surface of each pore on the coal sample. The adsorbate interacts with the active sites on the sample surface, generating intermolecular forces. The phenomenon of adsorbate molecules adhering to the solid surface is called adsorption. By measuring the adsorption amount corresponding to a series of pressures, a set of pressure-corresponding adsorption amount data is obtained, resulting in an adsorption isotherm. After the adsorption process is completed, the Langmuir volume constant V of the corresponding coal sample is obtained by fitting according to the Langmuir monolayer adsorption theory. L Langmuir pressure constant P L .

[0044] In this embodiment, isothermal adsorption test results were collected from 213 coal samples from coal-producing provinces in China. The physical adsorption instrument used was a Mack ASAP 2460 physical adsorption instrument. The adsorbate gas used in the isothermal gas adsorption experiment was methane gas with a purity of 99.99%. The isothermal adsorption temperature was 30 °C. The isothermal adsorption test results included: Langmuir volume VL. L Langmuir pressure P L V L and P L It was obtained by fitting the results of isothermal adsorption tests.

[0045] The Langmuir volume V L and Langmuir pressure PL The test was conducted according to the standard recommended by the American Society for Testing and Materials (ASTM), which involves conducting isothermal gas adsorption experiments under equilibrium water conditions and fitting the experimental data to the Langmuir monolayer adsorption theory. The equilibrium water condition refers to treating the coal sample to balance the moisture content before conducting the isothermal adsorption experiment, so that the moisture content of the coal sample is close to that of the original coal.

[0046] The relationship between Langmuir volume and Langmuir pressure is expressed by the following equation:

[0047]

[0048] Among them, V L P is the Langmuir volume constant; L V is the Langmuir pressure constant, which has the physical meaning of: L P represents the maximum adsorption capacity of coal. L The ratio of the desorption rate constant k1 to the adsorption rate constant k2 reflects the adsorption capacity of the coal's inner surface for gases. This ratio is given when the adsorption pressure P equals the Langmuir pressure P. L At that time, the amount of coal adsorbed is equal to half of the Langmuir volume, i.e., P = P L At that time, V = V L / 2. V L and P L The magnitude of the value depends on the properties of the coal and is obtained by fitting the results of isothermal adsorption experiments.

[0049] Step 3: Based on the maximum vitrinite reflectance R obtained in Step 1 o,max The samples were divided into three groups according to coal rank: anthracite sample group, bituminous coal sample group, and sub-bituminous coal sample group.

[0050] In this embodiment, coal samples are classified according to their maximum vitrinite reflectance according to the People's Republic of China Coal Industry Standard MT / T1053-2008, and the coal rank of each sample is determined. The vitrinite reflectance range for lignite is R. o,max <0.5, the vitrinite reflectance range of bituminous coal is 0.5≤R o,max <2, the vitrinite reflectance range of anthracite is 2≤R o,max <8;

[0051] In this embodiment, due to the maximum vitrinite reflectivity R o,max This step can reflect the changes in coal rank. Based on the maximum vitrinite reflectance of the samples, the coal samples are divided into three groups: anthracite sample group (93 samples), bituminous coal sample group (87 samples), and lignite sample group (33 samples).

[0052] Step 4: Establish a calculation formula for the change of gas content in coal seams with mining depth under different coal ranks, and verify it with the actual gas emission of the coal mine.

[0053] The calculation formulas for the variation of gas content in coal seams of different coal ranks with mining depth are as follows:

[0054]

[0055] Where d is the raw coal mining depth in meters; R is the Langmuir constant; V gas This refers to the gas content of the coal seam, expressed in cubic meters (m³). 3 / t; A and B are undetermined coefficients. Coal rank has a significant impact on the gas content of coal seams. For the calculation formula of gas content of coal seams of different coal ranks, the values ​​of coefficients A and B are different. The coefficients for anthracite are named A1 and B1; the coefficients for bituminous coal are named A2 and B2; and the coefficients for lignite are named A3 and B3.

[0056] The Langmuir volume V measured for the anthracite, bituminous coal, and lignite sample groups taken in step 3. L Langmuir pressure P L The data were simulated using the Monte Carlo method. First, it was assumed that the data from the anthracite, bituminous, and lignite sample groups conformed to a log-normal distribution, and the undetermined coefficients A1, B1, A2, B2, A3, and B3 were obtained. In this embodiment, the calculated values ​​were A1 = 40.80, B1 = 2.42; A2 = 23.17, B2 = 3.98; A3 = 10.94, and B3 = 4.12. The Anderson-Darling test was used to measure the goodness of fit of the probability distribution function. If the goodness of fit was above 99%, the values ​​of the undetermined coefficients were considered reliable. If the goodness of fit was less than 99%, a new probability distribution, such as a normal distribution or a discrete distribution, needed to be assumed for the corresponding sample groups until the goodness of fit was above 99%.

[0057] In this implementation, the probability distribution function fitting function in the Crystal Ball add-in of Microsoft Excel is used;

[0058] Substituting the undetermined coefficients A and B obtained from the above fitting into the formulas, we obtain the calculation formulas for the change of gas content in coal seams with mining depth for the three coal ranks as follows:

[0059]

[0060]

[0061]

[0062] The subscripts 1, 2, and 3 represent the anthracite sample group, the bituminous coal sample group, and the sub-bituminous coal sample group, respectively.

[0063] In summary, the method of this invention provides calculation formulas for the change of gas content in coal seams with mining depth under three coal ranks. The gas content in coal seams can be obtained based solely on the mining depth, eliminating the need for cumbersome isothermal adsorption experiments.

[0064] This embodiment collects isotherm test results of 213 coal samples from coal-producing provinces in China, including Langmuir volumes V of 93 anthracite samples, 87 bituminous coal samples, and 33 lignite samples. L and Langmuir pressure P L A corresponding database was compiled. Specific sample sources and calculation processes are shown in Table 1.

[0065] The actual gas emission volume of the coal mine is the measurement result obtained by the State Administration of Coal Mine Safety in accordance with the industry standard "Specification for Identification of Mine Gas Levels" (A01025-2006).

[0066] Table 1. Explanation of coal data used to calculate the parameters of the Langmuir isotherm model.

[0067]

[0068] Table 2 shows the measured V values ​​for the anthracite, bituminous coal, and lignite sample groups, respectively, processed using the Monte Carlo simulation method assuming the data conforms to a log-normal probability distribution. L P L The undetermined coefficients obtained from data fitting were used to establish calculation formulas for the variation of coal seam gas content with mining depth under three coal ranks, and to predict the gas content of coal seams at a given depth for a given coal rank with future mining plans. Figure 2 As shown.

[0069] Table 2 Undetermined coefficients for each coal rank used to predict coal seam gas content.

[0070] Coal rank A B anthracite 40.80 2.42 bituminous coal 23.17 3.98 lignite 10.94 4.12

[0071] The specific experimental measurements and calculation data are shown in Table 3.

[0072] Table 3. Data from coal sample experimental tests and calculations using the formula of this invention.

[0073]

[0074]

[0075]

[0076] Figure 3This is a comparison chart of the gas content of the coal seam calculated by the prediction method of this invention (in an embodiment of the invention) and the actual measured value. From... Figure 3 As can be seen from the results, the gas content of lignite seams predicted using this invention tends to be consistent with the actual measured values, demonstrating significant application effectiveness.

[0077] Figure 4 This is a comparison chart of the calculated gas content of the coal seam and the actual measured value using the prediction method of this invention for anthracite coal. From... Figure 4 As can be seen from the above, the error between the gas content of anthracite coal seams predicted by this invention and the actual measured value is small, and the application effect is obvious.

[0078] Figure 5 This is a comparison chart of the gas content of the coal seam calculated by the prediction method of this invention and the actual measured value. The error between the gas content of the coal seam predicted by this invention and the actual measured value is small, and the application effect is obvious.

[0079] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; therefore, these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A method of predicting gas content of a coal seam, characterized by, The method comprises the following steps: Step 1: Selecting coal samples, obtaining the maximum vitrinite reflectance R o,max ; Step 2: obtaining isotherm adsorption test results of each coal sample selected in step 1 by isothermal gas adsorption experiment, wherein the isotherm adsorption test results include: Langmuir volume V L , Langmuir pressure P L , V L and P L are fitted from the isotherm adsorption test results; where V is the Langmuir volume L and P is the Langmuir pressure L The isothermal gas adsorption experiments were carried out under equilibrium water conditions, and the experimental data were fitted according to the Langmuir monolayer adsorption theory; the equilibrium water conditions refer to the moisture content of the coal sample being close to that of the in-situ coal after the coal sample is subjected to moisture equilibrium treatment before the isothermal adsorption experiment. Step 3: Maximum vitrinite reflectance R obtained according to step 1 o,max The samples are divided into anthracite sample group, bituminous sample group and lignite sample group according to coal rank; Step 4: establishing a calculation formula of gas content of coal seams under different coal ranks changing with mining depth, and testing by using actual gas emission of coal mines; The calculation formula of gas content of coal seams under different coal ranks changing with mining depth is as follows: ； wherein d is the depth of raw coal mining, in meters; R is the Langmuir constant; V gas is the gas content of the coal seam, in meters 3 / t; A, B are undetermined coefficients; for the calculation formula of the gas content of the coal seam of different coal ranks, the values of the coefficients A and B are different, wherein the coefficients of anthracite are named as A1 and B1; the coefficients of bituminous coal are named as A2 and B2; and the coefficients of lignite are named as A3 and B 3； Step 4 is specifically that, for the Langmuir volume V L , Langmuir pressure P L Data, respectively, using Monte Carlo simulation method, first assume that the anthracite sample, bituminous coal sample and lignite sample group data conform to the lognormal distribution, fitting to obtain the undetermined coefficients A1, B1, A2, B2, A3, B3, and using Anderson-Darling test to measure the fitting degree of the probability distribution function, if the fitting degree is more than 99%, it is proved that the value of the undetermined coefficient is reliable; if the fitting degree is less than 99%, it is necessary to re-hypothesize the probability distribution that the corresponding sample group conforms to, until the fitting degree is more than 99%. The undetermined coefficients A1, B1, A2, B2, A3 and B3 obtained by fitting are respectively brought into the formula, and the calculation formula of gas content of coal seams under three coal ranks changing with mining depth is obtained as follows: ； ； ； Wherein, the subscripts 1, 2 and 3 respectively represent anthracite sample group, bituminous sample group and lignite sample group.

2. The method of predicting gas content of a coal seam according to claim 1, wherein The vitrinite random reflectance automatic measurement image analysis method in step 1 is specifically as follows: under the conditions of microscope oil immersion objective, vertical incident light and reflected light, microscopic images of the coal sample are sequentially collected by using a microscope camera and an automatic stage, and the gray scale of all measurement points of the microscopic images is determined, the vitrinite effective measurement points are identified by a vitrinite random reflectance automatic measurement system, the random reflectance values of the vitrinite effective measurement points are counted, and thus the maximum vitrinite reflectance of the sample is obtained.

3. The method of predicting gas content of a coal seam according to claim 1, wherein The isothermal gas adsorption experiment described in Step 2 is as follows: first, the sample is degassed, the coal sample is placed in a sample tube, then heated under vacuum, the sample tube is installed in the analysis station of the physical adsorption instrument, the adsorbate gas is gradually introduced into the vacuumed sample tube at a constant adsorption temperature, the adsorbate molecules entering the sample tube reach the surface of each pore of the coal sample, the adsorbate and the active sites on the sample surface interact to produce intermolecular forces, and the phenomenon that the adsorbate molecules are adhered to the surface of the solid is called adsorption; by measuring the adsorption amount corresponding to a series of pressures, a set of pressure-adsorption amount data is obtained, and an adsorption isotherm is obtained; after the adsorption process is completed, the Langmuir volume constant V L , the Langmuir pressure constant P L of the coal sample is obtained by fitting according to the Langmuir monolayer adsorption theory.

4. The method of predicting gas content of a coal seam according to claim 1, wherein The Langmuir volume V L and the Langmuir pressure P L are related by the following relation: ； where V L is the Langmuir volume constant; P L is the Langmuir pressure constant, when the adsorption pressure P is equal to the Langmuir pressure P L , the adsorption capacity of coal is equal to 1 / 2 of the Langmuir volume, that is, when P=P L , V=V L / 2; V L and P L are determined by the properties of coal and are obtained by fitting the results of isothermal adsorption tests.

5. The method of predicting gas content of a coal seam according to claim 1, wherein In step 3, the vitrinite reflectance interval of lignite is R o,max <0.5, the vitrinite reflectance interval of bituminous coal is 0.5≤R o,max <2, the vitrinite reflectance interval of anthracite is 2≤R o,max <8.

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