Method and system for determining content of free gas in deep coal rock based on carbon isotope characteristics

Abstract

The invention discloses a deep coal rock free gas content determination method and system based on carbon isotope characteristics, and belongs to the technical field of core analysis. The method comprises the following steps: obtaining the total gas content based on a plurality of groups of gas samples; obtaining carbon isotope values of a plurality of groups of gas samples; obtaining the methane carbon isotope distribution characteristics of the tight sandstone free gas at the same layer of the adjacent well; comparing the carbon isotope values of the plurality of groups of gas samples with the methane carbon isotope value of the free gas of the compact sandstone at the same layer of the adjacent well, determining the methane carbon isotope value limit of the free gas and the adsorbed gas of the coal rock, and obtaining the methane carbon isotope value limit of the free gas and the adsorbed gas of the coal rock; and analyzing the plurality of groups of gas samples based on methane carbon isotope threshold values of the free gas and the adsorbed gas of the coal rock to obtain the content of the free gas and the adsorbed gas in the plurality of groups of gas samples, obtaining the proportion of the free gas in combination with the total gas content, and determining the content of the free gas in the deep coal rock.

Description

Technical Field

[0001] The present invention belongs to the technical field of core analysis, and in particular relates to a method and system for determining the free gas content of deep coal rocks based on carbon isotope characteristics. Background Art

[0002] At present, the testing methods for free gas in coal rock are the on-site closed core sampling coal rock water saturation test method and the isothermal adsorption method. The on-site closed core sampling coal rock water saturation test method is used to test free gas. Due to the strong heterogeneity and developed cleats of high-order coal rock, the porosity and water saturation tested by block samples on site are not representative, and the calculated free gas has a large error; the isothermal adsorption method is used to determine the free gas in coal rock. It requires an indoor isothermal adsorption experiment to determine the adsorbed gas, and the water saturation and structure of the coal rock in the room have changed. In addition, the isothermal adsorption experiment uses methane gas, and the desorbed gas includes N2, CO2 and other gases in addition to methane. Therefore, the maximum adsorbed gas content of the coal rock is tested, and the free gas measured is the minimum free gas content, which cannot truly test the free gas content of the coal rock. Summary of the invention

[0003] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method and system for determining the free gas content of deep coal rocks based on carbon isotope characteristics, which can measure the gas content on site and directly test the free gas and adsorbed gas content of coal rocks, so as to provide a basis for accurately calculating the deep coal rock gas resources, evaluating favorable exploration areas and improving the success rate of deep coal rock gas exploration and development, thereby solving the problem of large errors in the free gas calculated by the on-site closed coring coal rock water saturation test method and the isothermal adsorption method due to the strong heterogeneity of high-order coal rocks and large structural changes in coal rocks.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] The present invention provides a method for determining the free gas content of deep coal rocks based on carbon isotope characteristics, comprising the following steps:

[0006] S1: Obtain the total gas content based on several groups of gas samples;

[0007] S2: Obtain carbon isotope values ​​of several groups of gas samples;

[0008] S3: Obtain the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells;

[0009] S4: Compare the carbon isotope values ​​of several groups of gas samples with the carbon isotope values ​​of methane in the tight sandstone free gas in the same layer of adjacent wells to determine the boundary between the methane carbon isotope values ​​of the free gas and the adsorbed gas in the coal rock, and obtain the boundary value of the methane carbon isotope values ​​of the free gas and the adsorbed gas in the coal rock;

[0010] S5: Based on the methane carbon isotope boundary value of free gas and adsorbed gas in coal rock, several groups of gas samples are analyzed to obtain the free gas and adsorbed gas content in several groups of gas samples. The free gas ratio is obtained by combining the total gas content to determine the free gas content in deep coal rock.

[0011] In the specific implementation process, in S1, the total gas content is the sum of the loss gas content, the pressure-maintaining gas content, the desorption gas content and the residual gas content.

[0012] In the specific implementation process, the steps for obtaining the pressure-maintaining gas content are:

[0013] S21: Acquire several groups of pressure-maintaining gas samples, and obtain the pressure-maintaining gas sampling time and pressure-maintaining gas volume data;

[0014] S22: when the pressure-maintaining gas volume data no longer increases, the total volume of the pressure-maintaining gas is obtained, and the pressure-maintaining gas analysis is terminated;

[0015] S23: Take out the core, divide it into analytical samples according to rock type, weigh the analytical samples to obtain the weight of the analytical samples, and obtain the pressurized gas content by dividing the total volume of the pressurized gas by the weight of the analytical samples.

[0016] In the specific implementation process, the steps for obtaining the content of the analyzed gas are:

[0017] S31: placing several groups of analytical samples in a water bath for desorption, obtaining analytical gas samples, and obtaining analytical gas sampling time and analytical gas volume data;

[0018] S32: When the desorption gas volume data no longer increases, the total volume of the desorption gas is obtained, and the desorption gas analysis is terminated; the total volume of the desorption gas is divided by the weight of the desorption sample to obtain the desorption gas content.

[0019] In a specific implementation process, the temperature of the water bath is the formation temperature.

[0020] In the specific implementation process, the steps for obtaining the residual gas content are:

[0021] S41: crushing the analytical sample, selecting a broken sample for analysis, obtaining a residual gas sample, and obtaining residual gas sample sampling time and residual gas volume data;

[0022] S42: when the residual gas volume data no longer increases, the total volume of the residual gas is obtained, and the residual gas analysis is terminated;

[0023] S43: Take out the broken core and weigh it to obtain the weight of the broken sample; divide the total volume of residual gas by the weight of the broken sample to obtain the residual gas content.

[0024] In the specific implementation process, the steps for obtaining the loss gas content are:

[0025] S51: Obtaining the gas loss time according to the time difference between the opening time of the pressure-maintaining inner cylinder and the core sealing time;

[0026] S52: The time difference between the core sealing time and the analysis completion time is used to obtain the analysis gas analysis time;

[0027] S53: By linearly fitting the analyzed gas content and the square root of the sum of the analyzed time and the lost time, the intercept of the straight line at time zero is obtained, and the intercept is the lost gas volume.

[0028] The present invention also provides a system for determining the free gas content of deep coal rocks based on carbon isotope characteristics, based on any one of the methods for determining the free gas content of deep coal rocks based on carbon isotope characteristics, comprising:

[0029] A carbon isotope analysis module for gas samples, used to obtain carbon isotope values ​​of several groups of gas samples;

[0030] The module for analyzing the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells is used to obtain the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells;

[0031] The module for obtaining the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock is used to compare the carbon isotope values ​​of several groups of gas samples with the carbon isotope values ​​of methane carbon isotope of free gas in tight sandstone of the same layer in adjacent wells, determine the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock, and obtain the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock;

[0032] The module for determining the free gas content and proportion in deep coal rocks is used to analyze several groups of gas samples based on the methane carbon isotope boundary value of the free gas and adsorbed gas in the coal rocks, obtain the free gas and adsorbed gas content in several groups of gas samples, and obtain the free gas proportion in combination with the total gas content to determine the free gas content in deep coal rocks.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] The present invention provides a method for determining the free gas content of deep coal rock based on carbon isotope characteristics, by means of on-site desorption experiments of the gas content of coal rock cores at surface temperature and formation temperature in turn, avoiding the use of high-temperature desorption higher than the formation temperature to cause δ 13 The C1 isotope value is artificially increased, which is contrary to geological reality; based on δ 13 C1 isotope fractionation theory, analysis of coal gas δ 13 The variation characteristics of C1 isotope value with fractionation time; the free gas δ 13By comparing C1 isotope values, the boundary between free gas and adsorbed gas in the desorbed gas is determined; based on the methane carbon isotope limit value, the corresponding free gas content is determined and the free gas ratio is calculated. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 is the coal rock desorption gas δ of the embodiment of the present invention 13 C1 change diagram;

[0036] Figure 2 The tight sandstone gas δ 13 C1 distribution map. DETAILED DESCRIPTION

[0037] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0038] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0039] The present invention provides a method for determining the free gas content of deep coal rocks based on carbon isotope characteristics, which specifically comprises the following steps:

[0040] S1: Based on several groups of gas samples, the pressure-maintaining gas content, the desorbed gas content, and the residual gas content are obtained, and the total gas content is obtained by combining the lost gas content; the total gas content is the sum of the lost gas content, the pressure-maintaining gas content, the desorbed gas content, and the residual gas content;

[0041] The steps of obtaining the pressurized gas content are as follows: obtaining several groups of pressurized gas samples, obtaining the pressurized gas sampling time and pressurized gas volume data; when the pressurized gas volume data no longer increases, obtaining the total volume of the pressurized gas, and ending the pressurized gas analysis; opening the pressurized inner cylinder, taking out the core, dividing it into analysis samples according to the rock type, weighing the analysis samples, registering the weight, and obtaining the pressurized gas content by dividing the pressurized gas volume by the analysis sample weight;

[0042] The steps of obtaining the desorbed gas content are as follows: placing several groups of desorbed samples in a water bath at the same temperature as the formation temperature for desorption, obtaining desorbed gas samples, obtaining the sampling time of the desorbed gas samples and the desorbed gas volume data; when the desorbed gas volume data no longer increases, obtaining the total volume of the desorbed gas, and ending the desorbed gas desorption; dividing the desorbed gas volume by the desorbed sample weight to obtain the desorbed gas content;

[0043] The steps of obtaining the residual gas content are as follows: crushing the analytical sample, obtaining the broken sample, weighing and analyzing it, and recording the weight of the broken sample; when the analytical gas volume data of the broken sample no longer increases, obtaining the total volume of the residual gas, and ending the residual gas analysis; dividing the residual gas volume by the weight of the broken analytical sample to obtain the residual gas content;

[0044] The steps for obtaining the lost gas content are as follows: based on the difference between the opening time of the pressure-maintaining inner cylinder and the core sealing time, the lost gas loss time is obtained; based on the difference between the core sealing time and the analysis completion time, the analysis gas analysis time is obtained; and by linear fitting the analysis gas content and the square root of the sum of the analysis time and the loss time, the intercept of the straight line at time zero is obtained, which is the lost gas volume.

[0045] S2: Gas sample SQ obtained 保 , SQ 解 , SQ 残 Conduct component and carbon isotope analysis to obtain carbon isotope values ​​of several groups of gas samples;

[0046] S3: Obtain the carbon isotope distribution characteristics of methane in the tight sandstone free gas in the same layer of adjacent wells; compare the carbon isotope values ​​of several groups of gas samples with the carbon isotope values ​​of methane in the tight sandstone free gas in the same layer of adjacent wells, determine the boundary of methane carbon isotope values ​​between coal rock free gas and adsorbed gas, and obtain the boundary value of methane carbon isotope between coal rock free gas and adsorbed gas;

[0047] S4: Based on the methane carbon isotope boundary value of free gas and adsorbed gas in coal rock, several groups of gas samples are analyzed to obtain the free gas and adsorbed gas content in several groups of gas samples. The free gas ratio is obtained by combining the total gas content to determine the free gas content in deep coal rock.

[0048] The present invention also provides a system for determining the free gas content of deep coal rocks based on carbon isotope characteristics, including a total gas content acquisition module, a carbon isotope analysis module for gas samples, a methane carbon isotope distribution characteristic analysis module for free gas in tight sandstones at the same layer of adjacent wells, a module for obtaining the limit value of methane carbon isotopes of free gas and adsorbed gas in coal rocks, and a module for determining the free gas content and proportion of deep coal rocks;

[0049] A total gas volume acquisition module is used to obtain carbon isotopes of several groups of gas samples;

[0050] A carbon isotope analysis module for gas samples, used to obtain carbon isotopes of several groups of gas samples;

[0051] The module for analyzing the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells is used to obtain the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells;

[0052] The module for obtaining the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock is used to compare the carbon isotope of several groups of gas samples with the carbon isotope of methane in free gas of tight sandstone in the same layer of adjacent wells, determine the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock, and obtain the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock;

[0053] The module for determining the free gas content and proportion in deep coal rocks is used to analyze several groups of gas samples based on the methane carbon isotope boundary value of the free gas and adsorbed gas in the coal rocks, obtain the free gas and adsorbed gas content in several groups of gas samples, and obtain the free gas proportion in combination with the total gas volume.

[0054] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0055] The present invention provides a method and system for determining the free gas content of deep coal rock based on carbon isotope characteristics. The experimental principle of the determination method is as follows:

[0056] During the desorption process of coalbed methane, 13 CH4 Ratio 12 CH4 is difficult to desorb, so isotope fractionation is likely to occur during desorption. The coal rock gas desorbed first is free gas, and δ 13 The C1 value is relatively light, and the coal rock gas desorbed later is adsorbed gas. 13 The C1 value is relatively heavy.

[0057] The specific steps of the method for determining the free gas content in deep coal based on carbon isotope characteristics described in the present invention are as follows:

[0058] Acquiring experimental data includes the following steps 1 to 16:

[0059] Step 1: When the pressure-maintaining core barrel reaches the ground, record the time of arrival and the surface temperature, and check whether the pressure maintenance is successful;

[0060] Step 2: Take out the pressure-maintaining inner cylinder and connect it to the analytical equipment;

[0061] Step 3: Open the outlet valve of the pressure-maintaining inner cylinder and record the time of opening the outlet valve;

[0062] Step 4: Take gas sample, number SQ 保 , record the sampling time, read the gas volume data, and register it as the pressure-maintaining gas volume V 保 ;

[0063] Step 5: The gas volume increment is 0, and the gas volume data is read and registered as the total volume of the pressure-maintaining gas V 总保 , the pressure-maintaining gas analysis is completed;

[0064] Step 6: Open the pressure-maintaining inner cylinder, record the opening time of the pressure-maintaining inner cylinder, take out the core, and divide the core into analytical samples according to rock type, numbered SY;

[0065] Step 7: Put the analytical sample into the analytical tank and record the sealing time;

[0066] Step 8: Connect the desorption tank to the desorption equipment and place it in a water bath maintained at formation temperature for desorption;

[0067] Step 9: Take gas sample, number SQ 解 , record the sampling time, read the gas volume data, and register it as the analyzed gas volume V 解 ;

[0068] Step 10: The gas volume increment is 0, and the gas volume data is read and registered as the total volume of the analyzed gas V 总解 , the analysis of the analytical gas is completed;

[0069] Step 11: Open the analysis tank, take out the analysis sample SY, weigh it, and record the weight; divide the pressure-maintaining gas volume by the analysis sample weight to obtain the pressure-maintaining gas content Q 总保 ; Based on the volume of the analyzed gas divided by the weight of the analyzed sample, the analyzed gas content Q is obtained 总解 ;

[0070] Step 12: Crush the analytical sample and select a core fragment, numbering it SY 碎 , put it into the analysis tank for analysis;

[0071] Step 13: Take gas sample, number SQ 残 , record the sampling time, read the gas volume data, and register it as the residual gas volume V 残 ;

[0072] Step 14: The gas volume increment is 0, and the gas volume data is read and recorded as the total residual gas volume V 总残 , residual gas analysis is completed;

[0073] Step 15: Open the analysis tank and take out the broken core SY 碎 Weigh and record the weight; divide the residual gas volume by the weight of the fragmented analyzed sample to obtain the residual gas content Q 总残 ;

[0074] Step 16: According to the time difference between the opening time of the pressure-maintaining inner tube and the core sealing time, the loss gas loss time is obtained. According to the time difference between the core sealing time and the analysis completion time, the analysis gas analysis time is obtained. By linearly fitting the analysis gas content and the square root of the sum of the analysis time and the loss time, the intercept of the straight line at time zero is obtained, which is the loss gas volume Q. 总损 , and the total gas volume Q 总 =Q 总保 +Q 总解 +Q 总残 +Q 总损 ;

[0075] Analysis steps:

[0076] For the gas sample SQ obtained above 保 , SQ 解 , SQ 残 Conduct compositional and carbon isotope analyses;

[0077] The free gas δ 13 Comparison of C1 values ​​to determine the carbon isotope boundary of methane between free gas and adsorbed gas in coal rocks;

[0078] Based on the carbon isotope limit value of methane between free gas and adsorbed gas in coal, the corresponding free gas content is determined and the free gas proportion is calculated.

[0079] Q85 Jingbenxi Group 8 # In the field desorption experiment of coal, the coal gas δ 13 The variation characteristics of C1 value with fractionation time, combined with the free gas δ 13 C1 value determines the carbon isotope boundary of methane between free gas and adsorbed gas in coal rock desorption gas.

[0080] See also Figure 1 , such as the embodiment of the present invention Q85 well Benxi group 8 # Coal rock desorption gas δ 13 As shown in the C1 value change diagram, with the extension of desorption time, methane δ 13 The C1 value tends to become heavier.

[0081] like Figure 2 As shown in the figure, the free gas δ 13The C1 value is in the range of -41.1‰ to -30.3‰. The maximum value of the methane carbon isotope of the free gas in the sandstone of the well area is used as the boundary between the free gas and the adsorbed gas in the coal rock desorption gas in the embodiment of the present invention, that is, the boundary between the free gas and the adsorbed gas in the coal rock desorption gas is -30.3‰.

[0082] As shown in Table 1 and Table 2, based on the methane carbon isotope limit value and combined with the total gas content data, the free gas content is determined to be 11.45m 3 / t, the free gas proportion is calculated to be 46.3%.

[0083] Table 1 Calculation table of free gas content

[0084]

[0085]

[0086] Table 2 Calculation table of adsorbed gas content

[0087]

[0088] As shown in Table 3, the measured results show that the free gas content of carbon isotope method is increased from 3.3m 3 / t~6.9m 3 / t increased to 11.5m 3 / t, the proportion of free gas increased from 13.2% to 24.3% to 46.3%. At the same time, due to the double errors of free gas calculation and adsorbed gas calculation in the water saturation test method, the total gas volume calculated was 28.56m 3 / t is greater than the measured total gas volume of 24.93m 3 / t, the accuracy of the data is questionable.

[0089] Table 3 Comparison of gas content test results of 8# coal of Benxi Formation in Q85 well using three methods

[0090]

[0091] The present invention provides a method and system for determining the free gas content in deep coal rock based on carbon isotope characteristics, by means of on-site desorption experiments of gas content in coal rock cores at surface temperature and formation temperature in turn, avoiding the use of high-temperature desorption higher than the formation temperature to cause δ 13 The C1 value is artificially increased, which is contrary to the geological reality; based on δ 13 C1 fractionation theory, analysis of coal gas δ 13 Variation characteristics of C1 value with fractionation time; free gas δ 13By comparing C1 values, the boundary between free gas and adsorbed gas in the desorbed gas is determined. Based on the methane carbon isotope limit value, the corresponding free gas content is determined, and the free gas ratio is calculated, providing a basis for accurately calculating the deep coal-rock gas resources, evaluating favorable exploration areas, and improving the success rate of deep coal-rock gas exploration and development.

[0092] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for determining the free gas content in deep coal based on carbon isotope characteristics, characterized in that: The following steps are involved: S1: Obtain the total gas content based on several groups of gas samples; S2: Obtain carbon isotope values ​​of several groups of gas samples; S3: Obtain the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells; S4: Compare the carbon isotope values ​​of several groups of gas samples with the carbon isotope values ​​of methane in the tight sandstone of the same layer in adjacent wells to determine the boundary between the carbon isotope values ​​of free gas and adsorbed gas in coal rock, and obtain the carbon isotope boundary value of methane in free gas and adsorbed gas in coal rock; S5: Based on the methane carbon isotope boundary value of free gas and adsorbed gas in coal rock, several groups of gas samples are analyzed to obtain the free gas and adsorbed gas content in several groups of gas samples. The free gas ratio is obtained by combining the total gas content to determine the free gas content in deep coal rock.

2. The method for determining the free gas content in deep coal based on carbon isotope characteristics according to claim 1, characterized in that: In S1, the total gas content is the sum of the lost gas content, the pressure-maintaining gas content, the desorbed gas content and the residual gas content.

3. The method for determining the free gas content in deep coal based on carbon isotope characteristics according to claim 2, characterized in that: The steps for obtaining the pressure-maintaining gas content are as follows: S21: Acquire several groups of pressure-maintaining gas samples, and obtain the pressure-maintaining gas sampling time and pressure-maintaining gas volume data; S22: when the pressure-maintaining gas volume data no longer increases, the total volume of the pressure-maintaining gas is obtained, and the pressure-maintaining gas analysis is terminated; S23: Take out the core, divide it into analytical samples according to rock type, weigh the analytical samples to obtain the weight of the analytical samples, and obtain the pressurized gas content by dividing the total volume of the pressurized gas by the weight of the analytical samples.

4. The method for determining the free gas content in deep coal based on carbon isotope characteristics according to claim 2, characterized in that: The steps for obtaining the analyzed gas content are: S31: placing several groups of analytical samples in a water bath for desorption, obtaining analytical gas samples, and obtaining analytical gas sampling time and analytical gas volume data; S32: When the desorption gas volume data no longer increases, the total volume of the desorption gas is obtained, and the desorption gas analysis is terminated; the total volume of the desorption gas is divided by the weight of the desorption sample to obtain the desorption gas content.

5. The method for determining the free gas content in deep coal based on carbon isotope characteristics according to claim 4, characterized in that: The temperature of the water bath is the formation temperature.

6. The method for determining the free gas content in deep coal based on carbon isotope characteristics according to claim 2, characterized in that: The steps for obtaining the residual gas content are: S41: crushing the analytical sample, selecting a broken sample for analysis, obtaining a residual gas sample, and obtaining residual gas sample sampling time and residual gas volume data; S42: when the residual gas volume data no longer increases, the total volume of the residual gas is obtained, and the residual gas analysis is terminated; S43: taking out the broken core and weighing it to obtain the weight of the broken sample analysis; dividing the total volume of residual gas by the weight of the broken sample analysis to obtain the residual gas content.

7. The method for determining the free gas content in deep coal based on carbon isotope characteristics according to claim 2, characterized in that: The steps for obtaining the lost gas content are: S51: Obtaining the gas loss time according to the time difference between the opening time of the pressure-maintaining inner cylinder and the core sealing time; S52: The time difference between the core sealing time and the analysis completion time is used to obtain the analysis gas analysis time; S53: By linearly fitting the analyzed gas content and the square root of the sum of the analyzed time and the lost time, the intercept of the straight line at time zero is obtained, and the intercept is the lost gas volume.

8. A system for determining the free gas content in deep coal based on carbon isotope characteristics, characterized in that: The method for determining the free gas content in deep coal based on carbon isotope characteristics according to any one of claims 1 to 7 comprises: A carbon isotope analysis module for gas samples, used to obtain carbon isotope values ​​of several groups of gas samples; The module for analyzing the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells is used to obtain the carbon isotope distribution characteristics of free gas methane in tight sandstone in the same layer of adjacent wells; The module for obtaining the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock is used to compare the carbon isotope values ​​of several groups of gas samples with the carbon isotope values ​​of methane carbon isotope of free gas in tight sandstone of the same layer in adjacent wells, determine the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock, and obtain the limit value of methane carbon isotope of free gas and adsorbed gas in coal rock; The module for determining the free gas content and proportion in deep coal rocks is used to analyze several groups of gas samples based on the methane carbon isotope boundary value of the free gas and adsorbed gas in the coal rocks, obtain the free gas and adsorbed gas content in several groups of gas samples, and obtain the free gas proportion in combination with the total gas content to determine the free gas content in deep coal rocks.

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