A method for predicting coalbed methane production capacity in undeveloped areas based on gas components in developed areas

Abstract

The present invention discloses a method for predicting the production capacity of coalbed methane in undeveloped areas based on the gas-containing components of coalbed methane in developed areas, and belongs to the technical field of coalbed methane mining. The method includes six parts: gas sample collection, gas-containing components and isotope testing, coalbed methane genesis classification, prediction parameter selection, production capacity prediction chart establishment, and prediction verification. The present application divides coalbed methane into two categories: thermogenic gas and mixed genesis gas: for thermogenic gas areas, components derived from the atmosphere (such as argon, etc.) are used as prediction parameters; for mixed genesis gas areas, components of a single origin and monotonically changing during the coalbed methane enrichment process are used as prediction parameters, and either a single component or a combination of multiple components can be selected; then, long-term gas production is used as a prediction indicator to establish a production capacity prediction chart for prediction verification. The present application proposes for the first time a method for predicting the production capacity of coalbed methane wells based on gas-containing components, which avoids the subsequent large-scale, long-term, and high-cost coalbed gas content testing in undeveloped areas, and improves the efficiency of coalbed methane development; and the method makes full use of the gas component test data in the coalbed methane and coal exploration stages, reduces the test cost of the subsequent development evaluation process, and has the significant advantages of high efficiency and economy.

Description

Technical Field

[0001] The present invention belongs to the technical field of coalbed methane exploitation, and specifically relates to efficient and low-cost prediction of coalbed methane well productivity, and in particular to a method for predicting the productivity of coalbed methane wells in undeveloped areas based on gas-containing components of coalbed methane in developed areas. Background Art

[0002] Coalbed methane is a high-quality clean energy, and the development and utilization of coalbed methane can effectively avoid gas outbursts and ensure safe production in coal mines. The coalbed methane reservoir has strong heterogeneity, which leads to large differences in the gas production of coalbed methane wells. The gas production of different gas wells in the same block can vary by more than 10 times. How to predict the production capacity of coalbed methane wells in undeveloped areas based on existing production data is a key issue that needs to be solved at present.

[0003] Gas content is a key parameter for coalbed methane development and evaluation. Usually, the gas production of coalbed methane wells in high gas content areas is significantly higher. At present, the field desorption method is mostly used to test the gas content of coalbeds. This method has two obvious shortcomings: 1) Coalbed methane is mainly in an adsorbed state, which leads to a long time for the coalbed methane to be fully desorbed (especially for coalbeds with high gas content and high adsorption saturation). The current industry standard stipulates that the termination test condition for desorption gas is: the desorbed gas volume is less than 10mL within one week, which undoubtedly takes a long time to complete the desorption process; 2) The gas content obtained by the desorption method test consists of three parts: loss gas, desorbed gas and residual gas. Among them, the loss gas is obtained by fitting the gas volume and time at the initial stage of desorption. The difference in the selection of the fitting section is easy to lead to large differences in the calculation results. In addition, since the desorption site is easily affected by human factors such as untimely core canning, it will also cause errors in the loss gas calculation results. Although the pressure coring technology currently proposed can improve the accuracy of loss gas calculation, the cost of pressure coring is significantly increased. Therefore, it is of practical significance to establish an efficient and low-cost coalbed methane well production capacity prediction method.

[0004] During the exploration phase of coalbed methane, it is often necessary to conduct multiple gas component tests. For example, the national standard "Method for Determination of Coalbed Methane Content" (GB / T 19559-2021) stipulates that during the gas content test, gas samples need to be collected at 1d, 3d and 5d to test the gas components; although the gas sample collection time specified in the industry standard "Method for Determination of Gas Content in Low-rank Coal Seams" (NB / T 10018-2015) is different, it is also necessary to collect three gas samples for gas component testing; therefore, a large amount of gas component test data will be obtained in this process. Compared with direct testing of gas content, testing of gas components is fast, accurate and low-cost. In addition, the difference in gas components is the result of the evolution of coalbed methane generation-migration-preservation, which has a strong indicative significance for coalbed methane enrichment. Therefore, gas components are considered to be potential parameters for predicting the productivity of coalbed methane wells.

[0005] However, most of the current methods for predicting the production capacity of coalbed methane wells are based on the change of parameters such as permeability and porosity to predict the change of gas production of a single well over time. For example, Chinese patent CN 101806224 B discloses a method for predicting the production capacity of underground coalbed methane extraction, which first measures the original parameters of the coal seam to be measured, and based on Darcy's seepage law, establishes a mathematical model for predicting the production capacity of underground coalbed methane based on the change of permeability during extraction, and then solves the model to obtain the average permeability after extraction, the coalbed methane pressure after extraction, the average coalbed methane content after extraction, and the coalbed methane extraction volume. Some technicians have also proposed methods for optimizing coalbed methane development areas, such as Chinese patent CN112529443A, which proposes a method for optimizing coalbed methane development target areas based on fuzzy pattern recognition, which comprehensively uses parameters such as coal seam vitrinite content, gas saturation and gas content to evaluate the development potential of coalbed methane in different regions. However, this scheme not only fails to directly predict the production capacity characteristics, but also requires the use of a large number of coal seam parameters in the optimization process, which significantly increases the difficulty and workload of test acquisition.

[0006] In summary, the prior art has not disclosed an efficient and low-cost method for directly predicting the production capacity of coalbed methane wells in undeveloped areas by using gas-containing components. Summary of the invention

[0007] The purpose of the present invention is to solve the problems existing in the prior art and to provide a method for predicting the coalbed methane production capacity of undeveloped areas based on the gas-containing components of the coalbed methane in the developed areas. This method can be used to efficiently predict the production capacity and avoid a large number of long-term and high-cost coalbed gas content tests.

[0008] In order to achieve the above technical purpose, the present invention is implemented by the following technical scheme: a method for predicting the coalbed methane production capacity of undeveloped areas based on the gas-containing components of coalbed methane in developed areas, comprising the following steps:

[0009] 1) Determine whether the data of the previous coalbed methane components and isotope test results are sufficient. If sufficient, jump directly to step 4); if insufficient, go to step 2);

[0010] 2) Collect gas samples from developed coalbed methane wells;

[0011] 3) Determination of coalbed methane components and isotopes;

[0012] 4) Classification of coalbed methane genesis: Based on the results of geochemical testing and the existing mature coalbed methane genesis identification charts (such as Whiticar charts, etc.), the genesis of different components in coalbed methane is analyzed, and then coalbed methane is divided into two categories: thermogenic gas and mixed genesis gas; the reason for the division into these two categories is that coalbed methane (CH4) is mostly biogenic gas and thermogenic gas. Biogenic gas is formed later and usually mixes with early thermogenic gas to form mixed genesis gas after generation. Although minor components such as N2 in coalbed methane also have atmospheric genesis, coalbed methane mainly focuses on CH4. Therefore, dividing coalbed methane into thermogenic gas and mixed genesis gas based on the genesis of CH4 can cover most areas of coalbed methane development;

[0013] 5) Selection of prediction parameters: For thermogenic gas areas, components derived from the atmosphere are selected as prediction parameters for coalbed methane well production capacity, such as argon and nitrogen. For mixed-genesis gas areas, the coalbed methane enrichment law is analyzed in combination with geological information, and components of a single origin and whose concentration changes monotonically during the coalbed methane enrichment process are selected as prediction parameters.

[0014] For thermogenic areas, the gas production of coalbed methane wells is mostly controlled by the formation sealing. The gas content and gas production of coalbed methane wells in areas with poor formation sealing are usually low. The higher the concentration of components from the atmosphere, the higher the atmospheric infiltration intensity and the worse the formation sealing. Therefore, for thermogenic gas areas, components from the atmosphere can be selected as prediction parameters for coalbed methane well production capacity. Argon is the most concentrated rare gas in coalbed methane and comes from the atmosphere. In addition, argon is a stable component and is not easy to react with groundwater, rocks, etc. The concentration change is mainly controlled by the atmospheric infiltration intensity. Therefore, areas with high argon concentrations indicate stronger connectivity with the atmosphere and weaker formation sealing. Although nitrogen in coalbed methane also comes from the atmosphere, nitrogen still has other sources such as thermogenic and biogenic. In some areas, nitrogen may have poor indicative significance for formation sealing.

[0015] 6) Establishing a production capacity prediction chart: using the parameters selected in step 5) to establish a relationship with the gas production of the coalbed methane well, and preferably establishing a gas production prediction chart with a stronger correlation;

[0016] 7) Prediction test: Use a small amount of gas content test results to test its correlation with the selected gas component concentration and gas production. If the gas content and the correlation between the two do not match, it is necessary to return to step 5) to reselect the gas component prediction parameters.

[0017] Furthermore, in step 2), the gas sample collected is the gas sample produced by the coalbed methane well with stable gas components and a production time of more than one year.

[0018] Furthermore, in step 2), if the drainage gas collection method is used in the sampling process, it is necessary to prevent the dissolution of soluble components such as CO2.

[0019] Furthermore, in step 3), mass spectrometry is used to determine the components and isotopes of coalbed methane. The isotopes measured are C, H and N. Isotopes are mainly used to assist in the analysis of the origin of coalbed methane, so only a small amount of samples need to be tested. Currently, coalbed methane components are mostly tested by gas chromatography. Some gas components are difficult to be effectively separated by the chromatographic column, resulting in low-concentration components in coalbed methane being difficult to detect (such as argon, etc.), but these trace components usually have a stronger indication of coalbed methane enrichment.

[0020] Furthermore, in step 5), for the mixed origin gas zone, a component with a single origin and a monotonically changing concentration during the coalbed methane enrichment process is selected as a prediction parameter. Here, only a single component can be selected as the prediction parameter, or multiple components that meet the conditions can be selected and combined as the prediction parameter.

[0021] Furthermore, in step 6), the gas production results of coalbed methane wells with a drainage time of more than one year are used.

[0022] The beneficial effects of the present invention are:

[0023] 1. This application is based on the theory that gas content is usually positively correlated with gas production, and uses gas-containing components to predict the production capacity of coalbed methane wells in undeveloped areas, avoiding the subsequent prediction of coalbed methane well production capacity by conducting a large number of long-term and high-cost coalbed gas content tests in undeveloped areas, greatly improving the efficiency of coalbed methane development;

[0024] 2. This application establishes the parameter selection process for thermal and mixed gas production capacity prediction, making the proposed method more universally applicable;

[0025] 3. Test the trace components such as rare gases in coalbed methane, and use the test results of a small amount of gas content as the test basis for prediction parameters to improve the prediction accuracy; and introduce argon gas (currently less attention is paid to this component in coalbed methane development) which is stable in nature and comes from the atmosphere as a prediction parameter, which can better reflect the degree of connectivity between different regions and the atmosphere (i.e., formation sealing);

[0026] 4. The long-term gas production is used as the prediction index, so that the gas production prediction result is mainly affected by the difference in gas content, which reduces the influence of engineering factors such as drainage system on the prediction result;

[0027] 5. Enhanced the utilization of coalbed methane component test results in the exploration stage and reduced the testing costs in the subsequent development and evaluation process.

[0028] 6. Use mass spectrometry instead of chromatography to detect coalbed methane components and improve test accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flowchart for predicting the production capacity of coalbed methane wells in undeveloped areas based on gas-containing components;

[0030] Figure 2 The geochemical test results of coal-bed methane in the thermogenic gas area in eastern Yunnan and western Guizhou;

[0031] Figure 3 It is a plate for identifying the genesis of coalbed methane;

[0032] Figure 4 The results of fitting and prediction of coalbed methane well production based on argon concentration are shown in Figure 1. (a) is the daily average gas production-argon concentration chart, and (b) is the correlation between gas content and argon concentration.

[0033] Figure 5 It is the coalbed methane production prediction result based on argon concentration;

[0034] Figure 6 It is the fitting and prediction result of coalbed methane well production based on nitrogen concentration;

[0035] Figure 7 It is the coalbed methane production prediction result based on nitrogen concentration;

[0036] Figure 8 It is the geochemical test result of coalbed methane in the mixed-genetic gas area in the western Fukang area on the southern edge of the Junggar Basin in Xinjiang;

[0037] Fig. 9 These are the plates for identifying the origin of coal-bed methane, where (a) is the plate for identifying the origin of CH4, and (b) is the plate for identifying the origin of CO2;

[0038] Fig.10 The results of fitting and prediction of coalbed methane production based on different components are shown in Figure 1. (a) The small figure is the daily average gas production-CO2 concentration chart; (b) The small figure is the daily average gas production-C 2+ Concentration chart; (c) small chart is the daily average gas production-biogas recharge intensity coefficient chart; (d) small chart is the correlation between gas content and daily average gas production and biogas recharge intensity coefficient;

[0039] Fig.11 It is the coalbed methane production prediction result based on the biogas recharge intensity coefficient. DETAILED DESCRIPTION

[0040] The following examples further illustrate the content of the present invention, but should not be construed as limiting the present invention. Without departing from the essence of the present invention, modifications and substitutions made to the methods, steps or conditions of the present invention all fall within the scope of the present invention.

[0041] Example 1: Thermogenic Gas

[0042] This example predicts the gas production in eastern Yunnan and western Guizhou, based on the gas production test results of 15 coalbed methane wells in Songhe (starting with GP), Laochang (starting with LC) and Enhong (starting with EH) (reference Figure 2 ) and daily average gas production statistics to establish a gas production forecast chart and verify the prediction accuracy (13 wells for fitting and 2 wells for prediction). The test results show that CH4 is a thermogenic gas (reference Figure 3 ). The argon in coalbed methane mainly comes from the atmosphere. The daily average gas production and gas content in the study area show a decreasing trend with the increase of argon concentration. The daily average gas production and argon concentration are mostly distributed on both sides of the fitting line (reference Figure 4 ), the average daily gas production error of the two wells based on argon concentration ((predicted value - actual gas production) / actual gas production × 100%) is about 15% (reference Figure 5 ). Although the average daily gas production and argon concentration of some wells deviate from the fitting line, it is extremely difficult to accurately predict the production of coalbed methane wells because the gas production of coalbed methane wells is not only controlled by the gas content, but also affected by engineering factors and geological conditions. The error of the average daily gas production of coalbed methane wells predicted based on argon concentration is acceptable. The prediction results indicate that it is reasonable to use the gas-containing components of coalbed methane in developed areas to predict the production capacity of coalbed wells in undeveloped areas, and this method has obvious advantages of high efficiency and low cost.

[0043] It is worth noting that the N2 to argon concentration ratio of the coalbed methane wells in Songhe area (lack of gas content test results) is less than 84, indicating that the N2 also comes from the atmosphere. The N2 to argon concentration ratio of some wells in other areas exceeds 84, indicating that some N2 comes from other sources (reference Figure 2 ). Based on the average daily gas production of 6 wells in Songhe area and the N2 concentration, a fitting formula was established and the gas production of one well was predicted (reference Figure 6 , Figure 7 ), with an error of 15.58%. The prediction results based on multiple parameters indicate that it is reasonable to select components from the atmosphere to predict gas production in thermogenic gas areas.

[0044] Example 2: Mixed Origin Gas

[0045] This example predicts gas production in the western Fukang area on the southern edge of the Junggar Basin in Xinjiang. Previous studies have shown that secondary biogas replenishment in the area increases the gas content of coal seams, resulting in higher gas production in coalbed methane wells. This time, based on the geochemical test results of 19 coalbed methane wells in the study area and the statistical results of daily average gas production, a gas production prediction chart was established. The prediction accuracy was tested using the gas content test results, and the gas production of two coalbed methane wells was predicted using the established chart. The test results show that CH4 in the area is a mixed gas, C 2+ is a thermogenic gas, CO2 is a biogenic gas, and N2 is a mixture of atmospheric and thermogenic gas (refer to Figure 8, Fig. 9 ).

[0046] Previous studies have shown that the origin of CH4 in the region changes from biogenic to mixed and thermal origins as the CO2 concentration decreases, indicating that the biogenic gas recharge intensity is higher in high-concentration CO2 areas. 2+ The enrichment process of coalbed methane also shows a monotonic change. For thermogenic gas, it is similar to C 2+ Compared with CH4, CH4 has the characteristics of small molecular diameter, low density and slightly smaller adsorption capacity. Therefore, during the migration of coalbed methane, CH4 is more 2+ The migration is fast and far. After a long distance migration, the coalbed methane is richer in CH4 and poorer in C than the original components. 2+ Therefore, C 2+ The area with low concentration is the enrichment area after the migration of thermogenic gas. On the other hand, the microbial gas production process C 2+ The concentration decreases, so C 2+ Low concentration indicates strong biological activity, which increases the intensity of biogenic gas recharge and coal seam gas content. 2+ Low concentration indicates high enrichment of coalbed methane.

[0047] The test results indicate that the daily average gas production increases with the increase of CO2 concentration and C 2+ The concentration decreases and shows an increasing trend. Most of the wells are distributed on both sides of the fitting line, indicating that the CO2 or C 2+ The rationality of concentration prediction of yield (reference Fig.10 It is worth noting that the above analysis shows that the CO2 and C 2+ The concentration changes are closely related to biogenic gas. Therefore, this embodiment proposes to use the biogas supply intensity coefficient (CO2 concentration / C 2+ The results showed that compared with a single component, the correlation between the daily average gas production and the biogas recharge intensity coefficient was further enhanced. The average error of the daily average gas production prediction of the two wells was about 12%, and the gas content was also positively correlated with the biogas recharge intensity coefficient and the daily average gas production (reference Fig.10 In the middle (c) and (d) panels, Fig.11 ). The prediction results based on multiple parameters indicate that for mixed-origin gas areas, it is reasonable to select a single-origin component that changes monotonically during the coalbed methane enrichment process to predict the average daily gas production.

[0048] It should be pointed out that when selecting a component with a single origin and monotonically changing during the coalbed methane enrichment process as a prediction parameter, you can either select only a single component that meets the conditions as a prediction parameter, or select multiple components that meet the conditions and form a corresponding combination relationship (such as the ratio of two components) based on these components as the prediction parameter. However, when multiple components are combined to construct prediction parameters, there must be a reasonable combination basis. For example, in this embodiment, CO2 concentration and C 2+ The concentrations were combined and their ratio was used as the prediction parameter because both components are related to biogenic gas.

[0049] The above shows and describes the basic principles, main features and advantages of the present invention. However, the above is only a specific embodiment of the present invention, and the technical features of the present invention are not limited thereto. Any other implementation methods derived by any technician in the field without departing from the technical solution of the present invention should be included in the patent scope of the present invention.

Claims

1. A method for predicting the production capacity of coalbed methane in undeveloped areas based on the gas-containing components of coalbed methane in developed areas, characterized in that: The steps include: 1) Determine whether the data of the previous coalbed methane composition and isotope test results are sufficient. If sufficient, jump directly to step 4); if insufficient, go to step 2); 2) Collect gas samples from developed coalbed methane wells; 3) Determine the components and isotopes of the collected coalbed methane; 4) Classification of coalbed methane genesis: Based on geochemical test results and existing mature coalbed methane genesis identification charts, coalbed methane is divided into two categories: thermal genesis gas and mixed genesis gas according to the CH4 component; 5) Selection of prediction parameters: For thermogenic gas areas, components from the atmosphere are selected as prediction parameters for coalbed methane well productivity; for mixed-genetic gas areas, components with a single origin and monotonically changing concentrations during coalbed methane enrichment are selected as prediction parameters. Specifically, a single component is used as a prediction parameter or multiple components that meet the conditions are selected and combined as prediction parameters; 6) Establishment of production capacity prediction chart: Use the parameters selected in step 5) to establish the relationship with the gas production of the coalbed methane well, and select the components with stronger correlation to establish the gas production prediction chart; 7) Prediction test: Use a small amount of gas content test results to test its correlation with the selected gas component concentration and gas production. If the gas content and the correlation between the two do not match, it is necessary to return to step 5) to reselect the gas component prediction parameters.

2. The method for predicting the production capacity of coalbed methane in undeveloped areas based on the gas-containing components of coalbed methane in developed areas according to claim 1, characterized in that: In step 2), the gas samples collected are those from coalbed methane wells with stable gas components and production time of more than one year.

3. The method for predicting the production capacity of coalbed methane in undeveloped areas based on the gas-containing components of coalbed methane in developed areas according to claim 1, characterized in that: In step 2), if the drainage gas collection method is used during the sampling process, it is necessary to prevent the soluble components from dissolving.

4. The method for predicting the coalbed methane production capacity of undeveloped areas based on the gas-containing components of coalbed methane in developed areas according to claim 1, characterized in that: In step 3), mass spectrometry is used to determine the components and isotopes of coalbed methane.

5. The method for predicting the coalbed methane production capacity of undeveloped areas based on the gas-containing components of coalbed methane in developed areas according to claim 4, characterized in that: In step 3), the isotopes determined by mass spectrometry are C, H and N.

6. The method for predicting the coalbed methane production capacity of undeveloped areas based on the gas-containing components of coalbed methane in developed areas according to claim 1, characterized in that: In step 5), for the thermogenic gas zone, components derived from the atmosphere are selected as prediction parameters for the production capacity of the coalbed methane well, and the components derived from the atmosphere include nitrogen and argon.

7. The method for predicting the coalbed methane production capacity of undeveloped areas based on the gas-containing components of coalbed methane in developed areas according to claim 1, characterized in that: In step 6), the gas production results of coalbed methane wells with a drainage time of more than one year are used.

Images