A method for predicting a favorable area of coal-bed methane with medium-high coal rank

Abstract

The application discloses a kind of middle-high coal rank coalbed gas favorable area prediction method, belong to oil and gas exploration and development technical field.The application first finds out the corresponding Lang's volume value when the lower limit value of middle coal rank vitrinite reflectance Ro according to the relationship between Lang's volume and vitrinite reflectance;Then the relationship between coal seam thickness (D), buried depth (H), overburden thickness (G) and gas content is established respectively, with the Lang's volume value when vitrinite reflectance Ro is 0.7% as the lower limit standard, to establish the intersection between coal seam thickness (D), buried depth (H), overburden thickness (G) and the Lang's volume value when vitrinite reflectance Ro lower limit value;Finally, the intersection value between the above three elements is determined as the standard, and the coalbed gas in a region is plotted with three elements, and the three-element superposition area is the favorable development area of coalbed gas.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for predicting a favorable area of coalbed methane with medium-high coal rank, and belongs to the technical field of oil and gas exploration and development. BACKGROUND

[0002] Coalbed methane is a kind of unconventional natural gas with methane (CH4) as the main component, which is generated from coal seams and mainly stored in the coal seams in an adsorbed state. The heat value of coalbed methane is equivalent to that of conventional natural gas, which is 2 to 5 times that of general coal gas. After combustion, it produces very little pollutants and belongs to high-quality clean gas energy. As a large coal resource country, China is rich in coalbed methane resources. According to the latest statistical data of the Ministry of Natural Resources in 2022, the geological resources within a depth of 2000 meters are about 30.05 trillion cubic meters, and the recoverable resources are about 12.5 trillion cubic meters. The geological resources within a depth of 2000 meters are 40.71 trillion cubic meters, and the recoverable resources are 10.01 trillion cubic meters. Among them, the low coal rank, medium coal rank and high coal rank coalbed methane resources are complete in structure, and each accounts for about 1 / 3 of the geological resources. China's coalbed methane resources are distributed in Ordos, Qinshui, Tuha, Junggar, Songliao, Tarim and Sichuan basins, with a total geological resource of nearly 26 trillion cubic meters. Among them, the coalbed methane geological resources in the exploration area of Sinopec are about 10 trillion cubic meters, accounting for 1 / 7 of the total coalbed methane geological resources in China, mainly distributed in Ordos, Sichuan and Junggar basins.

[0003] Coal rank is a parameter that affects the saturation state of coal seams and represents the maturity level that can be achieved in coalification. It is one of the important parameters for measuring the degree of coalification. With the increase of coal seam burial depth and the increase of geothermal temperature, the coal rank also gradually increases, and the coal changes from lignite to bituminous coal and anthracite. Generally, the thermal evolution degree of low-rank coal measures Ro is less than 0.7%, the thermal evolution degree of medium-rank coal measures Ro is between 0.7% and 1.9%, and the thermal evolution degree of high-rank coal measures Ro is greater than 1.9%.

[0004] In 2023, the production of coalbed methane in China was 117 billion cubic meters, and coalbed methane was mainly produced in Shanxi Province, mainly in the eastern margin of the Qinshui Basin and the Ordos Basin. 90% of the production is concentrated in high-rank coal seams, and the mining depth is mainly in the shallow layer less than 800m and the medium-deep layer of 800-1200m. However, for deep coalbed methane with a depth greater than 1200m, due to the unclear development geological background and enrichment main control elements, the development of deep coalbed methane in China is still in the exploratory stage, and there are great differences in the identification of enrichment areas by different geologists.

[0005] At present, the identification of middle-high rank deep coalbed methane enrichment areas is mainly based on a large number of drilling verification, which directly leads to low work efficiency and waste of a large amount of funds. Moreover, the development geological background and gas enrichment theory of middle-high rank deep coalbed methane are still unclear, which also directly leads to the phenomenon that explorers cannot accurately grasp a coalbed methane block, especially for a strange coalbed methane area, where to deploy the first exploration well, which directly leads to the process of subsequent exploration and development. If the deployment is unreasonable, it will lead explorers to lose confidence in the block, which will directly delay the discovery of coalbed methane in the area due to human misjudgment.

[0006] A coalbed methane reservoir enrichment recoverable evaluation method and device are disclosed in Chinese patent application CN118095644A published on May 28, 2024, and the evaluation method comprises: obtaining evaluation parameters of coalbed methane, calculating a hydrocarbon generation evaluation coefficient, an enrichment evaluation coefficient and a recoverable evaluation coefficient based on the evaluation parameters of the coalbed methane using a pre-set evaluation factor model, wherein the pre-set evaluation factor model is used to quantitatively characterize the condition of the coalbed methane from the three stages of hydrocarbon generation, enrichment and recoverability, obtaining a comprehensive evaluation result based on the hydrocarbon generation evaluation coefficient, the enrichment evaluation coefficient and the recoverable evaluation coefficient, and obtaining a coalbed methane reservoir enrichment recoverable evaluation result based on the comprehensive evaluation result. The evaluation method is a fine selection evaluation method, which can more comprehensively evaluate the coalbed methane and improve the accuracy of the coalbed methane reservoir enrichment recoverable evaluation. However, the research object of the coalbed methane reservoir enrichment recoverable evaluation method is an area where coalbed methane has been determined to be enriched and has been developed, and the problem it solves is how to further optimize the area rich in coalbed methane in the above research object, and it does not involve the problem of predicting a favorable area for a certain area.

[0007] Chinese patent application CN118941119A published on November 12, 2024 discloses a method suitable for coalbed methane resource potential evaluation in a low exploration degree area, which first determines the calculation boundary for carrying out coalbed methane resource evaluation, then divides the coalbed methane resource evaluation calculation unit, then selects the coalbed methane resource evaluation parameters, and sequentially performs coalbed methane geological resource evaluation and coalbed methane recoverable resource evaluation, and finally obtains the coalbed methane resource potential evaluation result. However, the evaluation method mainly evaluates the size of the coalbed methane resource in an area, which has nothing to do with the evolution stage of coal (low rank, medium rank and high rank), and it does not predict a favorable area for a certain area.

[0008] Therefore, it is of great importance to develop a simple and effective method for predicting a middle-high rank coalbed methane favorable area for the development of coalbed methane in China. SUMMARY

[0009] The application aims to provide a method for predicting a favorable area of coalbed methane of medium-high coal rank.

[0010] To achieve the above-mentioned purpose, the technical scheme of the method for predicting a favorable area of coalbed methane of medium-high coal rank is as follows:

[0011] The method for predicting a favorable area of coalbed methane of medium-high coal rank comprises the following steps:

[0012] (1) analyzing a coal rock sample in a to-be-predicted area to obtain a Lang's volume and a vitrinite reflectance Ro of the sample, establishing a fitting relationship between the Lang's volume and the vitrinite reflectance Ro of the sample, and determining a Lang's volume value corresponding to a lower limit value of the vitrinite reflectance Ro of medium coal rank according to the fitting relationship;

[0013] (2) determining lower limit values of a coal seam thickness, a coal rock buried depth and an overlying bedrock thickness based on a corresponding relationship between geological structure characteristic parameters and gas content in the to-be-predicted area and the Lang's volume value obtained in step (1);

[0014] (3) drawing isopleth maps of the three elements of the coal seam thickness, the coal rock buried depth and the overlying bedrock thickness in the to-be-predicted area respectively by using the lower limit values obtained in step (2), and fitting a regional range of a favorable area of coalbed methane in the to-be-predicted area.

[0015] The beneficial effects of the above technical solutions are that the prediction method of the high-rank coal seam gas favorable area in the application is an opening invention. The application analyzes a large amount of data on the characteristics of the high-rank coal seam gas, finds that there is a certain trend relationship between the gas content of the coal rock and the coal seam thickness (D), the coal rock buried depth (H) and the overlying bedrock thickness (G) of the coal seam, and determines that the three elements of the coal seam thickness (D), the coal rock buried depth (H) and the overlying bedrock thickness (G) can be used to predict the high-rank coal seam gas favorable area. Firstly, the Lan value corresponding to the lower limit value 0.7% of the vitrinite reflectance Ro of the medium-rank coal is found according to the relationship between the Lan volume and the vitrinite reflectance; then the relationship between the three elements of the coal seam thickness (D), the buried depth (H) and the overlying bedrock thickness (G) and the gas content is established, the intersection between the three elements and the Lan volume value of the vitrinite reflectance Ro of 0.7% is determined as the lower limit standard, and the intersection between the three elements and the Lan volume value of the vitrinite reflectance Ro of 0.7% is determined; finally, the intersection value between the three elements is determined as the standard, the coal seam gas in a region is plotted according to the three elements, and the superposition area of the three elements is the favorable development area of the coal seam gas. The data required by the prediction method of the application is simple to obtain, low in cost, high in practicability, easy to be widely applied, and can be used to guide the oil and gas exploration, development and research in a target basin, and has a high practical value for the strategic selection of the coal seam gas in each oil and gas basin, and has a very wide application prospect.

[0016] As a further improvement, the lower limit value of the vitrinite reflectance Ro of the medium-rank coal in step (1) is 0.7%.

[0017] As a further improvement, the corresponding relationship in step (2) is the corresponding relationship between the coal seam thickness, the coal rock buried depth and the overlying bedrock thickness and the gas content.

[0018] As a further improvement, the lower limit value in step (2) is the intersection value of the corresponding relationship and the Lan volume value obtained in step (1). BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 It is a flow chart of the prediction method of the high-rank coal seam gas favorable area in Example 1 of the application;

[0020] Figure 2 It is a correlation diagram of the thermal evolution degree Ro of the coal measure source rock and the Lan volume in the XX region in Example 1 of the application;

[0021] Figure 3 It is a correlation diagram of the coal rock buried depth (H) and the gas content in the XX region in Example 1 of the application;

[0022] Figure 4A correlation graph of coal seam thickness (D) and gas content in the XX region in Example 1 of the present application;

[0023] Figure 5 A correlation graph of overburden bedrock thickness (G) and gas content in the XX region in Example 1 of the present application;

[0024] Figure 6 A cross graph of coal seam thickness (D) and lower limit of gas content in the XX region in Example 1 of the present application;

[0025] Figure 7 A cross graph of coal seam buried depth (H) and lower limit of gas content in the XX region in Example 1 of the present application;

[0026] Figure 8 A cross graph of overburden bedrock thickness (G) and lower limit of gas content in the XX region in Example 1 of the present application;

[0027] Figure 9 A distribution graph of favorable area of coalbed methane in high coal rank in the Upper Paleozoic in the XX region in Example 1 of the present application. DETAILED DESCRIPTION

[0028] In the prior art, the identification of the enrichment area of the middle-high rank deep coalbed methane is mainly based on a large number of drilling to verify, which leads to low work efficiency and wastes a large amount of funds. Meanwhile, the existing evaluation method for the enrichment of the coalbed methane reservoir needs a large number of evaluation parameters, and it is difficult to obtain data, and the evaluation process is complex, which is not suitable for wide use. Through a large amount of investigation and data analysis, the present application uses the coal seam thickness, buried depth, overburden bedrock thickness and gas content to construct a correlation relationship, and uses the Lang's volume value corresponding to the lower limit value of the middle coal rank vitrinite reflectance Ro as the lower limit standard, respectively obtains the intersection of the coal seam thickness, buried depth and overburden bedrock thickness and the above lower limit standard, and uses the intersection value as the standard to make a three-element graph of the coalbed methane in the area to be evaluated, and the superimposed area of the three elements is the favorable development area of the coalbed methane.

[0029] The present application will be further described in combination with specific embodiments. It should be noted that, under the premise of no conflict, each embodiment described below or each technical feature can be combined to form a new embodiment. The equipment and raw materials used can be purchased from the market or are commonly used in the art. Unless otherwise specified, the methods in the following examples are conventional methods in the art.

[0030] In the following examples, unless otherwise specified, the experimental operations are conventional operations in the art.

[0031] In the following examples, unless otherwise specified, the raw materials used are conventional commercial products in the art.

[0032] A specific embodiment of the method for predicting favorable areas of medium- and high-rank coalbed methane according to the present invention:

[0033] This embodiment predicts favorable areas for medium- and high-rank coal seams in the XX region. The XX region is located in eastern China, covering the Dongpu Depression of the Bohai Bay Basin and some coal-producing areas in the adjacent North China Basin. The coal seams are developed in the Upper Paleozoic strata. The region has abundant seismic data, including two-dimensional and three-dimensional seismic data, and more than 20 coring wells.

[0034] Example 1

[0035] The process in this embodiment is as follows: Figure 1 As shown, the specific operation steps are as follows:

[0036] 1. Establishment of the correlation between the Langevin volume and vitrinite reflectance Ro of coal and rock samples, and acquisition of the Langevin volume corresponding to the lower limit of vitrinite reflectance Ro in middle-rank coal.

[0037] Actual measurements were performed on coal and rock samples from the XX region to obtain the Lambda volume and vitrinite reflectance of the coal and rock samples. Specific information about the samples is shown in Table 1.

[0038] Table 1. Specific values ​​of Lamellar volume and vitrinite reflectance of coal and rock in the XX region.

[0039]

[0040]

[0041] The data in Table 1 were fitted to establish the correlation between the Lancôme volume and vitrinite reflectance Ro of the coal and rock samples, as follows: Figure 2 As shown in the figure, there is a linear relationship between the Lambda volume of the coal and rock samples and the vitrinite reflectance Ro (y = 7.2147x + 13.607, R). 2 =0.7751; where y is the Langevin volume, m 3 / t;x is the vitrinite reflectance Ro (%). The larger the vitrinite reflectance Ro, the larger the Rand volume, and vice versa.

[0042] Based on the established correlation between the Lambda volume and vitrinite reflectance Ro of coal and rock samples, the Lambda volume corresponding to a lower limit of 0.7% for the vitrinite reflectance Ro of medium-rank coal was obtained. Figure 2 It can be seen that when Ro is 0.7%, the Lanz volume of the coal-bearing source rock is 14 m³. 3 / t.

[0043] 2. Establishment of the correlation between coal burial depth (H), coal seam thickness (D), overlying bedrock thickness (G), and gas content.

[0044] Based on the specific data of burial depth (H), coal seam thickness (D), overburden thickness (G) and gas content (part of the data from the papers published by Ji YP, 2021; Liu F, 2007; Li WQ, 2014; Chen HW, 2012, and part of the data from the coring wells in the area), the correlation between coal rock burial depth (H), coal seam thickness (D) and overburden thickness (G) and gas content was established respectively. The specific data of coal rock burial depth (H) and gas content in XX area are shown in Table 2, and the correlation is shown in Figure 3 The specific data of coal seam thickness (D) and gas content in XX area are shown in Table 3, and the correlation is shown in Figure 4 The specific data of overburden thickness (G) and gas content in XX area are shown in Table 4, and the correlation is shown in Figure 5

[0045] The content of coalbed methane is controlled by many factors, and each factor is difficult to control the content, but it is found that with the increase of burial depth, the content of coalbed methane shows a gradually increasing trend; with the increase of thickness and overburden thickness, it also shows a gradually increasing trend. But some points may be affected by geological factors, such as fault factors, and there are characteristics of decreasing coalbed methane content, which are regarded as abnormal points in subsequent analysis and do not participate in the analysis. Figures 3-5

[0046] Table 2 Specific data of coal rock burial depth (H) and gas content in XX area

[0047]

[0048]

[0049] Table 3 Specific data of coal seam thickness (D) and gas content in XX area

[0050]

[0051] Table 4 Specific data of overburden thickness (G) and gas content in XX area

[0052]

[0053] 3. Determination of the lower limit value of coal seam thickness, coal seam burial depth and overburden thickness

[0054] From the analysis results of step 2, there is a certain trend between the content of coalbed methane and the thickness of coal seam, the burial depth of coal seam and the overburden thickness, and further plotting analysis is carried out to obtain Figures 3-5 Figures 6-8 ​​​The data points within the dotted line are the analysis data points, and the data points outside the dotted line are the abnormal points, which do not participate in the analysis. The abnormal points can be affected by factors such as faults or cracks, causing coalbed methane leakage or natural gas from other places to enter, resulting in data being too large or too small. The solid line in the figure represents the sample evolution trend line located near the approximate middle region of the interval.

[0055] When the vitrinite reflectance Ro in step 1 is 0.7%, the corresponding Langmuir volume value is 14m 3 / t is the lower limit value standard, and the intersection value corresponding to 14m 3 / t is established for the three elements of coal seam thickness (D), burial depth (H), and overburden thickness (G), respectively, as shown in Figure 6 、 7 、8. According to Figure 6 、 7 、8, when the gas content is 14m 3 / t, the corresponding lower limit of the coal seam thickness is 4m, the burial depth of the coal seam is 1300m, and the overburden thickness is 300m (i.e., the intersection corresponding to 14m 3 / t is obtained by applying the horizontal line of ordinate 14m 3 / t to the solid line in Figures 6-8 , and the value represented by the abscissa at the intersection point is the lower limit of the coal seam thickness of 4m, the burial depth of the coal seam of 1300m, and the overburden thickness of 300m).

[0056] 4. Determination of favorable development area of coalbed methane

[0057] The upper Paleozoic medium-high rank coal seams in the XX area are plotted. First, the top surface of the coal seams in the XX area is structurally interpreted and plotted, and the burial depth of the coal seams is determined. According to the lower limit of the burial depth of the coal seams determined in step 3, which is 1300m, the area with a burial depth greater than 1300m and the area with a burial depth less than 1300m are determined. Then, the coal seam thickness in the area is plotted with a plane contour, and the distribution range of the area with a coal seam thickness greater than 4m is determined. Finally, the overburden thickness of the coal seams is plotted with a plane contour, and the area with an overburden thickness greater than 300m is determined. Finally, according to the superposition of the three principles, the area with a single coal seam thickness greater than 4m, a burial depth greater than 1300m, and an overburden thickness greater than 300m is the favorable development area of the medium-high rank coalbed methane.

[0058] According to the principle that the three elements of a single coal seam thickness greater than 4m, a burial depth greater than 1300m, and an overburden thickness greater than 300m are favorable areas, the three elements are superimposed, as shown in Figure 9The favorable area of coal bed gas is the overlapping area of the coal bed gas thickness greater than 4m, the buried depth greater than 1300m and the overlying bedrock thickness greater than 300m, and the area of the favorable area is 3500km 2 The area outside the favorable area does not meet the above three principles, or only meets one of the above principles, or meets two of the above principles.

[0059] According to the prediction method of the high-rank coal bed gas favorable area in the application, the favorable area of the coal bed gas is selected, and in recent years, xx well is arranged in the established favorable area of an oilfield, and the coal bed gas content of the well is 15.66m 3 / t, and the coal bed gas content of the exploration well outside the favorable area is little or no coal bed gas, which shows the effectiveness of the prediction method.

[0060] The above is only the preferred specific implementation of the application, but the protection scope of the application is not limited to this, any person skilled in the art can easily think of the changes or replacements within the technical range disclosed by the embodiments of the application, which should be covered in the protection scope of the application. Therefore, the protection scope of the application should be subject to the protection scope of the claims.

Claims

1. A method for predicting favorable areas for medium- and high-rank coalbed methane, characterized in that: The method comprises the following steps: (1) analyzing coal rock samples in a to-be-predicted area to obtain their vitrinite volumes and vitrinite reflectance Ro, establishing a fitting relationship between the vitrinite volumes and the vitrinite reflectance Ro of the coal rock samples, and determining a vitrinite volume value corresponding to a lower limit value of the vitrinite reflectance Ro of the medium coal rank according to the fitting relationship; (2) determining lower limit values of the coal seam thickness, the coal rock buried depth and the overlying bedrock thickness based on a corresponding relationship between the geological structure characteristic parameters and the gas content of the to-be-predicted area and the vitrinite volume value obtained in step (1); (3) drawing isograms of the coal seam thickness, the coal rock buried depth and the overlying bedrock thickness of the to-be-predicted area respectively by using the lower limit values obtained in step (2), and fitting out a regional range of a favorable area of the coalbed methane of the to-be-predicted area.

2. The method for predicting the sweet spot of coalbed methane with medium-high coal rank according to claim 1, characterized in that: The lower limit value of the vitrinite reflectance Ro of the medium coal rank in step (1) is 0.7%.

3. The method of predicting a sweet spot of coalbed methane with medium-high coal rank according to claim 1, characterized in that: The corresponding relationship in step (2) is a corresponding relationship between the coal seam thickness, the coal rock buried depth and the overlying bedrock thickness and the gas content.

4. The method according to any one of claims 1 to 3, characterized in that: The lower limit value in step (2) is an intersection value of the corresponding relationship and the vitrinite volume value obtained in step (1).

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