Study on fuzzy comprehensive evaluation method of geological conditions for coalbed methane drainage

Ni Xiaoming 1 Jie Mingxun 2 Wang Yanbin 1 Wu Jianguang 2.

(1. School of Resources and Safety Engineering, China University of Mining and Technology, Beijing100083; 2. China United Coalbed Methane Co., Ltd. Beijing 1000 1 1)

About the author: Ni Xiaoming, born in 1979, male, from Linfen, Shanxi, is a doctoral student, mainly engaged in gas geology and coalbed methane geology research. E-mail: nxm1979 @126.com.

In order to reduce the investment risk of surface drilling and underground drainage of coalbed methane, the influencing factors of coalbed methane mining are analyzed by analytic hierarchy process (AHP), and the hierarchical knowledge model of surface drilling and underground drainage of coalbed methane is established, and the fuzzy comprehensive evaluation system is established according to the multi-level fuzzy comprehensive evaluation method. On this basis, the evaluation results show that this evaluation method has certain guiding role in reducing the investment risk of coalbed methane mining.

Fuzzy evaluation; Ground drilling and production; Underground drainage of coalbed methane

Study on fuzzy comprehensive evaluation of coalbed methane development

Ni Xiaoming 1, Xie Mingxun 2, Wang Yanbin 1, Wu Jianguang 2.

(1. School of Resources and Safety Engineering, China University of Mining and Technology, Beijing100083; 2. Beijing Zhonglian CBM Co., Ltd. 1000 1 1)

Abstract: In order to reduce the investment risk of surface mining and underground drainage of coalbed methane, the influencing factors of coalbed methane mining are analyzed by analytic hierarchy process, and the hierarchical knowledge model of surface mining and underground drainage of coalbed methane is established. Based on the multilevel fuzzy evaluation method, a fuzzy comprehensive evaluation system is established. On this basis, the development of coalbed methane in Haizi Coal Mine in Huaibei Coalfield is evaluated by fuzzy method. The results show that the fuzzy evaluation method has important guiding significance for the development of coalbed methane.

Keywords: coalbed methane; Fuzzy evaluation; Open pit mining; underground drainage

introduce

The energy situation in China will be very severe in 2 1 century, and the contradiction between supply and demand of high-quality energy will be intensified. The environmental pressure caused by coal combustion also requires that the unreasonable energy structure with coal as the absolute advantage must be improved. China is rich in coalbed methane resources, with the total amount reaching 31.46×1.01.2m3 (Zhonglian CBM Co., Ltd., 2000), which is close to the total amount of natural gas resources in China and ranks third in the world. Therefore, the development and utilization of China's abundant coalbed methane resources can not only effectively increase the supply of efficient and clean energy in optimizing the energy structure, but also directly reduce the methane emission from coal mines and effectively alleviate the greenhouse effect. At the same time, it is expected to fundamentally curb the mine gas disaster and improve the safe production conditions in coal mines. However, the investment risk of coalbed methane is high, and reducing the investment risk is the key to the prospect of rolling development and utilization of coalbed methane. In order to reduce the investment risk, on the basis of analyzing the influencing factors of coalbed methane drainage by analytic hierarchy process, a fuzzy evaluation system of coalbed methane surface drilling and underground drainage is established by using multi-level fuzzy comprehensive evaluation method, and the effect of this system is tested by taking Haizi Mine in Huaibei as an example.

Analysis of influencing factors of 1 coalbed methane drainage

1. 1 Analysis of Influencing Factors of CBM Surface Drilling Production

Whether surface mining of coalbed methane has commercial value depends on gas production rate, competitive market price and large-scale output. At present, the contradiction between energy supply and demand in China is intensifying, and the coalbed methane market is promising. Therefore, the key to affecting the surface mining of coalbed methane lies in the output and final output of coalbed methane. There are two prerequisites related to gas production: permeability of coal reservoir and geological exploration ability; However, large-scale production is related to the gas-bearing property of coal reservoirs. On the basis of collecting and sorting out the previous research results, the author summarizes the knowledge model of influencing factors of open pit mining.

1. 1 "short-circuit condition" of coalbed methane surface drainage

The so-called "short-circuit condition of coalbed methane ground drainage" refers to the priority judgment condition, that is, as long as these conditions are met, the conclusion can be drawn directly without considering other factors [1]. Considering the technical level, economic benefits, geological conditions and resource conditions of coalbed methane development at present, it is considered that surface drilling of coalbed methane is not suitable under any of the following circumstances:

(1) storage and cap conditions. A certain coal seam thickness is the basis of coalbed methane reserves. The coal seam is too thin and the amount of coalbed methane is too small, which is not suitable for coalbed methane development. Because the coalbed methane industry is a new industry, there is no clear regulation on the thickness of coal seam. The minimum thickness of minable coal seam in China is 0.7m, so the coal seam thickness > 0.7 m is the lower limit of coalbed methane development. A good sealing layer can reduce the seepage and loss of coalbed methane. When the effective thickness of overlying strata is less than 200 m, it is not conducive to the preservation of coalbed methane, so the thickness of overlying strata is set at 200 m; Considering the current exploration and development level and exploration cost comprehensively, coal seams with buried depth of < 300 m or > 1500 m are not considered; The development of coalbed methane is realized by depressurization and desorption. If the pressure gradient is too low, with the current level of exploration and development technology, the recovery rate of coalbed methane is too low and the economic benefit is poor. According to statistical analysis, the lower limit of pressure gradient (MPa/ 100 m) is 0.4.

(2) Gas-bearing conditions. A certain gas content is the premise to ensure the development of coalbed methane. Considering the geological conditions, coal reservoir conditions and economic benefits in China, if the gas content of middle and high rank coal is less than 4m3/t or the gas content of low rank coal is less than 2m3/t, the surface mining of coalbed methane is not considered.

1. 1.2 CBM surface drilling and production knowledge engineering.

Whether coalbed methane has the commercial value of surface drilling depends on four key factors that help to develop this huge resource, which are: geological exploration ability; Gas-bearing strength of coal reservoir; Permeability and gas desorption ability.

(1) Geological exploration capability. Buried coal seam is too deep, which will not be conducive to the exploration and development of coalbed methane; Areas with complex structural conditions and developed faults are not conducive to the development of coalbed methane; Areas with frequent hydrodynamic activities are conducive to the migration and diffusion of coalbed methane, which is not conducive to its preservation; Therefore, buried depth, structural conditions and hydrogeological conditions are regarded as the influencing factors of coalbed methane exploration ability.

(2) Gas-bearing strength of coal reservoir. Thick coal seam and high gas content are necessary conditions for obtaining high gas content intensity [2]. Among the two parameters that determine the gas-bearing intensity, the thickness of coal seam is the most important, because the thickness of coal seam not only controls the gas-bearing intensity, but also controls the gas flow.

(3) Permeability of coal reservoir. The permeability of coal reservoir is the reflection of gas conductivity in coal seam. Coal reservoir is a dual porous medium, and the development of cleat directly affects its permeability. In the process of geological history development, coal seams are subjected to different degrees of tectonic stress, which makes coal bodies broken. Therefore, the structure of coal is closely related to the permeability of coal seam; It is generally believed that primary structural coal and fractured coal are ideal coal structure types for coalbed methane development, while fractured coal loses endogenous cracks, which makes permeability worse and is not conducive to coalbed methane development. Coal rank has a certain influence on the development of endogenous cracks in coal. Endogenous cracks generally appear in the long-flame coal stage, and gradually increase with the increase of coal rank until coking coal is the most developed, and then gradually close until anthracite coal stage is the lowest.

(4) Desorption ability of gas. The development of coalbed methane is realized by depressurization desorption, so adsorption time, temporary storage pressure ratio and gas saturation will all affect the desorption ability of gas.

According to the above analysis, the knowledge model of influencing factors of coalbed methane surface drilling and production is obtained, as shown in figure 1.

Figure 1 Knowledge Model of Influencing Factors of CBM Surface Drilling Production

1.2 Analysis of Influencing Factors of Coalbed Methane Underground Drainage

The investment of underground coalbed methane extraction project is large, so it is particularly important to evaluate the feasibility of underground coalbed methane extraction. When coalbed methane is pumped underground, the study of coal characteristics, gas occurrence and gas characteristics is the key index to determine whether coalbed methane can be pumped underground.

(1) coal body characteristic expression index. The damage degree of coal body will directly affect the development of coalbed methane; The firmness coefficient of coal will affect the gas outflow, that is, fracturing stimulation technology, which is an important indicator of coal characteristics.

(2) Gas occurrence indicator. Buried depth will affect the pressure, thus affecting gas desorption; The variation coefficient of coal thickness reflects the change of coal seam thickness in this area, thus inferring the enrichment degree of coalbed methane in this area; Structural conditions and hydrogeological conditions make the gas in coal seam gather or escape; Permeability reflects the difficulty of gas drainage in coal seam; These are signs of gas.

(3) Gas characteristic expression index. The gas pressure, initial gas emission velocity, gas permeability and gas resources of 100m borehole can reflect the gas quantity and duration [3] and can be used as indicators to characterize gas characteristics.

To sum up, we can get the knowledge model of influencing factors of coalbed methane underground drainage as shown in Figure 2.

2. Fuzzy comprehensive evaluation system for coalbed methane mining.

The fuzzy comprehensive evaluation method of coalbed methane mining is to apply the principle of fuzzy transformation and the principle of maximum membership degree, and consider the influencing factors of coalbed methane mining to comprehensively evaluate its mining risk [4 ~ 6]. On the basis of analyzing the influence of surface drilling and underground drainage of coalbed methane, due to the limitation of space, taking surface drainage as an example, this paper discusses the fuzzy comprehensive evaluation of coalbed methane mining risk.

Fig. 2 Knowledge model of influencing factors of underground drainage of coalbed methane

A set of factors affecting whether to use surface drilling and production can be summarized as follows:

U = {Earth exploration capacity, gas-bearing strength, permeability and desorption capacity} = {U 1, u2, u3, u4}

In order to make the evaluation results more reasonable, the surface drilling and production risks are divided into five grades, namely, the evaluation set is:

V = {good, good, average, poor, poor} = {v 1, v2, v3, v4, v5}

Technical progress in exploration, development and utilization of coalbed methane in China: Proceedings of the 2006 symposium on coalbed methane.

Any index affecting the concentration of drilling and production risk factors constitutes a fuzzy set, and its membership function is in the range of [0, 1]. According to the degree of membership, it is divided into five grades:

Let the single factor evaluation of the ith factor be Ri=(ri 1, …, rim), which is a fuzzy subset of U. For a region, the drilling and production index of coalbed methane in different regions is different, and it is set in different regions and measured n times. According to the principle of statistics, the number m of the membership degree of an index in μA(u) is determined, and then rik=m/n is determined. On this basis, the comprehensive evaluation matrix of single factor of surface drilling and production is established, namely

Technical progress in exploration, development and utilization of coalbed methane in China: Proceedings of the 2006 symposium on coalbed methane.

Determination of weight: In comprehensive decision-making, the weight of index factors is in a very important position, which reflects the role of each index factor in the process of comprehensive decision-making and directly affects the result of comprehensive decision-making. According to the expert scoring method and statistical principle, this paper establishes the weight distribution among various factors. Write the weight coefficient matrix as: M=(m 1, m2, m3, M4); Their sum is 1.

Then, the fuzzy comprehensive evaluation matrix of surface drilling and production is obtained by applying the composite operation of fuzzy matrix:

Technical progress in exploration, development and utilization of coalbed methane in China: Proceedings of the 2006 symposium on coalbed methane.

Normalization b, b' = (b 1/b, b2/b, b3/b, b4/b)

Comments are vague. For convenience, we should quantify them. In this paper, the percentage system is adopted, and the quantitative evaluation grades are shown in table 1.

Table 1 Standard Table for Ground Drilling and Production Grade of Coalbed Methane

The numerical value obtained by matrix product between b' and quantized value is compared with the grade standard table, and the multi-level comprehensive evaluation result is obtained.

3 example analysis

Haizi Coal Mine is located in Suixi County, Huaibei City, Anhui Province, and belongs to Linhaitong Mining Area in Huaibei Coalfield. The coal-bearing strata are Permian, and there are 6 minable seams from top to bottom, numbered 3, 4, 7, 8, 9 and 10. The total recoverable thickness is10.23m, accounting for 7 1.6% of the total coal-bearing thickness. Taking 7 # coal seam as an example, this paper briefly introduces the fuzzy evaluation process of surface drilling and underground drainage of coalbed methane in recoverable coal seam in Haizi Mine.

According to the expert's scores of the first-level evaluation index and the second-level evaluation index of the influencing factors of coalbed methane mining, these scores are quantified according to the weighted average method, that is, the subset of each factor and the weight of each factor are obtained, thus the fuzzy comprehensive evaluation index system of coalbed methane surface drilling and underground drainage in Haizi Coal Mine is established respectively, as shown in Table 2 and Table 3.

Table 2 Fuzzy comprehensive evaluation index system of coalbed methane surface drilling and production

Table 3 Fuzzy Comprehensive Evaluation System of Underground Gas Drainage

According to the measured values of each evaluation index at different points in Haizi Mine, the average value of each evaluation index is obtained by weighted average, and a single-level evaluation matrix is constructed according to the membership degree. See Table 4.

Table 4 Weighted average value of each evaluation index of coalbed methane mining risk in Haizi Coal Mine

According to the above data, using the fuzzy comprehensive evaluation method, the final evaluation scores of the first-class index of ground drilling and production of coalbed methane are 76.2, 29.25, 66.9 and 60. It can be seen that the gas intensity in this mining area is not high, but it is beneficial to the desorption and exploration of coalbed methane.

The score of comprehensive evaluation result is: 56.34. The comprehensive evaluation results show that the mine is not suitable for surface drilling and mining of coalbed methane in 7 # coal seam.

The final scores of the first-class index of gas drainage are: 13.75, 72.75 and 78.9. It can be seen that the characteristics of coal body in this mining area are extremely poor, but coalbed methane exists well, which is beneficial to the emission and diffusion of coal seam gas and is a dangerous coal seam with coal and gas outburst. The score of comprehensive evaluation result is: 68.83. The comprehensive evaluation results show that the mine is suitable for underground drainage of coalbed methane in 7 # coal seam.

4 conclusion

Multi-level fuzzy comprehensive evaluation method is used to evaluate the geological conditions of coalbed methane drainage, which can combine qualitative analysis with quantitative evaluation, reasonably reflect the suitability of surface drilling and underground drainage of coalbed methane in a region, facilitate each mine to take corresponding coalbed methane mining measures according to the evaluation results, and reduce the investment risk of coalbed methane. This method is simple, feasible and practical.

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