(School of Resources and Environment, Henan Polytechnic University, Jiaozuo, Henan 454000)
About the author: Zhang Xiaodong, born in 197 1, male, from wen county, Henan, Ph.D., lecturer. Mainly engaged in gas geology, coalbed methane geology and other research. E-mail: zwenfeng @ 163.com
Project Support: Doctoral Program Fund of Henan Polytechnic University (No.:6485 13), the project of "Investigation and Evaluation of Coalbed Methane Resource Potential in Jiaozuo Coalfield, Henan Province".
Taking Guhanshan mine field in Jiaozuo coalfield as the research area, through qualitative analysis and quantitative research, this paper discusses the control of coal seam gas-bearing by factors such as coal seam buried depth, roof and floor lithology, coal seam thickness and geological structure. Through the method of mathematical statistics, the regression equations of buried depth, effective buried depth, coal thickness and coal seam gas content are obtained, and the significance of the regression equations is tested. The research results show that the thickness of coal seam, fault structure and buried depth are the main factors affecting the occurrence characteristics of coalbed methane in Guhanshan mine field. The concrete manifestations are as follows: ① With the increase of coal seam thickness, the gas content increases linearly; (2) With the increase of buried depth and effective buried depth, the gas content increases sharply at first, and then slows down after a certain stage, and there is a logarithmic positive correlation between them; ③ The gas content at the fault tip is large, and the gas content in the fault zone is small; The gas content of the descending plate of the fault is greater than that of the ascending plate; In the same fault block, the farther away from the fault plane, the greater the gas content. The lithology of coal seam roof has certain influence on gas content, but it is not the main controlling factor that causes the change of gas content in coal seam in mine field.
Keywords: influencing factors of regression analysis of coalbed methane in Guhanshan mine field
Occurrence characteristics of coalbed methane in Guhanshan coal mine of Jiaozuo coalfield
Zhang Xiaodong, Wang Lili.
(School of Resources and Environment, Henan Polytechnic University, Jiaozuo 454000)
Abstract: Through qualitative and quantitative analysis, this paper probes into the influences of the buried depth and thickness of coal seam, lithology of roof and floor, geological structure and other factors on the gas-bearing property of coalbed methane. Through mathematical statistics, the regression equation between coal seam buried depth, effective buried depth, coal seam thickness and coalbed methane content is obtained, and the significance test is carried out. The result shows: 1. Buried depth, coal seam thickness and fault structure are the main factors affecting the occurrence characteristics of coalbed methane in Guhanshan Coal Mine. The actual characteristics are as follows: (1) With the increase of coal seam thickness, the coalbed methane content increases linearly; (2) Buried depth, effective buried depth and coalbed methane content are logarithmically positively correlated; (3) The gas content at the vanishing end of the fault is high, and the gas content in the fault zone is low. The gas content in the footwall of the fault is higher than that in the footwall; With the increase of the distance from the fault, the gas content in the same fault block is also higher and higher; 2. The lithology of roof and floor rocks has certain influence on gas content, but it is not the main factor to control the change of gas content.
Keywords: Guhanshan Coal Mine; CBM regression analysis; factor
order
Jiaozuo mining area is one of the production bases of high-quality anthracite in China, which is rich in coalbed methane resources. The mining area is adjacent to Qinshui coalbed methane field in Shanxi Province, and has made a major breakthrough in exploration and development. It is the first commercial-scale mining in China and has good conditions for coalbed methane exploration and development [1]. According to the investigation and research results of coalbed methane resources of China Coalfield Geology Bureau, the coalbed methane resources in the shallow weathered zone and the deep coal seam with gas content above 4m3/t in this area are165438+2000m108m3, and the resource abundance is 2.3 1× 108m3/km2. As one of the main production mines in Jiaozuo mining area, Guhanshan coal mine has experienced three coal and gas outbursts since mining. Since the well was built, the mine has invested a lot of energy and material resources to prevent and control coal and gas outburst every year. Transforming gas disasters into coalbed methane resources for utilization can not only fundamentally prevent coal and gas outburst accidents in this mine, but also have important practical significance for effective utilization of resources and local environmental protection.
According to the distribution and variation law of coal seam gas content in mine field, the controlling effects of geological structure, buried depth, coal seam thickness and roof and floor lithology on coalbed methane occurrence are systematically discussed, and the main controlling factors of coal seam gas content are found through mathematical statistical analysis, which provides guidance for gas outburst and exploration and development of coalbed methane resources in Guhanshan mine.
1 geological structure characteristics of mine field
Jiaozuo coalfield is located in the southeast of Taihang anticline uplift and the northwest of Jiyuan-Kaifeng depression. All kinds of structural features produced since Yanshan movement are widely developed in this area, mainly fault structures and weak fold structures. Magmatic activity in this area is very weak.
Guhanshan mine field is located between Guhanshan fault and Youfangjiang fault. The strike of coal measures strata is NE40, and the southeast dip angle is12 ~19. The large fault structures in the mine field are sparsely distributed, all of which are high-angle normal faults, belonging to the mine field with simple structure. The characteristics and distribution of small and medium-sized structures in the mine field are as follows:
Faults: The faults exposed in the mine field are all normal faults, and the strike is mostly east-west and northwest, with an inclination of 30 ~ 75. Vertical joints in rock and coal seams near faults are developed, and the roof is wrinkled.
Fold: The overall feature of fold is wide and gentle structural form, and the distance between syncline axis and anticline axis is about1.50m. Small folds appear locally, and the sliding surface of roof develops, which affects the strength of rock and coal seam and adversely affects coal seam mining.
2 Distribution law of gas content in coal seam
The main coal-bearing strata in the mine field are Carboniferous and Permian coal-bearing strata, with coal 13 layers, of which only 1 coal and one or two coal layers reach the recoverable thickness. The second layer 1 coal seam at the bottom of the Permian Shanxi Formation has a simple structure and an average thickness of 5.0m It belongs to a relatively stable medium-thick coal seam and is the target layer of this research project. In this study, 35 gas content data were collected, including 23 gas content data measured by drilling and coring, and gas content data measured by different parts of 12 faults. Within the range of buried depth 158 ~ 95 1m, the gas content is between 8.08 ~ 32m3/t.
As far as the whole mine field is concerned, the gas content shows an upward trend from east to west and a downward trend from south to north. In the same fault block, the closer to the fault zone, the smaller the gas content, but the larger the gas content near the fault tip. The gas content of deep fault block is higher than that of shallow fault block.
3 coal seam gas content control factors
Before discussing the relationship between related factors and gas content, it is necessary to analyze the applicability of the collected gas content data. In the data of 23 drilling cores, the methane content is less than 80%, which belongs to methane weathering zone. In this study, these data are not considered (* * * 5); There are 15 measured gas content data in different parts of the fault, and the gas content is greatly affected by the fault structure. Therefore, when discussing the influence of coal thickness, buried depth and other factors on gas content, this part of data can only be used as reference.
3. 1 Influence of coal seam thickness on gas content
According to the existing gas content data, the relationship between coal seam thickness and gas content is as shown in figure 1.
As can be seen from Figure 1, the thickness of coal seam is positively correlated with gas content, and the related equation is:
w = 6.9 178 * h- 14.262(R = 0.62)
Where: w-gas content (m3/t); H—— thickness of coal seam (m).
The significance test of the regression equation shows that for a given significance level α=0.05, T0.025 (11) = 2.2010, and from the data point, t=2.6200, t > T0.025 (/kloc-0).
Figure 1 Relationship between coal seam thickness (h) and gas content (w)
Fig. 2 Relationship between buried depth (h) and gas content (w)
3.2 Relationship between Buried Depth and Gas Content
Whether a large amount of gas produced in the process of coalification can be well preserved depends on the depth of coal seam, that is, the thickness of overlying strata. It is generally believed that with the increase of buried depth, the storage of coalbed methane is also increasing. According to the existing gas content data, the relationship between coal seam buried depth and gas content is shown in Figure 2.
As can be seen from Figure 2, with the increase of buried depth, the gas content tends to increase, and there is a certain positive correlation between them. The results of mathematical statistics analysis show that the correlation equation between them is:
w = 12.55 ln(H)-56.873(R = 0.69)
Where: w-gas content (m3/t); H—— Buried depth of coal seam (m).
The significance test of the regression equation shows that the calculated t=3. 195 1, t > t0.025 (11) = 2.2010, and the above regression equation is significant. Therefore, it can be considered that the buried depth is one of the main factors affecting the gas content of coal seam in Guhanshan mine field.
According to the trend line distribution of buried depth and gas content, when buried depth is less than 400m, gas content increases rapidly with the increase of buried depth; When the buried depth exceeds 400m, the gas content increases slowly with the increase of buried depth.
3.3 Relationship between Effective Buried Depth and Gas Content
The effective overburden thickness of coal reservoir refers to the thickness of coal seam overburden that controls the gas-bearing performance of coal seam in the stratigraphic section of coal-bearing basin or region [3]. It can be expressed by the stratum thickness from the coal seam to the first unconformity surface after a lot of gas generation, which truly reflects the influence of the tectonic movement after a lot of gas generation and the stratum uplift and denudation caused by it on the preservation conditions of coalbed methane [4]. Generally speaking, the greater the effective thickness of overlying strata, the better the preservation conditions; The thinner the effective formation thickness, the stronger the uplift and denudation caused by tectonic movement, and the lower the formation pressure, the easier it is for gas to desorb and dissipate.
At the end of Triassic, the uplift and denudation of strata in this area may reduce the gas content of coal seam. The thickness between the roof of Er 1 coal seam and the basement of Cenozoic strata reflects the thickness of the remaining caprock after the formation of unconformity, that is, the thickness of bedrock, and also reflects the effective buried depth of Er 1 coal seam gas [4]. The statistical results of this study show that the gas content in coal seam increases with the increase of effective buried depth, and there is a certain logarithmic relationship between them (Figure 3). The relevant equation is:
w = 9.86 ln(h 1)-34.87(R = 0.60)
Where: w-gas content (m3/t); H 1- buried depth of coal seam (m).
The significance test of the regression equation shows that the calculated t=2.4876, t > t 0.025 (11) = 2.2010, and the above regression equation is significant. Therefore, it is also considered that the effective buried depth is also one of the main factors affecting the gas content of coal seam in Guhanshan mine field.
Fig. 3 Relationship between effective buried depth (H 1) and gas content (w)
According to the trend line distribution of effective buried depth H 1 and gas content, it can be seen that when H 1 < 250 m, gas content increases rapidly with the increase of H 1; When h 1 > 250 m, the gas content increases slowly with the increase of effective buried depth.
3.4 Influence of fault structure on gas content in coal seam
Different types of geological structures have different structural stress fields and internal stress distribution characteristics during their formation, which will lead to differences in the occurrence, structure, physical properties, fracture development and groundwater runoff conditions of coal reservoirs and caprocks, and then affect the gas-bearing characteristics of coal reservoirs [2]. Generally speaking, normal faults are open, and poor sealing is conducive to the escape of gas, while reverse faults are compressive or compression-torsion, and good sealing is conducive to the preservation of gas. In addition, different parts of the same structure have different effects on the enrichment and migration of coalbed methane.
Guhanshan mine field belongs to monoclinic structure, and the main structural type in the area is normal fault, no reverse fault is found, and only folds appear locally. See table 1 for the distribution of gas content in different parts of main faults in the area, and table 2 for the data of gas content in boreholes closely related to the structure.
Table 1 data of gas content in different parts of the same fault structure
Table 2 borehole gas content data related to fault structure
According to the measured data of gas content in the upper and lower walls of different faults (see table 1), it can be found that the gas content in the rising wall of the fault is obviously smaller than that in the falling wall. The reasons are as follows: firstly, in the process of descending plate, the tectonic activity is relatively strong, and the temperature rises in the process of coal seam kneading, which leads to the pyrolysis of more coal seam macromolecular structures and side chains or branched chains, leading to the increase of local hydrocarbon generation; Second, the secondary faults associated with the footwall are relatively developed, which destroys the escape passage of coalbed methane. The kneading pressure of the structure enhances the damage degree of coal structure, weakens the permeability of coal seam, and forms a gas-gathering and gas-blocking structure, which is not conducive to gas escape and makes coalbed methane relatively rich.
As can be seen from Table 2, the gas content of 2308 boreholes near the fault tip can reach 32m3/t ... The main reason is that the stress concentration and coal fragmentation near the fault tip reduce the permeability of the coal seam, thus forming a gas accumulation structure and increasing the gas content. The gas content of borehole Guan 4 near the fault zone is small, only 9. 1.6m3/t, and the gas content of borehole 32- 10 is only 12.53m3/t because the coal breakthrough point is close to the cluster fault. It is also found that the farther away from the fault plane, the greater the gas content, mainly because the larger fault structures in the region are tensile normal faults, and these faults often form gas-conducting structures. The closer to the fault plane, the better the permeability, and the easier it is for the generated coalbed methane to escape, thus reducing the coalbed methane content.
3.5 Influence of roof and floor lithology on gas content
The roof of 2 1 coal seam in Jiaozuo coalfield is mostly interbedded with siltstone and fine sandstone, with dense siltstone and good cementation. 0. 12 ~ 1.0m dense carbonaceous mudstone develops between the roof and the coal seam. Mudstone with the thickness of11.2 ~ 21.5m is widely developed in the coal seam floor, which makes the coal seam in a better closed environment and is very beneficial to the preservation of coalbed methane, which is one of the main reasons why Jiaozuo mining area can enrich coalbed methane resources.
The roof of coal seam in Guhanshan mine field is gray and dark gray fine-medium grained sandstone, with a general thickness of 23.0 meters, and sometimes it becomes sandy mudstone. The porosity test results of coal seam false roof and direct roof rock samples are shown in Table 3.
Comparing the pore structure parameters of coal samples and roof rock samples, it can be found that the pore volume and specific surface area of roof rock are much smaller than that of coal, and so is the median pore size. Combined with the relatively stable distribution of the roof in the mine field, it can be inferred that the pore content of the roof rock in Guhanshan mine field is much smaller than that of the coal sample, and the macro-pore content of the roof rock is much smaller than that of the coal sample, mainly small pores. The floor rock of coal seam in the mine field is relatively stable mudstone and sandy mudstone, which has poor permeability and is beneficial to the preservation of coalbed methane and has relatively stable distribution characteristics.
Table 3 Pore structure parameters of coal samples and coal seam roof rock samples
It can be seen that the lithology of coal seam roof and floor in Guhanshan mine field is beneficial to the preservation of coalbed methane, but due to its relatively stable distribution characteristics, it can be inferred that the lithology of coal seam roof and floor in mine field is not the main factor causing the change of gas content.
4 conclusion
The thickness of coal seam in Guhanshan mine field (1) has great influence on the gas content in coal seam. The greater the thickness, the greater the gas content, and there is a linear positive correlation between them.
(2) The buried depth and effective buried depth of coal seam also have great influence. In the area with small buried depth and effective buried depth, the gas content increases rapidly with the increase of depth; When the buried depth is 400 meters and the effective buried depth is about 250 meters, the gas content increases with the depth, and the increasing trend slows down.
(3) The influence of geological structure on gas content is as follows: the gas content near the fault tip is large, while the Nevas content in the fault zone is small; The gas content of the descending plate of the fault is obviously greater than that of the ascending plate; In the same fault block, the farther away from the fault plane, the greater the gas content; And the farther away from the fault plane, the greater the gas content.
(4) The lithology of coal seam roof and floor in Guhanshan mine field is beneficial to the enrichment of coalbed methane, but it is not the main reason that affects the change of coal seam gas content in this mine field.
Acknowledgement: During the research of this paper, Mr. Jin Fugui, the chief engineer of Hydrogeological Exploration Company of Jiaozuo Mining Bureau, the chief of Guhanshan Mine Geology Department of Jiaozuo Coal Industry Group, and Mr. Yang, the deputy chief of Security Inspection Department, provided relevant information. With the help of Zhang Jing, a senior engineer, and Tang Jiaxiang, an engineer in the testing center of China University of Mining and Technology, the pore structure parameters were tested.
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