Fund Project: National Natural Science Key Fund Project (40730422); National Natural Science Foundation Project (40672101); Major national science and technology project (2008ZX05034).
About the author: Li Ming, born in 1987, male, from Suzhou, Anhui, is a doctoral student. Tel:13151981375, e-mail :cumtmingli@hot-mail.com.
(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 22 1 162. Key Laboratory of Coalbed Methane Resources and Accumulation Process, Ministry of Education, Xuzhou, Jiangsu 22 10083. Guizhou Liupanshui Faer Coal Industry Co., Ltd. 5530 17).
Through the analysis of geological background and gas-bearing data of Faer coal mine, the occurrence characteristics of coalbed methane and its distribution law in sequence and space are studied. Combined with isothermal adsorption experiment, mercury injection experiment and observation of coal reservoir deformation characteristics, the physical properties of coal reservoir are discussed. The results show that the coalbed methane in Faer coal mine has high methane concentration, methane content and gas content gradient. The gas content and gas content gradient fluctuate in turn, which is mainly caused by the difference of adsorption in each coal seam and the distribution of pressure system in coalbed methane reservoir. Influenced by bayberry syncline and topographic development, the gas content in coal seam shows a pattern of "high in the north and low in the south, showing ne distribution". Coal seam fracture system is mostly reformed by structure, which improves its development and connectivity, and also promotes the development of macropores and mesopores in coal. The theoretical gas saturation of coal reservoirs is mostly below 60%, which is in an undersaturated state. Faer coal mine has a good prospect of coalbed methane development.
Key words: characteristics of coalbed methane accumulation in Faer coal mine, physical properties of gas-bearing reservoir
Characteristics of coalbed methane reservoir in Faer coal mine
Li Ming 1, 2 jiang bo 1, 2 Lan Fengjuan 1, 2 Zhang Guishan 3.
(1. School of Resources and Earth Sciences, China University of Mining and Technology; Technology, Xuzhou, Jiangsu 22 1 1 16, China 2. Key Laboratory of Coalbed Methane Resources and Accumulation Process, Ministry of Education of China, Xuzhou 22 1008. Guizhou Faer Coal Co., Ltd., Liupanshui, Guizhou, 5530 17, China)
Abstract: Based on the analysis of geological background and gas content data of Faer coal mine, the occurrence characteristics of coalbed methane and its distribution in stratigraphic sequence and coal seam are discussed. We analyzed the methane adsorption isotherm experiment, reservoir deformation and mercury injection porosity (MIP), and further discussed the reservoir physical properties. The results show that the coalbed methane in Faer coal mine has high gas content, methane concentration and gas content gradient. Its gas content and gas content gradient fluctuate layer by layer, which is mainly caused by the difference of adsorption capacity of each coal seam and the distribution of coalbed methane reservoir pressure system. Yangmeishu syncline and topography are the main controlling factors affecting the current distribution pattern of coalbed methane, which is high in the north and low in the south and northeast. The gas saturation is generally lower than 60%, mainly for undersaturated coalbed methane reservoirs. Well-developed brittle cracks, macropores and mesopores formed by structural transformation. Faer coalbed methane exploration has a good prospect.
Keywords: Faer coal mine; Coalbed methane; Occurrence characteristics; Gas content; Reservoir physical properties
Qianxi coalfield has many coal seams, large cumulative thickness, high gas content, high abundance of coalbed methane resources and large resource reserves. Its development and utilization is conducive to alleviating the pressure of energy demand in southern China and reducing the occurrence of coal mine production accidents (Telly et al., 2008; Advanced, 2009; Jiang bo et al., 2009).
Faer Coal Mine is located in the south of Liupanshui City, Guizhou Province, with an area of about 92km2 and minable seam 19. The gas content in coal seam is high. It is estimated that the total amount of coalbed methane resources in this coal mine with buried depth 1000 m is 43.29× 108m3. This paper systematically studies the occurrence, distribution and reservoir-forming characteristics of coalbed methane in this area, which provides a certain theoretical basis and guiding significance for further development of coalbed methane and prevention of mine gas disasters.
1 geological conditions of coalbed methane occurrence
1. 1 strata and coal-bearing strata
The exposed strata in the mine field include Upper Permian Emeishan Basalt Formation (P3β), Longtan Formation (P3l), Lower Triassic Feixianguan Formation (T 1f), Yongningzhen Formation (T 1yn) and Quaternary System (q). Longtan Formation of Upper Permian is the main coal-bearing stratum in the mining area, with a thickness of 344 ~ 487 meters and 47 ~ 78 coal-bearing layers, with an average total thickness of 45.28 meters, in which the coal seam 19 is 1, 3,5-2,5-3,7, 10 respectively. See table 1 for the coal rock types and coal quality characteristics of minable seam.
Table 1 Faer Coal Seam and Coal Quality Characteristics Statistics Table
sequential
1.2 structure
Fa 'er Coal Mine is located in the northern section of Pu 'an structural deformation zone in Liupanshui fault depression of Qianbei Uplift of Upper Yangtze Platform (Wang Zhongtang, 1990), adjacent to Weining NW-trending structural deformation zone in the north. The SE wing of Myrica rubra syncline extends in ne direction (Xu Binbin et al., 2003). The NWW syncline in the north, the NWW syncline in the south, the Tucheng syncline in the west, the Baoshan syncline in the northeast and the Gesuohe anticline in the east are isomorphic, forming a hairy rhombic structure (Le Guangyu et al., 199 1, 1994). Among them, Baoshan Syncline, Dazhai Syncline, Yangmeishu Syncline and Gesuo River Syncline distributed in the northeast-southwest direction are characterized by the combination of closed syncline and wide and slow box syncline. However, influenced by the development of boundary faults, the Gemudi syncline and Taishaba anticline in the SW plate of Weining-Ziyun fault in the north, the Zhaozihe syncline and Tucheng syncline in the ne plate of Zhaozihe fault in the south all show relatively closed asymmetric folds, and the axial planes of the folds all point to the boundary fault, and the anticline shows tight, steep and sharp, even destroyed by the reverse faults developed in the core, and also shows a staggered fold combination; The central Yangmeishu syncline and Haqing anticline are relatively flat and broad, with asymmetric box shape, dip angle of 10 ~ 20 and span of about 17km. The fault structures in this area are mainly normal faults with strike of about NE30, followed by reverse faults with strike of about nw20 ~ 30°, and faults in other directions are also developed. The structural deformation of the edge zone of rhombic structure in the study area is stronger than that inside, and the NW-trending structure is stronger than the NE-trending structure. The structure with strong deformation is linear in strip shape, and the structure with weak deformation is massive, and the whole structure is a "block staggered and diamond combination" structure pattern.
Yangmeishu syncline is the main fold that affects the stratum distribution in the mine field. At the same time, there are Malong syncline and Bajiaotang anticline in the northwest, and Laofa 'er anticline in the northeast (Figure 1). The strata strike is generally distributed in NE-SW direction, with NW dip angle 10 ~ 15. Faults are mainly normal faults, which can be divided into two groups: northwest and northeast. The former is distributed in the southwest of the mine field, and forms a series of complex fault combination styles with the surrounding strike near north and south and near east and west. The latter is sparsely distributed in the northeast of the mine field, showing a broad and gentle horst and graben shape (Figure 2). Mine structure is mainly the product of Yanshanian tectonic activity (Mao et al., 1999).
1.3 Hydrogeology
Longtan Formation (P3l) is weak in water abundance, mainly fissure water, and some areas are under pressure. The lower member of Feixianguan Formation (T 1f) in the overlying strata contains almost no water, and belongs to the aquifuge. The upper section contains a small amount of fissure water, and its water abundance is weak. The underlying Emeishan basalt formation (P3β) contains fissure water, which is weak in water abundance and has a certain bearing capacity. The weak water content and certain bearing capacity of coal-bearing strata and their overlying strata in the study area are beneficial to the preservation and enrichment of coalbed methane in this area as a whole.
Figure 1 Faer Coal Mine 16 Contour Line and Structural Outline of Coal Seam Floor
Fig. 2 AA' structural section (see fig. 1 for the location of section line)
The study area belongs to middle-high mountain landform, with relatively open valleys or gentle slopes in the south and steep Table Mountain formed by Triassic strata in the north, with a relative height difference of 300~400m (Figure 2). Faer River and Beipan River flow through the mine field, and the complex terrain distribution and the development of surface water system will inevitably complicate the head distribution of Longtan Formation, thus affecting the distribution of coalbed methane.
2 coal seam gas
2. Distribution characteristics of1sequence
The gas content and gas composition of the study area 125 coal seam were tested. The statistical results show that the gas content in coal seam is 3.94~35.94m3/t, with an average of13.58m3/t; The composition is mainly CH4, and the average concentration of CH4 is 9 1.8 1% (Figure 3). The average methane content in each coal seam is above 65,438+1100m3/t, which generally tends to increase with the decrease of coal seam elevation (Figure 4). At the same time, the gas content of coal seam still fluctuates with the decrease of sequence, and the average gas content of 10, 15-2, 17, 23-2 and 29- 1 coal seam is relatively high, reaching15 ~/kloc-0. This fluctuation is more obvious in the change curve of methane content gradient in each coal seam, and its change trend is basically consistent with the change curve of methane content in coal seam (Figure 4), and the methane content gradient reaches the relative highest value in 10, 17 and 23-2 coal seams. The reservoir pressure of coal seam and the adsorption of coal seam itself are the key factors affecting gas content. It can be seen that the change of coal seam gas content with horizon fluctuation is mainly influenced by the adsorption difference of each coal seam and the distribution of coalbed methane reservoir pressure system.
Fig. 3 Relationship between methane content, coalbed methane concentration and buried depth
Figure 4 Statistical table of gas content and gas gradient in each coal seam
Figure 5-7 Relationship between Methane Content in Coal Seam and Buried Depth of Coal Seam
Fig. 6 Relationship between methane content gradient and buried depth in coal seam
2.2 Vertical distribution characteristics
With the increase of the buried depth of coal seam, the methane content and concentration of coalbed methane generally have an increasing trend (Figure 3 and Figure 5), but with the linear increase of buried depth, the dispersion is greater; This shows that other geological factors still have great influence. The test data at the stage of coal seam burial depth of 500 ~ 800 meters reflects the phenomenon that the coal seam burial depth is relatively large, but the gas content is relatively low (Figure 3). In the trend diagram that the methane content gradient of coal seam decreases with the increase of coal seam buried depth, the methane content gradient of this section of coal seam is lower than the overall trend value (Figure 6). Through the analysis of the original data, it is found that the abnormal points in this section are concentrated in the boreholes 10 12, J 1 106, J 1 107 and J 1406, J. However, the methane concentration at the abnormal point in this section is still very high, with an average value of 96. 17% (Figure 3), indicating that the coal seam is connected with the outside atmosphere and has not caused losses.
Gas-bearing gradients in deep coal seams 28, 29- 1, 29-3, 33 and 34 are relatively low (Figure 4). Although the metamorphic degree of these coal seams is relatively high, and more gas is produced in the process of coalification, the gas produced has migrated and dissipated into the overlying and underlying strata systems of coal seams because of their thin and unstable coal seams and low coal-rock ratio. Due to the steep terrain around the borehole in the high altitude area in the area and the drastic elevation change, on the one hand, the elevation of groundwater level is far from the surface elevation, which makes the reservoir pressure relatively reduced; On the other hand, when the coalbed methane migrates vertically to a low surface elevation, it will migrate laterally along the stratum, reducing the buried depth of effective sealing of the coal seam, which will lead to a decrease in gas-bearing gradient.
2.3 plane distribution characteristics
The gas content of coal seam shows obvious zoning characteristics on the plane (Figure 8). The gas content in the area south of Farr River is generally low, and the isoline of gas content is sparse. Only the central region has a gas content of 65438+ 100 m3/t, and the eastern and southern regions are surrounded by coal seams, which become a window for the loss of coalbed methane, resulting in a low gas content. The distribution of gas-bearing isolines in the west and north is controlled by Beipanjiang River and Faer River. The river flowing through the mine field is used as the drainage channel of groundwater, which leads to the decrease of groundwater level on both sides of the river and the relatively low content of coalbed methane on both sides of the river. The gas content in the north of Farhe River is relatively high, generally greater than 15m3/t, and the gas content in the northwest of the mine field is expected to reach about 35m3/t, with dense gas content isolines and large gas content change gradient on the reaction plane. There are relatively low gas content areas in boreholes 1204, J 1306 and J 1405 in the northeast. At present, the distribution pattern of coal seam gas content is "high in the north and low in the south, showing NE distribution", which is mainly controlled by the development of bayberry syncline and topography.
Fig. 7 Test results of gas pressure in partially drilled coal seam
3 coal reservoir physical properties
3. 1 reservoir porosity and permeability
The porosity, pore volume, pore specific surface area and pore structure of 1 4 coal samples in1and 3 coal seams were measured by mercury injection experiments (Table 2). The pore structure is divided by Khodot (1966) standard, 1000nm, 100nm, 65438. It can be seen that the pores in coal are mainly micropores, accounting for 4 1.29% (Figure 9); Macropore takes the second place, accounting for 24.86%, transition pore and mesopore account for 20.98% and 12.86% respectively.
Fig. 8 Isograms of gas content in coal seams of FAER Coal Mine 1, 3,5-2,7.
Table 2 Statistical table of data of mercury injection experiment and isothermal adsorption experiment
Note: the format of 3.81~ 7.01/5.16 is: minimum-maximum/average.
There are more than 2~3 groups of fractures in the coal seam, which are mainly caused by structure or structural transformation (Figure 10). The occurrence is X-shaped * * * yoke shear joints, oblique fractures and bedding fractures. 3, 5-2, 13- 1, 29- 1 and 29-3 coal seams are relatively developed, showing one or two groups of fractures perpendicular to the plane. The observation of micro-cracks shows that the same group of cracks can be stepped, trapezoidal or slow-wavy, and two or more groups of cracks often intersect and converge at a large angle. Unstable secondary cracks mostly come from the intersection area, and the fracture connectivity is good.
3.2 Coal seam adsorption
The isothermal adsorption experiment of coal seam (dry coal sample) in Faer coal mine shows that the Langmuir VL of 1, 3 and 5-2 coal seams is 23.55~27. 18m3/t, and the Langmuir pressure PL is 0.82~0.95MPa (Table 2, Figure1/kloc-0 3. The theoretical gas saturation of coal seam is 45.62%. The theoretical gas saturation of 5-2 coal seam is 52.36%. The theoretical gas saturation of coal reservoirs is mostly below 60%, which is in an undersaturated state.
Fig. 9 1 and 3 Pore Volume Distribution of Coal Seam at Different Stages
Figure 10 Macroscopic and Microscopic Deformation Characteristics of Coal Body
Figure 1 1 Faer Coal Mine 1, 3, 5-2 Isothermal Adsorption Curve of Coal Seam
4 conclusion
(1) Faer coal mine is rich in coalbed methane resources, with high methane concentration, methane content and gas content gradient, and its gas content and gas content gradient have sequential fluctuation changes, which are mainly caused by the difference of adsorption between coal seams and the distribution of coalbed methane reservoir pressure system.
(2) The phenomenon of low gradient of gas content in coal seam with vertical buried depth of 500~800m is mainly caused by the thin and unstable thickness of bottom 28~34 coal seam, low coal-rock ratio and lateral migration of coalbed methane at high altitude. Influenced by Yangmei syncline and topographic development, the gas content of coalbed methane shows a distribution pattern of "high in the north and low in the south, showing ne distribution" in the plane.
(3) The fracture system of coal seam is mostly reformed by structure, and its development and connectivity are improved, which also promotes the development of macropores and mesopores in coal. The theoretical gas saturation of coal reservoirs is mostly below 60%, which is in an undersaturated state. Faer coal mine has a good prospect of coalbed methane development.
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