Li Ming1,2 Jiang Bo1,2 Lan Fengjuan1,2 Zhang Guishan3
Funding projects: National Natural Science Key Fund Project (40730422); National Natural Science Foundation of China (40672101); National Major Science and Technology Project (2008ZX05034).
About the author: Li Ming, born in 1987, male, from Suzhou, Anhui, doctoral candidate. Tel: 13151981375, Email: cumtmingli@hot-mail.com
(1. School of Resources and Earth Sciences, China University of Mining and Technology, Xuzhou, Jiangsu 2211162. Key Laboratory of Coalbed Methane Resources and Accumulation Process, Ministry of Education, Xuzhou, Jiangsu 2210083 .Guizhou Faer Coal Mining Co., Ltd. Guizhou Liupanshui 553017)
Abstract: Through the analysis of the geological background and gas-bearing data of Faer Coal Mine, the occurrence characteristics of coalbed methane and its sequence and spatial distribution The distribution patterns were studied. Combining isothermal adsorption experiments, mercury injection experiments and observations of coal reservoir deformation characteristics, the physical properties of coal reservoirs were further explored. The results show that the coalbed methane in Faer Coal Mine has high methane concentration, methane gas content and gas content gradient. The gas content and gas content gradient have fluctuating changes in the sequence, which are mainly caused by the differences in the adsorption properties of each coal seam and the distribution of the coalbed methane reservoir pressure system. Affected by the bayberry syncline and topographic development, the current gas content of coal seams shows a distribution pattern of "high in the north and low in the south, spreading in the NE direction". The coal seam fissure system is mostly subject to structural transformation, and its development and connectivity have been improved. It also promotes the development of macropores and mesopores in coal. The theoretical gas saturation of coal reservoirs is mostly lower than 60, which means it is in an undersaturated state. Fa'er Coal Mine has good coalbed methane mining prospects.
Keywords: Characteristics of Coalbed Methane Reservoirs in Faer Coal Mine, Gas Content, Reservoir Physical Properties
Characteristics of Coalbed Methane Reservoirs in Faer Coal Mine, Southwest China
LI Ming1, 2 JIANG Bo1, 2 LAN Fengjuan1, 2 ZHANG Guishan3
( 1. School of Resource and Earth Science, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China 2. Key Laboratory of CBM Resources and Reservoir-forming Process, China Ministry of Education, Xuzhou 221008, China 3. Guizhou Faer Coal Co., LTD, Liupanshui, Guizhou 553017, China)
Abstract: Based on analysis of the geological background and gas content data in Faer coal mine, we dis- cussed the occurrence characteristics of coalbed methane (CBM) and its distribution along the stratigraphic sequence and in coal seams. We analyze the methane adsorption isotherm experiments, reservoir deformations and mercury intrusion porosimetry ( MIP), and further discuss the reservoir physical properties. The results show that CBM in the Faer coal mine is high in gas content, methane concentration and gas content gradient. Its gas content and gas content gradient fluctuate with the stratigraphic sequence that mainly caused by the difference in absorption capacity of each coal seam and distributions of CBM reservoir pressure systems. The Yangmeishu synclinorium and topography are the main controlling factors that affect the current distribution pattern of CBM, which is higher in the north th
an in the south and has a northeastern trend. The gas saturation is generally lower than 60, mainly undersaturated CBM reservoirs. Combined with well-developed brittle fractures, macropores, and mesopores, which formed by structural modification. The exploration prospect of Faer CBM is good.
Keywords: Faer coal mine; coalbed methane; occurrence characteristics; gas content; reservoir physical properties
The Qianxi Coalfield has many coal seams, large cumulative thickness, and high gas content. , coalbed methane has high resource abundance and large resource reserves, and its development and utilization is conducive to alleviating the pressure of energy demand in southern my country and reducing the occurrence of coal mine production accidents (Qin Yong et al., 2008; Gao et al., 2009; Jiang Bo et al., 2009 ).
Fa'er Coal Mine is located in the south of Liupanshui City, Guizhou Province, with an area of ??approximately 92km2 and 19 mineable coal seams. The coal bed gas content is relatively high. It is estimated that the total coal bed methane resources in this coal mine with a burial depth of more than 1,000 meters reach 43.29×108m3. This paper conducts a systematic study on the occurrence, distribution and accumulation characteristics of coalbed methane in this area, providing a certain theoretical basis and guiding significance for further coalbed methane development and the prevention and control of mine gas disasters.
1 Geological conditions for coalbed methane occurrence
1.1 Stratigraphy and coal-bearing strata
The exposed strata in the well field include the Upper Permian Emeishan Basalt Formation (P3β) , Longtan Formation (P3l), Lower Triassic Feixianguan Formation (T1f), Yongningzhen Formation (T1yn) and Quaternary (Q). The Upper Permian Longtan Formation is the main coal-bearing stratum in the mining area, with a thickness of 344~487m, containing 47~78 coal layers, and the average total coal seam thickness is 45.28m. Among them, there are 19 mineable coal seams, namely 1, 3, 5-2, 5-3, 7, 10, 12, 13-1, 13-2, 14, 15-2, 16, 17, 21, 23- The average thickness of coal seams 1, 23-2, 29-1, 29-3 and 33 is 26.82m. The coal rock types and coal quality characteristics of the mineable coal seams are shown in Table 1.
Table 1 Statistical table of coal seams and coal quality characteristics of Faer Coal Mine
Continued table
1.2 Structure
Faer Coal Mine is located on The northern section of the Pu'an rotational tectonic deformation zone in the Liupanshui fault depression of the Yangtze platform Qianbei uplift (Wang Zhongtang, 1990) is adjacent to the NW-trending tectonic deformation zone in Weining in the north. It is located on the SE wing of the bayberry syncline extending NE (Xu Binbin et al., 2003). The NWW-trending Gemudi syncline in the north of the study area, the NWW-trending Tucheng syncline in the south, the NNE-trending Baoshan syncline in the west, and the NNE-trending Gesuohe anticline in the east all form a hair-ear rhombus structure (Le Guangyu et al. , 1991, 1994). Among them, the Baoshan syncline, Dazhai anticline, Yangmeishu complex syncline and Gesuohe anticline running in the NE-SW direction show a combination of barrier-type folds that develop in parallel with tight anticlines and wide and gentle box-like synclines. feature.
Affected by the development of boundary faults, the grid bottom syncline and Taishaba anticline on the SW side of the Weining-Ziyun fault in the north, and the Zhaozihe syncline and Tucheng syncline on the NE side of the Zhaozihe fault in the south all show relatively Tightly closed, asymmetric folds, the fold axes all point to the boundary fault, the anticline appears as a tight steep, pointed top, and is even destroyed by reverse faults developed in the core, and also appears as a barrier-type fold combination; the bayberry complex in the middle The syncline and Haqingdi anticline are relatively gentle and broad, with an asymmetric box shape. The stratigraphic dip angle is generally 10°~20°, and the span is about 17km. The fault structures in the area are mainly normal faults trending around NE30°, followed by reverse faults trending NW20°~30°, and faults in other directions are also developed. The structural deformation in the edge zone of the rhombus structure in the study area is stronger than that in the inner area, and the NW-trending structure deforms more strongly than the NE-trending structure. The structures with stronger deformation are linearly produced in strips, while the structures with weaker deformation are block-like and overall It shows a structural pattern of "alternating blocks and diamond-shaped combination".
The Yangmeishu syncline is the main fold that affects the distribution of the minefield strata. At the same time, the NW-trending Malong syncline and Bajiaotang anticline and the NE-trending Laofaer anticline are also developed in the minefield (Figure 1) . The stratigraphic trend is generally NE-SW, leaning NW, with an inclination angle of generally 10°~15°. The faults are mainly normal faults and can be divided into two groups, trending NW and NE. The former is distributed in the southwest of the mine field and forms a series of more complex fault combination patterns with the surrounding ones trending near SN and EW; the latter is sparsely distributed. In the northeastern part of the mine field, there are relatively wide and gentle horsts and grabens (Figure 2). The mine structure is mainly the product of Yanshanian tectonic activity (Mao Jianqian et al., 1999).
1.3 Hydrogeology
The Longtan Formation (P3l) has weak water richness, mainly fissure water, and some areas have pressure-bearing properties. The lower section of the overlying Feixianguan Formation (T1f) contains almost no water and is a water-isolating layer; the upper section contains a small amount of fissure water and is weakly water-rich. The underlying Emeishan basalt formation (P3β) contains fissure water, which has weak water content and certain pressure-bearing properties. The weak water content and certain pressure-bearing properties of the coal-bearing strata and their overlying and underlying strata in the study area are generally conducive to the preservation and enrichment of coalbed methane in the area.
Figure 1 Contour line and structural outline of the 16th coal seam floor in Faer Coal Mine
Figure 2 AA′ structural section (see Figure 1 for the position of the section line)
The study area has a mid-alpine landform, with a relatively open valley or gentle slope terrain in the south, and steep table-like mountains formed by Triassic strata in the north, with a general relative height difference of 300~400m (Figure 2). The Fa'er River and Beipan River flow through the mine field. The complex topographic distribution and the development of the surface water system will inevitably complicate the distribution of hydraulic head in the Longtan Formation, which in turn affects the distribution of coalbed methane.
2 Gas content of coal seams
2.1 Sequence distribution characteristics
125 tests of coal seam gas content and gas components were conducted in the study area***. The statistical results show that the coal seam gas content is 3.94~35.94m3/t, with an average of 13.58m3/t; the main component is CH4, and the average CH4 concentration is 91.81 (Figure 3). The average methane content of each coal seam is above 10m3/t and has an overall trend of increasing as the coal seam level decreases (Figure 4). At the same time, the gas content of coal seams still fluctuates to a certain extent as the sequence decreases. Among them, the average gas content of coal seams 10, 15-2, 17, 23-2 and 29-1 is relatively high, reaching 15~19m3/t . This fluctuating change is more obvious in the change curve of the methane content gradient of each coal seam. Its change trend is basically consistent with the gas content change curve of the coal seam (Figure 4), and in the 10, 17 and 23-2 coal seams, The methane content gradient reaches a relative maximum. The reservoir pressure of the coal seam and the adsorption property of the coal seam itself are the key factors affecting the gas content. It can be seen that the change of the coal seam gas content with the layer fluctuation is mainly affected by the difference in the adsorption properties of each coal seam and the distribution of the coal bed methane reservoir pressure system.
Figure 3 Relationship between coal bed methane methane content and concentration and burial depth
Figure 4 Statistical chart of gas content and gas gradient of each coal seam
Figure 5 7 coal seams Relationship between methane content and coal seam burial depth
Figure 6 Relationship between coal seam methane content gradient and coal seam burial depth
2.2 Vertical distribution characteristics
With the coal seam burial depth With the increase, the content and concentration of coalbed methane methane generally have an increasing trend (Figures 3 and 5), but its linear increase with burial depth is more discrete; indicating that other geological factors still have a strong influence . The test data when the coal seam burial depth is 500~800m reflects the phenomenon that the coal seam burial depth is relatively large but the gas content is relatively low (Figure 3). The methane content gradient in the coal seam decreases with the increase of the coal seam burial depth. , it means that the gas content gradient of the coal seam buried deep in this section is lower than the overall trend value (Fig. 6). Through the analysis of raw data, it is found that the abnormal points in this section are concentrated in the 1012, J1106, J1107 and J1406 boreholes located in the high-altitude area and the 33 and 34 coal seams in the J1004 and J1403 boreholes (Figure 2). At the same time, the coal seam gas pressure test shows that : The 1012 borehole in the high-altitude area also has a relatively lower gas pressure gradient compared to the normal area (Figure 7). 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 no dissipation occurs due to the coal seam being connected to the outside atmosphere.
The gas gradients of the deep 28, 29-1, 29-3, 33 and 34 coal seams are relatively low (Figure 4). Although the degree of metamorphism of these coal seams is relatively high, they were formed during the coalification process. However, due to the thin and unstable coal seam and the low coal-rock ratio, the generated gas migrates and is lost to the overlying and underlying stratigraphic systems of the coal seam. Due to the steep surrounding terrain and sudden change in altitude of the boreholes in the high-altitude area, on the one hand, the groundwater level is further away from the surface elevation, causing the reservoir pressure to be relatively reduced. On the other hand, the coalbed methane migrates vertically to the surface. At low elevations, lateral migration and loss occurs along the formation, reducing the effective sealing and burial depth of the coal seam, which in turn leads to a reduction in the gas gradient.
2.3 Planar distribution characteristics
The gas content of coal seams shows obvious zoning characteristics on the plane (Figure 8). The gas content in the area south of the Faer River is generally low. The contour lines are sparse, and only the central area has a gas content of 10m3/t. The east and south are surrounded by coal seam outcrops, which become windows for coal bed methane to escape, resulting in low gas content. The distribution of gas content contours in the west and north is controlled by the Beipanjiang River and the Faer River. The rivers flowing through the mine fields serve as groundwater drainage channels, causing the groundwater levels on both sides of the rivers to decrease, resulting in a relatively high coalbed methane content on both sides of the rivers. Low. The gas content in the area north of the Fa'er River is relatively high, generally greater than 15m3/t. The gas content in the northwest of the well field is expected to reach about 35m3/t. The gas content contours are denser and the gas content change gradient is larger on the reaction plane. There is a relatively low gas content zone in the area around boreholes 1204, J1306 and J1405, which is mainly affected by the development of surface valleys and rivers. In the south, the gas content is also relatively low due to the influence of coal seam outcrops and the development of the Faer River. The current distribution pattern of coal seam gas content, which is "high in the north and low in the south, spreading in the NE direction", is mainly controlled by the development of the bayberry syncline and topography.
Figure 7 Gas pressure test results of some drilled coal seams
3 Coal reservoir physical properties
3.1 Reservoir porosity and permeability
Passed The mercury injection experiment measured the porosity, pore volume, pore specific surface area and pore size structure of 14 coal samples from coal seams 1 and 3 (Table 2). The standard of Hodot (1966) was used to classify the pore size structure. 1000nm, 100nm and 10nm are used as the dividing points to divide the pore diameter into macropores, mesopores, transition pores and micropores. It can be seen that the pores in coal are mainly micropores, accounting for 41.29 (Figure 9); macropores follow, accounting for 24.86; transition pores and mesopores account for 20.98 and 12.86 respectively.
Figure 8 Contour diagram of gas content of coal seams 1, 3, 5-2 and 7 in Faer Coal Mine
Table 2 Statistics of mercury injection experiment and isothermal adsorption experiment data
Note: The format of 3.81~7.01/5.16 is: minimum value-maximum value/average value.
There are usually 2 to 3 groups of fissures developed in coal seams, mainly those caused by structures or structural transformation (Figure 10), with "X" type ***yoke shear joints, oblique fissures, and bedding fractures. Fissures and other forms are produced; endogenous fissures in the 3, 5-2, 13-1, 29-1 and 29-3 coal seams are relatively developed, appearing as one or two groups of fractures perpendicular to the layer. Observation of microscopic cracks shows that the same group of cracks can be produced in the shape of steps, entrainments or slow waves. Two or more groups of cracks often intersect and merge at larger angles. The intersection area often derives unstable secondary small cracks, and the cracks are connected. Sex is better.
3.2 Coal seam adsorption properties
The coal seam isothermal adsorption experiment (dry base coal sample) of Faer Coal Mine shows that the Lambert volume VL of coal seams 1, 3 and 5-2 is 23.55~27.18 m3/t, Lang's pressure PL is 0.82~0.95MPa (Table 2, Figure 11). Combined with the gas content test data, it can be deduced that the theoretical average gas saturation S of coal seam 1 is 48.92; the theoretical gas saturation S of coal seam 3 is 48.92; The theoretical average S is 45.62; the theoretical average gas saturation of the 5-2 coal seam is 52.36. The theoretical gas saturation of coal reservoirs is mostly lower than 60, which means it is in an undersaturated state.
Fig. 9 Pore volume distribution diagram of coal seams 1 and 3 at each stage
Fig. 10 Macroscopic and microscopic deformation characteristics of coal body
Fig. 11 Faer Coal Mine 1. 3 and 5-2 coal seam isothermal adsorption curves
4 Conclusions
(1) Faer Coal Mine is rich in coal bed methane resources, and the coal bed methane has high methane concentration, methane gas content and Gas content gradient, its gas content and gas content gradient have fluctuating changes in the sequence, which are mainly caused by the differences in adsorption properties of each coal seam and the distribution of the coalbed methane reservoir pressure system.
(2) The phenomenon of low gas content gradient in coal seams at the vertical burial depth of 500~800m is mainly due to the thin thickness and instability of the bottom 28~34 coal seams, low coal-rock ratio and coal seams located in high altitude areas. Caused by lateral migration of gas. Affected by the bayberry syncline and terrain development, the current gas content of coalbed methane shows a distribution pattern of "high in the north and low in the south, spreading in the NE direction" on the plane.
(3) The coal seam fissure system is mostly subject to structural transformation, and its development and connectivity have been improved. It also promotes the development of macropores and mesopores in coal. The theoretical gas saturation of coal reservoirs is mostly lower than 60, which means it is in an undersaturated state. Fa'er Coal Mine has good coalbed methane mining prospects.
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