5.6. 1 metamorphic degree of coal
In the process of coalification, gas is constantly produced. The higher the degree of coalification, the more gas is produced. Therefore, other factors being equal, the higher the degree of coal metamorphism, the greater the gas content in the coal seam.
The metamorphic degree of coal not only affects the gas production, but also determines the gas adsorption capacity of coal to a great extent. At the initial stage of coal formation, lignite has loose structure and large porosity, and gas molecules can penetrate into coal, so lignite has great adsorption capacity. However, the amount of gas produced at this stage is small, which is difficult to preserve, and the actual gas content in coal is very small. In the process of coal metamorphism, due to the action of ground pressure, the porosity of coal decreases and the coal quality becomes denser. The porosity and internal surface area of long-flame coal are relatively small, so the gas adsorption capacity is greatly reduced, and the maximum gas adsorption capacity is between 20 and 30 m3/t. With the further deterioration of coal, under the action of high temperature and high pressure, many micropores are generated in the coal due to dry distillation, which makes the internal surface area reach the maximum when anthracite is used. According to the laboratory measurement, the micropore surface area of 1g anthracite can reach 200 m2. Therefore, the gas adsorption capacity of anthracite is the strongest, which can reach 50 ~ 60m3/t. However, during the transition from anthracite to ultra-anthracite, the micropores are reduced, and the gas adsorption capacity of coal drops sharply, and disappears when it reaches graphite.
The study shows that coals with different metamorphic degrees are often distributed in strips in regional distribution, forming different metamorphic zones. This metamorphic zoning controls the occurrence and regional distribution of gas to some extent.
Most of the three high metamorphic zones divided by Carboniferous-Permian coal-accumulating areas in North China are high gassy mines. In particular, Anyang, Hebi and Jiaozuo at the southern foot of Taihang Mountain are mainly anthracite, and most of the developed production mines are high gas mines and outburst mines. The minable seam of Longtan coal measures in the south section of Lianshao coalfield in Hunan Province has obvious metamorphic zoning, and the metamorphic degree gradually increases from east to west. Niumasi mining area is fat coal-coking coal, Aibeiqiao mining area is lean coal, Fengjiangxi mining area is lean coal, and Jiaozudi and Sanbitian mining areas are anthracite. Affected by metamorphic zoning, the mine gas grade in this area increases from east to west, and the degree of outburst danger increases.
Surrounding rock condition
The surrounding rock of coal seam refers to the rock stratum with a certain thickness range, including the direct roof, main roof and direct floor of coal seam. The influence of coal seam surrounding rock on gas occurrence depends on its gas barrier and permeability.
Generally speaking, when the lithology of coal seam roof is dense and complete rock, such as shale and oil shale, the gas in coal seam is easy to be preserved; When the roof is porous or brittle fracture rock, such as conglomerate and sandstone, gas is easy to escape. No matter whether it is the Lower Jurassic or Carboniferous-Permian coal seam in Beijing Jingxi Coal Mine, although the brand of coal is anthracite, it has good permeability because the roof of coal seam is medium-grained sandstone with a thickness of 12 ~ 16 m; At the same time, because the coal measures are denuded and only exist in the syncline core, in the long-term denudation process, the gas in the syncline escapes along the coal seam and sandstone roof, so the gas content in the coal seam is small and the gas emission in the mine is low.
The indexes related to the gas barrier and permeability of surrounding rock include porosity, permeability and pore structure. The argillaceous rock is beneficial to the preservation of gas, and its shielding ability will be greatly reduced if it contains impurities such as sand and silt. Different impurity contents of silt affect the size of dominant pores in argillaceous rocks. For example, when the silt content in mudstone is 20%, the pores of 0.025 ~ 0.05 micron are dominant; When the powder content is 50%, the dominant pore is 0.08 ~ 0. 16 μ m, and this pore size change is also reflected in the shielding performance of the rock. With the increase of pore size, the permeability will increase and the shielding ability of rock will be significantly weakened. Sandstone is generally beneficial to gas escape, but it is also a good shielding surface when sandstone porosity and permeability are low in some areas.
The permeability of surrounding rock of coal seam is not only related to lithologic characteristics, but also related to lithologic combination and deformation characteristics in a certain range. According to the mechanical properties of rocks, surrounding rocks can be divided into strong rocks (sandstone, limestone, etc. ) and weak rocks (fine clastic rocks and coal, etc. ). The solid rock stratum is not easy to deform plastically, but it is easy to fracture; Plastic deformation often occurs in weak strata.
Rock strata with different mechanical properties have different structural forms. Figure 5. 12 shows several deformation types of coal seam roof. Figure (a) shows the roof of surrounding rock broken by faults, mainly sandstone; (b) A tightly folded roof of surrounding rock composed of siltstone, mudstone and fine sandstone; (c) is another type, reflecting the lens phenomenon.
Fig. 5. 12 roof deformation of several different coal seams
(According to Gas Geology Laboratory of Jiaozuo Mining Institute, 1990)
Fig. 5. 13 Fracture characteristics of different lithology strata
(According to Gas Geology Laboratory of Jiaozuo Mining Institute, 1990)
1-timely sandstone; 2- mudstone; 3- coal; 4— Fine grained sandstone
In different types of rock strata, the development degree of cracks is also different. The solid rock stratum produces cleavage roughly perpendicular to the plane; The weak stratum produces dense flow cleavage, which is inclined or roughly parallel to the bedding plane; Refraction occurs in cracks of adjacent strong and weak strata (Figure 5. 13).
In order to reflect the difference of rock property combination in different mine fields or different blocks in the same mine field, the data of each borehole and crosscut within the research scope can be statistically analyzed. Select the top (bottom) interval of coal seam with a certain thickness, count the lithology and thickness of each interval in each borehole and cross-cut, and calculate the sand-mudstone ratio or sandstone ratio (the ratio of sandstone thickness in statistical interval to total thickness). According to the statistical data, draw the corresponding isoline or delineate the blocks with different gas preservation conditions.
geologic structure
The influence of geological structure on gas occurrence, on the one hand, causes the imbalance of gas distribution, on the other hand, forms favorable conditions for gas occurrence or gas discharge. Different types of structural features, geological structures in different parts, different mechanical properties and sealing conditions have formed different gas occurrence conditions.
5.6.3. 1 folding structure
The types, sealing conditions and complexity of folds all affect gas occurrence.
When the gas permeability of coal seam roof rock is poor and the structure is not destroyed, the anticline is favorable for gas storage, which is a good gas storage structure, and the gas in the axis of the anticline will gather relatively and the gas content will increase. In the mining area with syncline basin structure, when the roof sealing condition is good, it is difficult for gas to migrate along the vertical strata, and most of the gas can only flow to the surface along the two wings. However, at the edge of the basin, if the exposed area of coal-bearing strata is large, it is convenient for gas discharge. The natural gas content in tight fold areas is often high because these areas are subjected to strong tectonic action and stress concentration; At the same time, folded rock strata are often plastic, easy to fold and not easy to break, with good sealing, which is conducive to gas accumulation and preservation.
According to the data of Shanxi Province, Zhao Gao mining area in Shanxi Province is basically distributed in the axis of anticline, the inclined end of nose structure and the turning end of "S" anticline. For example, Niejiazhuang anticline in Datong coalfield has few faults, and the continuity of coal seam has not been destroyed, which can not only store gas but also prevent gas migration, and gas emission has occurred in four boreholes. Jinhuagong Mine and Xinzhouyao Mine are located at the turning end of anticline, and the gas content in coal seam is obviously higher than that in the two wings.
5.6.3.2 fault structure
Fault structure destroyed the continuity and integrity of coal seam and changed the conditions of gas migration in coal seam. Some faults are beneficial to gas emission, while others hinder gas emission and become escape barriers. The former is called open fault and the latter is called closed fault. The openness and sealing of faults depend on the following conditions: ① the nature and mechanical properties of faults. Generally, tensile normal faults are open, while compressive or compressive-torsional reverse faults are usually closed; (2) Connectivity between faults and surface or alluvium. Faults with large scale and connectivity with surface or alluvium are generally open; (3) After the coal seam is cut off by the fault, if the permeability is good, the rock properties of the coal seam in contact with the other plate of the fault are beneficial to gas emission; ④ The characteristics, filling, tightness and fracture development degree of the fault zone will affect the opening or sealing of the fault.
In addition, the spatial orientation of faults also affects the preservation or escape of natural gas. General strike faults prevent gas from escaping along the inclined direction of coal seam, while inclined and oblique faults cut coal seam into unconnected blocks.
Different types of faults form different structural boundary conditions and have different effects on gas occurrence. The east-west main structure of Jiaozuo mining area, Fenghuangling fault and Zhucun fault, has a drop of more than 100 meters, which makes the coal seam contact with Ordovician limestone developed in fractured karst caves, and it is an open fault, so the gas content near the fault zone is very small. However, for some medium-sized faults in the mining area, the lithology of the other fault in contact with the coal seam is mostly siltstone or argillaceous rock, which belongs to the closed fault and is the structural boundary of gas zoning.
5.6.3.3 tectonic assemblage
The combination types of structural characteristics that control the distribution of natural gas can be roughly summarized into the following three types:
1) Closed type of reverse fault boundary: In this type, compressive and compressive reverse faults are often the two-wing boundaries of mines or regions, and the fault planes are generally inclined to each other, making the whole section in a closed state. For example, in Daqingshan coalfield in Inner Mongolia, the north-south boundary is a reverse fault, and the fault planes tend to be opposite. Coalfield is located in the footwall of reverse fault, which forms a good sealing condition in structural combination. The gas content of coal seams in each mine in this coalfield is generally higher than that in Wuhai coalfield and Zhuozishan coalfield, which mined coal-bearing strata in the same era.
2) Sealing type of structural caprock: caprock condition originally refers to sedimentary caprock, and structurally, it can also refer to structural caprock. For example, a large overthrust fault pushes a large area of strata with poor permeability above or near the coal seam, which changes the original covering conditions and also plays a role in plugging gas. Although the northeast extensional fault of Tiechang No.2 well in Tonghua mining area of Jilin Province is beneficial to gas drainage, the overlying strata of coal seam are covered by the upper wall of reverse fault. Due to the sealing effect of fault plane and upper wall strata, a large amount of gas has accumulated in the footwall coal seam, and the gas content has increased significantly.
3) Fault block sealing type: this type consists of two groups of compressive and torsional faults in different directions, forming triangular or polygonal fault blocks on the plane, and the boundary of the fault blocks is trapped by closed faults. For example, in Fengfeng mining area, Hebei Province, the coal-bearing strata were cut by a series of high-angle normal faults generated by the late tectonic movement, forming a number of small graben or horst structures (Figure 5. 14), forming some closed areas favorable for gas storage. When these enclosed areas are far away from coal seam outcrops (such as Yang Yunhe Mine and Dajiaoshu Mine, etc.). ), even if the coal-bearing strata are raised and buried shallowly, the mine gas emission is still very large.
Fig. 5. 14 schematic diagram of geological profile of Fengfeng coalfield
(According to Gas Geology Laboratory of Jiaozuo Mining Institute, 1990)
1- gas weathering zone; 2- methane region
5.6.4 Buried depth of coal seam
Below the gas weathering zone, gas content, gas pressure and gas emission are all related to the increase of depth.
Generally speaking, the gas pressure in coal seam increases with the increase of buried depth. With the increase of gas pressure, the proportion of free gas in coal and rock increases, and the adsorbed gas in coal tends to be saturated gradually. Therefore, in theory, in a certain depth range, the gas content of coal seam also increases with the increase of buried depth. However, if the buried depth continues to increase, the increase rate of gas content will slow down. Table 5. 16 is a calculation example. As can be seen from the table, the methane content in coal seam increases with the increase of depth, and the proportion of free gas changes with the increase of depth.
Table 5. 16 Relationship between methane content and depth of coal seam
(According to Li Jin 1962)
With the increase of buried depth, the gas emission from coal seams in individual mines is relatively reduced. For example, Dahuangshan Mine of Xuzhou Mining Bureau belongs to a low gas mine, which is located in a shallow confined coal basin with a large dip angle of coal seam. There are thick and low permeability caprocks on the old and new unconformities, and natural gas mainly migrates to the surface along the coal seam. Due to the small coal basin and insufficient gas supply in the deep part, the gas emission decreases with the increase of mining depth when mining from shallow to deep around the basin.
5.6.5 Exposed degree of coalfield
The coal measures strata in the exposed coalfield are exposed on the surface, and the coal seam gas is easily discharged along the outcrop of the coal seam. In concealed coalfields, if the caprock is thick and the permeability is poor, coal seam gas will often accumulate and store; On the other hand, if the overlying strata have good permeability, it is easy to make the gas in the coal seam escape slowly, and the gas content in the coal seam is generally small.
When evaluating the exposure degree of a coalfield, we should not only pay attention to the current exposure degree of the coalfield, but also consider the exposure degree of coal measures strata and the duration of the weathering process in Nevas during the whole geological period after coal formation. Mining Carboniferous-Permian coal seam in No.3 well of Hongyang coalfield. The upper part of coal seam outcrop is covered by extremely thick Jurassic and Tertiary and Quaternary sedimentary strata. The outcrop depth of 13 coal seam is 700 ~1100 m. Although the burial depth is so large, the gas content of coal seam near the outcrop is still very small. The reason for this situation is that during the long geological period from late Paleozoic to late Jurassic, the crust in this area rose, coal-bearing strata exposed to the surface and suffered from strong gas weathering. The cover of late strata only retains the early natural gas distribution (Figure 5. 15).
Fig. 5. 15 Geological Profile of Well Hongyang 3
(According to Gas Geology Laboratory of Jiaozuo Mining Institute, 1990)
Many mining areas in Hunan province have red beds, that is, red rock series deposited from Cretaceous to Tertiary. This alluvial deposit dominated by fluvial facies has caused erosion and damage to coal-bearing strata. The distance between red beds and coal seams in some mining areas is very small, and some have become the direct roof of main coal seams. The extent of erosion varies from place to place. Some mining fields are eroded, and some are cut through by red beds in one or several mining fields. According to the survey, where there is red bed erosion, due to the long-term drainage conditions formed by erosion, the gas is not large, and most of them are low gas areas. For example, Longjiashan well in Hongwei coal mine in Baisha mining area and Tiejishan well in Niumashi mining area all belong to this situation.
Groundwater activity
Groundwater and gas * * * exist in coal seams and surrounding rocks, both of which are fluids, and their migration and occurrence are related to pores and fracture channels of coal strata. The migration of groundwater, on the one hand, drives the migration of gas in cracks and pores, on the other hand, drives the gas dissolved in water to flow together. Although the solubility of gas in water is only 1% ~ 4%, in areas where groundwater exchange is active, water can take away a lot of gas in coal seams, which obviously reduces the gas content in coal seams. At the same time, moisture is adsorbed on the surface of cracks and pores, which also weakens the ability of coal to adsorb gas. Therefore, the activity of groundwater is conducive to the escape of gas. The space occupied by groundwater and gas is complementary, and this inverse relationship is often manifested in the fact that the gas quantity is small in areas with large water quantity, and vice versa.
Hebei Fengfeng mining area is one of the large-scale water mining areas in northern China. On the west side of Gushan in this mining area, several large faults cut coal-bearing rock series, making the main minable coal seam contact with another Ordovician limestone of the fault, which is within the range of strong groundwater runoff zone. The mines to the west of Gushan are all low gas mines, and the gas content in coal seam is very small.
Longtan coal measures distributed in central and southeastern Hunan are obviously divided into "southern type" and "northern type" due to the difference of sedimentary environment during the formation process, and the dividing line is near 27 40 ′ north latitude. The north-south differentiation of Longtan coal measures also shows obvious differences in hydrogeological conditions. The underlying stratum of coal measures is Maokou limestone, which is a strong aquifer with developed karst fractures. When the gas barrier between coal seam and Maokou limestone is thin or missing, the mine water inflow is large, which creates conditions for easy gas discharge. The distance between Maokou limestone in the north and the upper coal seam is 0 ~ 10 m, forming some large water and small gas mines, such as Enkou and Coal Dam, which are all low gas mines, and the mine water inflow is greater than1000 m3/h; The distance between Maokou limestone and coal seam is increased to 300 ~ 400 m. Guanshanjing and Hongshandian mining area belong to "south type" production mines, both of which are high gas outburst mines with simple hydrogeological conditions, and the mine water inflow is less than100 m3/h. ..
magmation
The influence of magmatic activity on gas occurrence is complex. Magma invades coal-bearing strata or coal seams. Under the influence of magmatic thermal metamorphism and contact metamorphism, the metamorphic degree of coal increases, which increases the gas production and gas adsorption capacity. Under the condition of no gas barrier or poor sealing conditions, the high temperature of magma can strengthen the gas emission from coal seam and reduce the gas content in coal seam. Magmatic rocks sometimes partially cover or seal coal seams and become gas barriers. However, in some cases, due to the increase of cracks in the dike alteration zone and the strengthening of weathering, crack channels can be gradually formed, which is beneficial to gas discharge. Therefore, magmatic activity not only plays a role in generating and preserving gas occurrence, but also makes it possible for gas to escape under certain conditions. Therefore, when studying the influence of magmatic activity on coal seam gas, it should be analyzed in combination with geological background.
Generally speaking, it is quite common that magma intrudes into coal seams, which is beneficial to gas generation and preservation. For example, there are strong magmatic activities in the east and west wings of Beipiao coalfield in Liaoning Province. Magmatic intrusions are located in Sanbao Mine 1 well in the east wing of the coalfield and Taiji No.4 well in the west wing, and intrude into coal measures strata in the form of rock walls, bedrock, dikes and rocks. Among them, the intrusive body formed by intrusion along the bedrock of coal seam has great influence on gas occurrence and coal and gas outburst. First, it causes coal seam contact metamorphism and further produces gas; Second, the bedrock is located on the roof of coal seam, which plays a sealing role in gas emission; Third, the coal seam is broken by force, which leads to the destruction of coal structure. In No.9 coal seam in the first mining area in the west of Sanbao Coal Mine, the bedrock intrusion is located on the roof of the coal seam, with an area of 19.4× 104m2. This area is not only rich in gas, but also very serious outburst, which has happened twice. No.4 coal seam 19 outburst in Taiji No.4 well is distributed in the east wing of the mine field in the bedrock coverage area, and other minable coal seams have similar situations in the magma intrusion area.
In some mining areas and mines, due to magma intrusion into coal seams, gas escapes and gas content decreases. For example, Fujian Yong 'an mining area is a bare coalfield, and magmatic rocks invade coal seams in the form of dikes and dikes, baking and altering coal seams. The dike runs directly to the surface, and the roadway is drenched when exposed, which shows that the fractured roadway is well connected and is conducive to gas escape. The coal seam gas content in this mining area is generally very small, which belongs to low gas mine.
When studying the influence of magmatic activity on gas occurrence, we should also pay attention to the carbon dioxide produced by volcanism. The coal and surrounding rocks in some mining areas contain a lot of carbon dioxide gas, which may be related to volcanic eruption or magma intrusion. For example, the rocks and carbon dioxide outburst in the fifth well of Yingcheng, Jilin Province. The research shows that the gas source of carbon dioxide comes from the volcanic eruption after the coal measures are deposited, and a large amount of carbon dioxide in the eruption gas is poured into the coal measures alluvial sandstone along the fault zone, or absorbed by kaolinite rhyolite and coal seam.
Further reading is recommended.
1. Gas Geology Laboratory of Jiaozuo Mining Institute. 1990. Introduction to Gas Geology. Beijing: Coal Industry Press, 2 1 ~ 32.
2. Wang Dazeng. 1992. gas geology. Beijing: Coal Industry Press, 4 ~ 14, 28 ~ 37.
3. Zhang Zimin. 2005。 Gas geological law and gas prediction. Beijing: Coal Industry Press, 4 ~ 54.
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