Gas occurrence characteristics of surrounding rock mass in coal-bearing strata

1 Formation of gas in rock mass

The gas that flows into the work surface is partly from the coal seam and the other from the surrounding rock. Gas is different in the form of coal and rock. The former is mainly in the form of sucking, while the latter is mainly in the free state.

The gas content in the rock is related to the aggregate nature of the rock.

The main factor determining the gas content of rock in coal-bearing strata is porosity, and the adsorption gas content accounts for a small proportion. The amount of adsorbed gas in the rock depends mainly on the amount of carbonized matter contained. The rock composition and aggregation properties of the surrounding rock of the coal-bearing strata play different roles in the distribution of gas in the surrounding rock, and the gas in the limestone with cracks and the sandstone in the weakly cemented and well-ventilated sandstone The gas has better degassing than the gas in the dense rock layer containing clay . The gas content of the coal body is determined according to the thermodynamic conditions and the degree of metamorphism of the coal, and the gas content of the rock body is determined according to their respective gas gathering properties under the same conditions, and the amount depends on the surrounding rock mass. Porosity, gas pressure, temperature.

The porosity of the rock mass in the coal-bearing strata is generally low, and the amount of gas contained in the pores is small. Table 1 shows the results of porosity and permeability measurement of surrounding rock of coal-bearing strata in Yangquan mining area. The shale , sandstone and limestone samples were subjected to high-pressure volumetric isothermal adsorption measurement.

The adsorption measurement method is to take a fresh sample in the borehole and send it to the laboratory for the adsorption gas test. The composition of the methane gas adsorbed during the experiment is

99. 99%, the adsorption parameters were measured at a constant temperature of 30 ° C. The typical rock-like adsorption gas isotherm is shown in Figure 1 and Figure 2. 2 Gas occurrence characteristics and distribution law in rock mass In oil and gas geology, rock pore and permeability are the two most important physical parameters. These two parameters are also particularly important in studying the gas emission from the surrounding rock mass of coal-bearing strata.

It can be seen from Table 2 that the porosity and pore volume of various rocks are slightly reduced as the particle size becomes finer, and the porosity is poor or poor (Grade D or E). Among them, limestone has the smallest pore volume fraction, followed by sandy shale. The reason is that during the long geological time, most of them have become very dense due to compaction. In particular, the quartz contained therein has a mosaic-like contact between the particles due to the increased regeneration of the quartz , and almost all of the primary pores disappear.

单词 m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m m. According to the Central South Petroleum Geology Bureau and other units, the permeability of quartz sandstone can be as high as 27 800 m2/(MPa2·d), which is good or medium (II~V grade). Other rocks are <400 m2/(MPa2·d), which are weak or non-permeable (IV~V).

According to the rock properties of the surrounding rock mass of the coal-bearing strata, the following three categories can be summarized.

(1) The coarse sandstone has good permeability. As a gas permeable layer, its thickness, distribution range and distance from the gas-bearing coal seam control the gas accumulation in the coal seam.

(2) Fine sandstone, limestone and shale have poor gas permeability and seal the coal seam gas, which is not conducive to gas release.

The study of Huainan and Huaibei mining areas in East China shows that sandstone is the main reservoir in surrounding rock in the Permian coal-bearing strata, with many sandstone layers and large thickness. All of them are composed of clastic rocks, which are usually medium-fine-grained sandstones. According to the physical properties of 119 rock samples, the porosity of sandstone is 0. 292~3. 1 m2/(MPa2·d), of which <0. 4 m2/(MPa2) · d) accounts for 74% of the rock formations with poor permeability, and the distribution is uneven, but the physical properties become better in the deep. Another major factor in the sandstone seepage of gas is the degree of fracture development. It can be seen from the hydrogeological conditions that the geological conditions of each section of the mine are different, and the development of the fractures is quite different. Generally, the formation of the formation is gentle, and the sections with scarce faults have little or no water. The viscous water pressure of the sandstone layer of the coal-bearing strata is more than 1. 2MPa (- B6 coal seam above 235 m has been recovered), lower than coal seam gas pressure. In the 46 mining area - 318 m Shimen, there is gas emission from the local sandstone layer during the excavation.

(3) Mudstone. The mudstones in the coal-bearing strata in the Xie-Li area of ​​Huainan account for 40.99% of the thickness of the section. These mudstones are almost airtight and act as a barrier to gas in the underlying coal seams. They are good gas reservoir caps. The combination of the reservoir and cap features, in the coal-bed stratum histogram, each of the dark mudstone-coal-dark mudstone-sandstone-mudstone spiral deposition, that is, each coal segment is a good combination of reservoir and cap, both Good self-storing conditions. Each combination may form some high gas coal seams. For example, No. 8 coal in Luling Mine of Huaibei (equivalent to B5 coal seam in Huainan). The B8~B4 coal seam in the Li District of Xinzhuang Mine is a second coal section. C13 and B11b in the Xiejia area are the third and fourth coal sections. These coal seams have high gas content in various mining areas, and the amount of gushing is large, and the danger is prominent.

The occurrence of gas surges recorded during the mining process and during geological exploration is sufficient to demonstrate the presence of gas in the surrounding rock mass of the coal-bearing strata. During the mining process, the gas from the gushing dynamics in the rock masses shows that the well-developed joints or cracks in some hard sandstones and limestones are largely released by the deformation of the surrounding rock mass caused by mining. The gas bubbles that emerged from the mud during drilling exploration until a large amount of gas erupted strongly. 5 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 。 As the gas body continues to explode, the gas pressure is quickly reduced to below 0.1 MPa, and the gas production reaches about 13 m3/min. In the 8103 hole of the 8th sloping well of the mine, the third limestone hits the upper limestone. MPa。 The maximum amount of gas is 8. 2 m3 / min, the pressure is 0. 36MPa. When 5-24 geological boreholes were drilled in the area, gas was sprayed from the K4 limestone with water and gravel. The spray height was 20 m and the gas volume was 8 m3/min. Through multiple observations of surrounding rock gas, it is proved that the main source of gas is the rock inside the rock mass, and these rock layers generally do not have access to the ground, which is generally covered on these rock layers. Thick sand shale layers and mudstone layers. As a result of the influence of factors such as coal-bearing stratum geology and structural movements, the porosity and permeability of the surrounding rock mass of the coal-bearing strata vary greatly in the plane, and the gas in the rock body exists on the plane. Obvious regionality. In this area, the gas pressure of each layer is measured by long borehole source gas measurement method. The gas content of rock formations at different locations is obtained by indirect method, and the gas content of surrounding rock is determined according to the adsorption law.

3 Conclusion

(1) The rock has no adsorption, and the gas existing in the sandstone and limestone rocks exists in a free state, and the gas present in the carbonaceous shale and mudstone rock bodies exists in the adsorption and free state.

(2) The coarse sandstone body is used as the gas permeable layer. Its thickness, distribution range and distance from the gas-bearing coal seam control the gas accumulation in the coal seam. The gas content in the mortar is controlled by the porosity.

(3) Fine sandstone, limestone and shale have poor gas permeability and seal the coal seam gas, which is not conducive to gas release.

(4) The mudstone is almost airtight and acts as a barrier to the gas in the underlying coal seam. It is a good gas reservoir cap.

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