Research on the Stress Relief and Permeability Increase Mechanism and High-efficiency Gas Drainage Method on A Composite Coal Seam
Supervisor:yuan liang cheng yuan ping
Tectonic coal,a kind of extremely fractured and pulverized coal formed during long-time complex geologic tectonism of normal coal,has the characteristics of low strength,high gas content and poor permeability.It is difficult to extract gas from the tectonic coal,causing most coal and gas outburst accidents to happen in places with tectonic coal occurs.In Jiaozuo Mining Area of China,due to the complex multi-period geologic tectonic movement,a kind of special composite coal seam was formed then,which consists of an upper sublayer and a lower sublayer.The upper sublayer of the coal seam is hard coal(referred to as normal coal in this paper)which underwent less damage by geologic tectonism,and the lower sublayer of the coal seam is soft tectonic coal damaged by geologic tectonism.It is extremely difficult to extract gas because of its great variant coal properties in the coal seam.The application effect of many well-applied single coal seam gas drainage technologies in this composite coal seam turned out to be not so obvious,resulting in frequent coal and gas outburst accidents.In view of the problem of gas control in this composite coal seam,the 16 mining area of Guhanshan Mine was taken as the research background.Theoretical analysis,site investigation,and laboratory research were carried out to analyze the genesis of geologic structure and the coal gas occurrence characteristics of this kind of composite coal seam.In addition,the basic physical parameters,pore structure,gas adsorption/desorption ability,mechanical damage and permeability evolution characteristics of the composite coal seam,for normal coal and tectonic coal separately,were obtained and a hydraulic flushing gas drainage technology was proposed.The combination of numerical simulation,laboratory experiment and theoretical analysis was used to reveal the stress relief and permeability increase mechanism of the composite coal seam after hydraulic flushing gas drainage.A large number of engineering tests were conducted on site to investigate and analyze the practical application effect of the hydraulic flushing gas drainage technology on the composite coal seam.The main conclusions of this paper are as follows:(1)After undergoing special geologic tectonic movement,the microstructure of the normal coal is severely damaged,and the pore fracture system developed significantly.The tectonic coal’s micropore and minpore volumes are 8.20~9.48 times and 10.41~10.97 times of the normal coal,respectively.And the mesopore and macropore volumes are 97~180 times and 38~87 times of the normal coal,respectively.The ultra gas adsorption capacity of the tectonic coal is 47.29 m~3/t higher than 40.68m~3/t of the normal coal,and the initial gas emission velocity of the tectonic coal is as high as 33.8~44.4 mmHg,which is 1.6~2.2 times of the normal coal.The average value of the Protodyakonov’s coefficient of the tectonic coal is only 0.2 which is 7.5 times smaller than that of the normal coal.The tectonic coal sublayer in the composite coal seam is more likely to accumulate a large amount of gas energy,and at the same time due to its low mechanical strength and high initial gas desorption and release capacity,the risk of coal and gas outburst is dramatically increased.(2)The mechanical strength of the normal coal in the composite coal seam is much higher than that of the tectonic coal,and normal coal’s uniaxial and triaxial compressive strength is 3.63 and 2.05~2.70 times that of the tectonic coal,the modulus of elasticity and the cohesion 8.8 and 3.3 times that of tectonic coal and the Poisson’s ratio about62.5%of the tectonic coal.Under the stress path of loading axial pressure and unloading confining pressure,the average peak strength of normal coal and tectonic coal is reduced to 31%and 42%of the stress path of loading axial pressure and maintaining confining pressure,respectively.These two coal bodies are more prone to simultaneous break under the stress path of loading axial pressure and unloading confining pressure.(3)When the hydrostatic pressure is unloaded from 50 MPa to 2 MPa,the permeability of the normal and the tectonic coal increased by 792.2 times and 76.3times respectively.It shows that even if the coal body does not undergo macroscopic damage,the coal permeability can still be increased by unloading pressure.After the coal damage,the permeability of the tectonic coal did not increase significantly but the permeability of the normal coal increased to 65.9~117.0 times of the initial value under the stress path of loading axial pressure and maintaining confining pressure;the permeability of the tectonic coal and normal coal increased to 1.8~7.2 times and108.9~3127.5 times of the initial permeability respectively under the stress path of loading axial pressure and unloading confining pressure.The results of the permeability measurement show that the permeability increase rate of normal coal after the damage is significantly larger than that of tectonic coal,and the permeability increase path is more than that of tectonic coal.The simple“damage”cannot effectively enhance the permeability of the tectonic coal body,and therefore the effective“unloading”after the“damage”is actually the main means to enhance the permeability.(4)After the high-pressure hydraulic flushing on the composite coal seam,the tectonic coal can be flushed out effectively.In addition,the average flushed-out coal rate of the borehole is 0.48 t/m,and the borehole geometry of the composite coal seam has a diameter of 1.5 m in the lower tectonic coal,and a diameter of 0.1 m in the upper normal coal.Numerical simulation experiments show that the hole makes the stress disturbance range in the composite coal seam expand significantly,and the range of stress disturbance is increased by 4.8~8.5 times compared with ordinary drilling.The stress evolution path of the coal body around the borehole is a process of the maximum principal stress increasing and the minimum principal stress decreasing continuously,which corresponds to the stress path of loading axial pressure and unloading confining pressure in the triaxial mechanical experiment.Hydraulic flushing increases the volume of damaged coal around a single borehole by 78.5 times.On the whole,the range of permeability dramatically enhanced area around the borehole increases by 8 times,and the effective radius of gas extraction increases by 2.4~3.3 times.(5)The technology of hydraulic flushing gas drainage is greatly improved by adopting advanced high-pressure hydraulic flushing integrated equipment,improving the support of mining,separation,extraction and monitoring system and at the same time formulating a clear construction process flow.Field test results show that,compared with ordinary borehole technologies,hydraulic flushing can improve the gas extraction efficiency of boreholes significantly and ensure the safety of coal roadway driving.One year after applying hydraulic flushing borehole gas drainage on the composite coal seam,the permeability of the coal seam increased from the initial 0.0007mD to 0.06 mD,with an increase of 87 times;the holes in the tectonic coal provides ample space for the expansion of coal seam;the high concentration gas extraction period of hydraulic flushing boreholes increased by 4.9~10 times,and within 365 days,the average one hundred meter borehole gas extraction pure quantity increased from0.018 m~3/min.hm of common boreholes to 0.072 m~3/min.hm of hydraulic flushing boreholes;on the premise that the number of hydraulic flushing holes was only 89.7%of the common boreholes,the gas pre-drainage period was reduced from 1425 days of common boreholes to 336 days of hydraulic flushing boreholes.After the hydraulic flushing gas pre-drainage on the composite coal seam,the average tunneling speed of the coal roadway increased by 1.6 times to 4.4 m/d,and the outburst risk of the coal seam was significantly reduced during tunneling.This dissertation has 119 figures,22 tables and 164 references.