| Gas is not only an important factor on coal mine safety, but also a kind of non-renewable and clean energy. Gas extraction is the fundamental measure to prevent gas disasters, which is dominated by the coal seam permeability. High gas content, low-permeability and forceful adsorption increase the difficulty of gas extraction. The hydraulic stimulation technology is relatively simple, which can enhance permeability to improve gas drainage effect, but the theory of coal seam permeability improvement need more in-depth study, and the technique should be perfected in order to make it more extensively employed.By far, there are such ways to increase coal seam permeability of the single seam as loosing blasting of coal seam, fracture induced by phase transition of Carbon dioxide, shortening the borehole spacing and increasing the hole diameter, etc. These measures expend a great deal of engineering for the extraction, but their effect is finite. On the other hand, the hydraulic stimulation in the underground can solve this problem effectively.Taking Jiaozuo Zhongmacun coal mine and Hebi Zhongtai mining company as test area, this paper researches quantitative characterization of coal structure and gas migration in coal seam in order to reveal the law of permeability improvement induced by hydraulic stimulation technology, and to structure the theory of coal seam permeability improvement. According to the theory of coal seam permeability improvement, the technology for coal seam permeability improvement is study and determined, and the key equipment for coal seam permeability improvement is researched and developed. The effect evaluation methods of coal seam permeability improvement are brought up, which is validated and improved by the field test. The study was aimed at:(1) The characterization of coal structure and the mechanism of gas migration are researched, and the theory of coal seam permeability improvement is structured.Geological Strength Index(GSI) of Coal Bed is introduced to characterize quantitatively coal structure, which has not been realized yet. the recognition method and standard of flow state are discussed and analyzed, the flow state of gas migration is revealed, and the changing law of GSI before and after permeability improvement of coal seam is researched, and then the theoretical parameters including coal reservoir fracture pressure, stretch pressure, fluid loss coefficient, fracture parameters and the pressure drop equation are established base on the GSI, which improvement the theory of coal seam permeability improvement and provide theoretical support for permeability improvementThe relationship of GSI and the coal permeability is acquire by means of coupling test of solid mechanics, coal structure and permeability, the result displays that the permeability law of coal is normally distributed as the GSI of coal changes from 0 to 100, that the coal permeability is maximum value when GSI is 52.7, that pressure relief can be used to improve coal permeability When the GSI is less than 52.7, that hydraulic stimulation can be used to improve coal permeability when GSI is more than 52.7, that the migration types of gas in coal seam is seep and diffuse when GSI is between 100 and 65, that the migration types of gas in coal seam is seep when GSI is between 65 and 45, and that the mine migration types of gas in coal seam is diffuse when GSI is between 45 and 0.(2) The technology of coal seam permeability improvement are researched and formed on the bases of the theory of coal seam permeability improvement.According to the theory of coal seam permeability improvement and the principle of GSI optimization, the mechanism of permeability improvement and multiple-effect of the different types of hydraulic technology are discussed and compared. The comparison result shows that permeability improvement induced by fissure system could be obtained for hard coal, however stress relief could be used to improve permeability of both hard coal and soft coal. So it can be concluded that the corresponding process of permeability improvement is adopted according to the coal structure, and then the test of hydraulic stimulation is implemented in drilling holes along and crossing Seam. Finally, the technology of hydraulic stimulation is formed for specific seam, and the key equipment for the technology are researched and determined.(3) According to the technology of permeability improvement induced by hydraulic stimulation, the index system for effectiveness evaluation is determined, which is verified and improved by the test.Based on the theory of permeability improvement induced by hydraulic stimulation, the supporting equipment and technology programs for permeability improvement induced by hydraulic stimulation are developed, including the hydraulic stimulation equipment, drilling arrangement, sealing technology, stimulation type selection, effect evaluation methods etc. the gas extraction effect of the hydraulic stimulation is verified by experimental measurement of the hydraulic stimulation technology in Zhongmacun mine and Zhongtai mine, the results of which perfect the theory and technology of permeability improvement in coal seam... |
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