| For a single coal seam with low permeability and outburst risk,regional measures to increase permeability and eliminate outburst need to be taken to ensure coal mine safety production.The traditional regional outburst elimination measures are mainly concentrated underground,with low efficiency and high cost,and there is a contradiction with mine production.Based on the background of coal seam regional outburst elimination,this paper explores the new technology of coal seam gas control through engineering practice,underground anatomical observation,theoretical analysis and numerical simulation.In this paper,the outburst elimination technology of large fluid hydraulic fracturing and multi angle directional drilling hydraulic fracturing technology is studied,and the engineering practice and relevant theoretical analysis are carried out.Finally,a comprehensive evaluation system for outburst prevention effect of surface hydraulic fracturing is established in this paper.Firstly,based on the fine anatomical observation of coal structure and fracture system in the study area,the fracture system of coal seam in the study area is characterized on multiple scales,and the spatial model of large fracture system of coal seam in the study area is constructed to provide a basic for construction.Then,aiming at the problems of high risk of gas outburst,complex outburst prevention measures and slow construction progress in Xinyuan coal mine 3# seam,the engineering practice of ground large liquid hydraulic fracturing is implemented.The outburst prevention technology of high-volume hydraulic fracturing coal seam in surface vertical well is applied for the first time,which lays a foundation for establishing the three-dimensional outburst prevention and pre pumping technology system in outburst mine.Secondly,according to the actual situation of gas control and mining replacement in Jiulong coal mine,the comprehensive technology of ground multi angle and multi-level directional drilling and hydraulic fracturing is innovatively put forward.Directional drilling can guide the propagation direction of hydraulic fractures,increase the propagation length of fractures,form a largescale fracture making effect,and improve the uniformity of gas distribution,gas drainage efficiency and outburst elimination effect.Microseismic monitoring shows that this technology realizes the complex fracture network with radial covering the full thickness of coal seam centered on the borehole,and the treatment area is greatly increased.The drainage and production has been nearly one year,and the total annual gas production is nearly 600000 m3.The gas drainage and production of single wells and well groups has broken through the historical records of the research mining area,providing new methods and technologies for the prevention and control of coal and gas outburst hazards in the mine.On the basis of engineering practice,carry out relevant theoretical research.Firstly,using the extended finite element method,the effects of directional drilling number,vertical hole density,azimuth,directional drilling diameter,directional drilling length and other parameters on hydraulic fracturing fracture propagation guided by directional drilling are studied through numerical simulation.The smaller the azimuth and phase angle of multi angle directional drilling,the larger the aperture,length and vertical density,and the stronger the guiding effect of multi angle directional drilling.For vertical wells,the spatial arrangement of multi angle directional drilling with phase angle of 90°,azimuth angle of 45° and vertical density of no less than 2 holes/m is a suitable scheme.In the initial stage of fracturing,a large fracturing fluid injection displacement is required to improve the injection rate and fluid pressure,so as to play the role of simultaneous expansion and better guidance of multiple fractures.Then,the fully coupled hydrodynamics model is used to study the hydraulic fracture propagation and the stress disturbance and distribution during fluid injection.In the complete coal model,the hydraulic fracture is affected by the maximum and minimum principal stress,and a small deflection occurs at the crack tip,and the size is controlled by the boundary stress.After the fracturing fluid is injected into the complete coal model for 30 min,the stress change is concentrated in the range of 100 m from the crack tip,which increases with time and increases linearly with the crack length.The maximum stress difference increases less than 10 m from the fracture tip and then decreases slightly.The influence range of stress disturbance increases continuously in the process of hydraulic fracture propagation,and can reach 100 m after 30 min of fluid injection.The minimum principal stress,the influence range increases with time,increases linearly with the crack length,and the stress shadow area is slightly larger than the crack length.The increase of the minimum principal stress and the decrease of the stress difference indicate that the shear failure and tensile failure are restrained above and below the horizontal fracture,and the stress shadow area limits the development of effective permeability in the coal and rock body.In the connected fracture model,due to the low strength of natural fractures and the favorable direction of fracture groups,damage along natural fractures is more likely and less energy is required to open existing fractures.Finally,the evaluation system of outburst elimination effect suitable for surface hydraulic fracturing coal seam area is preliminarily established,mainly including fracturing fracture microseismic monitoring technology,fracturing fracture in-situ observation technology,fracturing fluid and tracer ion detection technology,and underground area and local outburst elimination effect verification.The organic combination of surface hydraulic fracturing effect evaluation and downhole outburst elimination effect evaluation system forms a complete and accurate comprehensive evaluation system suitable for surface hydraulic fracturing gas treatment,which is conducive to the popularization and application of this technology. |
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