| More than 70 percent of coal mines in China are high-gas mines with complex geological conditions,high gas content and high pressure.The main reason for the difficulty of gas treatment is the low permeability of coal seam and the difficulty of increasing permeability.With the deepening of coal mining depth and lower permeability of coal seam,how to greatly enhance the permeability of coal seam is the key to realize efficient extraction and safe mining of coal bed methane.It is difficult for the gas to be extracted by the traditional method of increasing the permeability of coal.At present,the hydraulic fracturing technology has been introduced in the field of oil and gas in the coal mine gas extraction,but cracks in the site construction disordered extension and serious destruction of the coal seam roof and floor,lead to small scope of anti-reflection and subsequent coal mining roof and floor supporting difficult.Aiming at the above problems,based on the research team for water jet cutting coal and rock breaking theory and technology research,on the basis of innovative put forward coal mine slotted compound new method for hydraulic fracturing increase coal seam permeability.Based on the theoretical analysis,numerical simulation and lab experiment and field experiment,the method of the jet slotting composite fracturing fracture mechanism,jet slotting composite fracturing antireflection mechanism and slotted compound fracturing techniques are studied,the main achievements are as follows:?1?Based on jet slotting pressure relief mechanism and hydraulic fracturing theory,innovative put forward coal mine slotted compound hydraulic fracturing new ways to enhance the permeability of coal seam,namely:even in the bottom of a coal seam roof,reasonable decorate,drilling of the coal seam by water jet cutting pressure relief,and then select the part of the hole as the pressure in the implementation of hydraulic fracturing.This method combines the jet slotting technology and hydraulic fracturing technology,the advantages of jet slotting to crack fracture and the orderly expansion,on the one hand,can significantly improve the fracturing scale,on the other hand can reduce the hydraulic fracturing pump pressure,reduce the damage to the coal seam roof and floor.?2?The mechanism of fracturing fracture propagation is revealed.Based on elastic-plastic fracture mechanics and mechanical structure characteristics of coal rock,combined with the stress analysis around the slot of jet cutting seam,the fracturing position and the crack pressure calculation model are established.Established the heterogeneous material seepage-damage model,and by using RFPA2D Flow numerical simulation software,further analyzes the cutting seam tank unloading fissure distribution range,and hydraulic fracturing geometric model is set up on this and get the slotted compound fracturing crack propagation law,and further analyzes the adjacent slot on the effects of hydraulic fracture,the in-situ stress difference,dip Angle of coal seam,coal lithological epicuticular slotted the influence law of compound fracturing effect.?3?The gas migration mechanism of fracturing and fracturing is revealed.The complex fracture of composite fracturing is simplified,and the equivalent seepage model of complex fracturing is established based on the improvement of the conceptual model.Based on the mechanism of water drive gas,the law of water and gas migration was analyzed in the process of hydraulic fracturing,so as to predict the existence of gas accumulation zone and establish its position model.Slotted compound fracturing through the field experiment,the change of moisture content and gas content of coal seam after fracturing and aperture of return water,gas migration regularity of jet slotting composite fracturing for validation,the results showed that the compound fracturing reconstruct regional gas reduction zone,gas,gas enrichment zone exists in the center of the pressure in 3050 m within the ellipse.?4?The field test of composite fracturing with jet slot was carried out.According to the underground environment of coal mine and the situation of existing equipment,the supporting equipment for the composite fracturing of jet slot in underground coal mine is developed,the process flow of the composite fracturing of jet slot is designed,and the calculation model of hydraulic system parameters is established.The field comparison experiment shows that the net amount of gas drainage and the concentration of gas drainage are increased by 11.8 times and 2.12 times respectively.Compared with conventional fracturing process,the pure quantity of gas drainage is 7.58 times higher,and the drainage concentration is 1.79 times higher.It is concluded that the complex fracture propagation can be induced by jet slot,and the effective pressure relief and gas extraction can be realized.The research results in this paper have made some contributions to the Fracture mechanics theory of seepage damage of heterogeneous coal rock,and contribute to the innovation application of hydraulic fracturing technology in mining engineering. |
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