| Water jet flushing cavity technology is currently recognized as a relatively good coal seam pressure relief and permeability enhancement method.It takes high-pressure water jet as the carrier and uses the kinetic energy generated by high-pressure water jet to expand the hole and hole the coal seam.With the continuous impact of high-pressure water jet on the coal seam,a large number of broken coal bodies are punched out of the hole,and more surface area is exposed inside the coal seam.At the same time,The formation of the cave destroys the original stress balance of the coal seam,redistributes the stress,and the stress concentration shifts to the deep part of the hole,which reduces the stress of the coal around the cave and plays a role of pressure relief to a certain extent;At the same time,the high-pressure water jet continuously impacts the coal body,and a large space cave is gradually formed in the coal seam.The stress concentration is transferred to the deep part of the coal seam along the hole,and the coal around the cave will expand and deform.The originally closed and closed pores and fractures in the coal seam will reopen and increase.Different pores and fractures extend outward,connect and penetrate,expanding the gas migration channel and increasing the permeability of the coal seam.However,the water jet punching measures mostly adopt the method of crossing boreholes,which is mainly used to enhance the permeability and strengthen the extraction in the tunneling area of coal seam roadway,while there is less research on the hydraulic punching and permeability enhancement measures for the drilling along the coal seam.With the progress of drilling and high-pressure water jet flushing cavity technology,water jet flushing caxity technology for this coal seam has been gradually applied and popularized in recent years.In this paper,aiming at the problems of long bedding drilling water jet hole making and coal washing section,complex coal seam geological structure conditions,easy hole collapse and hole plugging in the track grooving working face of the roadway side drilling yard,combined with the problems of low coal seam permeability,low gas drainage efficiency and stress concentration in some coal seams in huangyanhui coal mine of China Coal Xiyang Energy Co.,Ltd,The pressure relief effect and enhanced extraction effect of water jet cave making in this coal seam are studied.Firstly,this paper introduces the role of water jet cavitation technology in coal seam pressure relief and permeability enhancement from the theoretical aspect;Secondly,COMSOL multiphysics numerical simulation software is used to simulate and study the variation law of coal seam stress around the hole,the range of pressure relief area and the variation law of coal seam gas pressure under different hole radius in the process of water jet cavitation,which provides some theoretical and data support for the field test of water jet cavitation pressure relief and permeability enhancement technology;Finally,it mainly studies and analyzes the gas drainage concentration,net quantity,borehole gas flow attenuation coefficient and coal seam permeability coefficient of water jet hole making,so as to verify the pressure relief effect and enhanced drainage effect of water jet cavitation.The main conclusions are as follows:(1)Through the theoretical analysis of water jet pressure relief and permeability enhancement and the derivation of the formula of pressure relief zone,it can be concluded that the range of pressure relief zone increases with the increase of water jet cavitation radius.At the same time,according to COMSOL numerical simulation results and origin plot analysis,it can be concluded that the relationship between the radius of pressure relief area and the cavitation radius of water jet increases negatively exponentially,which also reflects that the water jet cavitation technology does play a role in pressure relief and permeability enhancement of coal to a certain extent.(2)According to COMSOL multiphysics numerical simulation research,it can be concluded that the coal seam gas pressure decreases at the center of the borehole and a certain distance around the borehole.The closer to the borehole wall,the smaller the coal seam gas pressure.At the same time,the lower the gas pressure around the cave near the coal wall of the roadway,the more obvious the effect of hydraulic cave building,pressure relief and permeability enhancement;As the cave gradually approaches the original stress coal seam area,affected by the original coal seam stress,the closer it is to the original coal seam stress area,the higher the gas pressure is relatively.After 30 days of drainage,the coal seam gas pressure around the hydraulic hole drilling decreased by about 17.8%.After 60 days of drainage,the coal seam gas pressure around the hydraulic hole drilling decreased by about 28.9%.After 90 days of drainage,the coal seam gas pressure around the hydraulic hole drilling decreased by about 40%.(3)After water jet cavitation,the first principal stress changes from the hole wall to the deep part of the hole,including stress relief area,stress concentration area and original stress area.Around the hole making hole,the distance from the center of the hole is different,and the pressure relief range is also different.The closer to the hole,the smaller the stress,and the stress value at the hole is close to 0.The pressure relief areas with cavity radius of 0.2m,0.3m,0.4m and 0.5m are 1.0m,1.6m,2.4m and 2.6m respectively.The pressure relief area around the hole will gradually increase with the increase of water jet cavity radius.Considering the factors such as economic cost and construction time cost,the water jet cavitation is the best when it is 0.4m.(4)After the water jet hole making measures,the gas flow attenuation coefficient of the hole making hole is between 0.0369～0.0382,which is about 15%of that of the ordinary drainage hole;The attenuation coefficient of borehole gas flow of ordinary drainage holes affected by hole making holes is between 0.082～0.113,which is 40%of that of ordinary drainage holes.The permeability coefficient of coal seam increases from 0.0708 m~2/(MPa~2·d)to 0.83～1.06m~2/MPa~2·d),which is about 11.7～21.2 times higher.These show that the water jet cavitation technology has a good effect on coal seam pressure relief and permeability enhancement.(5)After water jet hole making,the gas drainage concentration of the hole making hole is about 2.5 times higher than that of the surrounding holes affected by the hole making hole,and the net amount of gas drainage is about 0.81 times higher;Compared with ordinary drainage holes,the gas drainage concentration of hole making holes is increased by about 3.3 times,the net gas drainage amount of holes is increased by about1.09 times,the gas drainage concentration of holes affected by hole making is increased by about 0.23 times,and the net gas drainage amount of holes is increased by about 0.15times.(the paper has 31 figures,8 tables and 62 references)... |
PDF Full Text Request