| China is rich in coal resources,and at the same time contains considerable reserves of coal bed methane resources.Coalbed methane is a kind of high-quality clean energy and chemical raw materials.However,coal and gas outburst,gas explosion and other disasters may occur during coal mining.Gas extraction can not only make use of methane,but also eliminate gas disasters and provide safety guarantee for coal mining.However,due to the low permeability of coal seam in China,the complicated gas occurrence and the increase of in-situ stress with the increase of mining depth further increase the difficulty of gas extraction.This paper first studies the ultra-high pressure water jet slot technology and its permeability improvement mechanism,analyzes the ultra-high pressure water jet flow structure,mechanical characteristics,coal breaking mechanism,pressure relief effect and the evolution law of coal seam permeability,masters the water jet slot permeability improvement mechanism,and obtains the gas transport law of pressure relief coal seam.Based on FLAC 3D finite element analysis software,numerical simulation was carried out on hydraulic cutting seams of 81308 working face in Baode Coal Mine,Shanxi Province.According to the changes of coal displacement and stress before and after the cutting seams,the pressure relief effects under different cutting depths,different cutting widths and different cutting spacing were compared and analyzed.An effective cutting scheme with a depth of 1.0m,a width of 0.1m and a spacing of 3m was put forward and field tests were carried out.In the 81308 working face of Baode Coal Mine,there are slot group boreholes and contrast group boreholes,and the parameters such as chip discharge amount,borehole gas extraction concentration,gas purity and 100-meter gas extraction amount are selected as the investigation indexes for analysis.The results showed that under the same sealing technology,the maximum singlehole gas concentration in the slotted group was 6.62 times that in the control group,and the average gas concentration in the slotted group was 16 times that in the control group.The average gas extraction concentration of the combined drainage in the slotted drilling group was 3.6 times that of the control group,the average gas extraction flow rate was 1.9 times that of the control group,and the average 100-meter gas extraction flow rate was 1.7 times that of the control group.The test results show that the ultrahigh pressure hydraulic fracturing can improve the permeability of coal seam and the efficiency of gas extraction. |
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