| The health,stability and sustainable development of the coal industry is a major issue related to the national energy security.With the increasing depth of coal mining,coal gas content,gas pressure increased gradually.Part of the gas mine,high gas mine gradually upgraded to the outburst mine,gas disasters become increasingly serious.At this stage,the primary goal of gas drainage in China's coal mine is to ensure the safe production of coal mines,taking into account the development and utilization of coalbed methane.Therefore,to prevent the occurrence of gas accidents and ensuring the safety of mine productionn in the underground mining coal layer of the gas drainage is particularly important.For coal of low permeability,the permeability of coal is very poor,the measure of conventional drilling layout and parameters pre-pumping on coal gas can not achieve the required effection on gas drainage.Hydraulic fracturing in Virtual reservoirs is that uses the high pressure water as a driving force in the virtual reservoir resulting in the formation of the gap in the bottom floor of the coal,fissures or cracks to the existing cracks and even the movement of coal so that the pressure of coal can be released,the gas in the coal quickly desorbs and flows into the borehole space.Its role has two sides: the relief on a large area of coal pressure,the elimination of dynamic phenomena;another one is to increase the permeability of coal,improve the gas extraction rate,and ultimately achieve the purpose of prevention and conformity of mine gas disaster.Based on self-developed multi-field coupling test system for coal-bed methane(CBM)drainage in Chongqing University,and a hydraulic fracturing system to form a hydraulic fracturing physical simulation test system is added,the virtual reservoir hydraulic fracturing drill physical simulation test of hydraulic fracture of virtual reservoir under normal distance of 0mm,40 mm and 160 mm are carried out.The spatial distribution morphological characteristics of the fractured surface of hydraulic fracturing under different virtual reservoir thickness and the evaluation of hydraulic fracturing enhancement effect of virtual reservoir are analyzed.The main research results are as follows:(1)During the whole hydraulic fracturing process,the internal pressure of the coal and rock mass increases with the increase of the water injection pressure as the pressure water is injected,and the water pressure reaches the initiation pressure and the fracture is generated.The internal pressure of the measuring point is steep down with the water injection pressure.(2)The stress and strain evolution of coal and rock mass at different locations during fracturing is monitored by nine stress indenter and displacement sensor.The deformation law of coal and rock mass during hydraulic fracturing is simulated and the effect of ground stress and water injection pressure on coal seam deformation is discussed.The development of coal and rock mass deformation during hydraulic fracturing is demonstrated.(3)Under the same stress condition,the normal distance of the fractured borehole is changed from the coal seam.The fracture pressure values of the hydraulic fracturing test under different conditions are different.The crack pressure is 2.81 MPa when the normal distance of the fractured borehole being 40 mm,.In general,the peak pressure is 40mm> 160mm> 0mm.The thicker the thickness of the roof is,the greater the fracture pressure is,and the decrease of the water injection pressure is also different.(4)The fracturing drilling is located on the surface of the main fractured surface,which indicates that the fractures are generated near the fractured borehole and extend along the favorable surface of the hydraulic fractures,and the four fractures are different or less in the four groups.When the normal distance of the fractured borehole is 0mm and 40 mm,the main fractured surface is basically the same as the minimum When the normal distance of fractured borehole is 160 mm,the fracturing surface is the largest and the secondary fissure surface is complex,and the main fissure surface is not perpendicular to the minimum The horizontal stress extends to the coal seam and runs through the roof at an angle of inclination.(5)Taking the fractal drilling distance from the coal seam as the normal distance of 40 mm,the effect of the hydraulic fracturing on the permeability of the low permeability coal and rock mass is found by comparing and analyzing the extraction efficiency before and after the hydraulic fracturing.The results show that when the hydraulic fracturing is carried out under the stress control condition,the gas pressure evolution law of coal seam gas drainage under the three normal conditions of 0mm,40 mm and 160 mm is compared.The descending rate is 40mm> 160mm> 0mm,which indicates that The hydraulic fracturing effect is better when the fracturing drilling distance is 40 mm from the coal seam. |
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