Study On Gas Geology Characteristics And Coal Mine Gas Control With CO₂ Phase Transition Fracturing In Yangquan Mining Area

Yangquan mining area is a mining area that is more serious in coal and gas outburst in our country,with a large number of coal and gas outburst mines and serious coal mine gas disasters.CO₂ phase transition fracturing technology is an emerging coal mine gas control technology.It has been applied in many mining areas across the country,and the application effect is good.However,Yangquan mining area has unique gas geology characteristics,and the suitable CO₂ phase transition fracturing gas control model has not been fully studied.In view of this,with the help of CO₂ phase transition fracturing technology,this paper studies the gas geological characteristics of Yangquan mining area,comprehensively adopts the coal sample impact experiment,numerical simulation and gas control field experimental methods to complete the analysis of the change of pore and fracture scale,the research on the propagation law of coal seam fractures,the investigation on the gas control effect of the excavation roadway,and analyses the damage law of CO₂ phase transition fracturing on coal seam under different technical parameters.The main research conclusions are as follows:（1）The gas geological characteristics and gas control problems in Yangquan mining area are found out.Yangquan mining area is located on the regional monoclinic structure,in which small normal faults,anticlines and synclines are developed.The gas content,gas pressure and gas emission of coal seam are large,and increase with the increase of mining depth.The gas occurrence is uneven,and structural soft coal is developed.The permeability of coal seams is generally poor,prone to coal and gas outburst,and it is difficult for gas drainage and gas control.The effect of the intensive borehole technology for outburst prevention and excavation used in the mining area is not obvious,the gas drainage efficiency and excavation efficiency are low,the K₁ value and the gas gushing volume continue to exceed the limit,and the gas dynamic phenomenon occurs frequently.Which affect coal mine extracting joining seriously.（2）The effect of"fracture forming and pore expanding"increases with the increase of fracturing pressure.FESEM observation and high-pressure mercury intrusion experiment show that compared with the impacted coal sample under 120 MPa fracturing pressure,the size of fractures and pores of the impacted coal sample under 150 MPa fracturing pressure increases,and the maximum fracture width is increased from 3.29μm to 3.84μm,which is increased by 17%;the pore volume of pores above 100 nm is increased from 0.0141m L/g to 0.0165 m L/g,which is increased by 17%;the pore specific surface area of pores above 100 nm increased from 0.0200 m²/g to 0.0315 m²/g,which is increased by 58%,and the median pore size of the pore volume increased from 33.80 nm to 82.20 nm,which is increased by 143%.（3）The coal seam fracture propagation scale and damage area are large under the mode of"150 MPa double hole fracturing".With the help of the ANSYS platform,the numerical simulation of fracture propagation in coal seam induced by single-borehole/double-borehole CO₂ phase transition fracturing at different pressures was carried out.The results show that:Under the mode of"150 MPa double-borehole fracturing",the pressure relief range is 10.50 m,and the damage area is 26.82 m².the fractures formed are more developed,which is more practical for cover the excavation roadway and the roadway’s sides.（4）It is verified that under the mode of"150 MPa double-borehole fracturing",the effect of increasing permeability,pressure relief,homogenizing gas emission,and outburst prevention is remarkable.150 MPa double-borehole fracturing mode has remarkable gas control effect.Compared with intensive borehole technology,the average single-borehole gas concentration is 69%,with an increase of 0.9 times;the average pure flow rate of 100-meter drilling holes is 122 m³/d,with an increase of 1.9 times;the K₁ value exceeds the standard rate 1.9‰,with a decrease of 95%;the variance of the K₁ value is 0.001,with a decrease of 91%;the number of dynamic phenomena is 0.125 times,with a decrease of99%;the monthly excavation footage is 90 m,with an increase of 90%.（5）The coal in Yangquan mining area has poor permeability,prone to coal and gas outburst,great difficulty in gas drainage,low excavation efficiency,K₁ value and gas emission continuously exceed the limit,and frequent gas dynamic phenomena,which affect coal mine extracting joining seriously.Under the mode of"150 MPa double-borehole fracturing"mode,the coal seam fracture propagation scale and damage area are large,and increasing permeability,pressure relief,homogenizing gas emission,and outburst prevention is remarkable.It has been verified by engineering in Yangquan mining area.A gas control mode with efficient gas drainage,pressure relief,pressure equalization,homogeneous gas emission,outburst prevention,outburst elimination and rapid excavation has been formed,which is suitable for the outburst coal seam excavation roadway in Yangquan mining area.59 figures,26 tables,and 116 references.