Research On CO₂ Fracturing Enhanced Gas Pre-drainage Technology For Drilling Along Bedding In Low-permeability Outburst Coal Seams

The breathability of coal seam is an important content in the study of gas extraction,and it is an important index to mark the difficulty of gas extraction.In this paper,the influence of carbon dioxide phase transformation on gas extraction is investigated by theoretical analysis and numerical simulation,and then the phase change fracturing test of carbon dioxide is carried out in low permeability protruding coal seam,and the hole parameters of fracturing drilling and pumping drilling are determined by combining theoretical analysis,numerical simulation and field data.Firstly,based on the basic principle of carbon dioxide phase transformation,the damage range is divided into crushing ring,fracture development circle and fracture expansion ring by analyzing the deformation characteristics of coal body and the mechanism of coal body failure after phase change fracturing.According to the interaction between coal body deformation and coal body deformation caused by gas extraction,the fluid-solid coupling model of gas extraction after carbon dioxide phase transformation is established,which is combined with the seepage field model and the deformation field model.Combined with numerical simulation,the influence of the change of extraction negative pressure and initial permeability on the extraction effect after the phase transformation of carbon dioxide is analyzed,and the flow law of gas around the drilling borehole under different pumping time and the influence of extraction time on the pore spacing.The results show that the influence of negative pressure and initial permeability on gas extraction is different,which is beneficial to gas extraction in general,the radius of influence is larger in the early stage of extraction,the effective radius is small,and the spacing of cloth holes under different pumping time is obtained.Then,the gas flow rate,gas concentration and negative pressure of the suction hole in the crack propagation ring before and after the fracture and the crack expansion circle are investigated in the 20,917 back Wind groove,and it is proved that the penetration effect of the fracture expansion ring is very good after the phase transformation of carbon dioxide is carried out: After cracking,the average gas flow rate and concentration of the drilling group increased by 79% and 22% respectively,the average gas flow rate and concentration of the borehole were increased by 97% and 37% respectively after drilling in the fracture expansion circle,and 38% and 3% were increased after drilling fracture outside the fractured expansion ring.The results of the field test determined that the hole spacing of the borehole can reach 9 meters when the coal body is pumped for 180 days.Finally,the results obtained by theoretical analysis and numerical simulation are compared with the field test results,and the results are in good agreement.On this basis,the economic benefits of carbon dioxide phase change fracturing are analyzed,and the extraction effect after fracturing is evaluated.