Study Of Optimum Drilling Parameters In Low Permeability Coal Seam Based On CO₂ Phase Transition Fracturing Technology

According to the characteristics of coal seam with low permeability and high gas in Guizhou Province,this paper has completed the field test and numerical simulation of drilling gas extraction,field test and macro and micro analysis of CO₂antireflection technology.The research contents and conclusions are as follows:1.The experimental results of low temperature nitrogen physical adsorption static capacity method show that the pore structure and distribution of coal samples are greatly improved after fracturing.The content of micropores decreased first and then increased,while that of mesopores and macropores increased first and then decreased.In the B.E.T.model and FHH model,the pore structure changes in the same way,that is,CO₂induced cracking will simplify the pore structure of coal and make the surface more smooth.2.According to the 17#coal seam of Honghui coal mine,the experiment of gas drainage by drilling is carried out.After the basic parameters of coal seam gas are obtained,the effective influence radius of single hole and three hole extraction is analyzed by passive measurement.Results:under the condition of single hole,the effective influence radius of 17#coal seam is 4m～6m,while under the condition of three holes,the effective influence radius is10m～12m.3.By using COMSOL-multiphysics software,the numerical simulation test of gas drainage in17#coal seam of Honghui coal mine is carried out.The results show that:in a certain period of time,the gas drainage radius increases with the increase of time.In the stable stage,under the condition of three hole extraction,the radius of gas extraction is about twice that of single hole,and the effect of three hole extraction is better than that of single hole extraction.4.After the CO₂fracturing test,the attenuation coefficient of gas flow in the borehole decreased to 32%～64%of that before the fracturing;after the fracturing,the permeability coefficient of the coal seam,the average extraction concentration of the borehole gas and the average extraction purity increased by 1～9 times,6.64 times and 23 times respectively;after the fracturing,the residual gas content of the coal seam was 4.28m³/t～5.72m³/t;the gas extraction rate of the fracturing hole was more than 50%,more than 2.5 times of that before the fracturing;the gas extraction rate of the fracturing hole was more than 50%,more than 2.5 times of that before the fracturing.After that,the initial gas release rate of the coal sample decreased by36.9%～62.2%,which was lower than the critical value of outburst.Therefore,the experimental coal seam is changed from the difficult to be extracted coal seam to the minable coal seam,which is close to the critical value of the easy to be extracted coal seam.Therefore,CO₂fracturing technology can effectively promote the development of coal and rock pores,which is conducive to the coal seam with low permeability to increase the permeability,improve the gas drainage capacity,reduce the risk of gas outburst,and provide an effective reference for solving the problem of gas control in low permeability and high gas mines.