Experimental Study On Mechanism Of CO₂ Fracturing And Enhancing Permeability Based On Fractal Theory In Guizhou Soft Coal Seam

CO₂ fracturing antireflection technology,as a means of relying on the energy produced by liquid CO₂ phase change to impact the coal seam,has the characteristics of no spark during blasting,controllable pressure,and reusable.Compared with hydraulic antireflection technology,CO₂ better make the internal pores of coal rock crack and expand,and help to increase the permeability of soft and low-permeability coal seams in Guizhou.This paper uses a combination of laboratory tests and field experiments to collect coal samples before and after fracturing at the Dayun Coal Mine to conduct micro-physical property tests,and to test the macro gas flow and coal-rock physical property changes after fracturing in the field.The main research contents are as follows:First of all,this paper introduces the mechanical properties and pore structure of coal rocks,and introduces research methods for different pore sizes.The state of carbon dioxide phase transition is described.The principle of liquid phase carbon dioxide fracturing is analyzed from three aspects:the impact pressure generated by phase transition,the mechanical analysis of the effect of fracturing on rocks,and the cracking radius.the way.The fractal model to be applied to the low temperature nitrogen adsorption test,mercury injection test and scanning electron microscope test is introduced.Secondly,the basic parameters of raw coal are obtained through industrial analysis of raw coal in Dayun Coal Mine,and then mercury injection tests are performed on coal samples before and after cracking.Low-temperature nitrogen adsorption tests and scanning electron microscope tests are used to obtain the pore volume,average pore diameter,a series of micro-physical physical parameters such as specific surface and porosity have found that the coal pores have developed after fracturing.According to the low-temperature nitrogen adsorption curve and the mercury advance and retreat curve,it is found that the fracturing will develop the pore structure toward the through hole.The pore size distribution of the mercury intrusion test and the low temperature nitrogen adsorption test was comprehensively plotted,and the effect of carbon dioxide fracturing on the pore size distribution was visually compared.The fracturing results in a significant increase in pores,especially pores of 10,000 to 100,000 nm.According to the state of gas in the pores,the pores are divided into diffusion holes and seepage holes.After the cracking,both the diffusion holes and the seepage holes will increase,but in terms of percentage,the diffusion holes will decrease and the seepage holes will increase.Obtaining carbon dioxide fracturing will destroy gas storage sites in coal rocks and drive the gas state to desorb.Furthermore,the fractal dimensions of each model were drawn and calculated based on the low-temperature nitrogen adsorption curve,forward and backward mercury curve,and binary scanning electron microscope pictures.Compared with the fractal dimension of raw coal,the fractal dimension after fracturing has a tendency of decreasing first and gradually increasing.It was concluded that carbon dioxide-induced cracking tends to smooth the inner surface of pores.A new algorithm for calculating the comprehensive fractal dimension is proposed,which can calculate the comprehensive fractal dimension in the case of multiple models and multiple aperture sections,and correlate the comprehensive fractal dimension with the pore volume and average pore diameter to find the fractal dimension.It has an inverse relationship with pore volume and pore diameter.Finally,the natural gas flow rate of 1m,2m,3m and contrast holes 1 and 2 is calculated after cracking,and the corresponding permeability coefficient is calculated.Macro parameters such as changes in gas drainage before and after cracking are calculated.It is found that the natural gas flow rate of cracked holes is lower than that of control holes,and the permeability coefficient of cracked holes is larger than that of control holes,up to 3.5 times,and the gas extraction efficiency after cracking is higher than that of control holes,up to 5 times.The air permeability coefficient is combined with parameters such as the comprehensive fractal dimension and aperture to establish a macro and micro relationship.It is found that the smaller the fractal dimension at the micro level,the larger the average aperture,and the better the air permeability coefficient.Taken together,carbon dioxide cracking can indeed increase the permeability of coal seams and reduce the risk of coal and gas outburst.This paper provides support for the application of carbon dioxide column technology in soft and low-permeability coal seams through macro and micro studies.