Experimental Study On The Evolution Mechanism Of Coal Structure And The Seepage Law Of CBM Under Sound Field

China has abundant CBM resources.The area of reservoir have 4.15×10⁵km² and the resource reserves buried shallower than 2000m reach 3.68×10¹³m³ that is equivalent to 4.2×10¹⁰t standard coal.The amount of chinese CBM resourse is less than the amount of Russia and America,ranking the third in the world.CBM is a main harmful source of coal mine disasters and is a greenhouse gas,but it is also a kind of green and clean energy.Hence,it is imperative to research a safe and efficient method of CBM extraction.Considering the new method of sound field can be used to efficiently exploit CBM.Based on this theory and technology,this paper will analyses and test the pore and crack structure of coal before and after ultrasonic treatment by three nondestructive testing techniques that are SEM,BET and NMR.Experiment studied on the character of coal to methane adsorption,desorption and seepage under sound field by self-designed experimental equipments of methane adsorption,desorption and seepage under ultrasonic treatment.The main achievements of the study are as follows:（1）The results of SEM,BET and NMR show:Ultrasonic can clean the microparticles in surface and the crack channel of the water-bearing coal body,break the soft coal and form the new cracks.After ultrasonic treatment,the total pore volume,specific surface area,average pore diameter and porosity increases,and the fractal dimension D₁ decreases and D₂ increases,the T₂ spectrum peak increases and the connectivity of coal increases,which are beneficial to the desorption,diffusion and seepage of CBM.After ultrasonic treatment,the porosity is linear increasing with the power of ultrasonic treatment and the porosity empirical model of the coupling of sound field,temperature field,permeability field and stress field is established on this basis.（2）The adsorption law of coal to methane is consistent without sound field and under sound field,which can be described by Langmuir equation and D-R model.The adsorption capacity of methane decreases with the increase of ultrasonic power under sound field,the parameter ratio of adsorption model are linearly decreasing with the increase of ultrasonic power.The desorption law of methane is consistent before and after sound field treatment,the results for methane desorption data show the fitting curve of Diffusion model is the best effect,the fitting parameter ratio after and before sound field treatment are linearly increasing relationship with the increase of ultrasonic intensity.The D_F increase and the B_i’decrease after sound field treatment.The models of methane adsorption and desorption under sound field are established.（3）The permeability of methane decreases presenting negative exponential relationship with the increase of the effective stress before and after sound field treatment,and the permeability increases under sound field treatment.The permeability increase with the increase of sound field treatment time and then the change tend to be gentle in the same stress condition.The permeability is linear increasing relationship with the increase of ultrasonic power,and the empirical formula for the permeability of coal under sound field is obtained through experiments.（4）Based on the ultrasonic cavitation effect,mechanical vibration effect and thermal effect,analysing the influence of the three effects on the pore and crack structure of coal,the desorption and seepage characteristics,the mechanism of high efficiency extraction of CBM under sound field is revealed.