Experimental Study On Effect Of Liquid Nitrogen Cryotherapy On Physical Properties Of Coal And Gas Drainage

With the increase of mining depth,the permeability of coal seam decreases gradually.Permeability of coal seam directly affects the difficulty of gas extraction,in order to increase the permeability of coal seam,cryotherapy of low temperature liquid nitrogen has been emerged at the right moment.Therefore,it is of theoretical and practical significance for the implementation of this technology to study the change law of the physical characteristics of coal after cryotherapy.Based on rational determination of NMR parameters,this thesis studied the micro-pore and fracture characteristics of cryogenic coal,compared the macroscopic mechanical properties of cryogenic coal with liquid nitrogen,compared gas desorption capacity,established a nitrogen-gas migration model and predicted the gas emission displacement effect in coal seam under nitrogen injection.The following research results were obtained:?1?In order to ensure the accuracy of the low field nuclear magnetic resonance experiment for testing the coal pore and fracture,the thesis compared the effects of waiting time,echo number,echo interval and repetition times on the experimental results,determined the transformation coefficient of nuclear magnetic-pore size,calculated the original pore distribution of coal,obtained the T₂ cutoff values and compared the pore and fracture development characteristics of coal with different metamorphism.?2?The thesis compared and studied the variation law of fracture volume of anthracite and coking coal with cryotherapy time of liquid nitrogen,water saturation of different samples and cryotherapy times.It was obtained that the influence of liquid nitrogen cryogenic on fracture volume growth rate of anthracite is greater than that of coking coal,the fracture volume of frozen coking coal with low water saturation increased less.Numerically simulated the cooling process of liquid nitrogen in saturated coal samples,investigated the mechanism of fracture expansion and fracture form of coal with liquid nitrogen.Detected and obtained the variation law of acoustic emission characteristics in the process of cooling and heating.?3?Based on the uniaxial mechanical experiment,the thesis compared the stress-strain curves of anthracite and coking coal with and without liquid nitrogen cryotherapy,obtained the elastic modulus,compressive strength and brittleness of cryogenic coal,and analysis the essential reason for the change of mechanical properties of coal.According to the damage mechanics theory,determined the law of damage variable evolution,and established a statistical damage constitutive model with full stress-strain characteristics,and verified the rationality of the model,obtained the effects of LN₂ cryogenic on model parameters.By comparing the fracture images,acoustic emission,and the energy dissipation characteristics,the fracture characteristics of cryogenic coal samples during loading were obtained.?4?Through the desorption experiment of non-cryogenic and cryogenic coal with liquid nitrogen,the thesis compared the gas desorption capacity of cryogenic coal with liquid nitrogen It was obtained that the desorption amount and diffusion coefficient of cryogenic coal samples increased to different degrees According to diffusion-seepage equation,the gas pressure distribution in the pore and fracture system in the process of coal particle desorption is obtained by numerical simulation A method for calculating gas desorption quantity is presented and the results are verified Based on the fractal theory,the fractal dimension of pore and fracture in cryogenic liquid nitrogen coal samples were obtained,and the correlation between fractal dimension and diffusion coefficient was analyzed.?5?Based on theoretical analysis,a nitrogen-gas permeability model of coal seam was established,and the effects of fracture expansion on coal permeability were obtained.Combined with Darcy's law and the conservation equation of mass.Finally,the thesis established the control equation of nitrogen and gas seepage flow in coal seam,obtained the distribution of gas pressure in coal seam under nitrogen injection condition was obtained through numerical simulation,the effects of liquid nitrogen cryotherapy on gas displacement in coal seam were compared.