Experimental Study On The Effects Of Liquid Nitrogen Freezing And Thawing Damage In Coal With Different Water Content

Coalbed methane is one of the main factors affecting the normal and safe mining of coal mines.Coalbed methane extraction is the main method for controlling coal seam gas.However,many coal seams in China have problems such as low permeability and difficult gas drainage.In recent years,the use of coal seam gas displacement to solve the problem of coal seam gas drainage has been widely concerned.The use of coal seam liquid nitrogen flooding gas technology can not only use the low temperature characteristics of liquid nitrogen to freeze and thaw the coal body,increase the coal body fissure,and the liquid nitrogen phase can also be more effective to drive the gas in the coal seam after the damage.come out.Therefore,this paper studies the effects of freezing and thawing damage of coal body considering the water content.Through experiments and theoretical analysis,the effects of coal body pore structure after coal-water freeze-thaw damage and the coal body pair are studied through experiments and theoretical analysis.The variation of gas adsorption,desorption and seepage characteristics,theoretical analysis of the mechanism of nitrogen flooding gas in coal seam injection and the mechanism of liquid nitrogen freeze-thaw damageThe anthracite coal from the Yucheng coal mine in Shanxi was selected to produce coal samples with different water content,and then the liquid nitrogen was subjected to liquid nitrogen freezing and thawing treatment.For the coal samples before and after freezing and thawing damage,the low-temperature nitrogen adsorption method was used to study the change of pore structure in coal and the effect of water in the coal body on freeze-thaw damage,and the fractal theory was used to study the change of coal pore structure.law.After the freeze-thaw damage treatment,the pore structure in the coal body is destroyed,and the specific surface area and micropore volume increase rapidly with the increase of water content,and the total pore volume and average pore diameter increase.The pore structure is extended and expanded to become a pore with a larger pore diameter.The freeze-thaw damage has the effect of increasing pores and reaming the coal.The larger the moisture content of the coal sample,the more the pore structure changes after freeze-thaw treatment.obvious.The experimental isothermal adsorption experiment was carried out on the experimental coal body.The common adsorption model was used to fit the experimental data,and the gas adsorption isotherm formula of the experimental coal sample was obtained.The thermodynamic theory was used to calculate the surface free energy change value of each coal sample.The effect of freeze-thaw damage on the adsorption characteristics of coal with different water content.The changes of adsorption constant a and b values were compared and analyzed.The a value of coal increased after freeze-thaw damage,that is,the saturated adsorption amount increased,but the adsorption constant b value decreased,and the gas adsorption amount under the same gas pressure was low;After the gas is absorbed,the surface free energy of the coal body increases,and the methane adsorption per unit area increases,but the specific surface area of the coal decreases.Therefore,the gas adsorption amount decreases under the same gas pressure,and the water content is higher.The more obvious the change.The gas isothermal desorption experiment was carried out to obtain the gas isotherm desorption curve,and the influence of freeze-thaw damage on the gas desorption and desorption rate of coal was analyzed.After the treatment,the limit desorption amount of the coal sample decreases.During the period of desorption start,the gas desorption amount of the treated coal sample is larger than that before the treatment.As the desorption time increases,the desorption amount of the coal sample after treatment is gradually smaller than The pre-treatment coal sample,and the higher the water content of the coal body,the greater the desorption amount of gas under the same desorption time.The gas percolation experiment was carried out to analyze the influence of freeze-thaw damage on the gas permeability in coal.After the damage treatment,the permeability of the coal body is greatly increased.After the freeze-thaw damage treatment,the pore structure of the coal body is simplified,the gas migration channel is increased,and the gas is easier to flow.The mechanism of freezing and thawing damage of coal by liquid nitrogen is analyzed,and the process of nitrogen flooding in coal seam is analyzed.The effect of temperature and stress,frost heaving force and expansion force is the combined effect of nitrogen and flooding.As a result,the liquid nitrogen flooding gas is a comprehensive action process under the combined action of nitrogen gas enthalpy,gas partial pressure diffusion and displacement and desorption.The pore structure and storage and transportation characteristics of the coal after the freezing and thawing of liquid nitrogen cause the coal body to change.Gas is easier to be displaced.