Experimental Study On The Effect Of Freeze-thaw Action On Coal Damage And Gas Dispersion

Cryogenic Fracturing technique is an effective method for potentially modifying coal reservoirs and promoting coalbed methane production.Cryogenic Fracturing technology usually refers to the injection of liquid nitrogen liquid,carbon dioxide or other low-temperature medium in the coal seam to modify the coal reservoir,thereby enhancing the permeability of the coal seam.In1998 United States Kentucky,McDaniel B W and others used liquid nitrogen as a fracturing fluid on the four coalbed methane wells and a tight sandstone gas wells,obtained a good yield effect.At present,the research on the cryogenic fracturing of coal seam at home and abroad is focused on the ground stress,temperature field and fissure distribution of low temperature fluid injected into coalbed methane well.The freeze-thaw damage on coal induced by cryogenic fracturing is less studied,under the action of freeze-thaw damage,the study of coal damage,pore structure and permeability is not deep.The related research of coal gas dispersion characteristics have not been carried out.In view of the above problems,the paper discusses the influence of freeze-thaw damage on the pore structure,mechanical properties and gas dispersion ability of coal under the method of theoretical analysis and experiment.In this paper,the cyogenic fracturing technique is used as the research background,and the freeze-thaw damage is the research entry point.On the basis of the relevant information,the author analyzes the damage mechanism of coal under freeze-thaw action.At the same time limit the following a number of experimental conditions: water content,coal are saturated samples;freeze-thaw temperature,range-20?-20 ?.In this paper,the using of coking coal and lean coal are mainly investigated the effect of freezing and thawing times.A study on the macroscopic damage of coal caused by freeze-thaw was carried out.In this paper,the surface failure characteristics of coal after different freeze-thaw cycles are analyzed,and the compressive strength of the coal and the failure characteristics of the acoustic emission signal are measured by uniaxial compression and acoustic emission.The kinetic parameters such as Poisson 's ratio and Young' s modulus of coal rock under different freeze-thaw cycles are obtained by measuring the ultrasonic wave in the room.The coal porosity and permeability influced by freeze–thaw action was studied.In the experiment,nuclear magnetic resonance(NMR)and scanning electron microscopy(SEM)were used to study the porosity and permeability of coking coal and lean coal.The total porosity,producible porosity(PP),irreducible porosity(IP),permeability,and the proportion of adsorbent pores and seepage holes in coal are mainly tested.These tests discussed in depth the changes in the microstructure of coal by freeze-thaw damage.Finally,the influence of freeze–thaw action on the gas dispersion characteristics of coal was studied,and the variation of gas dispersion parameters(dispersion,dissipation velocity and dissipation coefficient)of coal before and after freeze-thaw action was analyzed.The results show that the freeze-thaw damage of coal can help to simplify the pore structure of coal,reduce the diffusion resistance,increase the seepage channel of coal and reduce the seepage resistance,which is beneficial to the migration of gas(coalbed methane)from the coal seam.