Modification Of Coal Pore Structure With Nano Fracturing Fluid And Its Effect On Gas Adsorption And Desorption Characteristics

In order to study the gas desorption law of different destructive types of coal in the process of freezing coring,the adsorption experiments of liquid nitrogen at low temperature are carried out to explore the pore structure,and then the equivalent average temperature of coal core is calculated by establishing coal core heat transfer model based on COMSOL numerical simulation.Finally,the freezing core simulation experiments under different adsorption equilibrium pressure,different external heat temperatures and different destructive types of coal are carried out by relying on the test platform of freezing response characteristics of coal containing gas independently developed by the author team,and the similarities and differences of sucking back,gas desorption amount and gas desorption rate of different destructive types of coal are analyzed from the pore structure of coal and equivalent average temperature of coal core.The main research conclusions are as follows:（1）The adsorption and desorption curves of different destructive types of coal accord with type II of IUPAC by analyzing the pore structure with liquid nitrogen at low temperature method,indicating that pore width of coal exists in every stage,and the open pore is the main pore in the smaller pore width range.With the increase of the degree of the destructive types of coal,the phenomenon of adsorption loop becomes more obvious,and the maximum adsorption amount increases.（2）The pore structure of coal is tested with Micromeritics Company ASAP2020specific surface area and pore width distribution tester.The test results show that the pore volume of the three destructive types of coal increases sharply in the pore width within 0-10 nm,and reachs the peak value at 50 nm,and the minipore volume is the main contributor to the total pore volume.The minipore volume of II,III and IV destructive types of coal accounts for 36%,44.8%and 54.3%respectively.The pore volume and pore specific surface area of coal increase with the increase of the degree of the destructive types of coal,which is mainly reflected in the increase of micropore and minipore in coal.（3）Based on the characteristics of freezing coring,ignoring the axial temperature conduction of coal core,the thermal conduction model of coal core during freezing coring is established by using COMSOL,and the temperature field of coal core radial section is analyzed.With the characteristics that the enthalpy is not affected by the nonuniformity of temperature distribution of the body,the radial temperature of coal core at different time is numerically analyzed by using area division,obtaining the equivalent average temperature of coal core at different external thermal temperatures and different time in the process of freezing coring.The relationship between the equivalent average temperature and time is consistent with the equation:（?）=T₀exp（-αt）+h.（4）Aiming at the phenomenon of sucking back in the freezing coring process,the simulation experiments of gas-free coal freezing coring are carried out and the compression factor is used to correct the influence of sucking back on gas desorption data.By analyzing the sucking back curve,it is found that the limit sucking back volume will be restrained by the external heat temperature,but the influence on sucking back rate is not obvious,which is mainly because the increase of the external heat temperature will slow down the decrease of the coal core temperature.According to the ideal gas state equation,it can be seen that the decrease of pressure in the unit will be restrained if the cooling rate of the coal core becomes sluggish.And the relationship between the amount of sucking back and the degree of the destructive types of coal is positive,which is because the higher the degree of the destruction types of coal belongs to,the larger the pore volume and pore specific surface area it has,providing more"space"for the sucking back.Moreover,the superposition effect of the micropore wall on the potential energy promotes the interaction between the coal pore surface and the gas molecules,increasing the coal sucking back amount.（5）The curves of gas desorption under different gas pressure and different external heat temperature all conform to the law that increase rapidly in the early stage,then increase slowly,and finally tend to be stable.While the curves of desorption rate show a rapid decrease in the early stage,then decrease slowly,and finally tend to be stable.Gas pressure and external heat temperature are positively correlated with gas desorption amount and gas desorption rate.Meanwhile,the increase of gas pressure and external heat temperature will accelerate coal core gas desorption into the stable stage.（6）The laws of gas desorption curves of different destructive types of coal are similar,and the gas desorption amount and desorption rate of coal core are positively correlated with the degree of the destructive types of coal,which means that the higher the degree of the destructive types of coal belongs to,the larger the gas desorption amount and desorption rate will be.This is mainly due to the fact that the higher the degree of the destructive types of coal belongs to,the greater the pore volume of its macropore and mesopore will be,which provides a passage for gas migration and promotes gas desorption,and at the same time enables coal to store more gas which means greater desorption potential.It also explains that when the gas pressure and external heat temperature increase,the higher the degree of the destructive types of coal belongs to,the greater the increase of the gas desorption amount and gas desorption rate will be.（7）By observing the gas desorption curves of different destructive types of coal in freezing and non-refrigerating environments,it is found that freezing has a significant inhibitory effect on gas desorption of coals with different destructive types,and the higher the degree of destruction types of coal belongs to,the more significant the inhibitory effect will be.This is because the gas desorption curves of different destruction types of coal in non-refrigerating environment are similar,while the gas desorption curves in freezing environment show great difference due to the difference of the destructive types of coal,thus resulting in the difference of gas desorption inhibition rate.The dissertation includes 38figures,22tables and 86 references.