Study On Pore Structure And Gas Desorption Characteristics Of Frozen Coal

In China’s coal and gas outburst mines,particularly those in Guizhou Province,the permeability of coal seams is typically low.The inefficiency of coal mine gas drainage is now caused by the insufficient universality of technologies to increase the permeability of coal seams,such as CO₂fracturing and hydraulic fracturing.The safe and effective operation of China’s coal and gas outburst mines is constrained by this circumstance.Studying novel technology to improve coal seam permeability is urgently needed.Many researches have demonstrated that low-temperature freezing can alter the coal’s pore structure,which offers a fresh perspective on the problem of raising the coal seam’s permeability.This research focuses on a variety of engineering application backgrounds,including the precise measurement of gas content loss in coal seams during coring under the condition of a low-temperature frozen coal body,and the technology of increasing the permeability of a frozen coal body to improve the extraction effect.The evolution of pore damage and mechanical characteristics of frozen coal under low-temperature freezing are investigated experimentally in this work using low-field nuclear magnetic resonance（NMR）,industrial CT scanning technology,and a triaxial hydraulic servo device.The law of gas adsorption and desorption of frozen coal is then analyzed by simulation software using the experimental platform of gas adsorption and desorption of high and low-temperature coal,the coupling effect of"Three fields,"and the effect of freezing on coal core temperature.The following are the key findings:（1）The change rule of the coal sample after freezing was quantitatively analyzed using the absolute area change rate and relative area change rate of the T₂spectrum.Following low-temperature freezing,the fraction of the coal sample’s adsorption pores declines as the freezing temperature drops,while the proportion of the seepage pores steadily rises.The interior cracks of the coal body are encouraged to begin,spread,extend,and link by the low-temperature freezing.Throughout cooling,the pore structure is continually changing.At-5°C,the coal sample’s pore cracks become more intense,the number of seepage holes and total holes significantly increases,and the growth of the seepage holes generally remains stable.The expressions of stress and displacement of the coal body under the influence of fracture frost heaving force under the condition of bidirectional unequal pressure are derived using the above evolution law of fracture structure damage of coal body with freezing temperature,in conjunction with the frost heaving force model of water bearing fractures under freezing action and using the maximum circumferential stress compound fracture criterion of fracture mechanics.（2）The compressive strength,acoustic emission count,and energy properties of coal samples are all significantly impacted by the freezing temperature.The uniaxial compressive strength of samples of bituminous coal and anthracite recovered to normal temperature after freezing treatment（0°C,-5°C,-10°C,-15°C,and-20°C）decreases exponentially with the decrease of temperature,whereas acoustic emission counts and energy characteristics significantly increase with the increase of temperature.The triaxial compressive strength and elastic modulus of coal samples decrease with increasing temperature,while Poisson’s ratio rises,according to the deformation and failure characteristics of coal samples under triaxial compression at different temperatures（20°C,0°C,and-20°C）.（3）The gas cumulative desorption test of the coal body performed under the same freezing conditions reveals that,at low gas adsorption pressures（0.58 MPa,1.13 MPa,and1.62 MPa）,the gas escape rate of the frozen coal sample is negative,indicating that the cumulative desorption gas of the frozen coal sample is greater than the gas absorbed by the coal sample and that some free watts are converted into adsorbed gas under the low-temperature freezing condition.The gas escape rate of the frozen coal sample reaches0.40%and 31.57%,respectively,when the gas adsorption pressure increases to 2.03 MPa and2.63 MPa.（4）With the same gas adsorption pressure,the cumulative amount of desorption gas in coal samples rises as the freezing temperature falls,and low temperatures that fall within a specific range can effectively stop desorption and gas escape.The coal sample’s accumulated desorption gas exceeds the total gas retained when the freezing temperature falls to-5°C,and the gas escape rate is negative.As a result of the formation of frozen fractures,the gas escape rate increases by 36.00%and 27.72%,respectively,when the freezing temperature falls to-10°C and-15°C.The reason is the development of frozen fissures,frost-heaving cracks begin to form in the coal sample,which creates a pathway for gas to move through and diffuse throughout the coal body and easily escape during the exposure process.For coal samples,the essential temperature for fracture initiation and gas release because of frost heaving is-5°C.（5）The gas permeability,volume stress and gas pressure of coal body affect each other,and they are related to each other in an asymmetric paraboloid shape.The gas permeability of a frozen coal mass gradually decreases with an increase in stress under various gas pressure situations.The permeability reduces with increasing stress and tends to remain steady when the gas pressure changes to 1.5–2.0 MPa.Permeability and gas pressure have a nonlinear relationship that manifests a stage change and clear slip effect.There are two stages to the shift in the rate of gas permeability loss and the effective stress of the frozen coal body.The permeability loss rate is approximately 43%,which drastically changes when the effective stress is below the threshold value of 5.6 MPa.On the other hand,it loses permeability at a20%rate.（6）The experiment examining the"Radius-Temperature-Time"relationship of low-temperature frozen coal demonstrates that the temperature increases linearly with the increase of radius,and the temperature change is most noticeable close to the cold source wall.As the freezing time progressed,the overall temperature of the freezing medium showed a downward trend,and the freezing radius gradually increased.Thus,by fitting R=0.16571t^0.39691,the relationship between the freezing duration and the freezing radius was discovered,revealing a strong power function growth relationship between the two.（7）The distribution characteristics of the coal stress field,seepage field,and temperature field around the borehole under various temperatures,gas pressures,and burial depths are revealed by simulating the coupling effect of the gas seepage field,temperature field,and stress field in frozen coal.When the temperature drops,the free gas pressure drops,the supporting influence on the coal body’s pores diminishes,and the coal body’s deformation increases.In the process of extracting gas,the fall in gas pressure decreases with the decrease in temperature.When the borehole wall’s temperature is higher than the coal seam’s initial temperature,the heat is transported from the wall to the coal seam’s depth;otherwise,the heat is transmitted from the coal seam to the wall.The coal permeability curve intersects at specific temperatures.The permeability at the borehole wall is maximum when the coal seam temperature is 20°C,and it is smallest when the coal seam temperature is between-20°C and 0°C.With a rise in coal seam temperature,the gas pressure in the coal body surrounding the borehole gradually drops.After 20 days of extraction,the analysis reveals a positive correlation between coal seam temperature and the width of the gas drainage belt;additionally,under conditions of constant gas pressure and burial depth,the coal seam’s gas content increases as temperature decreases while the single hole gas drainage volume decreases.（8）The simulation study on the impact of freezing temperature on the coal core reveals that coal core temperature gradually decreases as freezing time increases,and the temperature of each measuring point in the coal core has a significant correlation with the radial distance,while the correlation effect with the axial direction is not obvious.When the temperature is always below freezing,the effect is better the faster the freezing speed.The entire freezing process has gone through a process of first speeding up and then progressively slowing down.The properties of gas desorption and the evolution law of the pore structure of frozen coal bodies are the major topics of this work.These results can provide a theoretical basis for precise assessments of the gas content in coal seams,for the freezing and permeability augmentation of outburst coal seams,and for the freezing method of coal crossheading coal uncovering.This research,which also has important practical ramifications,can effectively address the theory and technology of safe mining in coal and gas outburst coal seams.