Study On Influence Factors And Mechanism Of Coal Cracking Under Freeze-thaw Cycle

Coal seams in China are generally characterized by high metamorphic degree,low permeability and low reservoir pressure,which leads to poor gas drainage effect and prone to gas disaster accidents.Therefore,new technology of coal seam permeability enhancement with high safety and efficiency is urgently needed.Low temperature freezing is a new technology of coal seam permeability enhancement,and the effect of coal seam permeability enhancement is obvious.Scholars at home and abroad have carried out experimental and theoretical research on low-temperature frozen sandstone and shale,and studied the influence of freezing time and water saturation on the fracture characteristics of rock samples,but the research on coal cracking is relatively small,many aspects need to be further studied.In this paper,the nuclear magnetic resonance technology is used for coal samples with different metamorphic degrees to study the fracture propagation characteristics and influencing factors of coal samples under the conditions of different cold sources and different cycles of freeze-thaw cycles.The research results can provide theoretical basis and technical support for the new technology of low-permeability coal seam,such as low-temperature freeze-induced fracture,enhanced extraction,and low-temperature frozen crosscut quick uncovering.On the basis of previous studies,this paper obtains the following research results through research:(1)the effect of freezing and thawing on the pore structure of coal is obvious.The pore structure of anthracite and coking coal before and after freeze-thaw was studied by NMR.Through the comparative analysis of T₂ map area changes of coal samples before and after freeze-thaw,it is considered that the freeze-thaw cycle can promote the development of internal pore structure of coal samples,and the degree of freeze-thaw damage of coking coal is greater than that of Anthracite;with the increase of freeze-thaw times,the T₂ map area of coal samples increases,and the increase of map area decreases gradually;during the three freeze-thaw cycles,the freeze-thaw of freezer changes the pore structure of coal samples The result is better than that of liquid nitrogen freezing and thawing.The effect of liquid nitrogen freezing and thawing on the pore structure of coal is greater than that of freezing and thawing in freezer.(2)the experiment of gas emission of coal sample proves that the freezing and thawing cycle can improve the pore structure of coal.Through the measurement of the parameters of gas emission of anthracite and coking coal before and after freezing and thawing,the results show that the freezing and thawing damage can improve the pore structure of coal body,effectively increase the permeability of coal body,reduce the resistance of gas diffusion,contribute to the desorption of gas,increase the limit gas desorption of coal body,increase the desorption speed and gas effective diffusion system The number increases.(3)the cracking mechanism of coal sample under freeze-thaw cycle was discussed.It is considered that the fracture of coal sample is caused by thermal stress produced by coal matrix and frost heaving force produced by water ice transformation.The thermal stress intensity factor of crack tip produced by liquid nitrogen freezing and thawing is greater than that of cold box,and the thermal stress intensity factor of anthracite is greater than that of coking coal;the frost heaving force of coal sample produced by cold box freezing and thawing cycle is greater than that of liquid nitrogen freezing and thawing cycle,and the frost heaving force of coking coal is greater than that of anthracite,which is consistent with the experimental results.Based on the above study,it is established that the cold source is the main factor affecting the cracking of freeze-thaw cycle coal body,the coal sample metamorphism degree is the second,and the freeze-thaw times are the least.