Study On Permeability Characteristics Of Coal Rock Under Cold Loading

Gas is not only a by-product of coal mining but also an important clean energy source with high utilization value and broad application prospects.Based on effective prevention and control of gas outbursts,efficient gas extraction has become a research focus in energy production.In terms of coal and gas outburst prevention and control,scholars have proposed low-temperature freezing technology,which can not only increase the permeability of coal seams but also improve the efficiency of gas extraction.To effectively apply this technology to gas mining,cold loading is used to cause damage to the pores and fracture structures of coal rock,thereby achieving the goal of increasing permeability and production.The paper is based on the research background of cold loading induced cracking and permeability increasing coal rock technology and adopts a combination of experimental research and theoretical analysis research methods to study the pore and fracture structure damage and failure rules,physical and mechanical properties,and permeability characteristics of coal rock samples with different cold loading cycles,different cold loading temperatures,and different combinations of coal rock height ratios.A wave velocity permeability model suitable for coal rock has been established,and from the perspectives of variable temperature stress damage and cold loading,the mechanism of coal rock permeability under cold loading was summarized,revealing the permeability characteristics of coal rock under cold loading.The main research results of the paper are as follows:(1)As the cold loading cycle increases,the width and rate of crack propagation on the coal rock surface gradually increase;With the increase of the cold loading period,new cracks with larger lengths and widths are formed after the original cracks of coal rock are destroyed.The expansion of cracks seriously hinders the propagation of ultrasonic waves,leading to the attenuation of longitudinal wave velocity,an increase in wave velocity attenuation rate,and pore volume;In the process of the cold loading cycle,the micro and macro damage of coal rock is a cumulative process,and a large number of pores and cracks gather together to form a fracture network,which eventually causes the macro damage of coal rock.(2)The permeability of coal rock increases with the increase of the cold loading period;Under the same cold loading cycle,the permeability and permeability growth rate of coal rock increase with the continuous decrease of cold loading temperature;As the cold loading cycle increases,the compressive strength and elastic modulus of coal rock gradually decrease.The decrease in mechanical strength of coal rock leads to an extension of the compaction and yield stages during uniaxial compression,and a shortening of the elastic deformation stage.(3)Based on the Hudson fracture medium model,a wave velocity pore volume model for coal rock is established with pore volume as the intermediate variable.Combined with the generalized Kozeny Carman equation,a wave velocity permeability model suitable for coal rock is established;By substituting the experimental results into the theoretical model for verification,it was found that the theoretical model can better reflect the relationship between coal rock wave velocity and permeability.(4)The variable temperature stress damage and cold loading cycling are the key factors causing the damage and failure of coal rock under cold loading;The damage degree of coal rock can be judged by establishing the damage and failure discriminant and fracture damage criterion of coal rock soaked in liquid nitrogen;The permeability of coal rock is influenced by axial load,and the trend of the permeability strain curve of coal rock under cold load is generally consistent;Before entering the yield failure stage,the permeability of coal rock slowly decreases with the increase of axial load.