| Mine gas is not only a valuable resource,but also a serious threat to coal mine safety production.Under the background of the improvement of coal mine machinery and automation,the mining depth of coal mine is deepened,the mining intensity is strengthened,so that the emission of gas is increasing.Fully mechanized mining with large mining height is characterized by large mining height and large development range of overburden fractures,which greatly enhances the pressure relief gas migration and storage capacity of goaf,and also increases the gas prevention and control pressure of working face.Therefore,a three-dimensional large-scale physical similarity simulation experimental system for gas transportation and storage is developed in this paper.Starting with the movement and failure law of coal seam mining overburden and the characteristics of mining overburden fractures,the development and distribution characteristics of mining overburden fractures are studied by using the methods of theoretical analysis,experimental research,numerical simulation and field practice,and the mathematical model of overburden fracture gas transportation and storage channel is established;On this basis,the law of pressure relief gas transportation and storage under the condition of large mining height fully mechanized mining is studied,and the mechanism of pressure relief gas transportation and storage in large mining height fully mechanized mining is revealed.The main contents and achievements of this paper are as follows:(1)The similarity criterion of gas transportation and storage is established based on elasticity and fluid mechanics.Based on the similarity criterion,a new physical similarity simulation laboratory material with paraffin and cement as cement is successfully developed.The material is highly consistent with the prototype in elasticity and seepage mechanics,meets the requirements of gas-solid two-phase physical similarity materials,and plays a good role in promoting the study of gas transportation and storage law by means of physical similarity simulation.(2)Using the method of system construction,a three-dimensional large-scale physical similarity simulation experimental system for gas transportation and storage is independently designed and developed.The system consists of seven sub units:box and base,automatic hydraulic mining,flexible loading,automatic ventilation,gas emission and gas drainage,and comprehensive data acquisition and control.According to the similarity ratio of 1:100,the maximum mining depth of 2105m,mining height of 0～12m and advance distance of 200m can be simulated;The ventilation unit can simulate various ventilation modes such as U-type,U+L-type and Y-type,and realize ventilation with different air volume;The extraction unit can simulate many kinds of three-dimensional extraction methods,such as high-level roadway,high-level drilling,ground extraction and so on;The gas emission unit adopts the independent injection mode to realize the gas emission of different gas emission quantities and different positions.The experimental system can simulate the functions of coal seam mining,mine ventilation,gas emission and gas drainage,and realize the integrated and synchronous research on scientific problems such as overburden fracture evolution,mine pressure distribution,pressure relief gas migration and gas drainage in the process of coal seam mining,so as to provide experimental platform support for this research.(3)Based on the geological conditions of a typical fully mechanized mining face with large mining height in Shanxi,the dynamic distribution characteristics of microseismic events and the variation characteristics of microseismic parameters in overburden space during mining are obtained through large-scale three-dimensional physical similarity simulation experiments.Using the method of model cutting,the distribution of three zones of overburden fractures in fully mechanized mining with large mining height is obtained,the relationship between overburden fracture and subsidence,as well as the characteristics of subsidence,fracture density and fracture penetration of each overburden are studied,and the distribution characteristics of dominant channels for gas transportation and storage are obtained.(4)By means of three-dimensional large-scale physical similarity simulation experiment,the law of gas transportation and storage in three-dimensional fracture network space of fully mechanized goaf with large mining height under U-shaped ventilation is obtained.In the three-dimensional space,the gas concentration distribution along the working face from the inlet side to the return side shows an increasing trend;The gas concentration increases gradually in the depth direction of the goaf until the gas concentration in the deep goaf is stable and maintained at a high level.Due to the influence of the porosity change of the mining overburden in the goaf and the characteristics of gas rise,float and diffusion,there are obvious stratification characteristics in the height direction of the goaf.The gas concentration at the top of the fracture zone is 66-68%.The gas rises,floats,diffuses and seeps in the mining fracture,and finally enriches at the top of the fracture zone.(5)Using the methods of physical similarity simulation and numerical simulation,the distribution and migration law of gas concentration in mining overburden fractures under the influence of different air volume levels and different ventilation methods are studied,and the influence degree of gas by various factors in the direction of working face,goaf depth and height is obtained.(6)According to the fracture time sequence of overburden mining fracture evolution process and goaf filling characteristics of large mining height fully mechanized coal seam,the migration characteristics of pressure relief gas in mining fractures in active area,transition area and compaction area are given;The formation of gas transportation and storage channel and the distribution characteristics of void ratio in goaf are analyzed,and the distribution equation of void ratio in goaf is established;The comprehensive control model of pressure relief gas seepage,rising and floating diffusion in the mining fracture ellipsoidal zone is established,and the dynamic evolution relationship between the pressure relief gas migration and the longitudinal fracture fracture and transverse separation fracture of overburden is obtained.According to this model,the gas transportation and storage mechanism in the mining fracture ellipsoidal zone can be clarified,which provides a certain theoretical basis for pressure relief gas drainage technology.The mechanism of pressure relief gas transportation and storage in overburden of large mining height fully mechanized mining revealed in the study has been well verified and applied in the gas disaster control of a typical high gas mine face in Shanxi.After the implementation of the pressure relief gas drainage measures of strike high roadway and long borehole,the better pressure relief gas drainage effect is achieved,the safe mining of the working face is guaranteed,and the scientific basis for mine gas control under similar geological conditions is provided. |
