Study On Seepage Characteristics Of Coal Samples Under Water-Thermal-Mechanical Coupling

In recent years,with the rapid development of the national economy,the demand for coal resources is increasing rapidly.It is the general trend for coal resources to move from the shallow to the deep.Compared with the shallow,the deep coal mining is faced with the “five high two disturbances”(high geostress,high gas,high ambient temperature of working face,high rock burst tendency and high permeability pressure).The complex engineering geological environment under the action of strong mining stress disturbance and strong time sequence disturbance of roadway group excavation),and the mining disturbance activity is easy to cause the activation of coal and rock fault fissures.Under the water-thermal-mechanical coupling effect,the pore structure and permeability of coal and rock mass are further changed,and the probability of water permeability is increased,which brings great challenges to coal mine safety production.Therefore,the study of permeability characteristics of coal seam under complex stress environment has important engineering significance.Based on this,the seepage characteristics of coal samples under multi-field coupling were systematically studied by combining experimental and theoretical methods.Firstly,the seepage test of raw coal samples under multi-field coupling was carried out to explore the influence of stress,temperature and osmotic pressure on the permeability characteristics of coal samples,and reveal the permeability characteristics of coal and rock under the coupling of stress,temperature and osmotic pressure.Then,based on the basic theory of rock mechanics,the relationship between volumetric strain,thermal expansion coefficient,porosity and temperature is deduced,and the influence of porosity on temperature is further discussed,which provides a theoretical basis for coal mining.The main research contents are as follows :(1)The porosity of coal samples was tested by Ani MR-150 rock nuclear magnetic resonance imaging analysis system,and the T2 spectrum and pore size distribution map were drawn.(2)Using RYL-600 shear rheometer and MTS815 electro-hydraulic servo rock mechanics test system,uniaxial compression test and conventional triaxial compression test are carried out on saturated coal samples.The basic mechanical parameters such as compressive strength and deformation of coal samples under uniaxial and triaxial compression under different stress levels are obtained,which provides a theoretical basis for the design of multi-field coupling seepage test under triaxial compression.(3)The MTS815 electro-hydraulic servo rock mechanics test system was used to conduct the seepage test of coal samples under the multi-field coupling of water,heat and force by the steady-state method.The test data were analyzed,and it was found that the increase of temperature caused the volume expansion of coal samples,and the internal pore volume of coal samples was compressed and the seepage channel was reduced,which inhibited the occurrence of seepage process and led to the decrease of permeability.The higher the temperature,the greater the volume expansion of coal samples,the more obvious the inhibition of coal permeability.Under triaxial stress,the increase of water pressure does not significantly improve the permeability.The increase of confining pressure will make the volume compression of coal samples more dense and inhibit the generation of seepage.The greater the confining pressure,the smaller the permeability.(4)Approximate evolution analysis of coal sample porosity was carried out to establish the relationship between volumetric strain,thermal expansion coefficient,porosity and temperature.Through theoretical analysis,it is concluded that the porosity of coal samples decreases and the pore volume decreases with the increase of temperature,which inhibits the occurrence of seepage behavior and reduces the permeability.