Characteristics Of Gas Permeability And Mechanism Causing Disaster In Soft Roof-Floor Seam

The soft roof-floor seam had been suffered from tectonic movements and dominated by sliding structures in many stages in western Henan Province.The coal seam structure was seriously damaged.The coal seams were low in strength,gas content was high,gas pressure was high,and gas disasters were prone to occur.This particularity of soft roof-floor seam structure had become a crucial factor affecting the gas migration of coal seams and even the occurrence of coal and gas outburst.In order to study the gas permeability characteristics and the disaster-causing mechanism of soft roof-floor seam,the pore structure characteristics,raw coal and three different particle sizes of different types of raw coal in soft roof-floor seam were been investigated by on-site investigation,laboratory tests and numerical simulation.And than the isothermal ad-/desorption characteristics of coal samples with different sizes?2?6mm,0.18?0.25mm and<0.11mm?under different temperature conditions?25°C,35°C and45°C?,physical mechanics and seepage characteristics of four kinds of briquettes with different particle sizes,stress,porosity and plastic distribution characteristics of coal seams with different conditions of the soft roof-floor seam,and the occurrence mechanism of gas disasters in soft roof-floor seam had been studied.?1?Based on the scanning electron microscopy results,the spatial geometric models of the pore structure of the single-grain coal sample,the contact form between the coal particles,the single-grain coal pile,and the mixed-size coal particle pile had been separately constructed.The contact between coal particles was usually three kinds of situations:point contact,line contact and surface contact.In the case of natural accumulation,the gaps between coal particles of various particle sizes were filled with smaller particles,and then a more complex void structure had been formed.Then the geometric space for gas storage and movement of loose coal bodies had been constituted by the pores and fissures of the coal particles themselves.?2?The results of pore structure measurement by mercury injection method and low temperature liquid nitrogen adsorption method showed that the four types of coal had the largest contribution to the specific surface area of micropores.Most of the pores were open pores,which was conducive to the gas desorption.?3?Pressure,temperature and particle size have a significant effect on the ability of coal samples to ad-/desorption CH₄.When the pressure was less than 2MPa,the ad-/desorption rate of CH₄was higher,and then the adsorption rate decreases until the saturation was reached.With the increase of temperature,the maximum Langmuir volume of CH₄had a significant downward trend.The adsorption amount of raw coal to CH₄was smaller than that of coal samples with different particle sizes.As a whole,the smaller the particle size was,the shorter the CH₄diffusion distance was.And the more the gas molecules entered the interior of the coal,the more ad-/desorption volume was under the same conditions.And the amount and the ad-/desorption rate were larger.At the same gas content,the critical desorption pressure decreased with the decreasing particle size.Therefore,for the coal seams with the smaller particle size,the treatment standard should be raised the standard value of gas pressure and gas content should be lowered to a lower level.?4?The uniaxial compressive strength,elastic modulus and deformation modulus of briquette increased with the decrease of particle size,and the cohesion,internal friction angle and ultimate strength increased with the decrease of particle size.The"loose particles"generated in the voids between the coal particles were the key factors affecting the plastic characteristics.The mechanical properties mainly depended on the meshing force between the particles.The smaller the particles,the greater the meshing force.?5?Under the conditions of loading and unloading confining pressure and axial pressure,the permeability of different particle size coals tended to decrease with the decrease of particle size,2?6 mm>raw coal>0.18?0.25 mm>less than 0.11 mm.The maximum loss rate of permeability had a good correlation with the cohesion.The greater the cohesive force of the briquette,the greater the loss rate of permeability.The loss rate of permeability increased with the decrease of the particle size,and the permeability loss rate during loading and unloading under axial pressure was higher than the permeability loss rate under loading and unloading confining pressure.?6?The process of gestation,stimulation,development and termination of soft roof-floor seam gas dynamic disaster had been analyzed.It proposed that the“fluid-like zone”was the area that causes the gas dynamic disaster.The zone had been formed in the geological tectonic movement,controlled by the surrounding coal and rock mass,and always maintained a dynamic equilibrium state without mining activities.Based on the numerical simulation results,the minimum distance for detecting the location of the"fluid-like zone"had been given.