Preparation And Initiation Of Coal And Gas Outburst And Its Numerical Modelling Study

As the main energy resource in China, coal and gas outburst always is a main factorrestricting the safe and high-efficiency production of coal. As the technology development ofoutburst control and the increased demand of coal mines for security, higher requirement forcoal and gas outburst mechanism is presented. The qualitative understanding of outburstbased on comprehensive hypothesis is limited to guide the control technology pointedly andspecifically. The quantitive research of outburst mechanism should be enhanced.Using rock mechanics, mechanics of flow through porous media and adsorption theory,the governing equations of coalmass-gas evolution before outburst initiation were establishedbased on double porosity media model. Coupling terms such as effective stress,adsorption-induced deformation and permeability, showed the interaction between coal gasfield and coal/rock stress-strain field. With the energy for outburst initiation used forrepresenting the outburst tendency, the effect of coal gas, ground stress and mechanicalproperties of coal on energy for outburst initiation and the effect of distribution and evolutionof energy for outburst initiation on coal and gas outburst were analyzed. The main researchwork were listed in the following.(1) Micropore in coal is the dominant factor of adsorption property. When the poreaperture is small, with the magnitude of gas molecular diameter, the adsorption potential ofpore wall would overlap. The adsorption force increases in this case, and the adsorptionreaches saturation at low gas pressure, meaning low Langmuir pressure. Therefore, besidesthe maximum adsorption capacity of coal, the Langmuir pressure is affected by themicropores as well, especially the small ones. Due to the micropores increase as the coal rank,Langmuir pressure shows the declining trend as the coal rank increase.(2) After the study of permeability increase with the softening modulus after stress peakof coal mass, combing the permeability model of double porosity medium the permeabilityevolution equation in the process of excavation was established. The study of permeability infront of working face showed the zonal distribution characteristics that from the original zoneof coal seam to the coal wall the permeability changed though the decrease zone, increasezone and surge increase zone. This characteristics was closely related to the stress zonaldistribution characteristics. As the permeability of coal, the coal gas was zonally distributed aswell. The coal mass near the coal wall had high permeability and low gas content, as a safetybelt preventing the outbursts. And then the effect of different forms of coal gas ondeformation and failure of coal was studied. Besides the effect of pore pressure from the free gas, the change of adsorbed gas affected the stress state and failure of coal mass. The strengthenhancement after coal gas discharging could reduce the outburst risk.(3) From the perspective of permeability distribution, the formation of large-scaleoutburst was studied. Abnormal abundance of coal gas occurs around the location oflarge-scale outburst and an annular low-permeability zone in coal seam was necessary forabundant gas preservation. With a simplified one-dimensional flow model, the results shownthat permeability decrease and length of the low-permeability zone were the key factorsaffecting the high-pressure gas preservation. The high gas pressure and gradient increased theoutburst tendency when the excavation entered into the low-permeability zone. Thehigh-pressure gas preserved by the low-permeability zone could provide enough energy for alarge-scale outburst.(4) To analyze the tendency of coal and gas outburst, the conditions required by theunstability of coal-gas system were studied. Yield failure and energy accumulated in coal-gassystem over the storage capacity were required for an outburst, and the accumulation degreeof energy for outburst could show the outburst danger. Based on the energy condition foroutburst, the ratio between accumulated energy for outburst initiation and the energyconsumed in the process was utilized as outburst criterion. The corresponding calculationequations of accumulated energy and consumed energy were established.(5) Coal gas, ground stress and mechanical properties of coal were the main factors forenergy for outburst initiation. Coal gas was the main factor for energy accumulation. Thetectonic stress loaded on coal seam had an effect for energy accumulation as well, increasingthe outburst tendency. The effect of mechanical properties of coal was mainly due to thestrength and crushing work.(6) Through the outburst simulation experiment, it was verified that coal gas and stressload were the main source for energy for outburst initiation, and the energy for outburstinitiation had decisive effect on outburst. Besides, through different engergy conditions, therequied energy for outburst initiation with certain mechanical property of coal mass could beobtained based on the outburst results. This provides a method for analyzing the energystorage capacity of coal-gas system.(7) Because of the heteropic coal seam conditions in the exploitation process, thedistribution of energy for outburst initiation in the broken coal in front of the working faceevolved. When the excavation met the conditions such as low permeability, high gas, highstress and tectonic coal, the accumulated energy for outburst initiation would beyond thestorage capacity. The system energy became into unbalanced state, and released energy resulting in outburst. Hence, the distribution and evolution of energy for outburst initiationcontrolled the outburst initiation.