| Coal seam gas extraction can prevent mine gas disaster and reduce greenhouse gas emission.However,the low coal permeability affects the extraction efficiency.Hydraulic slotting can effectively improve the permeability of gas.Meanwhile,it also draws extra water into the coal seam.As the coal seam contains water originally,there is little research on how water existing in coal seam affects flow characteristics of gas in hydraulic slotted coal.In this research,four aspects are examined to explore the interaction mechanism of water-gas and the seepage characteristics in hydraulic slotted coal through physical experiment,theoretical model building and data analyzing,which is of great significance for understanding the flow rule of gas in hydraulic slotted coal and improving the gas extraction efficiency.The four aspects are:microcosmic mechanism of gas adsorption in water-bearing coal,mechanical characteristics and damage mechanism of water-bearing coal after hydraulic slotting,the permeability of gas in water-bearing coal,the fluid-solid coupling properties of water-bearing coal after hydraulic slotting.The main research findings are:(1)Three kinds of coal samples collected from the work sites are classified into meta-bituminous coal,ortho-bituminous coal and para-bituminous coal.By electron microscope scanning experiment,the surface structure features of the three kinds of coal samples are measured.It turns out that:cracks develop in low rank bituminous coal,and the pore connectivity is good.The development degree of cracks in bituminous coal dust with medium metamorphic grade is low,while the microspores develop.Most are small pores.The surface of bituminous coal with high metamorphism degree is smooth and densified.The development degree of cracks is low,while the microspores develop.The analysis of the surface functional groups of coal shows that the content of-OH group decreases and hydroscopicity decreases with the increase of coal ranks,the oxygenic groups decrease and hydrophilic decreases with the increase of coal ranks.(2)Coal adsorption of water mainly includes physical absorption and hydrogen-bond interaction on mineral surface.Absorbed water mainly exists in mesoporous and large microporous.The coal adsorption capacity of gas is mainly affected by the surface area of porous coal.The bigger surface area is,the better adsorption capacity is.When moisture content increases to a certain value,extra water exists in the dissociation form in the space between coal particles and macrospore.As the extra water has little effect on the methane adsorption of pore surface,there is critical moisture content.(3)With the increase of moisture content,there are more cracks on the coal sample,the locations of the principal cracks are more dispersive and there are more secondary cracks and remote cracks.When the moisture content is low,the main reason for the complete destruction of the coal sample is that the principal cracks run through the coal sample.When the moisture content is high,coal samples lose their beating capacity because of the generation,extension and cut-through of the secondary cracks.The change of moisture content also affects the initiation location of initial cracks.(4)When gas pressure is constant,the negative exponent relation appears between the permeability and the surrounding rock stress.The higher water content percent of the coal sample is,the more significant effect the surrounding rock stress has on the permeability.There is negative exponential function between the permeability and the moisture content.When the surrounding rock stress is constant,with the rise of gas pressure,it needs less time for gas in the coal sample to reach steady seepage and the more unevenly gas distributes in the sample gas,When the gas pressure is constant,with the rise of the surrounding rock stress,there are different change features of gas under the influence of pore and fracture aperture change.(5)Fluid-solid coupling properties of water-bearing coal after hydraulic slotting under the true stress environment by physical experiment simulations.In the experiment,the process can be divided into five stages:vacuum pumping,methane injection,water injection,loading and methane desorption.Three parameters brought up to describe the coal dynamic response feature.ε_v andε_m describes the deformation and deflection at ultimate load of the loaded coal sample separately.KB describes the diffusion coefficient of gas.The influence rule on the three parameters for the gas pressure and moisture content was analyzed.(6)Based on the anisotropy of coal structure,cross-coupled equation considering coal mass deformation,matrix methane diffusion and gas flow characteristics of cracked is built.By analyzing the data,the rule of how initial water saturation,diffusion attenuation coefficient,initial gas pressure and initial permeability affect gas extraction is concluded.The numerical simulation based on the field trial data of drill site 3#and4#below 20912 cutting hole of Lin Hua Coal Mine shows that the gas production rate of the normal hole is lower than the holes in which the coal gas has been slotted in the same extraction time.It means that hydraulic slotting improves the extraction efficiency.In addition,the gas production rate in worksite coincides with the simulated results of the model.It proves the rationality and engineering applicability of the model.The findings in this research provides solid theoretical and practical foundation on the project design,parameter setting,process optimization of permeability enhancement under hydraulic slotting,investigation of gas extraction effect and the revelation of hydraulic slotting permeability enhancing. |
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