Study On The Fractal And Seepage Characteristics Of A Coal Sample Meso-structure Of Coal Seam Water Infusion

Coal seam water infusion is the comprehensive prevention and control of mine disasters at present widely used in the world.One of the theoretical foundations of this technology is the seepage characteristics of pressurized water in coal,and coal as a typical porous medium with a double fracture-porosity structure inside.Therefore,the seepage of water in the coal body is closely connected with the distribution of the intermal structure of the coal body.In order to establish the relationship between seepage characteristics and pore structure of coal,this paper tests the characteristics of pore structure and seepage characteristics of coal under different confining pressures and pore water pressure during the water infusion process of the coal seam;At the same time,the distribution of pores in coal samples before and after water infusion was tested by the mercury injection test.By analyzing the results of NMR experiments,it is found that the intermal fracture-pore structure of water-infusion coal samples is staggered and the pore size distribution is significantly different,and the proportion of the volume occupied by transitional holes is the largest,followed by macrospores,and the proportion of micropores and mesopores 1s the smallest;In the process of water infusion,the pore expansion of high-pressure water can communicate with the pore network,and the increase of confining pressure will increase the skeleton stress of the coal sample,compress the coal sample and reduce the pore volume.The water infusion process will eventually form pore water storage-fissure diversion phenomenon.Mercury injection experiment results show that in the coal seam water infusion process,the water seepage process will cause the coal sample skeleton to compressively deform,thus changing the intermal pore distribution of the coal sample,but it will not cause brittle damage to the coal sample skeleton.When the coal seam water infusion is finished,the deformation produced inside the coal sample will be restored to its original shape under the effect of elastic energy.Based on the NMR experiments and the mercury injection experimental data,the fractal characteristics of coal pore structure are analyzed in combination with the fractal geometry theory.The results show that the fractal dimension calculated from the results of NMR experiments can well represent the pore structure in the full-scale range,while based on the fractal dimension of high-pressure mercury injection experiment results,there is an excellent fractal feature in the seepage pore section,but in the adsorption pores,segment fractal characteristics are not obvious,so the mercury intrusion method can only better characterize the pore structure of the seepage pore section.Therefore,it is preferable that the internal pore structure parameters and related fractal dimension of the coal sample obtained through NMR experiment and theoretical calculations be substituted into the fractal capillary beam seepage model to obtain the theoretically measured seepage,and the results are compared with the experimental results.The theoretical values and measured values of the rate are consistent with the variation law of confining pressure and pore water pressure,that is,the water pressure increases,the seepage increases,the confining pressure increases,and the seepage decreases.However,because the large-diameter seepage channel of the measured coal is blocked by the filling material,the theoretical calculation result is higher than the liquid measurement value.Therefore,improving the connectivity of the large-pore percolation channel through the coal seam water infusion will significantly improve the water infusion effect of the coal seam.The results demonstrate that the theory pore area fractal dimension and the measured pore volume fractal dimension have some changes.At the same time,because of the different meanings of quantitative characterization of the pore structure,the change trend is opposite.