| Coal seam infusion is widely used in coal mine safety production,however,there is a lack of quantitative research on the valid wetting range of injection holes under different water injection conditions,leading to a lack of scientific basis for the design of injection hole layout spacing and reasonable water injection volume.Based on this,in order to study the spatial and temporal distribution of coal moisture during the spontaneous imbibition of the coal-water coupling field and the influence of the internal pore structure characteristics on the spatial and temporal distribution of moisture,and to reveal the interaction mechanism between the pore structure characteristics of coal and moisture transport,the Dissertation selected Liulong coal as the experimental coals.Eight kinds of columnar remolded coal samples with different pore structures and one kind of nonstandard remolded coal sample with eccentric holes were prepared,which represented the low metamorphic strong destructive coal and high metamorphic weak destructive coal commonly encountered in coal seam water injection projects,respectively.Two types（low-metamorphic strong-destructive coal and high-metamorphic weak-destructive coal）were used.Research was carried out on the spontaneous imbibition pattern and the spatial and temporal distribution of moisture in degassed-coal,starting from the pore structure and the dynamic process of spontaneous imbibition in the coal.The pore structure characteristics of coal bodies were studied by mercury pressure,low temperature nitrogen adsorption,micron CT scanning,3D digital core and low field NMR techniques,respectively,to finely characterise the 3D structure of pores and fissures at different scales in coal bodies with different pore structures.The mass method,water-sensitive colour indicator method,slice drying method and low-field NMR technique were used to investigate the imbibition law and water spatial and temporal distribution characteristics of coal with different pore structures and different water supply conditions and the following research results were obtained.（1）Adopting hanging drop methods,the overall porosity of the eight coal samples decreases with increasing pressing pressure,and the porosity of Yong Hong coal is significantly higher than that of Liulong coal under the same pressing conditions due to its stronger skeleton effect;the contact angle of all eight coal samples measured by the hanging drop method is less than 90°,which belongs to the range of wettability,and the higher the pressing pressure,the smaller the contact angle is,and the contact angle of Yong Hong coal is smaller than that of Liulong coal,which has better wettability.In the nanometer pore size segment,the pore volume increases with the increase of pressing stress of remolded coal,but the pore morphology will not be changed.（2）The results of the mercury intrusion method and low-temperature nitrogen adsorption method show that there are a large number of semi-closed pores with poor connectivity in the tiny pore size section,accompanied by a certain amount of fine-necked bottle-type pores,and gradually change to open pores in the medium pore size section and above of Liulong coal.While the pore shape of Yonghong coal is consistent in all pore size sections,with mainly open pores and good connectivity overall;both coal samples have fractal characteristics,and the fractal dimension of Yonghong coal is larger than that of Liulong coal,in line with the rule that the fractal dimension of high-order coal is larger than that of medium-order coal.（3）Micro-CT test results show that the remolded coal composed of low-metamorphism and strong-destruction coal and high-metamorphism and weak-destruction coal decreases with the total porosity,and the equivalent radius,pore volume,specific surface area,average pore volume and pore connectivity of micro-pores gradually decrease,while the fractal dimension slightly increases,but the change range is obviously different,and the change of relative pore number shows an opposite trend.With the decrease of porosity,the maximum and average values of throat length and throat radius of the two types of coal show an obvious exponential decreasing trend.（4）Secondary destruction of coal particles occurs in both remodelled coal reconstructions,and differences in the degree of secondary destruction of the two types of coal and in the evolutionary pattern of the aforementioned pore structure characteristics are caused by differences in the degree of destruction of the original coal sample,the solidity coefficient and other factors.（5）According to the different spatial water saturation of the coal sample,the coal body of the moisture distribution of the coal sample during spontaneous seepage can be divided into four zones:filling zone,near-filling zone,peak surface zone and drying zone,and the moisture distribution curve formed by the four zones shows an inverse"S"shape and conforms to the Boltzmann function law.On the time scale,the range of the four water saturation zones can change dynamically with the growth of time.The moisture distribution gradually changes from an inverse’S’-shaped curve to an approximately linear distribution when the sample is completely wetted by moisture.（6）The experimental results of the cumulative seepage volume,seepage height and moisture distribution during the spontaneous seepage of the coal body show that the relationship between the cumulative volume and height of seepage per unit area and the seepage time are in accordance with the power function;in the early stage of seepage,the exponent of the power function is approximately equal to 0.5,i.e.the square root relationship,and in the middle and late stage of seepage,the exponent gradually decreases as the seepage inertia force decreases and the gravitational and viscous forces increase.The porosity,fractal dimension,tortuosity and pore connectivity of the coal during the seepage process all have an effect on the cumulative seepage volume and the height of seepage.The square root relationship of the seepage process is the same as the classical capillary seepage model,but the spontaneous seepage of water in the coal body has a hysteresis effect,and the classical capillary force theory is less applicable to the seepage process of water in the coal body.（7）NMR T2relaxation spectroscopy,imaging and one-dimensional frequency coding techniques were used to test the quantitative water supply seepage characteristics of large size remodeled coal samples with eccentric pores.The test results showed that:the quantitative water supply seepage process was divided into two stages,wetting and diffusion,and the adsorption of water by the coal body was carried out throughout the seepage process;under the same water content,the larger the wetting range,the greater the adsorption of water by the coal body;the adsorption capacity of the coal body for water was not only related to the physical parameters of the coal itself,but also closely related to the spatial distribution of water.（8）A mathematical model without considering gravity and gravity factors is constructed.Without considering the gravity factor,the mathematical model has the advantages of simple form,less parameter demand and convenient use.It is suitable for small-size coal samples or short-time imbibition conditions.The index（n）of the diffusion coefficient equation in the model is 5.7;The spontaneous imbibition mathematical model considering gravity based on the pore structure parameters in coal has good reliability and applicability,and is suitable for large-scale coal or long-term imbibition conditions;Using the spontaneous imbibition mathematical model considering gravity factor,the law of water distribution in the process of saturated imbibition and unsaturated imbibition of large-scale coal model is analyzed,and the mechanism of water transport in the process of spontaneous imbibition of coal is revealed. |
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