| Coal is the major energy in China. With mining depth deeper, the topic of coal and gas simultaneous extraction is significant. Cracks in coal are classified into different types of gas channel according to the gas flow condition. In this paper, the criterion of crack width were proposed to determine the type of gas channel. Fracture mechanics model was established and field monitoring, numericial simulation and lab experiment were employed to partition the abutment pressure area and study the evolution law of gas channel, permeability, porocity and gas velocity, pressure and concentration distribution at different distance ahead of working face, which provides support for coal and gas simultaneous extraction.1) Gas channelCoal is a material with lots of micro-pore and crack. Crack is the main channel of gas. The crack in which gas flow comply with the Darcy law is defined as micro-gas channel, in which gas flow is turbulent is defined as macro-gas channel and in which gas flow is between darcy flow and turbulence flow is defined as transition gas channel.Also, Crack width is calculated to determine the type of gas channel according to Renod equation. It shows that the crack width is not a constant, it is concerned with permeability, and original pressure of gas in the coal.2) Mechanical model of crack development and closureThe mechanical model of crack development and crack closure are established to study the evolution rules of crack in the abutment pressure area. The results show that, in the elastic zone of abutment pressure area, the original crack experiences the shear slide-self extension–bending extension-shear extension. At the peak point, the crack extends to macro-failre and in the limit equilibrium zone, the crack extension mode is reverse shear slide; The closure of crack is related to the original rock stress and the changing rate of stress. When the original rock stress is big, the original cracks are all closed and gradually open in the process of approaching the working face. In the limit equilibrium zone, all the cracks are open. When the original rock stress is small, the original cracks are all open and gradually closed near the peak point. In the limit equilibrium zone, all the cracks are open.3) Experimental studyIn this part, Macro and micro experiments are conducted to study the crack evolution rules in front of working face and the process of crack evolutiondamage-crack-failure is illustrated. Moreover, the second and original cracks are studied separately and the bending point in the complete stress-strain curve is found after which the transverse crack begins to occur.(1) According to the results of the micro-experiment:In the early elastic stage of the complete stress-strain curve, the natural damage has already varied significantly under optical microscope, though it is intact in vision; The formation of second crack depends on the development of the natural damage and he damage near the original crack is severe which explains the phenomenon that the second crack always occurs near the original crack; the crack connect with the damage nearby in its extending way and tends to branch towards the damage in the coal.(2) According to the results of the macro-experiment:(1) There is always a bending point before plastic point in the complete stress-strain curve after which transverse second crack begins to occur. The slope of bending point is slightly smaller than the plastic point and its strss is nearly 0.77 of the peak point.(2) In the complete stress-strain curve under uniaxial compression experiment, as the vertical stress increases, original crack experiences shear slip, shape ? and stable bending extension respectively before plastic point and after plastic point it switches to shear extension. The shear extension results in macro-failure at peak point; It branches towards the original damage in coal matrix in its extension way and extends towards the vertical direction in the end.(3) Second crack occurs due to the original damage evolution. Before the plastic point, the transverse second crack is rare and inclined one occurs isolatedly and extends slowly. After bending point, transverse second cracks begin to develop and after plastic point, large amount of second cracks in all directions occur and extend rapidly; Second crack tends to self-similar extension and prefers to connect with other cracks nearby; It always occurs near the original crack and always occurs with the slip of original crack.4) Partition of the abutment area and the field monitoringBased on theoretical and experimental research, the abutment area is divided into several partitions in which the crack develops in different mode; The partitions are: initial crack zone, shear slide and self-similar extension zone, bending extension zone, transverse crack occurrence zone, shear extension zone, macro-shear failure zone, and reverse shear slide zone. Combined with the formula of abutment pressure, the distance from working face of different zones are determined.Also, cracks in front of the working face are observed in the field, the results fit well with the theoretical and experimental research. It shows that in the process of approaching the working face, the amount of crack increases. The combination type of crack is from isolation to connection. Transverse second crack occurs near the working face and the amount of inclined crack is rare compared with the vertical crack. The permeability is the biggest near the structural crack. Moreover, the borehole gas extraction is employed and proves the gas channel evolution.5) Gas flow condition and permeability evolutionBased on the partition of the abutment area, the gas flow condition the permeability in different zones is studied. According to the results:(1) The gas flow condition can be determined by Renod criterion. Generally, in the elastic zone of the abutment area, the gas flow complys with the Darcy law and in the limit equilibrium zone, it complys with the Forhheimer law, that is non-linear flow.(2) Permeability decreases in the initial stage of compression in the complete stress-strain curve and increases after plastic point and reaches the biggest value after the peak stress. In the plastic flow stage of coal and rock, it is concerned about the surrounding stress; Permeability is significantly related to the effective stress, rock property and direction of crack and its variation has the scale effect. In the field scale, there is no decreasing stage of the initial compression and after the peak stress, it increases rapidly.6) Numerical simulationThe coupled model of gas and coal is established and the software of comsol is employed to study the evolution rules of permeability, porosity, gas velocity, concentration, and pressure in front of the working face. The results show that:(1) With the advance of working face, the coefficient of stress concentration increases, the area affected by the mining activity bigger, the plastic area ahead of the working face moves and the plastic strain increases.(2) In the area of 10-15 m ahead of working face, the permeability increases significantly and its distribution changes with time. Permeability in the area of 15m-25 m ahead of working decreases with time, but the permeability increasing area does not change with time.(3) After excavating, the gas flows towards the working face and the velocity of gas flow is bigger near the working face. The gas pressure is smaller and the gas concentration is bigger near the working face than far from the face. The gas pressure decreasing area and the gas concentration increasing area develops towards the deep coal body with time.(4) In the area of 10-15 m ahead of working face, the gas flow is nonlinear flow, complying with the Forhheimer equation. |
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