| Porosity and fissure structures directly influence the gas absorption and desorption properties in a coal reservoir.The change law and damage mechanism of pores and fissures are important to improve the permeability of a hypotonic coal reservoir.In order to investigate the damage progress of the original pores and fissures,cyclic cold loading experiments on coal samples with various water-saturation levels by using liquid nitrogen(LN)as an ultralow temperature refrigerant to gain the initial temperature,confining pressures and various low temperatures were done.The relationships between the structural damage of coal samples and factors including water-saturation levels,temperature,confining pressures and cold loading cycles were studies.A laser confocal microscope,an ultrasonic detective analyzer and a CT tester were used to investigate the change of the surface fissure width,the sound wave speed,degree of porosity,the inner structural damage extension and porosity change law,and to analyze the structural damage and accumulative damage laws of coal samples under the effect of cold loading.The effect mechanism of swelling stress initiated from the ice wedge and temperature stress on the structural damage of coal samples was studied;the structural damage mechanical models and damage criterions were suggested under effects of water-saturation,temperature,confining pressure,and the coupled effect of those factors.Taking the fluctuating rate of sound wave speed as a parameter of the damage factor,verification was complemented between the theoretical extension of structural damage and corresponding experimental results.The main results are listed as follows.(1)When ice wedge swelling stresses of coal samples with various water saturation levels is generated,local structural damage occurs.While the damage accumulates cyclically,the dimensions of holes and fissures in coal samples increase from a micro-scale level to a macro-scale one until they fracture.(2)The soaking effect of LN makes the temperature of coal samples decrease,which generates temperature stress,and stress concentration occurs at the tip of holes and fissures.When the stress strength factor exceeds the fracture strength of a coal sample,the structure of the coal sample is damaged and new fissures are generated.(3)For coal samples with the equivalent LN cold loading cycles,the structural damage increases as the confining pressure increases;With equivalent confining pressure,the structural damage increases as the loading cycle number increases.(4)With the same low temperature environment,the surface fissure width of water-saturated coal samples accumulatively increases as the cold loading cycle number increases.With an equivalent cold loading cycle,the entire structural damage aggravates with the decrease of environment temperature value.(5)The effect of LN cold loading on the damage of the initial fissure of coal samples is significant.With one cycle of cold loading,damage occurs in micro scale,rather than macro scale;As cyclic cold loading goes on,the micro structure experience gradual penetration,and the damage accumulates to macro-scale level.Those results show that the usage of LN on a coal sample under cyclic cold loading can lead to the structural damage expansion and even bring about fracture breakage;Under the condition of low temperature,the swelling stress originated from the ice wedge and the temperature stress are the main factors for structural damage of coal samples;for cold loading,the minimum cycle numbers for the destabilizing fracture of coal sample under different water-saturation values,temperature values and confining pressures can be predicted via our functions.The achievement of this research can provide a theoretical basis for the fracturing modification technique by the LN injecting method on coal seam with low permeability,and can develop new approaches for the disaster prevention of rock burst,the control of gas outburst and gas extraction in the case of low permeability. |
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