我国大部分矿区煤层透气性较差，煤层瓦斯常规钻孔的预抽效率较低。因此，如何提高煤层渗透率已成为低透煤层瓦斯防治及提高抽采效率的关键。液氮致裂煤体已成为一种潜在的高效无水压裂增透技术，应用前景广泛。研究表明，煤体液氮致裂增透主要是在温度应力与冻胀力等作用下，能量耗散导致孔隙损伤、裂隙演化，从而提高煤层渗透率。但煤体冻融全过程微细观结构动态演变及能量耗散难以实时监测，本文通过定制波导杆传输信号，利用声发射无损检测技术，分析液氮冻融煤体全过程能量变化特征。

论文选择焦作赵固一矿二₁煤层煤样，通过自主研发的液氮冻融煤体全过程声发射监测实验系统，得到不同含水率煤体冻融过程能量变化时域特征。冻结过程能量变化呈现三个时期：陡增期、波动期、平静期；融化过程能量变化呈现两个时期：增长期、平静期。煤体冻融全过程能量耗散随含水率增加呈线性增大，含水率5.96%煤体能量为8032847 mV·μs相比干燥煤体的2445596 mV·μs提高了228.46%。

通过计算关联维数分析不同含水煤体冻融过能量变化规律，得到随着煤体含水率增加，冻结过程的陡增阶段关联维数呈正指数型增加，缓慢增长、稳定阶段关联维数呈线性增加关系，融化过程的增长、稳定阶段关联维数皆呈正指数型增加。通过不同裂隙产生能量波差异对各含水煤体液氮冻融过程裂隙类型判别，得到冻结过程以拉伸裂隙为主，煤体干燥时拉伸裂隙占比为75.85%，随含水率增加而减小。融化过程以剪切裂隙产生为主，并随含水率增加而增大。通过对煤体冻融全过程产生裂隙的三维定位，得到定位事件随冻融时间在煤体内部空间变化规律，事件数随着煤体含水率增大而增加。

利用高频冲击仪、PCI-II声发射系统及XTDIC系统对煤体冻融前后动态压缩过程中的力学性质、不可逆耗散能和表面应变进行采集，得到液氮冻融使得煤体抗压强度、弹性模量分别下降5.72%、13.26%，并随含水率增加煤体力学损伤加剧。测定了煤体压缩破坏过程中能量变化规律，随时间变化将振铃计数分为发育期、增长期和衰弱期3个阶段。冻融后含水率最大煤体动态压缩破环过程的累计振铃计数是6458973仅为干燥煤体19674171的32.83%，耗散能量明显减小。随煤体含水率增加，压缩过程表面裂纹发育更明显，裂纹数量和应变集中带增加，破坏模式属于拉剪复合破环，破坏形状呈“X”状，最终破环完整性增强，煤体能量释放强度降低。

采用能量震级b值变化描述单轴压缩全过程不可逆耗散能动态演化规律，随着煤体含水率的增加，累计能量耗散呈负线性减小，b值呈正线性增加。得到了压缩过程中裂隙占比变化规律，拉伸裂隙占比呈负指数减小，剪切裂隙占比呈正指数增大。综合分析能量参数，得到了冻-融-压缩过程损伤量与煤体含水率的关系，随着含水率增大，冻融过程的损伤量D_t变大，含水率5.96%时损伤量为0.3644是干燥煤体0.0630的5.78倍，同时压缩过程损伤量逐渐减小并且三维定位事件数量呈负线性减小。反演了冻-融-压缩过程煤体弹性模量变化模型，冻融过程中干燥煤体弹性模量减小0.1205 GPa，含水率5.96%煤体弹性模量减小量为干燥煤体的5.79倍。即液氮冻融煤体过程对其物理性质改变越大能量耗散增加，单轴压缩破坏其结构所需能量越小，研究结果有利于揭示能量耗散诱导冻融煤体孔裂隙改造效应，以期对液氮致裂煤层增透技术的适用性进一步理解，指导液氮致裂工艺参数优化和现场应用。

The permeability of coal seams in most mining areas in China is low, which makes the effect of pre-drainage of coal seam gas by conventional drilling unsatisfactory. Therefore, how to enhance the permeability of coal seams has become the key to the prevention and control of gas in low permeability coal seams and the improvement of extraction efficiency. Liquid nitrogen fracturing coal has become a potential high-efficiency water-free fracturing anti-reflection technology with wide application prospects. The research shows that the liquid nitrogen fracturing of coal is mainly under the action of temperature stress and frost heaving force. The energy dissipation leads to pore damage and fracture evolution, thus improving the permeability of coal seam. However, it is difficult to monitor the dynamic evolution of microstructure and energy dissipation in the whole process of coal freezing and thawing in real time. In this paper, by customizing the waveguide rod to transmit signals, the acoustic emission nondestructive testing technology is explored to analyze the energy change characteristics of the whole process of liquid nitrogen freeze-thaw coal.

In this paper, the coal sample of II₁ coal seam in Jiaozuo Zhaogu 1 Coal Mine is selected. Through the self-developed acoustic emission monitoring experimental system for the whole process of liquid nitrogen freezing and thawing coal, the time domain characteristics of energy change in the freezing and thawing process of coal with different water content were obtained. The energy change of freezing process presents three periods: steep increase period, fluctuation period and calm period. The energy change in the melting process presents two periods: growth period and calm period. The energy dissipation of the whole process of freezing and thawing of coal increases linearly with the increase of water content. The moisture content of 5.96% coal is 8032847mV·μs, which is 228.46% higher than that of dry coal 2445596mV·μs.

By calculating the correlation dimension, the variation law of freeze-thaw energy of coal with different water content is analyzed. With the increase of coal moisture content, the correlation dimension of the steep increase stage of the freezing process increases exponentially, and the correlation dimension of the slow growth and stable stage increases linearly. The freezing process is dominated by tensile cracks. The proportion of tensile cracks during coal drying is 75.85%, which decreases with the increase of water content. The melting process is dominated by shear cracks and increases with the increase of water content. Through the three-dimensional positioning of the cracks generated in the whole process of coal body, the variation law of the positioning event with the freeze-thaw time in the internal space of coal body is obtained. The number of events increases with the increase of water content of coal.

The mechanical properties, irreversible dissipation energy and surface strain of coal during dynamic compression before and after freezing and thawing were collected by high frequency impact instrument, PCI-II acoustic emission system (AE) and XTDIC system. The liquid nitrogen freeze-thaw makes the compressive strength and elastic modulus of coal decrease by 5.72% and 13.26% respectively, and the mechanical damage of coal increases with the increase of water content. The energy variation law in the process of coal compression failure was measured. The ringing count was divided into three stages: development stage, growth stage and weakening stage. After freezing and thawing, the cumulative ringing count of 6458973 is only 32.83% of the dry coal 19674171, and the dissipation energy is obviously reduced. With the increase of water content of coal, the development of surface cracks during compression is more obvious, and the number of cracks and strain concentration zone increase. The failure mode belongs to the tensile-shear composite failure mode, and the failure shape is “X” shape. Finally, the integrity of the broken ring is enhanced and the energy release intensity of the coal body is reduced.

The variation of energy magnitude b value is used to describe the dynamic evolution of energy in the whole process of uniaxial compression. With the increase of water content of coal, the cumulative energy dissipation decreases linearly. The b value increases linearly. The variation law of the proportion of cracks in the compression process is obtained. The proportion of tensile cracks decreases exponentially, and the proportion of shear cracks increases exponentially. The relationship between the damage amount and the water content of coal in the freezing-thawing-compression process is obtained by comprehensively analyzing the energy parameters. As the water content increases, the damage amount D_t of the freeze-thaw process increases. When the water content is 5.96%, the damage amount is 0.3644, which is 5.78 times that of the dry coal 0.0630. At the same time, the amount of damage in the compression process gradually decreases and the number of three-dimensional positioning events decreases linearly. The elastic modulus change model of coal during freezing-thawing-compression process was inverted. The elastic modulus of dry coal decreased by 0.1205 GPa during freezing-thawing process. The decrease of elastic modulus of coal with water content of 5.96 % is 5.79 times that of dry coal. The greater the change in the physical properties of the liquid nitrogen freeze-thaw coal body, the greater the energy increases, and the smaller the energy required for uniaxial compression to destroy its structure. The results are helpful to reveal the effect of energy dissipation induced pore fracture change. Further understand the applicability of liquid nitrogen fracturing coal seam penetration enhancement technology and guide the optimization of liquid nitrogen fracturing process parameters and field application.