低渗透煤体循环脉冲气爆增透裂纹扩展规律研究

Permeability is the key factor which dramatically affects the gas drainage of low permeable and gassy coal seam. The process and criterion of crack propagation and fracturing mechanism of coal induced by high pressure gas shock are fully studied through synthetic methods of field investigation, experimental research and theoretical study, considering both positive and negative factors which influence the crack growth, such as shock wave, air pressure, cyclic number of blasting, geo-stress and coal strength. The information of crack propagation, blasting stress wave and gas pressure is captured during coal blasting by differ monitoring systems. The attenuation of stress wave, crack propagation velocity and variation of shock gas pressure are revealed by analyzing laboratory results. A crack growth model of coal induced by high pressure air blasting is set up on the basis of dynamic wave theory and gas-solid coupling principle through introducing a nonideal gas state equation. And criterions are proposed to describe the shear and tension failure of crack at different extension states. Base on the experimental results and analytical solution of theoretical model, the fracturing mechanism of coal by high pressure gas shock is fully studied and revealed. The permeability enhancement of coal by high pressure air blasting technology is compared to that of other technology, for example the long-hole blasting. The conclusions of this work could be used as a primary guidance and provide theoretical foundation for the gas drainage design and practical engineering application of single low permeable and highly gassy coal seam,especially the coal seam that are deeply buried.

煤层透气性是影响低透气性高瓦斯煤层瓦斯抽采的关键因素。项目采用现场调研、实验研究和理论分析相结合的方法，考虑气爆应力波、气体压力、气爆次数、地应力和煤体强度的影响，对不同类型低渗透煤体进行循环脉冲高压气体冲击气爆增透实验。利用高频动态应变和气体压力测试系统、环形导电线圈等测试设备，监测气爆全程煤体的动应变、位移、加速度、裂纹扩展、爆腔气体压力等时程信息，分析气爆应力波在煤体内传播的衰减规律和爆腔气体压力随裂纹扩展的衰减规律，研究煤体裂纹起裂、扩展、止裂的过程、条件和裂纹扩展速度、长度的变化规律，提出煤体裂纹扩展的准则。基于固体波动理论、气固耦合理论，引入非理想气体状态方程，建立煤体循环脉冲气爆裂纹扩展的理论模型，求解得到煤体气爆裂纹的扩展长度。揭示煤体气爆增透机理和效果，为气爆技术在单一高瓦斯低透气性煤层，尤其是深埋高应力低透本煤层增透和瓦斯抽采的工艺设计和工程实践提供理论支持和实验基础。

煤层透气性是影响低渗透瓦斯煤层瓦斯抽采的关键内因。采用理论分析、室内实验、数值模拟和现场实验相结合的方法，开展了煤体液态二氧化碳相变气爆致裂过程和关键影响因素的研究。基于气固耦合理论，引入非理想气体状态方程，建立了煤体气爆致裂裂纹扩展的气固耦合理论模型，获得了煤体气爆裂纹扩展长度的半解析解。煤体液态二氧化碳相变气爆致裂的理论分析、室内实验和数值模拟研究结果具有较好的一致性，研究结果表明，煤层地应力是煤体气爆裂纹扩展的主要抑制因素，二氧化碳气体压力是裂纹扩展的驱动外因，随煤层地应力的增加，煤体气爆致裂裂纹扩展范围随之近幂指数形式变化而减小，随气爆气体峰值压力的增加，煤体气爆致裂裂纹扩展范围随之近线性地增大；爆破孔预裂缝能较大幅度上提高煤体气爆裂纹的扩展范围；煤体二氧化碳气爆致裂是依靠气爆气体持续冲击和尖劈致裂的结果，气爆气体对爆破孔壁的冲击在煤体内产生的应力波主要起到产生初始裂纹的作用，后续裂纹继续扩展主要依靠爆腔气体流入裂纹的尖劈压裂效应，多次循环气爆可在一定程度上提高煤体裂纹数量和扩展范围。基于国内外液态二氧化碳气爆设备的调研基础上，对关键部件进行改制，研制出适合煤矿井下作业环境的气爆致裂器，在山西晋煤矿区进行的液态二氧化碳相变气爆致裂增透的现场实验表明，煤层实施二氧化碳相变气爆致裂增透技术后，煤层瓦斯抽采量可提高1.25～2.91倍，初步验证了液态二氧化碳相变气爆致裂增透新方法的有效性，为气爆新技术在单一低渗透瓦斯煤层的增透和瓦斯抽采的工艺设计和工程实践推广应用提供理论和实验基础及现场应用经验。