开挖卸荷条件下钻孔周围煤体破裂损伤与瓦斯流动协同响应机制研究

Coal mass around boreholes under the condition of excavation unloading turns to be fractured and damaged, which leads to the structural changes of pore and fracture network in coal mass, and gas transport rules tend to be complex. At present, there is still a lack of scientific and quantitative analysis and expression for the synergetic response mechanism of fracture damage and gas transport of excavation unloading coal mass. Therefore, the coal mass around boreholes in working face of high gas coal mine is regarded as the research object. Mine rock mechanics, fracture damage mechanics, porous medium seepage mechanics and computational fluid mechanics are synthetically used for the design of coal deformation destruction and gas transport characteristics tests under the condition of excavation unloading. And the research on multi physics behaviors of coal mass around boreholes is carried out considering the unloading effect. The real physical properties of coal mass around boreholes, spatial distribution of pore and fracture network and the rule of gas permeability evolution are obtained respectively. Besides, the fracturing damage mechanism of coal mass around boreholes, and the multi physics coupling mechanism of stress field, seepage field, damage field and temperature field are determined. The control mechanism of gas transport on pore and fracture evolutional patterns is analyzed. Most importantly, the synergetic response mechanism of fracture damage and gas transport in excavation unloading coal mass is discussed, and the quantitative evaluation system of gas permeability of coal mass around boreholes is established, which can provide a theoretical basis for the efficient gas extraction and the safety production of coal mine.

开挖卸荷条件下钻孔周围煤体出现破裂损伤，引起煤体孔隙-裂隙网络结构性改变，导致煤层瓦斯运移规律趋于复杂；当前，开挖卸荷条件下钻孔周围煤体的破裂损伤与瓦斯流动协同响应机制尚缺乏科学化、定量化的分析和表达。为此，以高瓦斯矿井采煤工作面钻孔周围煤体为研究对象，综合运用矿山岩石力学、断裂损伤力学、多孔介质渗流力学和计算流体力学等理论，针对性地设计开挖卸荷应力路径下煤体变形破坏和瓦斯流动特性试验，开展考虑卸荷作用的钻孔周围煤体多物理场行为特征研究，获取真正的钻孔周围煤体力学特性、孔隙-裂隙网络空间展布及煤体渗透率演化规律，明确钻孔周围煤体破裂损伤机制及应力场-渗流场-损伤场-温度场等多物理场耦合作用规律，分析钻孔周围煤体孔隙-裂隙演化模式对瓦斯流动的控制机理，探讨开挖卸荷钻孔周围煤体破裂损伤和瓦斯流动的协同响应机制，建立钻孔周围煤体渗透能力定量评价体系，为煤矿瓦斯高效抽采和煤矿安全生产提供理论支撑。

为了全面系统地分析开挖卸荷条件下钻孔周围煤体破裂损伤与瓦斯流动协同响应机制，本项目采用实验室试验、理论分析、数值模拟和工程试验相结合的方式，以高瓦斯矿井采煤工作面钻孔瓦斯抽采为研究背景，以钻孔周围煤体为研究对象，深入探讨了开挖卸荷条件下钻孔周围煤体变形破坏机制和瓦斯流动规律。首先，开展了不同加卸载条件下含瓦斯煤体渗流试验，研究了含瓦斯煤体孔隙-裂隙演化模式及煤体渗透率演变规律，发现不同应力路径下，煤样渗透率均随着轴向应变的增大呈先减小后增大的趋势，但较常规三轴加载应力路径，加轴压卸围压下煤样渗透率增大系数大幅提升，煤样峰值强度和轴向变形均较小，但径向变形较大；煤样失稳破坏后，多有主裂隙通道产生，煤样裂隙发育整体上呈纵向裂纹劈裂型发育和横向扩容叠加复合形态；加轴压卸围压效应加快了含瓦斯煤体主裂隙通道形成进程，对于峰后阶段煤体渗透率的持续改善具有促进作用。结合CT检测结果，还发现受载条件下煤样裂隙演化规律性阶段式变化明显。其次，在进行理论分析的基础上开展了开挖卸荷条件下含预设孔洞煤体瓦斯渗流特性数值模拟，发现相同孔隙压力、围压和轴压条件下，有孔洞时煤体瓦斯渗透率远大于无孔洞；煤体瓦斯渗透率随着孔洞深度的增大呈先减小后增大，随着孔洞半径的增大而增大；宏观孔洞的存在对瓦斯流动控制作用明显，为此可利用高压水射流技术，形成钻孔周围煤体裂隙贯通网络，达到钻孔轴向疏导畅通-径向扩流增渗立体化促抽效果。再次，基于煤体裂隙率计算公式和影响因素，推导了钻孔周围煤体裂隙率演化方程和钻孔周围煤体渗透率演化方程；结合CT数变化规律，定义了开挖卸荷条件下钻孔周围煤体破裂损伤变化量计算公式，进而推导了钻孔周围煤体破裂损伤演化方程。最后，分析了钻孔周围煤体孔隙-裂隙演化模式对瓦斯流动的控制机理，探讨了建立钻孔周围煤体渗透能力定量评价体系的可行路径。本项目研究成果可为矿井瓦斯高效抽采和灾害防治提供理论支撑。