西部富水中生代煤系地层冻结凿井中冻融劣化机理及渗害预控

The exploitation of coal resources in Northwest China satisfies the national strategic demands, artificial freezing method is almost the only effective construction method in the northwest coalmine shaft constructions, which bore through several hundred meters Cretaceous, Jurassic coal-bearing strata rich in water. Construction in these strata becomes a problem because of its late diagenesis, poor cementation, low strength, argillization, lack of stable aquiclude and good water conductivity. A number of serious coalmine shaft submergence and freezing wall collapse failure occurred recently. Hence, CT real-time mechanical scanning test and acoustic emission monitoring on frozen rocks in freezing and thawing process were conducted to clarify the change of mineral components and content, microstructures and micropores of the rocks in these strata. The influence of richness in water, high confining pressure and high seepage pressure on microscopic and macroscopic physical and mechanical properties of these strata in freeze-thaw process, and the mechanism of freeze-thaw deterioration was ascertained. The change of strength, deformation and permeability of Mesozoic coal strata under freeze-thaw deterioration were investigated. Mining shaft freezing simulations, permeability test and field monitoring of freezing wall after thawing were conducted to reveal the mechanism of spatio-temporal variation of stress and seepage field around the shaft after the freezing wall thawed out, and to explain the mechanism of freeze-thaw deterioration of freezing wall which results in gushing water in work place. All these research can provide scientific basis for the prediction, prevention and control of seepage disasters in construction and operation of coalmine shaft with artificial freezing method.

西北煤炭资源开发是国家能源战略需求,冻结法是西北矿井建设中穿越富含水数百米厚白垩系和侏罗系煤系地层主要的几乎唯一有效的工法。该类地层成岩晚、胶结差、强度低、遇水泥化、无稳定隔水层、层间导水性强等特点成为工程建设难题;近期施工已发生多起严重涌水淹井和垮塌事故。为此,进行富水弱胶结岩层冻融过程细观CT实时力学试验及声发射测试，通过微细观试验手段弄清冻岩融化过程对弱胶结岩石矿物成分、含量及微结构、微孔隙变化影响；探求富含水、高围压、高渗透压对冻岩解冻后微细观和宏观物理力学特性及冻融劣化的影响机理；获得中生代煤系地层冻结凿井中岩石冻融劣化的强度、变形、渗透性变化规律；进行立井冻结模拟和冻结壁解冻后渗透性试验和现场监测,揭示冻结壁解冻以后井筒周围应力场和渗流场发生时空变异的机理,阐明冻结壁冻融劣化导致沿冻结管全深地层串水诱发工作面渗害形成机制，为井筒建设渗害预控和使用安全运行提供科学依据。

我国西部地区白垩系和侏罗系煤系地层具有成岩晚、胶结差、强度低、遇水泥化、无稳定隔水层、层间导水性强的特点，在煤炭矿井建设中特别容易产生渗害问题。近年来施工已发生多起严重涌水淹井和垮塌事故。基于三轴压缩试验，充分考虑冻融过程中地应力的影响，利用围压模拟红砂岩受到的地应力环境。试验结果表明：白垩系红砂岩冻融后的力学参数显著下降。冻融过程中，地应力的作用可认为是提高了孔裂隙的约束能力，使冻结作用可以尽可能地向次级微孔隙发展。由于冻结程度的加深，冻胀力进一步增加，红砂岩的损伤加剧，故而融化后红砂岩的力学参数相对于无围压冻融进一步降低。基于三维细观可视化技术计算分析了冻融作用对中砂岩孔隙率以及内部孔隙结构的影响。结果表明：随着冻融时围压的增加，中砂岩损伤加剧，不同围压下中砂岩CT尺度孔隙率增加了3.57%~9.51%。中砂岩CT尺度孔径主要分布在0~240μm区间，其中0~60μm孔径区间占比最大，超过70%。中砂岩冻融损伤主要是0~40μm孔径范围的小孔隙数量的增加，其中0~20μm与20~40μm孔径的孔隙所占比例增加幅度分别超过25%与10%，而40-60μm孔径的孔隙所占比例增加不明显甚至略有减小。基于三轴渗流试验并结合声发射技术研究了泥质砂岩经历一次冻融后三轴渗流加载过程中的力学与渗透特征。试验结果表明，研究所用泥质砂岩经历冻融后胶结性能劣化，表面最大次生裂隙宽度达到0.028mm。当围压小于25MPa时，随着围压的增加，泥质砂岩的峰值强度提高了约64%，破坏模式从脆性破坏向延性破坏发展。随围压增加，泥质砂岩初始渗透率和峰值渗透率均呈负指数形式下降，起裂水平随围压线性增加，但扩容水平呈指数变化趋势。最终获得了中生代煤系地层冻结凿井中岩石冻融劣化的强度、变形、渗透性变化规律；揭示了冻结壁解冻以后井筒周围应力场和渗流场发生时空变异的机理,阐明了冻结壁冻融劣化导致沿冻结管全深地层串水诱发工作面渗害形成机制，研究成果为井筒建设渗害预控和使用安全运行提供了科学的依据。