考虑外水侵入效应的水力压裂煤体瓦斯渗透性响应机制研究

Gas drainage is an important measure to inhibit gas accidents and improve China's energy structure.But the coal seam permeability is a key factor in the effect of the impact on gas drainage. According to the key scientific issues as understanding unclear and mechanism unknown of impaction of external water intrusion to hydraulic fracturing improving the effects of coal permeability, this study made use of permeability under"coal-water-gas three-phase medium state basis of modified mechanical testing machine, starting pressure gradient test apparatus","simulated hydraulic fracturing drilling seepage experimental device", to carry on the effect experiment of different water saturation of coal samples slippage before the finale and after uninstalling of high-pressure water load under the condition of fixing confining pressure, starting pressure gradient test, gas permeability coefficient test, Reynolds test, to discuss coupling relationships between water saturation of its coal samples and starting fracture gradient and permeability coefficient; It researched the stress-strain test of coal on the basis of simulated drilling loading of high-pressure water, response characteristics of permeability coefficient of coal samples to the injection pressure changes and to the changes in water flow, it also analyzed of evolutionary mechanisms of hydraulic gradient or coal structural transformation or permeability. This study identified its response mechanism of coal gas permeability to the external water intrusion, also revealed its combined effect and controlling mechanism of hydraulic fracturing superimposed on extracellular fluid invasion to seam gas permeability. Its result will provide the basis for improving the application effect of hydraulic fracturing measures.

瓦斯抽采是抑制瓦斯事故、改善我国能源结构的重要举措，而煤层渗透性是影响瓦斯抽采效果的关键因素。针对外水侵入影响水力压裂改善煤体渗透性效应认识不清、机制不明这一关键科学问题，利用力学试验机基础上改装的“煤-水-气三相介质状态下渗透率、启动压力梯度测试装置”、“模拟钻孔水力压裂渗流实验装置”，开展基于固定围压轴压条件下高压水载荷前、卸载后不同含水饱和度原煤样滑脱效应实验、启动压力梯度测试、瓦斯渗透系数测试、雷诺数测试，探讨煤样含水饱和度–启动压裂梯度–渗透系数之间的耦合关系；进行基于模拟钻孔高压水载荷下煤体应力应变测试、煤样渗透系数对注水压力变化响应特征、煤样渗透系数对注水流量变化响应特征研究，分析水力梯度–煤体结构改造–渗透性之间的演化机制。查明煤体瓦斯渗透性对外水侵入的响应机制，揭示水力压裂叠加外液侵入对煤体瓦斯渗透性带来的综合效应及其控制机理，为提高水力压裂措施的应用效果提供应用基础。

有效的瓦斯抽采是抑制瓦斯事故、改善我国能源结构的重要举措，而煤层渗透性是影响瓦斯抽采效果的关键因素。本项目针对外水侵入影响水力压裂改善煤体渗透性效应认识不清、机制不明这一关键科学问题，采用实验室物理模拟和现场试验相结合的方法，对不同含水饱和度条件下瓦斯解吸量、解吸速率、瓦斯放散性进行了物理模拟，对钻孔瓦斯涌出初速度q以及钻屑瓦斯解吸指标K1进行了现场注水测试，明确了外加水分对瓦斯解吸、释放的抑制效应，同时揭示出外水侵入煤体条件下所测试的校检指标，一定程度上掩盖了煤与瓦斯突出危险的本质；利用自主设计、改造的煤-水-气三相介质状态下气体渗透率和启动压力梯度实验装置以及高压水载荷下煤岩气体渗流实验平台，相继完成了未受载煤样不同含水率状态下瓦斯渗透性以及高压水载荷前后煤样瓦斯渗透性对比分析实验研究，不但进一步查明了外水侵入对煤体瓦斯渗透性带来的抑制效应，而且揭示了水力压裂叠加外水侵入对煤层瓦斯渗透性改善起到的综合效应，研究表明：含水率并不是影响启动压力梯度的主要因素，当煤层渗透率增大到一定程度，启动压力梯度就将消失，揭示了对于透气性较好的高渗煤层，水分的增加对瓦斯抽采的影响是有限的，提高瓦斯渗透性的关键是控制好水力压裂的裂隙改造效果；从“双重效应”的概念，在机理、机制上对水力压裂的增透效果进行了探讨；构建了含瓦斯煤体水力压裂流场评价数学模型，开展了基于瞬变电磁法和示踪剂跟踪法的水力压裂流场分布特征理论和实验研究，提出了用于改善水力压裂效果的“钻、割、压、卸、注”一体化集成技术工艺。研究成果对深刻认识外水对瓦斯运移的抑制效应、优化水力压裂技术工艺、提高瓦斯治理效果具有重要的理论和应用价值。