低透气性煤层脉动液氮致裂煤体孔隙演化规律及损伤力学特性

It has increasingly been of great challenge for enhanced permeability for efficient extraction of gas in low permeability coal seam, for the entire coal-mining industry. Artificially strengthened cracks are the key approach to increase permeability of low permeability coal seam. The applicant proposes a novel approach of enhancing reservoir permeability by pulsating liquid nitrogen fracturing (PLNF) during which liquid nitrogen is injected into low permeability reservoirs. The dynamic evolution of coal microscopic fractures measured by NMR three-dimensional imaging and CT scanning technology is to reveal fine pore distribution of coal in three-dimensional visualization which uncovers porosity changes influenced by PLNF. The researches on the coal permeability variation under different conditions are conducted to establish porosity-permeability model based on the new permeability-enhancement method. The failure law of coal fatigue damage, caused by PLNF, is studied to establish the coupling relationship between variables of PLNF and fracturing parameters of coal. This work illustrates mechanics process of PLNF in coal seam to build the mechanics criteria and uncover the coupling mechanism of freezing, expanding and cryogenic fracturing caused by liquid-gas and solid-liquid phase change to analyze the key factors of fissures extending and connecting during the PLNF process. Another research point of this work is to get the evolution laws of coal porosity and permeability of combining the PLNF with artificially inducing fissures and come up with an evaluation approach for PLNF.

如何提高低透气性煤层渗透率、高效抽采瓦斯成为矿业安全学科面临的严峻挑战。人工强化造缝增透是低透气性煤层瓦斯高效抽采的关键，申请人提出了低透气性煤层脉动液氮致裂增透方法。本项目通过NMR核磁三维成像和CT扫描技术对脉动液氮致裂煤体微观裂隙进行动态分析，实现孔隙分布三维可视化精细表征，揭示脉动液氮致裂煤体孔隙演化规律；研究不同脉动条件下煤层渗透率变化特征，建立脉动液氮致裂煤体孔隙度-渗透率变化关系模型；研究煤体液氮脉动致裂疲劳损伤破坏规律，建立脉动液氮注入参量和煤体破裂特征参变量之间的耦合关系，描述脉动液氮致裂煤体损伤致裂的力学过程并建立力学准则；揭示液氮注入煤体过程中液气相变、水冰相变所产生的冻结致裂、膨胀致裂、低温致裂三重力学作用机制；分析脉动液氮致裂裂隙发育、连通的关键控制因素；研究脉动液氮致裂和人工诱导裂隙结合下的煤体孔隙度和渗透率变化规律，提出脉动液氮致裂效果评价方法。

煤矿瓦斯抽采不仅可以减少矿井灾害事故，而且对改变中国能源结构具有积极影响，但我国煤层普遍具有微孔隙、高吸附、低渗透的特点，煤层渗透率成为制约瓦斯抽采的主要瓶颈。本项目提出了低透气性煤层多循环脉冲液氮致裂方法，系统研究了煤体孔隙演化规律及损伤力学特性，主要研究内容：（1）通过对煤样进行液氮循环冲击，发现随着脉冲次数的增加，煤样宏观损伤明显，介观孔裂隙数量、面孔隙度增加，微观孔隙尺寸扩展，数量增加；有效孔隙度、自由流体占比增加；（2）利用红外温度测量及三维数字散斑全场应变测量装置，得到液氮冲击过程中煤样端面的温度场及应变场变化规律，建立了液氮冷冲击热应力损伤模型，得到了热应力随冲击距离的变化关系；（3）利用低场核磁共振检测技术，得到了液氮循环冲击下煤体未冻水含量的变化规律，以及液氮注入后在煤体孔隙中赋存状态变化特征，揭示出液氮注入煤体过程中的多重力学作用机制及煤体物理力学参数变化特征；（4）揭示了煤体液氮致裂疲劳损伤的破坏规律，建立了液氮注入参量和煤体破裂特征参变量之间的关系，描述了脉动液氮煤体损伤致裂的力学过程并揭示不同荷载形式下裂隙演化规律；（5）分析得到液氮注入煤体过程中循环脉冲参量、煤体力学特征和裂隙空间扩展的耦合关系，揭示了循环脉冲液氮致裂裂隙发育、连通的关键控制因素，研究了液氮循环脉冲致裂裂隙控制与致裂效果评价方法。