低渗煤层注入液氮致裂机理研究

Fracturing low-permeability coal seam and enhancing permeability is a key issue to recovery methane and prevent its disaster. The synthesized methods including experiment, theoretical analysis and numerical modeling, are adopted to research the mechanism of fracturing low-permeability coal bed through liquid nitrogen injection. The submersion tests, semi-submersion tests and borehole thermal shock tests of three raw coal types are performed. The area change of pores and micro-fractures and the change of sound velocity and amplitude, between initial state and after being cooled in liquid nitrogen, are measured. The effects of coal temperature, saturation rate and water phase change, freezing-thawing cycles, temperature gradient, injection amount and gasification pressure of liquid nitrogen, confining pressure on propagation and extension of the inner micro pores and fractures are investigated. Combined with the effective stress principle, tensile and shear strength theory, ice-water phase transition theory and fracture mechanics, the mechanical criterion of the micro fissures’ extension and fracturing of coal are presented. The criterion of progressive fracturing subjected to freezing-thawing cycles and damage accumulation is proposed. Combined with Lagrange element method and discrete element method, the fracturing and micro fissures’ extension processes of coal during submersion tests, semi-submersion tests and borehole thermal shock tests are modeled and reproduced. Parameters analysis is performed. Combined with the conclusions from the theoretical analysis and experiments, the mechanism of fracturing low-permeability coal bed through liquid nitrogen injection is presented. The research will provide a new roadway and theoretical basis to fracture low-permeability coal bed and enhance permeability, and help to recovery methane and decrease gas disaster.

低渗煤层的致裂增透是煤层气抽采和瓦斯灾害防治的关键。项目采用实验研究、理论分析和数值模拟相结合的研究方法，研究低渗煤层注入液氮的致裂机理。项目对三种原煤开展液氮溶浸实验、半溶浸实验和注入钻孔煤实验，测试液氮作用前后煤孔隙和裂隙面积的变化、波速和波幅的变化，提出煤温度、饱水度及水相变、液氮冻融循环、温度梯度、围压、液氮注入量及气化压对煤内孔隙和微裂隙缺陷结构扩展的影响规律。结合有效应力原理、拉剪强度理论、冰水相变理论和断裂力学理论，建立液氮冷冻下煤破裂和内部微裂隙扩展的力学判据；建立液氮冻融循环下煤损伤积累、渐进破裂的力学判据。结合拉格朗日元和离散元数值方法，再现溶浸实验、半溶浸实验、液氮注入钻孔煤实验中煤破裂和微裂隙扩展的动态过程。利用数值模型进行参数分析，结合理论和实验研究成果，揭示液氮注入低渗煤层的致裂机理，为低渗煤层致裂增透提供一种新方法和理论基础，指导煤层气抽采和瓦斯灾害防治。

低渗煤层的致裂增透是煤层气抽采和瓦斯灾害防治的关键科学问题。项目采用实验研究和理论分析等综合研究方法，研究了低渗煤层注入液氮的致裂机理。项目自制了液氮半溶浸煤试验装置和钻孔煤注液氮试验装置；从辽宁阜新矿区、山西晋城矿区、内蒙古纳林河矿区取煤，制作煤样，利用自制设备开展了液氮溶浸煤实验、液氮半溶浸煤实验、钻孔煤注液氮实验，利用非金属超声检测分析仪、相机、显微相机、激光共聚焦显微镜、XZY-90型X射线数字成像检测系统、uCT225kVFCB型高精度CT试验分析系统等测试了液氮作用前后煤样的宏微观结构变化；研究了煤型、饱水度及水相变、煤样初始温度、围压、冷冻温度、冻融循环周期等因素对煤样纵波波速、微裂隙扩展和损伤演化的影响规律；建立了考虑饱水度及水相变、温度应力和液氮冻融循环周期的液氮冷冻煤微裂隙扩展力学判据；测试了不同条件煤样液氮作用前后其单轴抗压强度、破坏应变、脆性模量、循环荷载作用应力应变曲线滞回环的演化特征，分析了饱水度、冻融循环次数和冷冻温度等因素对液氮作用煤力学特性的影响规律；分析了液氮作用煤温度应力、水相变冰楔效应和冻融循环损伤积聚的致裂机制，提出了液氮注入低渗煤层的致裂机理；以钻孔煤注液氮为例，分析了液氮冷冻致裂的影响半径及时间效应。通过项目研究，获得了低渗煤层注入液氮致裂的机理，明晰了影响致裂效果的影响因素及影响规律，为低渗煤层致裂增透和高效抽采提供了新的技术方法，也为冲击地压灾害、瓦斯突出灾害的防治提供了新的技术思路。