冷热冲击循环下低渗透煤体的结构演化特征与增透机制研究

Permeability enhancement of coal seams is the key to the efficient extraction of methane in low-permeability coal seams and the prevention of methane dynamic disasters. With the development of coal seam permeability enhancement technology, anhydrous permeability enhancement measures has been applied and gained widespread attention. On the basis of the research results of crushing coal caused by high-temperature nitrogen, the applicant considers the superposition of cold-hot shock erosion and fatigue-fracturing coupling, and proposes a method for crushing coal with circulating liquid nitrogen and high-temperature nitrogen (cold-hot shock). The project takes the fracture and permeability enhancement mechanism of coal under the cold-hot shock cycle as the starting point of the research and takes the internal crack evolution law as the research link. Besides, the true triaxial cold-hot shock test system on coal is adopted to study the heat and mass transfer characteristics of coal and the evolution characteristics of coal surface macro-crack, internal micro-crack and pore structure, and clarifies the relationship between heat and mass transfer and structural evolution of the coal. Based on the energy criterion of fracture propagation and seepage theory, a mechanical model of fractured coal is established related to the thermal shock parameters, coal fracture evolutions and permeability changes. And combined with the mechanical characteristics of coal damage and the results of the permeability evolution test, the permeability enhancement mechanism of coal is revealed. On this basis, the core influencing factors are determined, and the condition criteria for the formation coal fractures and determination method of the reasonable parameters are given, laying a theoretical foundation for the future engineering application of this method.

煤层增透是低渗透煤层瓦斯高效抽采和瓦斯动力灾害防治的关键。随着煤层增透技术的发展，无水增透措施得到应用并取得广泛关注。申请人在已有高温氮气致裂煤体研究成果的基础上，考虑冷热冲击侵蚀与疲劳致裂耦合叠加作用，提出了液氮与高温氮气（冷热冲击）循环致裂煤体的方法。项目以冷热冲击循环下煤体的破裂与增透机制为研究的切入点，把内部裂隙演化规律作为研究的纽带，采用真三轴冷热冲击煤体试验系统研究煤体的传热传质特性与煤体表面宏观裂隙-内部微观裂隙-孔隙结构的演化特征，阐明煤体传热传质与结构演化的关系；基于裂隙扩展能量准则与渗流理论，建立冷热冲击参数-煤体裂隙演化-渗透性变化相关联的致裂煤体力学模型，结合煤体损伤力学特性与渗透率演化试验结果，共同揭示煤体的增透机制；在此基础上，确定其核心影响因素，给出形成煤体破裂的条件准则和合理参数的确定方法，为该方法今后的工程应用奠定理论基础。

煤层的低渗透性严重影响瓦斯抽采效果和利用率，煤层增透成为了瓦斯高效抽采的关键。随着煤层增透技术的发展，无水增透措施得到应用并取得广泛关注。项目负责人通过前期研究与调研，提出了高温氮气和液氮冷热冲击联合致裂煤体的方法。为探索高温氮气、液氮无水致裂煤体的可行性与时效性，本项目研究了高温氮气热冲击、液氮冷冲击和冷热冲击循环下煤体的传热传质特性、结构演化特征和破裂增透机制。研究结果表明：基于热-流-固耦合数学模型数值模拟发现，氮气温度、注气时间和钻孔直径对氮气渗流的温度效应影响显著，而氮气压力和煤体初始渗透率对温度效应影响甚微。高温氮气作用下煤体的抗压强度、残余强度和弹性模量随氮气温度和压力升高而下降；氮气温度越高、氮气压力越大，煤体的最大渗透率比越大；多循环氮气注入有利于煤体的裂纹再次扩展，其增加渗透率效果强于单次氮气注入。液氮冷冲击下煤样的抗压强度、弹性模量显著提高，泊松比、抗拉强度均降低。液氮冷冲击煤样的破坏方式、裂纹扩展方向和破碎程度等均有明显改变。液氮可导致微裂纹沿晶或穿晶扩展，诱发煤体孔隙结构破坏及基质断裂；基于结构分形得出，液氮冷冲击煤体孔隙结构分布更加均匀，连通性更好，裂纹扩展在空间上发生的无序性增强，导致破坏后碎片尺寸均匀性增加、数量增多及断裂面粗糙度增加。较常温煤体，热冲击后煤体的液氮破裂压力下降20%-55%；随着热冲击温度增加，煤样端面和侧面的破裂程度增加。冷热冲击循环煤样的破裂压力降低48.45%。随着冷热冲击循环次数增加，煤体的渗透率均增加，钻孔周围分布的孔隙压力逐渐增大，使得裂纹尖端应力强度因子增大，煤体抗裂性能逐渐降低。研究成果为冷热冲击循环致裂增透方法的工程应用提供理论和工程参数指导，对丰富我国无水增透的理论内涵、提高瓦斯利用率与助力“双碳”目标的实现具有重要的现实意义。