基于核磁共振与应力分析的液氮冷浸致裂煤岩研究

Study on coal cracking under liquid nitrogen soaking based on nuclear magnetic resonance and stress analysis

为探究液氮冷浸作用下煤岩孔隙结构的变化规律,以干燥煤岩为试验对象,分别对烟煤和无烟煤试样进行不同冷浸时间处理,通过核磁共振(NMR)试验对比分析液氮冷浸前后煤岩孔隙变化特征;并利用COMSOL软件对液氮冷浸过程进行数值模拟,在得到煤岩温度场分布的基础上进行热应力的计算与分析。结果表明:液氮冷浸作用对煤岩中的各尺寸的孔、裂隙均有不同程度的破坏作用,液氮冷浸30 min时,烟煤与无烟煤的微小孔隙体积增加幅度分别为86.43%和20.28%;液氮冷浸煤岩过程中,低温由煤岩表面逐渐向内部传递,煤岩内部各点产生的热应力随时间的延长逐渐增大,直至最大应力值9.84 MPa,热应力的产生及变化是干燥煤岩主要的增透机制。

In order to study variation law of pore structure of coal under liquid nitrogen soaking,with dry coal as an experimental object,bituminous coal and anthracite were treated under different impacting time respectively.Then,pore variation characteristics of coal before and after liquid nitrogen soaking was analyzed through NMR experiment,and numerical simulation of soaking process was conducted by using COMSOL software before thermal stress was calculated and analyzed on the basis of distribution of coal temperature field.The results indicate that liquid nitrogen soaking has different degrees of damage to pores and fissures of various sizes of coal,and micropore volumes of bituminous coal and anthracite will increase by 86.43%and 20.28%,respectively when they are soaked for 30 minutes.Under liquid nitrogen soaking,low temperature gradually transfers from coal surface of to its inside while thermal stress generated at various points inside coal gradually increases with time until the maximum stress value reaches 9.84 MPa.Generation and change of thermal stress is the main mechanism for permeability increase of dry coal.