液氮溶浸时间对烟煤渗流特性的影响及传热过程模拟

Effect of liquid nitrogen immersion time on seepage characteristics of bituminous coal and simulation of heat transfer process

低透气性一直限制着我国煤层气的高效抽采,近年来,液氮致裂技术因其绿色环保、增透效果显著等优势成为研究热点。低温液氮在冷冲击致裂煤层的同时,热量也在发生迁移,液氮对煤层的致裂效果是随时间逐渐发生衰减的。为了保证液氮致裂全程的高效性、节约工艺时间,有必要对液氮致裂过程进行细化研究,精确评价其时间效益。为此,选择安徽省淮北煤矿中阶烟煤分别进行了1,5,10,30 min的液氮溶浸处理实验,并对传热过程进行数值模拟。结果表明:1)液氮溶浸处理可显著提升煤体的渗透率,处理后的煤样较初始状态渗透率增幅可达208.82%~432.50%。2)液氮溶浸处理后,拟合参数"a"值明显增大,即煤体渗透率对孔隙压力的敏感性增大;随着围压的上升,拟合参数"a"值呈减小趋势,即煤体渗透率对孔隙压力的敏感性逐渐减小。3)实验结果表明随着溶浸时间的增长,液氮的致裂增透效果是在逐渐衰减的;根据数值模拟结果,将液氮溶浸致裂煤体分为3个阶段:0~158 s为高效致裂阶段,158~450 s为正常致裂阶段,450 s后为低效致裂阶段。

Low permeability of coal seams has always restricted efficient extraction of coalbed methane in China. In recent years, liquid nitrogen fracturing technology has become a research hotspot due to its advantages of environmental protection and significant anti-reflection effect. While low temperature liquid nitrogen impacts on the coal seam, heat also migrates and fracturing effect of liquid nitrogen on coal seam gradually attenuates with time. In order to ensure high efficiency of liquid nitrogen fracturing process and save process time, it is necessary to conduct detailed research on liquid nitrogen fracturing process and accurately evaluate its time benefit. The middle-rank bituminous coal from Huaibei Coal Mine in Anhui Province was selected for 1, 5, 10, 30 min liquid nitrogen immersion experiments, and heat transfer process was numerically simulated. The results have shown that, 1) liquid nitrogen immersion can significantly improve permeability of coal mass, and permeability of treated coal samples can increase by 208.82%-432.50% compared with the initial state. 2) After liquid nitrogen immersion, value of fitting parameter a increases obviously, that is, sensitivity of coal permeability to pore pressure increases. With the increase of confining pressure, the value of fitting parameter a reduces, that is, sensitivity of coal permeability to pore pressure gradually decreases. 3) The experimental results have shown that fracturing and anti-reflection effect of liquid nitrogen gradually decreases with the increase of liquid nitrogen immersion time. According to numerical simulation results, coal cracking by liquid nitrogen immersion can be divided into three stages: high-efficiency fracturing stage with 0-158 s, normal fracturing stage with 158-450 s, and low-efficiency cracking stage with 450 s.