Analysis of mechanical properties, permeability and fracturing mechanism of coal samples at different fracturing time of liquid nitrogen
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摘要:
为研究液氮致裂时间对煤样的力学性能和渗透率的影响，利用自主研发的WYS-800三轴瓦斯渗流试验装置及声发射检测系统，对4组不同液氮致裂时间处理的煤样分别进行三轴力学渗流试验并采集声发射信号，对三轴力学渗流试验中各组煤样的力学性能、渗透率的变化进行了分析，描述了声发射信号的特征；根据沸腾换热理论、一维圆柱导热理论、热应力理论分析了致裂机理，计算了不同致裂时间下产生的热应力，通过数据拟合揭示了平均温度降、平均热应力、初始渗透率与致裂时间的关系。研究结果表明：①液氮致裂时间对煤样的力学性能产生不同影响，抗压强度和弹性模量随致裂时间的增加呈现先减小后增大的趋势，泊松比则呈现先增大后减小的趋势，煤样三轴加载时轴向应力-轴向应变曲线的阶段性演化具有明显差异，与力学参数的改变相关。②不同液氮致裂时间煤样在三轴加载过程中的渗透率均呈U型变化，煤样的初始渗透率、最小渗透率、试验测得最大渗透率随致裂时间的增加而增大，致裂30 min时增幅分别为119.05%、437.5%、146.49%；声发射信号在压密和弹性阶段不活跃，主要产生于屈服阶段和破坏阶段，致裂后煤样的声发射振铃计数峰值产生于破坏点附近，均大于20000次。③煤样与液氮之间的膜态沸腾换热系数为570.4 W/(m2·K)，煤样平均温度降与致裂时间相关，对平均热应力和初始渗透率起主导作用，致裂30 min时煤样内部产生的平均温度降可达213.63 K，平均热应力可达5.850 MPa。④液氮处理后煤样的初始渗透率与平均温度降之间呈线性关系，与致裂时间呈负指数分布关系。改变参数取值，可推广至其他类似处理的煤样或实际生产评估。
Abstract:In order to study the influence of liquid nitrogen cracking time on mechanical properties and permeability of coal samples, the independently developed WYS-800 triaxial gas seepage test device and acoustic emission detection system were used to conduct triaxial mechanical seepage tests on four groups of coal samples treated with different cracking time and collect acoustic emission signals. The mechanical properties and permeability of coal samples in triaxial mechanical seepage experiment were analyzed, and the characteristics of acoustic emission signals were described. Based on boiling heat transfer theory, one-dimensional cylinder heat conduction theory and thermal stress theory, the cracking mechanism was analyzed, and the thermal stress under different cracking time was calculated. The relationship between average temperature drop, average thermal stress, initial permeability and cracking time was revealed by data fitting. The results show that: ① Liquid nitrogen fracturing time have different influence on the mechanical properties of coal samples, the compressive strength and elastic modulus with the increase of the crack time shows the tendency of increase with the decrease of the first, the poisson's ratio are increased after the first decreases, coal sample triaxial loading when the axial stress and axial strain curve of the periodic evolution has obvious difference, associated with the change of mechanical parameters. ② The permeability of coal samples with different cracking time changes in u-shape during the triaxial loading process. The initial permeability, minimum permeability and maximum permeability increase with the increase of cracking time, and the increase rate is 119.05%, 437.5% and 146.49% when the cracking time is 30min. Acoustic emission signals are not active in compaction and elastic stages, and are mainly generated in yield and failure stages. The peak value of acoustic emission ringing count of coal samples after cracking is generated near the failure point, which is more than 20000 times. ③ The film boiling heat transfer coefficient between the coal sample and liquid nitrogen is 570.4 W/(m2·K). The average temperature drop of the coal sample is related to the cracking time, and the average thermal stress and initial permeability play a leading role. The average temperature drop of the coal sample can reach 213.63 K and the average thermal stress can reach 5.85 MPa after 30 min of cracking. ④ There is a linear relationship between initial permeability and average temperature drop, and a negative exponential relationship between initial permeability and cracking time of coal samples after liquid nitrogen treatment. Changing the parameter value can be extended to other similar coal samples or actual production evaluation.
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图 2 不同致裂时间煤样轴向应力−轴向应变曲线
Figure 2. Axial stress-axial strain curves of coal samples at different cracking times
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图 3 不同致裂时间煤样轴向应力、渗透率、振铃计数−轴向应变曲线
Figure 3. Axial stress, permeability, ring count - axial strain curves of coal samples at different cracking times
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图 4 煤样平均温度降、平均热应力-致裂时间曲线
Figure 4. Average temperature drop and average thermal stress-cracking time curves of coal samples
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图 5 液氮致裂煤样初始渗透率与致裂时间的关系
Figure 5. Relationship between initial permeability and cracking time of coal samples induced by liquid nitrogen
表 1 煤样显微组分、工业分析和镜质组反射率参数
Table 1 Coal sample maceral, industrial analysis and vitrinite reflectance parameter
煤样 显微组分体积分数/% 工业分析/% Ro,max/% 煤阶 镜质组 惰质组 壳质组 矿物 Mad Aad Vad FCad 新景矿3号 76.9 20.8 0 2.3 1.74 15.87 8.25 74.14 2.38 高阶煤 
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表 2 不同致裂时间下的试验方案
Table 2 Experimental scheme under different cracking time
试样编号 液氮致裂
时间/min围压/MPa 瓦斯压力/MPa 加载方式
及速率Y0 0 5 1 力加载0.02 kN/s Y1 10 5 Y2 20 5 Y3 30 5
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表 3 不同致裂时间煤样三轴加载下的参数
Table 3 Parameters of coal samples under triaxial loading with different cracking times
试样编号 致裂时
间/min密度/
（g·cm−3）抗压强
度/MPa弹性模
量/MPa泊松比 Y0 0 1.855 44.31 4 279 0.110 Y1 10 1.827 25.73 2 499 0.198 Y2 20 1.873 37.74 2 623 0.351 Y3 30 1.869 39.78 3 645 0.175
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表 4 不同致裂时间下煤样的特定渗透率
Table 4 Specific permeability of coal samples at different cracking time
试样编号 致裂时
间/min初始渗透率/
10−15 m2最小渗透率/
10−15 m2试验测得最大
渗透率/10−15 m2Y0 0 0.0 105 0.0 008 0.0 271 Y1 10 0.0 196 0.0 020 0.0 319 Y2 20 0.0 217 0.0 026 0.0 391 Y3 30 0.0 230 0.0 043 0.0 668
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