School of Safety Engineering, China University of Mining and Technology
National Engineering Research Center for Coal Gas Control

低温流体液氮向煤层钻孔内注入过程中的传热传质规律及煤损伤分析，是目前液氮在煤层增透领域应用中亟待研究的关键问题。通过利用流量计、内视镜、热电偶、质量天平和超声探测仪，可视化分析了液氮注入过程中煤钻孔内液氮的初始累积过程和液位状态变化，探究了不同注入速率下钻孔内液氮的实际液位、净液位和注入效率随注入时间的变化规律，并对3个不同时刻下的煤体进行超声损伤检测。研究发现，液氮初始累积时会在钻孔底部煤的交界面处形成明显的莱顿佛洛斯特效应，随着钻孔底部煤温的不断降低，辐射换热系数不断减小，进而蒸汽膜厚度随之降低。不同注入速率下，1 m长钻孔内液氮的净液位最大增加量均为35 cm左右，这意味着，向长钻孔内注入液氮时，应实施分段注入，仅增加液氮注入时间会造成液氮的浪费。实际液位与净液位比值的变化规律均为先降低然后趋于稳定，稳定状态下2者的液位比相近。不同注入速率下液氮注入效率随注入时间的变化规律，均为降低—升高—稳定—降低。低速注入速率下的液氮注入效率更高，提升注入速率后单位体积液氮所相变的氮气，在钻孔内换热时间更短、向钻孔外排出温度更低，进而增大了液氮损耗，使注入效率变低。超声波测试结果表明，液氮能够渗入裂缝内部，使裂缝两侧在冷冲击的作用下冻缩，增加原有的裂缝宽度并使缝尖受到裂缝两侧冻缩力的作用拉伸扩展变形，形成新的裂缝。

Currently the law of heat and mass transfer and the analysis of coal damage in the process of injecting low-temperature liquid nitrogen (LN2) into the borehole of coal seam are the key problems to be solved urgently for the application of LN2 in the field of coal seam permeability enhancement. By using flowmeter, endoscope, thermocouple, mass balance and ultrasonic detector, the initial accumulation process and liquid level changes of LN2 during its injection into a coal borehole were visualized and analyzed. On this basis, the variation laws of the actual liquid level, net liquid level, and injection efficiency of LN2 in the borehole with time at different injection rates were investigated, and the coal body was ultrasonically detected for damage at three different times. It was found that the initial accumulation of LN2 will form an obvious Leidenfrost effect at the interface of the coal at the bottom of the borehole. With the continuous reduction of the coal temperature at the bottom of the borehole, the radiation heat transfer coefficient decreases, and then the thickness of the vapor film decreases. At different injection rates, the maximum increment of the net level in a 1 m borehole is about 35 cm, which means that when LN2 is injected into a long borehole, it should be injected segmentally, and the increase of injection time of LN2 will lead to the waste of LN2. The ratio of actual liquid level to net liquid level decreases first and then tends to be stable. The ratio of two liquid levels is similar in stable state. The change rule of LN2 injection efficiency with time at different rates is reduction-increase-stability-reduction. The injection efficiency of LN2 is higher at low injection rate. When the injection rate is increased, the nitrogen gas with phase change per unit volume of LN2 will have shorter heat exchange time in the borehole and lower discharge temperature outside the borehole, thus increasing the loss of LN2 and lowering the injection efficiency. Ultrasonic detection results show that as LN2 penetrates the fractures, the cold shock effect can freeze and shrink the fractures in both sides. As a result, the original fractures grow wider and their tips are stretched and deformed by the freezing and shrinking forces on both sides. Consequently, new fractures are formed.

国家杰出青年科学基金资助项目(51925404)；国家自然科学基金项目资助项目(51774278)；江苏省研究生科研与实践创新计划资助项目(KYCX22_2657)

丛钰洲，翟成，丁熊，等. 煤层钻孔内注入液氮过程中的传热传质规律及煤损伤分析[J]. 煤炭学报，2023，48(8)：3128−3137.

CONG Yuzhou，ZHAI Cheng，DING Xiong，et al. Analysis on heat and mass transfer law and coal damage during liquid nitrogen injection into coal seam borehole[J]. Journal of China Coal Society，2023，48(8)：3128−3137.