基于氮气吸附法和压汞法低温液氮冻结煤体分形特征研究

Research on fractal characteristics of coal freezing with low temperature liquid nitrogen using nitrogen adsorption method and mercury intrusion method

液氮冻结是实现煤体孔隙贯通,提高煤层气抽采效率的有效方法。为定量化研究低温液氮冻结煤体孔隙结构特征,通过自主搭建液氮冻结煤体试验平台,分别对褐煤进行单次冻结和循环冻结,利用高压吸附仪和压汞仪测试液氮冻结过程中的煤体孔隙改造规律,分析液氮冻结煤体孔隙分形维数。试验结果表明:经过液氮冻结,煤体氮气吸附量和进汞量均增加,循环冻结煤体氮气吸附量与进汞量增加率分别是单次冻结煤体氮气吸附量和进汞量增加率的1.08倍和1.62倍。D_(1)(渗流孔分形维数)和D_(2)(吸附孔分形维数)均在2~3,孔隙分形特征显著。D_(1)、D_(2)随绝对冻结时间增加而减小,表明孔隙贯通性增强,内部结构复杂性降低。循环冻结煤体孔隙分形维数减小量大于单次冻结煤体孔隙分形维数减小量。D_(1)/D_(2)与绝对液氮冻结时间正相关,循环冻结过程D_(1)/D_(2)增长速率是单次冻结过程D1/D2增长速率的2.06倍。

Liquid nitrogen freezing is an effective method to achieve coal pore penetration and improve coalbed methane extraction.To quantitatively study the pore structure characteristics of low-temperature liquid nitrogen frozen coal,the self-developed liquid nitrogen frozen coal test platform was used to conduct the single freezing and cyclic freezing tests for lignite,respectively.The pore transformation law of coal in the process of liquid nitrogen freezing was examined by the high-pressure adsorption instrument and mercury injection instrument,and the pore fractal dimension of liquid nitrogen frozen coal was analyzed.The test results showed that after liquid nitrogen freezing,the nitrogen adsorption capacity and a mercury input capacity of coal increased.The increased rates of nitrogen adsorption capacity and mercury input capacity of circulating frozen coal were 1.08 times and 1.62 times that of single frozen coal,respectively.D_(1)(fractal dimension of seepage hole)and D_(2)(fractal dimension of adsorption hole)were ranging between 2-3,showing significant pore fractal characteristics.D_(1) and D_(2)decreased along with the increase of absolute freezing time,indicating an enhanced permeability of pores and a reduced complexity of internal structure.The reduction of the fractal dimension of circulating frozen coal was greater than that of single frozen coal.D_(1)/D_(2)was positively correlated with the freezing time of absolute liquid nitrogen.The growth rate of circulating freezing D_(1)/D_(2) was 2.06 times of the single freezing D_(1)/D_(2).