Influence of liquid nitrogen immersion on seepage characteristics of different rank coal samples

Coal seams with low permeability are widespread and the span of coal rank is large in China. How to choose a reasonable liquid nitrogen fracturing method for coals with different ranks and properties is an urgent problem to be solved. For this purpose,the coal samples of three coal ranks including lignite,bituminous coal,and anthracite are se- lected for liquid nitrogen fracturing under different conditions. Cameras are used to photograph (fixed point) and observe the evolution of macro-cracks on the coal sample surface. The gas flow tests with the Gas Flow and Displacement Testing Apparatus (GFDTA) and the nitrogen of 99. 999% purity show the change pattern of coal sample permeabili- ty. The trend of percolation curves of coal samples obtained from the experiments is briefly discussed,and the permea- bility values of coal samples at the different stages of liquid nitrogen treatment are compared and analyzed. The experi- mental results show that:① under the same confining pressure conditions,the permeability of coal samples shows a “U”-shaped change trend with gas pressure increases,and the fitting effect using the quadratic function formula is good;② a single liquid nitrogen immersion treatment can produce a significant permeability enhancement effect on lig- nite coal samples;③ by increasing the number of liquid nitrogen cycle treatments,the effect of liquid nitrogen on bitu- minous coal can be effectively improved. The maximum increase of bituminous coal permeability reaches 514. 89% af- ter one liquid nitrogen immersion treatment, and the maximum increase can reach 1 129. 79% after two treat- ments;④ the improvement of anthracite permeability by repeated liquid nitrogen treatments is not obvious,and will only gradually reduce its tensile strength. The liquid nitrogen immersion treatment method by increasing the tempera- ture gradient of heat and cold shock can effectively improve the permeability of anthracite. At confining pressure-gas pressure of 2. 00-1. 25 MPa,the permeability increase of anthracite with liquid nitrogen treatment and oven treatment of 50 ℃ is only 2. 44% ,while the permeability increase of anthracite with liquid nitrogen treatment and oven treatment of 100 ℃ reaches 125. 43% ;⑤ based on the experimental results,a liquid nitrogen cracking and permeability en- hancement method was proposed,which is suitable for different rank coal seams.