NMR study on the distribution pattern of pore water in the process of freeze-thaw permeability enhancement of coal

Freezing and thawing damage of slope rock is one of the main hazards of surface coal mines in cold regions, which can easily cause rock damage and slope instability and landslides. However, this pore space due to freeze-thawing can increase the production of coalbed methane when mining deep coal seams. Water is one of the most important factors in slope destabilization, but is the key to fracturing coal seams by liquid nitrogen freeze-thaw. In this paper, we carry out liquid nitrogen (LN₂) cycling freeze-thaw experiments based on nuclear magnetic resonance (NMR) technology to study the characteristics of pore water changes. The results of the study show that: (1) Pore bound water mostly exists in small and medium-sized pores, and pore free water mostly exists in large pores; with the increase of the number of freeze-thaw cycles of LN₂, the pore expansion causes the migration of pore water, and the relaxation time of the first peak appears to be overtaken and lagged; (2) Coal samples had the best effect at 15 freeze-thaw cycles. The number of freeze-thaw cycles was positively correlated with water content volume fraction, pore water accumulation, effective pore water percentage, and effective pore throat percentage before 15 cycles, and the opposite was true after 15 cycles, and BBC coal sample had the best fracturing effect; (3) The initial pore connectivity of the coal is poor, the degree of mobilization is high, and the pore water is endowed with less pore water, and the degree of pore water mobilization decreases and then increases by freeze-thaw cycles, and the difference is the smallest in the 15th cycle; (4) LN₂ low temperature and pore water phase change caused by pore water “water wedge and ice wedge” mutual transformation caused by pore damage.