Study on the Evolution of Pore Structure of Anthracite Coal under Liquid-Nitrogen Freeze-Thaw Cycles

To analyze the evolution characteristics of the internal pore structure of anthracite by liquid-nitrogen (LN₂) freeze-thaw cycles, nuclear magnetic resonance was used to test the water-saturated samples, which were frozen-thawed with LN₂ for 0-9 exposure. The pore size distribution, development degree, and variation characteristics of different pores were examined from the changes of parameters related to T ₂ spectra, which increased the macropore frequency: First, LN₂ freeze-thaw cycles are beneficial to the development of cracks and have the greatest promoting effect on macropores. Second, the crack development rate of coal samples decreased with the increase of the number of freeze-thaw cycles. The crack development rate was the fastest in the first freeze-thaw cycle, and the crack development did not increase significantly after the third freeze-thaw cycle. Third, the pore structure of anthracite is under the influences of thermal stress and frost heaving force (the force caused by the expansion of water as it freezes). Therefore, it can be determined that three times of LN₂ freeze-thaw cycles has the best effect in the LN₂ injection to increase coal permeability and recover coalbed methane. The results provide theoretical support for the field application of coal freezing cracking and antireflection promotion pumping.