The changes in pore characteristics and mechanical properties of coal rock of water-saturated coal and rock after freeze-thaw cycles

Liquid nitrogen freeze-thaw has a positive effect on increasing the permeability of coal and rock.In order to study the pore characteristics and mechanical characteristics of water-saturated coal rock under liquid nitrogen cycle freezing and thawing, nuclear magnetic resonance, triaxial loading and seepage experiments were carried out on water-saturated frozen coal rock.The results show that: (1) After the freeze-thaw cycles, various types of pores in the coal and rock gradually develop. This is because liquid nitrogen freeze-thaw can promote the generation and development of micro-pores, and expand and fuse them into medium and large pores, thereby forming a fracture network; (2) After five freeze-thaw cycles, the porosity of the coal and rock increased from 1.089% to 2.571%, and the permeability increased from 0.771 mD to 19.884 mD. Compared with the initial freeze-thaw, the number of adsorption pores in the coal and rock increased from 1.15 times to 2.82 times, and the number of seepage pores increased from 1.80 times to 6.23 times; (3) After multiple freeze-thaw cycles, the internal fracture structure of the coal and rock gradually expands and fuses to form new fractures. The generation of new fractures releases the bound water in the adsorption pores to larger pores and transforms it into free water, resulting in a significant increase in the porosity and permeability of the coal and rock; (4) With the increase in the number of freeze-thaw cycles, the rate of energy accumulation in the coal and rock before the peak gradually decreases.