Effect of Liquid Nitrogen Freeze–Thaw Cycles on Pore Structure Development and Mechanical Properties of Coal

[Image: see text] To improve the mining efficiency of coalbed methane, liquid nitrogen freeze–thawing experiments were performed to improve coal seam permeability and to study its influence on coal pore structure development and mechanical properties. Mechanical properties and nuclear magnetic resonance tests of coal samples were performed with 0, 5, 10, and 15 freeze–thaw cycles of liquid nitrogen. The results show that the number of freeze–thaw cycles caused the change of uniaxial compressive strength and elastic modulus of coal, and the change effect decreased significantly after 11–15 freeze–thaw cycles. Between 0 and 5 freeze–thaw cycles, the base growth rate of the transverse relaxation time T(2) spectral area of the full pore of coal is 44.1%, and that of the transverse relaxation time T(2) spectral area of adsorption pore is 71.5%. After 6–10 freeze–thaw cycles, the fixed base growth rate of the transverse relaxation time T(2) spectral area of the full hole of coal is 269.0%, and the chain growth rate is 156.2%. In this stage, the chain growth rate of the transverse relaxation time T(2) spectral area of the seepage hole is 198.4%, which is mainly the growth of seepage hole volume. After 11–15 freeze–thaw cycles, the chain growth rate of the full pore of coal transverse relaxation time T(2) spectrum area is 20.1%, the chain growth rate of adsorption pore is 4.8%, the chain growth rate of seepage pore is 22.2%, and the growth rate of the pore volume is greatly reduced. Comparing the changes of pore and coal mechanical properties in different pore sizes, it can be seen that the change of adsorption pore volume has a greater impact on coal mechanical properties.