Experimental study on mechanical properties, acoustic emission characteristics and energy evolution of coal samples after freezing with liquid nitrogen

• LN 2 freezing and freeze–thaw can damage the pore and fracture structure. • LN 2 freezing and freeze–thaw can reduce the P-wave velocity of coal sample. • The expansion process of pores and fractures was analyzed by MRI. • The mechanical property is analyzed by acoustic emission and energy evolution. Liquid nitrogen (LN 2 ) fracturing technology is of interest to the petroleum/energy industry because of its limited water consumption and non-pollution compared with conventional hydraulic fracturing. Currently, the usefulness of LN 2 fracturing as a new reservoir stimulation technology in coal seams is not yet clear. To investigate the effect of LN 2 freezing and freeze–thaw on the mechanical properties of coal, the expansion of pores and fractures of coal samples during LN 2 freeze–thaw was analyzed by MRI. Uniaxial compression and acoustic emission tests were also carried out on the coal samples for each type of LN 2 freezing condition. The P-wave velocity, uniaxial compressive strength, acoustic emission, and energy evolution characteristics of coal samples for different LN 2 freezing conditions were analyzed, and the mechanisms causing damage during LN 2 freeze–thaw were discussed. The results show that: (1) LN 2 freezing and freeze–thaw can damage the pore and fracture structure of the coal samples, causing the expansion and connection of the original fracture, which lead to the decrease in the P-wave velocity and the relative attenuation rate of the coal samples. (2) The bearing capacity of the coal samples decreases after LN 2 freezing and freeze–thaw, causing a decrease in the uniaxial compressive strength of the coal samples. (3) LN 2 freezing and freeze–thaw do not change the period of acoustic emission events of coal samples during loading during uniaxial loading. The maximum acoustic emission ring counts and accumulative acoustic emission ring counts of the coal samples decrease with an increase in the LN 2 freezing time and the freeze–thaw cycle. (4) LN 2 freezing and freeze–thaw can initially damage the coal sample, reducing the total energy and elastic energy of the coal sample. The higher the degree of initial damage, the lower the secondary damage required for the failure of the coal sample, which results in a decrease of dissipated energy.