Experimental study on the mechanical properties of coal and rock under heating- liquid nitrogen cooling impact cycling

As a new type of anhydrous fracturing method, liquid nitrogen (LN2) fracturing provides a new way for the efficient exploitation of coalbed methane (CBM). In order to reveal the physical and mechanical properties and fracture characteristics of low permeability coal rock under LN2 fracturing, the heating and LN2 cold shock cycling test system was used to carry out LN2 fracturing tests, and the fracture pressure, acoustic emission (AE) characteristics and section morphology of coal rock under different LN2 cold shock cycling treatments were obtained. On this basis, the effect of LN2 cycling treatment on coal rock cracking was discussed. The results show that the average velocity of longitudinal wave is 1.421 km/s for the coal samples treated with 0 LN2 freeze-thaw cycles, and the average velocity of the coal samples treated with 15 LN2 freeze-thaw cycles is 0.256 km/s, and the decrease of wave velocity is 81.98%. The size and number of internal defects in coal samples increased significantly due to LN2 freeze-thaw. The tensile strength of coal samples in the 0-cycle group decreased from 0.943 MPa to 0.346 MPa in the 15-cycle group, with a decrease of 63.31%. After 15 freeze-thaw cycles of the coal sample and multiple LN2 cycles, the internal structure of the coal mass was fully damaged, resulting in full deterioration of the internal structure of the coal sample and the largest decrease in tensile strength. Under 0 cycles of fracturing, the original initial cracks were pressed and rubbed with each other, and the loading damage was small at this time. Multiple LN2 freeze-thaw cycles induce fracture initiation, development and connectivity of original fractures, and eventually form macroscopic fractures and begin to destroy, which is most obvious for coal sample defects. The box dimension and roughness of the 0-cycle coal sample section are 2.876 and 1371.376 μm, respectively, and the box dimension and roughness of the 15-cycle coal sample section are 2.993 and 2899.494 μm, respectively, and the growth rate is 4.07% and 111.43%, respectively. There is a positive correlation between roughness and box dimension, both of which increase with the increase of freeze-thaw cycle.