Experimental study on coal permeability and cracking characteristics under LN2 freeze-thaw cycles

Liquid nitrogen (LN2), a new and effective reservoir stimulating medium, has been widely used to boost coal bed methane (CBM) production. In this study, the damage mechanism and permeability evolution of the coal during LN2 freeze-thaw cycles are explored. The permeability tester was used to obtain the permeability of coal under different confining pressure and freeze-thaw cycles; the camera and scanning electron microscope (SEM) were respectively employed to observe cracks in the coal from the macroscopic and microscopic perspectives, and the ultrasonic wave was used to monitor the wave velocity of the coal. The experimental results showed that initial defects in the coal were firstly destroyed, forming the main fracture network; with the increase of cycles, the cracks were generated in coal matrix and gradually expanded and intersected with the main fracture network, then a complex fracture network was formed. The above results are consistent with the results of wave velocity. The wave velocity decreased about 40% after 10 cycles. Thus, the permeability of coal increased with the increase of cycles. Under the confining pressure of 9 MPa, the permeability reached 1.216 mD with an increase of 1200 times after five cycles. Besides, the coal sample with the small initial wave velocity had the most obvious improvement of permeability. The increase of confining pressure led to the decrease of permeability, and the sensitivity of permeability to confining pressure increased with the increase of cycles. Especially, under the low confining pressure, there was the biggest decrease of 3.44mD on the permeability for 1 MPa increment of confining pressure. During the freeze-thaw cycles, the temperature stress was the main factor of the damage on cracks and cementation structures, and LN2 gasification can cause the secondary propagation of cracks. Moreover, the uncoordinated deformation of matrix and mineral particles was the main factor promoting the initiation of cracks.