Study on the surface crack propagation mechanism of coal and sandstone subjected to cryogenic cooling with liquid nitrogen

Conventional hydraulic fracturing often causes problems such as reservoir damage, water consumption, and pollution. Recently, liquid nitrogen (LN2) fracturing, an environmentally friendly fracturing technology, has attracted more attention. In this paper, the mechanism of surface crack propagation in coal and sandstone induced by LN2 is explored through laboratory tests and numerical simulations. An ultrasound detection analyser is employed to investigate the longitudinal wave velocity (V-p) changes in specimens before and after LN2 cooling. The micromorphology of specimens is observed through scanning electron microscopy (SEM). Furthermore, the heat transfer characteristics and damage zone of specimens induced by LN2 are determined. The experimental results indicate that LN2 cooling is more effective for improving coal permeability than it is for improving sandstone permeability. After LN2 cooling, a complex fracture network is observed from the coal surface. However, limited damage to the sandstone surface is observed. V-p decreases by 24.7%-38.1% for coal specimens after cooling but by less than 0.06% for sandstone. The simulation results indicate that the low-temperature region and the tensile area gradually expand into the interior of the specimens with time. However, the temperature of sandstone drops faster than coal when specimens contact LN2. The maximum tensile stress induced by the instantaneous contact between the specimens and LN2 can be generated at the outer surface of the specimens. Moreover, the maximum damage area of coal extends 3.3 mm from the outer surface, while the damage zone of the sandstone is almost zero.