Action scope and mechanical damage mechanism of liquid nitrogen cold shock on coal

The range of the cold shock of low-temperature liquid nitrogen on coal, the variation law of thermal stress in coal caused by temperature difference and the mechanical action mechanism of coal damage are the key issues to be studied urgently in the application of liquid nitrogen in the field of coal and rock permeability enhancement. Using the non-contact full-field strain measuring device, infrared temperature distribution measuring device and the thermocouple temperature measuring device, the variation laws of the coal temperature field and strain field around the borehole with time are measured after the liquid nitrogen cold shock on the borehole of coal samples in the open environment. By establishing a thermal-stress model including elastic modulus, Poisson’s ratio, thermal expansion coefficient, temperature difference of measuring points and radius parameters of measuring points, the experiments are designed to measure the parameters of models and the parameters are input into the models for solution. The thermal stress at different positions in coal and rock and its variation law with cold shock distance and time are calculated, and the above laws of three types of coals with different metamorphic degrees are compared. The results show that in the process of liquid nitrogen cold shock, the temperature at each measuring point of three types of coal samples all shows a variation law of rapid decrease-slow decrease-stability, and the thermal stress at each measuring point has experienced three stages: rapid growth-slow growth-zero growth. The closer to the cold shock borehole, the greater the temperature difference between adjacent measuring points, and the greater the thermal stress. After 1 800 s of liquid nitrogen cold shock, the thermal stress at each measuring point of three kinds of coal samples basically reached a stable stage, and the thermal stress at 1 cm from the cold shock drilling hole was ≥ 1.84 MPa. The cold shock of liquid nitrogen can cause four kinds of deformation in the coal, and then damage the coal. Among them, the type II, III, IV deformation caused by “pressure-induced tension” and the compound deformation of three are the fundamental reasons for the formation of complex cracks in the coal. However, although the range of cooling and deformation of different coals is different, the overall range of action is limited, and it can only produce damage around the borehole. Therefore, the liquid nitrogen cold shock technology is suggested to be applied in combination with fracturing technologies, and the damage around the borehole after cold shock can effectively reduce the initiation pressure and make the fracturing fracture more complicated.