Effect of liquid nitrogen thermal shock on structure damage and brittleness properties of high-temperature marble

As a new waterless fracturing technology, liquid nitrogen (LN2) fracturing is expected to be applied in the exploitation of geothermal energy. Therefore, understanding the thermomechanical features of hot rock under LN2-cooling is essential. In this study, a battery of laboratory-scale experiments on the marble with the temperatures of 25-300 degrees C are conducted to reveal the changes in their structure damage and brittleness properties after both the air-cooling and LN2-cooling treatments. Following LN2 cooling, the P-wave velocity, compression strength, elastic modulus, and brittleness index of the marble reduce, while the complexity of pores, macro-pore volume, and porosity increase. The higher the marble's temperature is, the more significant the changes in these properties are. The extensive changes in the thermomechanical behaviors of the marble are induced by LN2-cooling compared with air-cooling. The results indicate that LN2-cooling can produce greater damage than air-cooling and the influence of LN2 thermal shock can be strengthened by increasing the marble temperature. After the thermal shock, the brittleness properties of the marble are closely related to its structural damage degree. During LN2 thermal shock, there are two primary contributions to the structural damage. The first is the temperature gradient inside the heated marble, which can generate surface macro-fractures, while the second is the heterogeneity and anisotropy of thermal expansion coefficients of minerals. Inter-granular cracks can be readily produced by the difference in thermal expansion of adjacent minerals and trans-granular cracks can be produced due to the difference of thermal expansion coefficients in different crystallographic directions of the mineral.