Thermal Damage and Acoustic Emission Characteristics of High-Temperature Granite under Liquid Nitrogen Cooling

Abstract

Liquid nitrogen fracturing is an efficient stimulation technique for exploiting hot dry rock geothermal energy. Understanding the physical and mechanical damage characteristics of high-temperature reservoir rocks under liquid nitrogen cooling is crucial for the application of liquid nitrogen fracturing technology. Therefore, nuclear magnetic resonance technology, acoustic wave velocity measurement technique, acoustic emission (AE) technology, and 3D scanning technology were used to explore changes in the physical and mechanical properties of high-temperature granite under liquid nitrogen cooling from macroscopic and microscopic perspectives. Our research findings show that, as treatment temperature increased, the internal pore structure of the sample changed gradually, with decrease in proportion of micropores and increase in proportion of macropores. The number of pores of various sizes increased gradually. In particular, after treating the granite to a treatment of 600℃, there was a significant increase in the quantity of pores within the granite, primarily manifested by an increase in macropores. From 25 to 600℃, the compressive strength decreased from 160.79 to 68.44 MPa, a reduction of 57.44%; the tensile strength decreased from 11.13 to 6.02 MPa, a reduction of 45.91%. The fractal dimension of the fracture surface of Brazilian disk samples was calculated using the box-counting method, and the results indicated that an increase in treatment temperature would lead to an increase in roughness of the sample’s fracture surface. During the uniaxial compression tests, the AE parameter rise angle (RA) suddenly increased near the peak load. The straight line relationship (average frequency = 11RA + 60) was used to classify the AE signals generated during uniaxial compression of samples. With increase in treatment temperature, the shear signal increased gradually, which is highly consistent with the macroscopic failure characteristics of the samples.