Evolution of mechanical properties of granite at high temperature and high pressure

Abstract

Rock engineering works, such as geothermal and deep hydrocarbon resource development and deep underground geological disposal of nuclear waste, are closely related to the mechanical properties of rocks at high temperature and high pressure. This study reports on the use of the micro-CT technique, acoustic emission (AE) technique and the 600 °C 20MN servo-controlled trixial compression system for rock testing under high temperature and high pressure to study the evolution of mesostructure, AE characteristics and the evolution of macro-mechanical properties of granite under high temperature and high pressure. The results obtained from the experiments show that: (1) Very few micro-cracks occur when granite is at 200 °C, while mylonitic crystal granular structures appear at 500 °C. (2) AE characteristics indicate that the thermal cracking of granite is intermittent and multi-stage. Under the influence of thermal cracking, the permeability of granite also presents several peaks. (3) Under the stress state equivalent to 1000 m buried depth, the thermal deformation and the thermal expansion coefficient both have different stages from room temperature to 600 °C. The thermal expansion coefficient under triaxial pressure is approximately 20 times less than the coefficient without confinement, showing that this parameter is profoundly affected by the confining pressure. The failure mode of granite under high temperature is shear failure, just as under room temperature. The stress–strain curve, however, presents different characteristics compared to those under room temperature. (4) When subjected to confining pressure, the elastic modulus of granite has different periods. Overall, it decreases with the increase of temperature.