Experimental investigation of thermal cycling effect on physical and mechanical properties of heated granite after water cooling

Abstract

Geothermal energy is recommended as a clean and renewable energy resource. During the exploitation process, hot rocks in a geothermal reservoir will be subjected to cyclic water cooling. Laboratory tests were conducted to investigate the physical and mechanical behaviors of water-cooled granite specimens subjected to various thermal cycles with temperatures ranging from 20 °C to 500 °C. Meanwhile, scanning electron microscope (SEM) images were used to capture the development of the microcracks within the specimens. It was found that volume increases with thermal cycle and mass, that density and P wave velocity decrease with thermal cycle, especially after the first thermal cycling, and their change rates diminish with increasing thermal cycle. Both uniaxial compressive strength (UCS) and elastic modulus (E) of granite decrease with thermal cycle. The decreasing extents gradually tend to constant with thermal cycle, and they remain almost unchanged once the thermal cycle reaches 20 iterations. The deterioration mechanisms of the physical and mechanical behaviors of granite are mainly caused by the generation and propagation of microcracks. Microcrack density increases slowly with the thermal cycle rising from 10 to 30, corresponding to the decrease of the physical and mechanical properties. The results are expected to provide an important contribution to the stability of well boreholes during the exploitation of deep geothermal energy.