The role of multiple heating and water cooling cycles on physical and mechanical responses of granite rocks

Abstract

Investigating the coupling effects of temperature levels and number of heating-water cooling cycles on thermal damage, cracking failure mechanisms, and mechanical responses of HDR (hot dry rock) is a vital issue during the exploitation of geothermal energy. In this study, a large number of granite samples exposed to high temperature treatment (25, 150, 300, 450, 600\xa0°C) with multiple heating and water cooling cycles (1, 5, 10, 15) were carried out micro observations of X-ray diffraction (XRD) and scanning electron microscopy (SEM), and uniaxial compression tests with acoustic emission (AE) signals monitoring. The results show that, as the temperature level or number of heating and water cooling cycles increases, uniaxial compressive strength, elastic modulus, crack damage stress, and the damage index all present a reduction, while the peak axial strain, dilatancy volumetric strain and Poisson’s ratio increase. A genetic algorithm is established to evaluate the coupling influences of temperature and number of heating-water cooling cycles on the uniaxial compressive strength and dilatancy volumetric strain. The AE activities tend to be active due to crack initiation, development and coalescence. For a large temperature level or number of cycles, the AE signals remain active during the whole deformation process, while increase rate of the accumulated AE counts with time gradually slows down. The granites undergo a typical splitting failure mode, characterized by several tensile cracking planes at a low temperature or number of cycles, but present complicated crack networks of a turtle shape with increasing temperature or number of cycles, resulting in a larger fractal dimension of the induced surface cracks. The failure mechanism of rock gradually transfers from brittleness-dominated to ductility-dominated due to accumulated thermal damage. Then, the micro thermal damage mechanism of uncompressed granites is analyzed through microscopic observations of XRD and SEM. A power function is proposed to describe the relations between longitudinal wave velocity and uniaxial compressive strength. With increasing temperature or number of cycles, the maximum diffraction intensity of quartz decreases by 25.82%–38.89% and 13.42%–31.93%, respectively, and the micro thermal defects gradually develop, resulting in weakened macro bearing capacity.