Microcrack formation and fracture characteristics in granite under supercritical water conditions

In this study, the triaxial compression experiments were conducted to investigate the temperature and pore pressure effects on fracture strength of a granite. The experiments were conducted at temperatures and pressures up to 600°C and 150MPa both under water (supercritical) and air-dry environment. Experimental results show that the strength of the granite under air-dry conditions is temperature insensitive within the temperature range of this study. The strength of the granite was also nearly constant up to 350°C under water-saturated conditions. In contrast, under temperatures above the critical point of water (374°C and 22MPa or more temperature and pressure), the strength decreases rapidly with increasing temperature. We observed the micro-structural change around fracture surface using l00gm thick thin section of granite samples. Optical observation results around the fracture surface revealed that the crack density gave a higher value in supercritical water condition than in sub-critical water conditions. It was shown that the strength decrease under supercritical water conditions was due to the formation of micro-fractures induced by the effect of supercritical water as well as the stress effect, which may be referred to as supercritical water enhanced stress corrosion cracking. These experimental results suggest that it may be possible to generate a volume-like (microcrack predominant) reservoir in supercritical rock masses utilizing the stress corrosion cracking phenomena under supercritical water conditions, which is more advantageous to extract the heat energy compared with a crack-like reservoir.