Experimental investigation of mechanical behavior and thermal damage of hot dry rock exposing to different cooling conditions

The exploration of underground geothermal energy requires investigating the cooling effects on the mechanical behaviors and damage mechanisms of high-temperature granite due to injection-production activities. The present work focuses on the influence of preheating and subsequent cooling treatments on the thermal damage of mechanical behavior/properties of a fine-grained granite, with special attention to the cooling method. The studied granite, drilled from a depth of 2000 m in Sichuan Province, China, was heated from room temperature to 800 °C and cooled by air and water cooling methods. It is observed that the cooling method has an important impact on the mechanical and damage behavior of the studied granite once the pre-heating temperature exceeds 500 °C. The dramatic transition of crack threshold is also observed as a function of temperature and cooling rate. Moreover, the experimental analysis exhibits that thermal conductivity is capable of providing a satisfactory estimation of thermal damage level of granite, compared to P-wave velocity. Finally, the influence of crystal particle sizes on the thermal damage of granite is also performed. Compared to fine-grained (FG) and medium-grained (MG) granites, one observes that during the heating phase, the most important thermal damage is observed in coarse-grained (CG) granites while CG granites are less influenced by the cooling methods. The obtained experimental results and analysis can help understand the mechanical and failure behavior of granite exposed to different cooling regimes encountered in deep geothermal projects.