Temperature-dependent mechanical behaviour of Australian Strathbogie granite with different cooling treatments

Understanding the mechanical behaviour of reservoir rock under different temperatures with different cooling conditions is necessary for safe and effective deep geo-engineering applications, including geothermal energy extraction, deep geological disposal of nuclear waste, deep mining and coal gasification projects. The aim of this study is, therefore, to investigate the effect of increasing temperature (from room temperature to 800 °C) followed by two cooling methods (both rapid and slow) on the mechanical behaviour of Australian Strathbogie granite under uniaxial conditions. Further, a separate experimental program was conducted under continuous heating conditions without cooling the samples to compare the results of cooled samples. In order to investigate the strain developments in granite subjected to heating following slow and rapid cooling, ARAMIS photogrammetry technology was adopted, and the corresponding fracture propagation patterns were investigated using an acoustic emission (AE) system. Optical microscopic imaging technology was used to identify the corresponding micro-structural alterations and crack-formation patterns. According to the results, once the rock mass is subjected to higher thermal stresses, strength and elastic characteristics are significantly reduced, mainly due to thermally-induced damage in terms of both inter-granular and intra-granular cracks. The stress-strain response revealed that the failure mode of granite is changed from brittle to quasi-brittle fracturing with increasing temperature. The following cooling causes the strength and elastic characteristics of the granite to be further decreased through the enhancement of crack density, and the influence of rapid cooling is much greater than that of slow cooling, due to sudden thermal shock. This is evidenced by the AE results, according to which both high pre-heated temperatures and high cooling rates cause much quicker crack initiation and propagation in granite with lesser seismicity in the quasi-brittle region.