Failure Behavior of Hot-Dry-Rock (HDR) in Enhanced Geothermal Systems: Macro to Micro Scale Effects

Evaluations of the mechanical properties and failure modes of granite at high temperatures are important issues for underground projects such as enhanced geothermal systems and nuclear waste disposal. This paper presents the results of laboratory experiments that investigated the physico-mechanical failure behavior of granites at high temperatures. The results allowed several important conclusions to be drawn. Both the uniaxial compressive strength (UCS) and tangent modulus decrease with increasing temperature. Specifically, the UCS-temperature curve can be divided into three sections: a section (20-200°C) where UCS shows a slight decrease, a section (200-300°C) where the UCS decreases significantly, and a third section (300-500°C) where the rate of UCS decrease stabilizes. However, in the entire temperature range from 20 to 500°C, the tangent modulus decreases exponentially. The number of acoustic emission (AE) counts decrease and the counts occur less frequently at higher temperatures. Individual grains are surrounded by a large number of microcracks at 200°C and the crack length increased significantly when heating to 300°C. Specifically, the length of micro-cracks in the granite at 300 °C could be 10 times longer okthan that at 200°C. Quenching or injecting cold water into HDRs would further weaken the rock and induce thermal damage to the rock structure. The strength of rock would be further quench-weakened by 10%, 20% and 30% at 200°C, 300°C and 500°C, respectively. Therefore, in Enhanced/Engineered Geothermal Systems (EGS), quenching is much more destructive than normal thermal stress.