Experimental study on the physical and mechanical variations of hot granite under different cooling treatments

Geothermal energy arrested in HDR (hot dry rock) is commonly harvested from the reservoir fractured by artificial stimulations. Understanding the physical and mechanical behaviors of HDR after different cooling treatments is critical for selecting most efficient fluid to fracture the rock mass. Here we experimentally examine the physical and mechanical variations of a typical type of HDR, granite, after natural, water, and liquid-nitrogen cooling treatments, followed by a multi-scale investigation on the evolution of cracks. We find that the effect of cooling treatment on the physical and mechanical properties of heated granite specimens is increasingly remarkable as the peak temperature grows. The difference among the influences of different cooling treatments is unnoticeable when the specimens are heated to peak temperatures of 200 °C and 400 °C, but becomes highly pronounced as the peak temperature ascends to 600 °C and 800 °C. Liquid-nitrogen cooling treatment induces most alterations in the granite properties. The cracks in the granite specimens cooled by liquid-nitrogen coalesce together and form complex fracture networks, which are more intensive than those by natural and water cooling treatments. Previous studies claimed that the liquid-nitrogen cooling treatment had weaker influence on the granite specimen than water, possibly due to the Leidenfrost effect that impeded the heat transfer between rock and liquid-nitrogen. Our findings demonstrate that liquid-nitrogen is superior in cracking and thus enhancing the permeability of the granite specimens, particularly for those heated to a temperature over 400 °C. The good performance of liquid-nitrogen is attributed to that the granite specimen is easily cracked by the high tensile stress created jointly by the high temperature difference caused by its ultra-low temperature and large convective heat transfer coefficient induced by violent boiling.