140 / 2024-03-01 16:20:02

Experimental study on the tensile strength and fracture roughness in thermally treated granite specimens with different cooling ways

Granite, thermal damage; cooling in water, tensile strength, roughness

The study on the mechanical properties of rocks after high-temperature treatment is crucial for ensuring the engineering stability and enhancing the efficiency of deep geothermal exploitation of hot dry rock (HDR). During the HDR exploitation process, the high-temperature reservoir rocks are cooled down by water, and understanding the strength and damage characteristics of these rocks post-cooling is important. To investigate the effects of different cooling ways on rock strength and failure patterns, this study presents an experiment of thermal granite specimens subjected to natural cooling and water cooling. Brazilian split tests were conducted on the thermally treated granite to observe the evolution of tensile strength under different cooling conditions. Additionally, using a laser scanning system, three-dimensional profiles of the fracture surfaces were obtained, and their roughness was calculated through programming to analyze the relationship between tensile strength and surface roughness. Finally, combining SEM observations of the fracture surfaces, the thermal cracking mechanisms in granite, including thermal cracks, grain size, and water weakening effects, were explored. The results indicate that: (1) The tensile strength of granite decreases with increasing temperature, and the strength of specimens cooled by water is lower than those naturally cooled. (2) The roughness of the Brazilian split fracture surfaces increases with temperature, but the cooling way does not significantly affect the surface roughness. (3) As the temperature rises, mineral grains grow larger, the degree of thermal damage increases, and the microstructure of the granite deteriorates further. This transition from transgranular to intergranular fracture leads to a reduction in granite strength and an increase in surface roughness. The conclusions of this study are expected to provide theoretical insights into the damage and fracture mechanisms of reservoir rocks during the deep geothermal exploitation of HDR.