Thermal Effects on the Physical and Mechanical Properties and Fracture Initiation of Laizhou Granite During Hydraulic Fracturing

Laboratory experiments are performed on Laizhou granite samples after heating and rapid water-cooling treatment to investigate the thermal effects on the physical and mechanical properties and fracture initiation of granite during the utilization of hot dry rock. Experimental results show that P-wave velocity and tensile strength monotonously decrease because of the generation of thermally induced microcracks when the thermal treatment level increases from 25 to 500 degrees C. Scanning electron microscopy analysis indicates that few and isolated microcracks are observed before 300 degrees C, whereas microcrack networks are generated at 400 degrees C and 500 degrees C. Porosity and gas permeability sharply increase with the enhancement in the density and connectivity of microcracks. The variation trend of compressive strength tends to show a transition from increasing to decreasing at the threshold temperature. Uniaxial compressive strength and cohesion increase and the Young's modulus slightly changes before 300 degrees C. Afterward, the three parameters decrease, indicating that the dominant mechanism transforms from thermal hardening to thermal cracking. Failure process of uniaxial compression shows that the proportion of crack closure region in the pre-peak stage gradually increases, whereas those of the elastic and stable crack growth regions decrease after 300 degrees C with the increase of the thermal treatment level. Experimental results of laboratory fracturing tests show that the decreasing tendency of breakdown pressure is similar to that of tensile strength. The pressure build-up rate substantially decreases because of the enhancement in fluid leak-off, and hydraulic fracture initiation occurs before the breakdown of granite samples when the thermal treatment levels are higher than 300 degrees C. Shear failure becomes dominant based on the hypocenter mechanism analysis of acoustic emission events.