Effect of high temperature on the mixed mode I-II fracture characteristics of hot-dry rock

Abstract

To investigate the variation law of rock fracture characteristics with temperature in hot-dry reservoirs, the pure mode I, pure mode II and mixed mode I-II loading experiments were carried out on sandstone and granite at room temperature and after high-temperature (100 °C, 300 °C, 500 °C and 700 °C) treatment by using the central cracked Brazilian disk (CCBD) specimen. The changes of parameters such as failure load, fracture toughness and fracture envelope with temperature were discussed, and the changes of rock mineral composition and fracture surface morphology were analyzed by X-ray diffractometer (XRD) and scanning electron microscope (SEM), respectively. The results showed that the influence of temperature on the fracture characteristics of sandstone and granite presents different evolution laws.With the increase of temperature, the failure modes of sandstone and granite all change from brittle failure to ductile failure, but the critical temperature of the transition is different. Granite shows obvious ductile failure characteristics at 500 °C, while sandstone shows slight ductile failure characteristics at 500 °C.The failure load and fracture toughness of sandstone increase first and then decrease with the increase of temperature, which indicates that within a certain range, the temperature has a toughening effect on sandstone, and can improve its failure resistance. However, the failure load and fracture toughness of granite always decrease with the increase of temperature, which indicates that the temperature only has a deteriorating effect on granite. In the range of 100 °C to 500 °C, the temperature enhances (sandstone) or weakens (granite) the mode I fracture toughness of rocks more than it enhances or weakens the mode II fracture toughness. However, when the temperature is 700 °C, the temperature weakens the mode II fracture toughness more than the mode I fracture toughness. The area enclosed by the fracture envelope of sandstone first increases and then decreases with the increase of temperature, while the area enclosed by the fracture envelope of granite decreases with the increase of temperature. As the temperature increases, the tortuosity of the crack paths in sandstone and granite increases, the cross-section morphology becomes rougher, and the internal fissures and fragmentation increase.