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The stress threshold of deep reservoir shale subjected to fracturing fluid immersion is an important factor affecting fracture initiation and propagation during fracturing. However, little information has been reported on the effect on shale of soaking at high temperature and high pressure (HTHP). In this study, immersion tests and triaxial compression tests were carried out at reservoir temperature and in-situ stress on the downhole cores with different mineral compositions. The characteristics of stress thresholds, i.e., crack initiation stress ( σ c i ), crack damage stress ( σ cd ), and peak deviator stress ( σ p ), of shale affected by the different times of soaking with low-viscosity fracturing fluid (a) and the different viscosity fracturing fluids (a, b, and c) were investigated. The results show that hydration at HTHP has a significant softening effect on the stress thresholds ( σ c i , σ cd , σ p ) of reservoir shale, but the softening rate varies for samples with different mineral compositions. The crack initiation stresses of quartz-rich and clay-rich shales treated with different soaking times and different soaking media remain almost unchanged in the range of 47 to 54% of the corresponding peak strength, while the crack initiation stresses of carbonate-rich shales are significantly affected. The ratio σ cd / σ p of quartz-rich shale is significantly affected by the different viscosity fracturing fluids (a, b) and the different times of soaking with low-viscosity fracturing fluid (a), while clay- and carbonate-rich shales are less affected. The results of this study can provide a reference for the fracturing design of deep shale gas development. [ABSTRACT FROM AUTHOR]

The high temperature and high in-situ stress geological environment can significantly affect the mechanical properties, failure modes, and deformation characteristics of deep shale reservoirs. In this study, real-time high temperature triaxial compressive tests simulating the deep shale formation environment (temperature: 25–150 °C, confining pressure: 0–100 MPa) are carried out. The GSI-strength degradation and constitutive models are derived based on the Hoek–Brown criterion. The results show that in low confining pressure conditions, the mechanical behavior of shale is greatly influenced by temperature. Compared with shale at 25 °C, the compressive strength of shale at 150 °C decreases by up to 13.7%, and the elastic modulus decreases by up to 36.9%. The peak strain was increased by a factor of up to 1.4, and the yield stress level was advanced by as much as 7.4%. However, in high confining pressure conditions, the shale plasticity characteristics are significantly enhanced and the failure mode is relatively single. The GSI-strength degradation model can well characterize the variation law of shale strength with confining pressure under high temperature conditions. The statistical damage constitutive model matches the actual stress–strain curve very well, and it can fully reflect the deformation and failure characteristics of deep shale. The findings of this study can help us better understand the variation of mechanical properties of deep shale. [ABSTRACT FROM AUTHOR]

• The mode-I fracture toughness of granite at high temperature was tested in real time. • The increase in temperature in the low-temperature range universally toughens the rock. • Heat treatment method affects the mode-I fracture toughness of granite. • The temperature threshold of granite fracture toughness at real-time high temperature is 200 °C. • Fracture characteristics of granite at high temperature are similar to those after cooling. The mode-I fracture toughness of semicircular bend granite specimens at high temperature was tested in real time, and the failure specimens were examined using X-ray computed tomography. The results showed that with increasing temperature, the fracture toughness increased from room temperature (20 °C) to 200 °C, then decreased beyond 200 °C. Quasibrittle failure was observed up to 500 °C, followed by plastic characteristics at 600 °C. The cracks tended to propagate along the boundaries of mineral crystals. With increasing temperature, the main cracks became rougher and tended to propagate perpendicular to the tensile stress, and more branch cracks formed. The increase of temperature within a certain range had toughening effect on a variety of rocks and temperature-treatment method affected the mode-I fracture toughness of granite. Compared with the mode-I fracture toughness after cooling to room temperature, the temperature threshold of mode-I fracture toughness of granite at real-time high temperature was higher. The results provide guidance to determine fracture toughness and stability for high-temperature rock engineering. [ABSTRACT FROM AUTHOR]