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High temperature can damage rocks and change their mechanical properties. In this study, some basic and vital properties of granite heated at different temperatures were investigated. Combined with previous research results, the effect of temperature on some physical and mechanical parameters of granite and the relationships among P-wave velocity (vp), tensile strength (σt), and mode-I fracture toughness (KIC) were analyzed. The results showed that the mass, vp, and σt of granite decreased gradually with increasing heat-treatment temperature. The KIC increased slightly from room temperature (20 °C) to 100 °C and decreased gradually when it exceeded 100 °C. The ratio α of σt to KIC increased with increasing temperature; α is related to brittleness of the rock and it can be used as index for measuring its brittleness. The vp of granite had a strong linear relationship with the mechanical properties. The two damage factors based on vp can characterize well the degradation of the mechanical properties of granite below 400 and above 400 °C. The results provide an important guide for evaluating the damage degree of high temperature on rocks using the ultrasonic non-destructive testing technology and characterizing quantitatively the ductility and brittleness of rocks. [ABSTRACT FROM AUTHOR]

The safe and efficient development of ultra-deep shale gas deeper than 4500 m is closely related to the comprehensive evaluation of the mechanical behavior, brittleness and anisotropy characteristics of reservoir rocks under in-situ stress conditions. However, the above-mentioned information about ultra-deep shale is rarely reported. This study aims to fill this gap by systematically investigating the mechanical and anisotropic characteristics of cores, with 0° and 90° bedding orientations, from different layers in the vertical depth of 4900–4930 m, under confining pressure of 110 MPa. Furthermore, a new brittleness index with three parts related to brittle failure, i.e., the energy storage performance in the pre-peak stage, the performance of elastic energy used to maintain the post-peak failure, and the severity of the energy released during the post-peak stage, all of which are physically meaningful, was proposed. Subsequently, the index was validated and applied to the brittleness evaluation of ultra-deep reservoir rocks. The brittleness was evaluated in the following descending order: black shale of Longmaxi Formation ＞ gray-black shale of Wufeng Formation ＞ limestone of Baota Formation. Overall, the Wufeng- Longmaxi Formation shale still exhibits brittle failure under such high confining pressure, while the Baota Formation limestone exhibits quasi-ductile failure, which is consistent with the phenomenological behavior of cores. Further, various experimental techniques have been employed to study the brittle and quasi-ductile failure mechanisms of reservoir rocks. The anisotropy evaluation results show that the anisotropy degree of the mechanical and brittleness parameters corresponding to the deep formation is significantly lower than that of the shallow formation. The anisotropy degree of strength and deformation parameters (Young's modulus, Poisson's ratio) is closely related to the content of quartz and clay minerals, respectively. The above results are expected to provide data support and theoretical guidance for the development of deep and ultra-deep shale gas in the southern Sichuan region of China. • The mechanical characteristics of ultra-deep reservoir rocks in the vertical depth range of 4900–4933 m are reported. • The influence of the bedding effect on the mechanical behavior of reservoir rocks under high confining pressures is studied. • A new three-part brittleness index in which each part has a clear physical meaning related to brittle failure is proposed. • The brittle and quasi-ductile failure mechanisms of reservoir rocks under high confining pressures are revealed. [ABSTRACT FROM AUTHOR]