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The cement sheath is an annular structure between casing and formation, which is crucial to the integrity of the wellbore system. Considering that the temperature and pressure environment is changing continuously with increasing burial depth, the micro-structure and macro=mechanical properties of the in-situ cement sheath will change accordingly. To investigate the variation of burial depth on the evolution of the tensile mechanical behavior of oil cement stone, five temperature-pressure curing and testing conditions (25 °C—0 MPa, 50 °C—10 MPa, 80 °C—20 MPa, 110 °C—30 MPa, and 140 °C—40 MPa) are set to approximately simulate an in situ temperature-pressure environment at five typical burial depths (0 m, 1000 m, 2000 m, 3000 m, and 4000 m). The in situ tensile behavior, micro-structure and pore size distribution of the cement stones at each condition are tested and comparatively analyzed. Results show that with increasing temperature and pressure, the brittleness of the cement stone reduces and its ductility strengthens accordingly. The tensile strength experiences rapid growth at first, then increases at a slower rate and finally decreases. The failure mode of the cement stone gradually transforms from tensile splitting to tensile-shear composite fracture, accompanied by increasing fracture surface roughness. Microscopically, with increasing curing temperature and pressure, the pore structure of cement stone gradually transforms from closely stacked laminated sheets to interconnected fiber networks. The dense structure of cement stone gradually becomes loose and porous. The porosity also increases from 15.96% to 29.46%. [ABSTRACT FROM AUTHOR]

Continental shale is abundant with unconventional oil and gas and has considerable potential for exploitation. However, due to differences in the deposition environment, continental shale presents unique characteristics in terms of mineralogy and mechanical properties compared to marine shale. Furthermore, the mechanical influence of fracturing fluid immersion must be considered because of the dual effects on the productive shale system caused by fracturing fluids remaining in the formation. In this study, tri-axial compression tests were performed on Jurassic continental shale specimens collected from an exploratory well (2778–2816 m deep) in the Sichuan Basin of China before and after exposure to fracturing fluids. Meanwhile, two quantitative indicators, the roughness R , which characterizes the widely distributed yield deformation, and the complexity F , which evaluates the complex multi-fractures, were used to distinguish the different brittle-plastic characteristics of specimens before and after fracturing fluid immersion. The results showed that the immersion of fracturing fluids weakened the mechanical properties of continental shale. Two types of fracturing fluid immersion effects were distinguished by the statistics of the failure characteristics of the different layers: lubrication-dominated and softening-dominated. Unlike conventional smooth tri-axial stress-strain curves, many of the post-peak curves were in a 'step-like' shape, corresponding to two sources: one part came from the formation of multi-fractures (brittle characteristics), and the other part came from the softening deformation of main failure surface (plastic characteristics). A new brittleness index, BI new , was proposed considering failure characteristics with different brittleness effects, which could effectively distinguish the brittleness difference before and after exposure to fracturing fluids. • Mechanical variations of continental shale along longitudinal profile were studied. • Different effects of fracturing fluids were distinguished by failure characteristic. • Different sources of additional energy dissipation were recognized. • New brittleness index was proposed incorporating different failure characteristics. [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]