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The time-dependent characteristics of reservoir performance, where deep shale (vertical depth > 3500 m) interacts with fracturing fluid at high temperature and pressure (HTHP) during or after hydraulic fracturing, are closely related to the optimal design of fracturing in adjacent sections. However, the available information on the effect of hydration under HTHP on shale properties is insufficient, especially since the interaction between fracturing fluid and shales with different mineral compositions has rarely been considered comprehensively. In this study, fluid–shale interactions under HTHP were investigated for typical marine shales with different mineral contents (clay-rich, quartz-rich, and carbonate-rich) collected from the Wufeng–Longmaxi Formation in the southern Sichuan Basin, China. The time-dependent characteristics of the mechanical, brittleness, and fracture behaviors of immersion-treated samples at different soaking times were characterized by triaxial compression tests, and X-ray microcomputed tomography (CT) scans of post-test samples. The results show that the evolution of the mechanical parameters with soaking time is different for the three types of shales. The effect of hydration under HTHP on the strength of clay-rich shale and carbonate-rich shale except for the 15-day soaking case ranges from 39.24% to 54.88% and 24.05% to 33.99%, respectively, while the effect on quartz-rich shale is relatively slight, within 20%. The effect of hydration on the shale brittleness was evaluated based on a three-part energy-based brittleness index, in descending order, clay-rich shale > carbonate-rich shale > quartz-rich shale. The crack initiation and propagation during the deformation and failure of the soaked sample are more likely to extend along the direction of the hydration crack. For cases exhibiting quasi-ductile characteristics, hydration-induced microcracks may extend over almost the entire sample, allowing locally concentrated stresses in the immersed sample to be dispersed into each crack during deformation. The experimental results can provide data support and theoretical guidance for fracturing design in deep shale gas exploration in southern Sichuan. Highlights: The evolution of the mechanical and brittleness parameters of THMC-treated shale with soaking time was characterized quantitatively. A three-part energy-based brittleness index, where each part has a well-defined physical meaning related to brittle failure, was used to characterize the shale brittleness at different soaking times. The effect of soaking time and mineral composition on the hydration-induced deterioration of shale was investigated. The deterioration mechanism of THMC-treated shales with different mineral compositions (clay-rich, carbonate-rich, and quartz-rich) was revealed. [ABSTRACT FROM AUTHOR]

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 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]