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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]

Sandstone oil–gas reservoirs in the Junggar Basin, China have great development potential. However, their ultra-deep formation depth leads to high crustal stress and high breakdown pressure. Therefore, in this research, we studied the cyclic hydraulic fracturing of tight sandstone with different combinations of "high-pressure duration + low-pressure duration" under high-stress conditions. Through laboratory experiments, the pump pressure curves, hydraulic fracture morphology, acoustic emission counts, and peak frequency of the samples were obtained. The results showed that: (1) Compared with conventional hydraulic fracturing, the breakdown pressure of cyclic hydraulic fracturing was reduced by more than 30%, the minimum threshold of cyclic pump pressure required for sample breakdown was between 60%Pb and 70%Pb, and cyclic hydraulic fracturing more easily formed complex and diverse hydraulic fractures. (2) In cyclic hydraulic fracturing, under the same upper limit of cyclic pump pressure, the shorter the high-pressure duration, the fewer the cycles required for sample breakdown. (3) Under the same "high-pressure duration + low-pressure duration" condition, the lower the upper limit of the cyclic pump pressure, and the greater the number of cycles required for sample breakdown. (4) The AE cumulative counts curves fluctuated greatly during cyclic hydraulic fracturing, rising in an obvious step-wise manner and the AE peak frequency was banded and mainly divided into three parts: low frequency, medium frequency, and high frequency. [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]