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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 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]

Increasing upper limits of cyclic loading tests at different temperatures were carried out to reveal the influence of temperature and stress level on the mechanical properties and energy characteristics of granites. The mechanical properties of granite were altered by temperature and cyclic loading, which is manifested by the observation of rock hardening, i.e., the elastic modulus increases gradually in an approximately logarithmic relationship with increasing stress levels and the relative residual strain shows a decreasing trend. The energy evolution of granite changes from nonlinear characteristic to linear characteristic with increasing temperature. The input energy density and elastic energy density, at 25 °C and 100 °C, both increase nonlinearly with increasing stress levels as an approximately exponential relationship, whereas those, in the range of 200 to 600 °C, increase almost linearly as the unloading stress level is less than 0.9. The energy distribution of granite subjected to cyclic loading was revealed, i.e., the energy accumulation is dominant in the pre-peak stage. The mathematical model of elastic energy density and input energy density at different temperatures was proposed, which is critical for the energy-based brittleness index that can be used to evaluate the stability and compressibility evaluation of the reservoirs. This generalized equation of different types of rocks merits further study. [ABSTRACT FROM AUTHOR]

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]