Search

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]

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]

In this paper, a reconfigurable dual-band bandpass filter, using memristive switches, is proposed. The applied memristor is non-volatile and has low-power consumption, which could be potentially used as an RF switch. The dual-band bandpass filter is based on the dual-mode resonator with a memristor. The odd-mode and even-mode method is utilized to analyse the proposed reconfigurable filter. By loading a memristor to the optimum position of the resonator, the proposed dual-band bandpass filter's responses can be switched. The memristor is equivalent to a low-impedance resistor in ON state and a high-impedance capacitor in OFF state. The frequency responses of the proposed reconfigurable dual-band bandpass filter can be tuned by transforming the state of two memristors. The alternative test method is used in this paper, and measured results are consistent with simulated results. [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]

This paper investigates the variation of mechanical properties of granite during temperature and stress cycling, which is an important part of evaluating the long-term thermal and mechanical stability of thermal energy storage. Cyclic temperature and loading tests were conducted where the upper limit of cyclic temperature was 100–600 °C, and the upper stress limits were 70% and 85% of the average uniaxial compressive strength (UCS) at the corresponding temperature. The response of stress–strain characteristics of the granite samples to changes in temperature, and cyclic load upper limit, while the number of temperature and loading cycles was comprehensively analyzed. The results show that the temperature and stress cycles have significant effects on the mechanical properties of granite (i.e., stress–strain curve, strength, elastic modulus, and deformation). The elastic modulus of the sample during loading increases gradually. The strain corresponding to the upper loads of the granite samples decreases with an increasing number of cycles. Additionally, the UCS of samples after 10 cycles at 70% loading stress is greater than that at 85% loading stress. The mechanical properties of samples change dramatically during the first and second cycles at 85% loading stress, whereas at 70% loading stress, the mechanical properties change gradually in the first few cycles, and then tend to stabilize. Cyclic hardening is observed at temperatures below 500 °C, where post cyclic UCS is greater than the uncycled average UCS. This phenomenon requires further research. [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]

Highlights • Two different thermal cycling treatments were applied to granite samples. • Both slow cooling and rapid quenching induced damage to granite samples. • Rapid quenching has a more significant effect on granite samples due to thermal-gradient cracking. • The thermal deterioration mechanisms due to different thermal cycling were revealed by optical microscope observations. Abstract Two different types of thermal cycling treatments (slow heating followed by slow cooling or rapid quenching) were carried out on the granite samples in the present research. The responses of physical, mechanical and transport properties of the granite after various thermal cycling treatments were compared and analyzed. Non-destructive tests (mass, volume, longitudinal wave propagation and permeability) and destructive tests (uniaxial compression and Brazilian splitting) were utilized to characterize these properties. Variations of all the properties demonstrate that thermal cycling (heating and cooling) can induce damage in the samples. Mass, density, P-wave velocity, UCS, Young's modulus and tensile strength decrease, whilst volume and permeability increase with rising temperature. Additionally, the results also demonstrate the importance of cooling method. Rapid quenching has a more significant effect on granite samples, showing lower density, P-wave velocity and strength, but greater permeability, which is due to the generation of thermal gradient cracks. Moreover, the difference between P-wave velocity and UCS of slow cooling and those of rapid quenching increases with temperature up to 500 °C, which can be attributed to the more significant thermal shock cracking induced by higher temperature-gradient stress in the samples during rapid quenching. The microstructural analyses from thin sections show that microcracking in granite subjected to thermal cycling treatments was induced in different severity depending on peak temperature and cooling method. [ABSTRACT FROM AUTHOR]

• The mode-I fracture toughness of granite at high temperature was tested in real time. • The increase in temperature in the low-temperature range universally toughens the rock. • Heat treatment method affects the mode-I fracture toughness of granite. • The temperature threshold of granite fracture toughness at real-time high temperature is 200 °C. • Fracture characteristics of granite at high temperature are similar to those after cooling. The mode-I fracture toughness of semicircular bend granite specimens at high temperature was tested in real time, and the failure specimens were examined using X-ray computed tomography. The results showed that with increasing temperature, the fracture toughness increased from room temperature (20 °C) to 200 °C, then decreased beyond 200 °C. Quasibrittle failure was observed up to 500 °C, followed by plastic characteristics at 600 °C. The cracks tended to propagate along the boundaries of mineral crystals. With increasing temperature, the main cracks became rougher and tended to propagate perpendicular to the tensile stress, and more branch cracks formed. The increase of temperature within a certain range had toughening effect on a variety of rocks and temperature-treatment method affected the mode-I fracture toughness of granite. Compared with the mode-I fracture toughness after cooling to room temperature, the temperature threshold of mode-I fracture toughness of granite at real-time high temperature was higher. The results provide guidance to determine fracture toughness and stability for high-temperature rock engineering. [ABSTRACT FROM AUTHOR]