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At present, hydrofracture technology is mainly used for shale gas exploration and exploitation, and the effect of volume fracturing is tightly linked to the mechanical properties (including anisotropy and brittleness) of shale. Meanwhile, Sichuan Basin is also an important area for unconventional natural gas exploration and development in China. This work analyzed mineral composition and microstructure of outcrop shale matrix in Lower Silurian Longmaxi Formation through XRD test, slice observation, and scanning electron microscope (SEM). How anisotropy affects shale deformation, strength and failure characteristics at different bedding angles (0°, 30°, 45°, 60°, and 90°) under diverse confining pressures (0 MPa, 25 MPa, 50 MPa, 75 MPa, and 100 MPa) in conventional uniaxial/triaxial experiments were discussed. The acoustic emission (AE) evolution was introduced to reveal the cracks closure and propagation of Longmaxi Formation shale and qualitatively characterized the brittleness of the shale. What's more, based on the existing brittleness index, this study proposed an indicator BInew that considers the shape of the stress–strain curve before and after the peak stress to evaluate the brittleness of the Longmaxi Formation shale. The index BInew can better describe the changes of brittleness with confining pressure and bedding angle. Finally, this work elaborately explained the relationship between AE characteristics, brittleness, and failure modes of shale. AE characteristics can not only qualitatively characterize shale brittleness but also show the failure process and modes of shale samples. Change of failure modes rests with the difference in brittleness and also reflects the level of brittleness. The change of brittleness with failure modes can be expressed as shear along bedding plane (Sh-Al) > splitting through bedding plane (Sp-Th) > conjugate shear failure (Co-Sh) > shear along bedding plane (Sh-Al) > shear through bedding plane (Sh-Th). Highlights: Uniaxial/triaxial compression and acoustic emission (AE) monitoring of Longmaxi formation shale were carried out. Influence of anisotropy of shale on mechanical properties under different confining pressures was analyzed. Different brittleness indexes were used to evaluate the brittleness of shale, and a new brittleness index was established. Relationship between AE characteristics, brittleness, and failure modes of shale was discussed in detail. [ABSTRACT FROM AUTHOR]

• Mechanical responses and failure characteristics of Longmaxi Formation shale were revealed. • Acoustic emission (AE) evolution of Longmaxi Formation shale during triaxial compression test was described. • Micro-mechanism of improving ductility by high confining pressure and real-time high temperature is different. For the safe and effective extraction of deep shale gas, it is essential to analyze the mechanical responses and failure characteristics of shale under real-time high temperature and high-stress coupling. Herein, triaxial compression experiments on Longmaxi Formation shale were carried out with various bedding angles under different confining pressures and real-time temperatures. The experimental results demonstrate that high confining pressure can transform the mechanical performance of shale from brittle to plasticity. Real-time high temperatures not only can aggravate this transition process but also cause discontinuous micro-rupture before shale failure. The acoustic emission (AE) evolution of shale under various confining pressures and real-time temperatures is generally similar and can correspond with the division of stress-strain stages. However, changes in confining pressure and real-time temperature have opposite effects on AE activity rate and associated parameters. In the deep environment, the real-time high temperature can also drive additional pores and microcracks, which change the microstructure of shale samples. Under high confining pressure, the impact of real-time high temperature on macroscopic failure modes diminishes, resulting in shale samples typically exhibiting the single shear failure mode. Furthermore, high confining pressure and real-time high temperature have different micro-mechanisms for improving shale ductility, as evidenced by their distinct effects on AE evolution. [ABSTRACT FROM AUTHOR]

To clarify the shear-slip process and characteristics of fault, we conducted triaxial shear tests driven by displacement on shale samples with prefabricated sawcut and natural fractures, and the acoustic emission (AE) technology was used to monitor the shear-slip process in real-time. According to the test results, the shear-slip process and mechanical characteristics of sawcut and natural fractures were analyzed. The test results indicate that the shear-slip process of fractures can be divided into four stages, improving the confining pressure will cause more serious plastic failure in the shear-slip process, and the increase of roughness will make the phenomenon of "stress drop" more obvious, which is closely related to the earthquake occurrence. Confining pressure and roughness all have implications for the stress thresholds of each stage. With the transition of shear-slip stages, the effects of confining pressure and roughness on the shear-slip of fractures are more drastic. The maximum static friction coefficient μ 0 increases with initial roughness increasing and is independent of confining pressure. Due to the strain-strengthening effect of fracture slip, the average value of dynamic friction coefficient μ d under simple shear deformation is greater than the maximum static friction coefficient μ 0. Based on AE response characteristics, microcracks development, and rupture scale during the shear-slip process were also explored. AE events are mainly distributed along the inclined fracture. The AE response in different shear-slip stages displays pronounced differences, and the AE evolution and macro mechanical characteristics of fractures show significant time-synchronization. The variation of peak frequency can reflect the rupture scale transformation and the influence of confining pressure and roughness. Additionally, the differences in the morphological evolution of sawcut and natural fractures are mainly reflected in the wear pattern, degree and the rock debris attach amount. Finally, we also discussed the micro-deformation mechanism of asperities during the shear-slip process. This work may provide a reference for the research of fault activation and slip in shale reservoirs. • Triaxial shear tests driven by displacement on shale samples with prefabricated sawcut and natural fractures were carried out. • Shear-slip process and acoustic emission (AE) evolution of sawcut and natural fractures in shale were analyzed. • Morphological evolution and characteristics of fracture surfaces before and after the test were revealed. • Micro-deformation mechanism of asperities during the shear-slip process was clarified. [ABSTRACT FROM AUTHOR]

• Triaxial compression coupled with permeability tests of limestone were carried out. • Mechanical properties of limestone under stress-seepage coupling were analyzed. • Effects of gas pressure and confining pressure on the mechanical properties, permeability and failure modes were studied. • Relationship between cracks propagation and permeability evolution of limestone were clarified. Limestone rock widely exists in various rock engineering, its mechanical behavior and permeability evolution have a great influence on the safety and stability of related engineering projects. To study the mechanical behavior, permeability and failure modes of limestone rock under stress-seepage coupling, triaxial compression coupled with permeability tests were carried out. Experimental results reveal that the failure of limestone still conforms to the Mohr-Coulomb criterion under stress-seepage coupling, and effective confining pressure σ 3 eff can indicate the effect of gas pressure P G and confining pressure σ 3 on the deformation parameters of limestone samples. Peak stress σ P and residual stress σ R increase with the enhancement of confining pressure and decrease with the ascent of gas pressure. Permeability evolution of limestone under different gas pressures and confining pressures is generally similar, and the trend of initial permeability k I , peak permeability k P and residual permeability k R decreasing with effective confining pressure σ 3 eff can be expressed by the exponential function. The increase of confining pressure and decrease of gas pressure will retard the acoustic emission (AE) response and inhibit the propagation of microcracks. More importantly, limestone samples always show composite failure modes dominated by tensile cracks under low confining pressure, while under high confining pressure, the limestone samples show shear failure modes. The difference is limestone samples are composed of tensile microcracks on the microscale under stress-seepage coupling, and the proportion of shear microcracks grows as gas pressure P G and confining pressure σ 3 increases. This research can provide a basic reference for relevant engineering applications. [ABSTRACT FROM AUTHOR]

This paper focuses on analyzing the mechanical properties, permeability and energy characteristics of limestone from coal measures strata of through-coal seam (TCS) tunnel in the context of the Taoziya tunnel as the engineering background. The improved MTS815 Flex test GT rock mechanics system and acoustic emission (AE) technology were utilized for the triaxial compression with permeability test of limestone from coal measures strata of TCS tunnel. The results indicate that gas pressure and confining pressure significantly affect the mechanical properties and permeability of limestone from coal measures strata of TCS tunnel. The gas pressure promotes the development and propagation of cracks, whereas increasing confining pressure strengthens the resistance of limestone samples to crack propagation. When limestone samples transition from the initial compaction stage to the unstable cracks propagation stage, the possibility of gas leakage during tunneling in the coal measures strata should be recognized, and the stability of surrounding rock should focus on the failure stage. In the process of progressive failure of limestone samples, AE events with larger amplitude are distributed near the main macroscopic fracture zone, and the distribution of AE events closely corresponds to the macroscopic failure mode of limestone samples. Exponential functions can be used to describe the trend of initial permeability k i , peak permeability k p and residual permeability k r decreasing with effective confining pressure σ e. The closure and propagation of microcracks are the fundamental reason for the corresponding relationship between the stress variation process, AE characteristics and permeability of limestone samples. Finally, the energy characteristics of limestone under triaxial compression were analyzed based on the stress-strain curves of limestone. This study may give fresh insights and necessary references to ensure the safe construction and development of TCS tunnels. • Mechanical properties of limestone from coal measures strata of TCS tunnel were studied. • Initiation and propagation of cracks in limestone samples were studied. • Relationship between cracks development and permeability of limestone was revealed. • Energy characteristics of limestone under triaxial compression were clarified. [ABSTRACT FROM AUTHOR]