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The frictional stability of rocks is fundamental in controlling the instability and activation of faults. Given the strong correlation between seismic events in the Sichuan Basin of China and hydraulic fracturing for shale gas extraction, we conducted a series of velocity-stepping friction experiments with Longmaxi shale from the Changning shale gas extraction block under fluid injection. The experimental results indicate that the frictional process of Longmaxi shale can be divided into four distinct stages and the effective normal stress can demonstrate the influence of injection pressure. Additionally, the phenomenon of a decreasing friction coefficient μ with increasing effective normal stress can be explained by the frictional adhesion theory and a competitive mechanism between the deformation of asperities and microscopic dilatant slip. As the injection pressure increases, the friction state of Longmaxi shale transitions from velocity-strengthening to velocity-weakening, which is beneficial for potential seismic slip. After friction, the fracture surface experiences wear, and the surface morphology of the left and right parts exhibits a symmetrical distribution. With increasing effective normal stress, the morphological evolution of fractures becomes more prominent, which is related to the variation in the effective contact area. Based on the experimental results, we propose that the uneven distribution of fluid pressure on the Longmaxi shale fracture may also contribute to the transformation of frictional stability. When studying the frictional properties of rocks under fluid injection, it is also necessary to focus on the evolution of fluid pressure distribution on the fracture surface during the frictional process. • The frictional process of Longmaxi shale under fluid injection can be divided into four stages. • Pressurized fluid can accelerate the transition of Longmaxi shale from the V-strengthening state to the V-weakening state. • The morphological evolution of Longmaxi shale fracture is related to the variation in the effective contact area. • Uneven distribution of fluid pressure is also a presumable factor changing the frictional stability of Longmaxi shale. [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]

• Roadways deformation with deep soft surrounding rock is serious and difficult to control. • The full-section layer-double arch synergy reinforcement technology was proposed. • The mechanism of layer-double arch synergy reinforcement technology was shown. • The deep soft rock roadway can be effectively controlled by the composite support scheme. In order to research the surrounding rock control technology of deep soft rock roadway, the air returning cross-cut of Xinhu Coal Mine in Bozhou City was taken as an example to study the failure characteristics, roadway deformation mechanism and surrounding rock repair technology. Before the surrounding rock of the roadway is repaired, the roof of the roadway sink seriously, the floor heave is obvious, and the surrounding rock strength is low, showing characteristics of soft rock. Therefore, The composite support method of " shotcreting + grouting anchor bolt + anchor bolt + grouting anchor cable + anchor cable " is proposed by the full-section layer-double arch synergy reinforcement technology. The technical parameters of the scheme are determined by theoretical analysis, engineering analogy and engineering practice. The numerical simulation is adopted to verify the rationality of the layer-double arch collaborative reinforcement technology. And, the on-site monitoring results show that the deep soft rock roadway can be effectively controlled by the composite support scheme, which can provide reference value for other similar engineering. [ABSTRACT FROM AUTHOR]

Previous prediction models have faced challenges in characterizing the three-dimensional (3D) surface subsidence of salt caverns with different shapes. In order to overcome these limitations, this study proposes a novel theoretical model based on stochastic medium theory. This model provides a theoretical formula for characterizing the 3D surface subsidence of salt caverns with different shapes (oblate spherical, spherical, and vertical ellipsoidal). By employing the Particle Swarm Optimization (PSO) algorithm to address the theoretical model, it can generate a three-dimensional settlement surface and subsequently compare it with the numerical results. The results demonstrate a strong consistency between the theoretical and numerical results. By conducting the parameter sensitivity analysis, it becomes evident that the shape of salt caverns holds paramount significance and should not be disregarded in the study of 3D surface subsidence. As the main influence angle increases and the burial depth decreases, the maximum settlement value of the salt cavern continues to increase while the maximum influence radius continues to decrease. The maximum settlement value and maximum influence radius of salt caverns gradually increase with the increase of the span. This suggests that the proposed theoretical model can effectively and intuitively describe the 3D surface subsidence characteristics of salt caverns. • A theoretical model of 3D surface subsidence has been proposed based on stochastic medium theory. • The proposed model can accurately predict 3D surface subsidence of salt caverns with different shapes. • The proposed model can effectively describe the 3D surface subsidence characteristics of salt caverns. [ABSTRACT FROM AUTHOR]