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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]