Search

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]

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]