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Reservoirs characterized by shale–sandstone interbedded structures are extensively dispersed throughout China, and understanding the propagation mode of hydraulic fractures is a prerequisite for shale gas exploitation engineering. In this study, hydraulic fracturing tests were conducted using horizontal wells subjected to true triaxial compression conditions to elucidate the mechanism by which the initiated layer lithology and stress state influence the fracture morphology. Furthermore, the propagation mode of the fractures and the consequential effects on fracture network formation were investigated. The findings demonstrate four distinct interaction modes between the shale–sandstone interfaces and hydraulic fractures. The lithology of the initiated layer influences the fracture propagation trajectory, thereby affecting the propagation modes at the shale–sandstone interface and resulting in varying effects on layer penetration. The stress difference exerts a significant controlling influence on the fracturing behavior, enhancing the possibility of interconnecting multiple rock layers characterized by substantial stress differences. Highlights: Hydraulic fracturing tests of shale–sandstone interbedded structure were conducted.Four interaction modes between fracture and lithology interface are noted.Fracture propagation modes are explored using acoustic emission parameters.Formation effect of the fracture network was evaluated using fractal dimension method. [ABSTRACT FROM AUTHOR]

With the increase of mining depth and intensity, coal and gas outburst dynamic disasters occur frequently. In order to deeply study the macroscopic fracture mechanism of coal body and evolution characteristics analysis of impact force, taking the outburst coal seam of Pingmei No. 11 Coal Mine and Sunjiawan coal seam of Hengda Coal Mine as the research objects, the simulation roadway test system of self-developed true triaxial coal and gas outburst is applied to carry out the simulation test of deep coal and gas outburst with buried depths of 1000 m, 1200 m, 1400 m and 1600 m. During the test, the overlying strata stress is simulated by axial compression, the surrounding rock stress is simulated by confining pressure, the gas pressure is simulated by pore pressure, the impact force and acoustic emission monitoring technology are introduced, and the coal seam gas pressure is simulated by mixture pressure of 45% CO2 and 55% N2. From the viewpoint of fracture mechanics, the crack propagation mechanism of coal in the outburst launching area is discussed, the evolution characteristics of impact force and gas pressure are analyzed, and the influence law between acoustic emission signal and impact force is revealed. From the viewpoint of energy conversion, the transformation character of gas internal energy to impact kinetic energy (gas pressure to impact force) are analyzed. The results show that the generation of I-type crack is a prerequisite for outburst catastrophe. With the crack propagation, I-type and II-type cracks intersect and penetrate, resulting in internal structural damage and skeleton instability of coal. Gas wrapped fragmentized coal body thrown, outburst occurs. There is obvious negative pressure in the roadway after outburst. The occurrence of negative pressure is greatly affected by the physical and mechanical properties of coal, ground stress and gas pressure. Impact kinetic energy is mainly provided by gas internal energy. Part of the gas pressure is converted into impact force. The strength and duration of the impact force are determined by the gas pressure. Under the condition of deep working conditions (high ground stress and low gas pressure), the propagation of impact force in the roadway is more hindered. Both impact force and acoustic emission signals can monitor the occurrence of outburst. The peak point of acoustic emission ringing count is earlier than the impact force. The acoustic emission signal can monitor the outburst hazard earlier. The impact force can more specifically reflect the coal fracture. [ABSTRACT FROM AUTHOR]

• A set of coal and gas outburst simulation experimental system was self-developed. • Deep coal and gas outbursts have been studied and found that the incubation can be divided into the early incubation and the later incubation. • The relationship between the cumulative AE energy during the whole incubation and the energy released during the outburst is obtained. • A new prediction index for coal and gas outburst based on acoustic emission parameters was proposed. Coal and gas outburst is the most serious dynamic disasters in coal mining. The essence of mechanics is the instability and destruction process of coal and rock caused by energy accumulation and release. With mining depth increase, complex geological and stress conditions make coal and gas outburst present new characteristics, which severely restricts the reliability and effectiveness of disaster warning. For this study, the coal and gas outburst tests at different simulation depths were carried out to explore more effective prediction and prevention measures. The results show that a two-phase flow of coal and gas is ejected from the outburst mouth at high speed for 0.6–1.5 s. As the simulation depth increases, both the critical gas pressure and relative outburst intensity decrease, but the unit outburst intensity increases. The deep coal and gas outburst was characteristic of low threshold of gas pressure, easy to trigger and high intensity. The evolution of acoustic emission (AE) energy has experienced "steady-rise-peak" process. The cumulative AE energy in the early incubation is relatively small which is gradually increases at the later incubation. The cumulative AE increases sharply exposed strong outburst risk at the excitation. The propagation of impact force in the roadway has experienced "rise-peak-decrease" process, showing a "crest effect". The cumulative AE energy increases exponentially with the peak impact force. According to the range of changes in the outburst indicators N 1 and N 2 , the outburst risk is divided into five levels: no risk, low risk, medium risk, high risk and strong risk. A new prediction method for coal and gas outburst based on acoustic emission parameters is proposed, and its applications are discussed. [ABSTRACT FROM AUTHOR]