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The effect of inter-particle friction on the macroscopic and microscopic mechanical behaviors of powder particle pressing system is an important research topic to improve the density of particle assembly. However, due to the discreteness and complexity of particle system, this problem has not been fully understood. In this study, a numerical model of triaxial powder particle pressing system was established by the DEM method, and the influence of inter-particle friction on the mechanical behavior of particle assembly was comprehensively investigated from the macroscopic and microscopic perspectives. The simulation results show that the friction between particles has a significant effect on the macroscopic mechanical properties of this powder particle pressing system, such as stress-strain response, dilatancy behavior and peak strength. In addition, the evolution of microscopic parameters such as coordination number, microstructure, sliding rate and force chain of this powder particle pressing system is also markedly influenced by the friction between particles. This research work also reveals how the friction between particles affects the movement of particles and the rearrangement of the system. At the same time, it can provide basic theoretical guidance for effectively improving the density of powder metallurgy pressed blanks. [ABSTRACT FROM AUTHOR]

Due to the extremely low permeability of shale reservoirs, large-scale reservoir fracturing is required. Hydraulic fracturing is one of the most important technologies in shale gas exploration and development. In this paper, the acoustic emission energy and the number of location and fracture surface morphology of specimens before and after fracture are studied through hydraulic sand fracturing test. The test results show that: (1) the energy ratio obtained during hydraulic fracturing without proppant is the smallest, and increasing the confining pressure, as well as reducing the displacement and viscosity of the fracturing fluid will cause the energy ratio to decrease. From the perspective of acoustic emission energy, the proppant play an important role in the generation of fractures during hydraulic sand fracturing; (2) when the confining pressure increases, the number of shale specimens before and after rupture is the largest, but the total number of locating events is smaller than the sanding ratio increased; there is no proppant hydraulic fracturing, the number of specimens before and after the rupture is the largest. And the total number reached the minimum, indicating that the proppant can play an important role in the hydraulic sand fracturing test; (3) the sand is relatively large, the specific surface and standard deviation both reach the maximum, indicating that the fracture surface roughness is the largest under the test condition, and the fracturing effect is the best, but the specific surface and standard deviation are the minimum when fracturing without proppant, so indicating that the fracture surface fracturing effect is the worst at this time. [ABSTRACT FROM AUTHOR]

The effect of mining-induced stress–fracture–seepage field coupling (SFSC) is critical to the safety and productivity of the underground mining. This study investigated the spatiotemporal effect of SFSC at the Bulianta Colliery panel #31401 of the Shendong coalfield in China, characterized by ladder key stratum and composite aquifer. An integrated method including borehole and working face observation, in-site water leakage, and real-time video imaging was adopted to examine the behavior of overburden movement, water level change, and groundwater inrush. Simultaneously, the evolution of the SFSC was visualized by the FLAC3D–CFD (fluent) simulator with a developed non-Darcy model, and the dynamic characteristics of the SFSC were analyzed. The results indicated that the macro-stress shell, fractured field distributed beneath the macro-stress shell with a 48 m height difference in the lower and higher key stratum zone, and seepage field including initiation zone, acceleration zone, deceleration zone, and drainage zone were identified. The confined aquifer was affected by the fracture field with a height of 140–154 m, and groundwater invasion was triggered and developed beneath the lower key stratum. The confined aquifer was kept stable by protection of the higher key stratum by controlling the propagation of the fracture field at the height of 90–100 m. Highly permeable channels were distributed beneath the micro-stress shell as a result of the release of in-site stress, especially at the rupture region of the micro-stress shell between the lower and higher key stratum, with the maximum fluid discharge. Furthermore, the SFSC response resulted in large and medium groundwater invasion and strong variation of abutment stress peak, when creating rupture at the micro-stress shell and main roof. Finally, based on the mechanism of the SFSC, the developed optimization strategy could efficiently mediate the effect of the SFSC by lowering the mining height, increasing the advance rate, and enhancing the support resistance of the longwall mining face. [ABSTRACT FROM AUTHOR]

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Based on microfocus X-ray computed tomography analyses, the relationship between the characteristics of pore structure and fluid flow behavior was investigated through the single-phase flow experiment. The macroscopic bulk fluid seepage behavior was fundamentally explained by the microscale flow mechanism considering all the porous microstructure analysis of artificial cores. The results indicate that the pore size and connectivity have significant effects on the initial permeability of the rock cores. The permeabilities obtained from different methods have a linear law relationship with porosities in tested artificial cores. The results also suggest that the permeability of core decreases exponentially with the increase in effective stress. The inner different pore structures have an important influence on the stress-dependent fluid flow in synthetic porous rocks. The polynomial equation yields well fittings of the artificial core which has the poor pore sorting characteristic. The permeabilities of the artificial core are more significantly affected by changes in the low effective stress range. The larger the pore channel of the artificial core is, the greater influence on permeability the pressure will have. The stress sensitivity of artificial core increases as the grain diameter decreases. The heterogeneous coefficient and the stress sensitivity of permeability are positively correlated. [ABSTRACT FROM AUTHOR]