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Abstract With the deep extension of coal mining in China, fault water inrush has become one of the major disasters threatening the safety production of coal mine. Based on the control equations of steady state and non-Darcy seepage in fractured rock mass, the multi-parameter nonlinear dynamic seepage equations of fractured rock mass are established in this paper. Based on the nonlinear dynamics theory, the function of the state variable in the system is derived, and the influence of the gradual change of non-Darcy flow factors on the structural stability of seepage system is studied. The research achievements show that there are three branches in the equilibrium state of the seepage system. Specifically, the stability of the equilibrium state changes abruptly near the limit parameter. The seepage dynamic system of fractured rock mass has the delayed bifurcation, and the coal mine disaster such as fault water inrush occurs easily at the bifurcation point. The research results are of great significance to enrich the theory of fault water inrush in coal mine, and to reveal the disastrous mechanism of fault water inrush and guide its prevention and control technology in coal mine, which can provide the theoretical reference for predicting the water seepage stability in fractured rock mass.

Confining stress response is considered an accompanying behavior of hydraulic fracturing. Along these lines, an evaluation model of confining stress response was presented in this work. It was established on a rock representative volume element (RVE) and based on the hydraulic volumetric opening model, which stems from the theories of poroelasticity, breakdown damage, and hydraulic fracture mechanics. From the extracted outcomes, it was demonstrated that the confinement of the stress response depends on the matching among the characteristic parameters (ε b , ε s , m) of the rock breakdown, the volumetric opening, and channel flow regimes of the fracturing fluid. Examples in four limiting fracturing regimes show that (1) the confinement of the stress response is strongly determined by the existence of various fracturing regimes and takes place in a different manner during fracture initiation and opening. More specifically, during fracturing initiation, the ratio of the confining stress response to the far-field stress ( P c m a x / σ h ) is 2.0500 in the M regime, 1.9600 in the M ˜ regime, 2.7126 in the K regime, and 1.7448 in the K ˜ regime, while when the fracture is opened, these values ( P C / σ h ) are 1.8994, 1.8314, 1.6378, and 1.2846, respectively. (2) The impact of the confined stress response to the fluid pressure is also affected by the fracturing regimes; e.g., in both M and M ˜ regimes, the peak confinement stress responses lag behind peak pore pressures, but in the K and K ˜ regimes, lag off disappears. (3) The pore volumetric opening ( V p e ) leads to an increase in the confining stress response, while the fracture opening ( V p d ) leads to a reduction in the confining stress response. [ABSTRACT FROM AUTHOR]

The existing reservoir stimulating technologies are only applicable to hard coal but helpless for soft coal, which is one of the main factors hindering the CBM industrialization in China. Therefore, it is urgent to develop a universal stimulating technology which can increase the permeability in various coal reservoirs. Theoretical analysis and field tests were used to systematically analyze the mechanical mechanisms causing the formation of various levels and types of fractures, such as radial tensile fractures, peripheral tensile fractures, and shear fractures in hydraulic fracturing, and reveal the mechanism of permeability enhancement by fracture network stimulating in surrounding beds and coal reservoirs. The results show that multi-staged perforation fracturing of horizontal wells, hydraulic-jet staged fracturing, four-variation hydraulic fracturing and some auxiliary measures are effective technical approaches to fracture network stimulation, especially the four-variation hydraulic fracturing can stimulate the fracture network in vertical and cluster wells. It is concluded that the fracture network stimulating technology for surrounding beds has significant advantages, such as safe drilling operation, strong stimulation effect, strong adaptability to stress-sensitive and velocity-sensitive beds, and is suitable for coal reservoirs of any structure. Except for the limitation in extremely water-sensitive and high water-yield surrounding beds, the technology can be universally used in all other beds. The successful industrial tests in surface coal bed methane and underground coal mines gas extraction prove that the theory and technical system of fracture network stimulating in surrounding beds and coal reservoirs, as a universally applicable measure, will play a role in the CBM development in China.

The hidden collapse column has the characteristics of concealment, suddenness and connection with karst water, which pose a serious threat to safe production in coal mines. In this study, a numerical model of collapse columns with a random distribution of pores was constructed by a finite difference calculation program. Numerical simulation analyses of the coal mining face advance were carried out to explore the response characteristics of rock in the collapse column area under the influence of four factors: mining impact, lateral pressure coefficient, pore water and confined groundwater. The results show that the abutment pressure reaches its maximum when the working face advances to the boundary of the elastic stress elevated zone of the collapse column. The plastic zone above the collapse column shows a "Λ" shape and keeps growing during the advance of the working face. The height of the plastic zone in the top and bottom of the coal seam increases with the increasing lateral pressure coefficient. An increase in the lateral pressure coefficient can amplify the effect of mining on the vertical displacement and plastic zone distribution of the collapse column. When the collapse column is connected to the confined groundwater, the pore water pressure will increase significantly with the advancement of the working face. Under the joint action of confined groundwater and pore water, the extent of the plastic zone around the collapse column will be larger, and the development of the plastic zone inside the collapse column is especially obvious. This study will provide a basis for revealing the rock response rules in the area of the hidden collapse column during mining. [ABSTRACT FROM AUTHOR]

The air jet nozzle structure has a significant impact on the heat transfer uniformity and cooling rate of glass during the glass tempering process, and there is a lack of research on the discrepancy of different jet structures. The effects of double air knife, single air knife structures, and air hole structures on the cooling performance of glass in air-cooled tempering were investigated using numerical simulation and simplified analytical models. The calculated results are in agreement with the experimental results. The results show that the DK structure has a significant effect on improving the overall heat exchange performance of the glass, with lower surface temperatures for the same quenching time. The temperature inhomogeneities of DK, SK, and AH structures in the glass axial direction were 2.09%, 2.53%, and 3.33%, respectively, and the heat transfer uniformity of the DK structure was significantly better than the latter two. In comparison to the AH structure, the average heat transfer coefficients of the DK and SK structures increased by approximately 17.82% and 8.13%, respectively, while the heat transfer rates increased by approximately 7.05% and 4.51%. The study further improves the insufficiency of glass tempering research and provides theoretical guidance for the actual production of tempered glass. • Effects of double air knife, single air knife and air hole jet nozzles on glass heat transfer uniformity were investigated. • The double air knife jet structure has a significant effect on improving the overall heat exchange performance of the glass. • l The double air knife increases the average convective heat transfer coefficient of the glass by approximately 17.82%. • l Research to provide theoretical guidance for the practical production of tempered glass. [ABSTRACT FROM AUTHOR]