Experimental Microscopic Permeability Characteristics And Cross Combination Effect Of Fracture On Coal

Fractures are the major pathways for seepage in underground rock masses, their permeability and mechanical characteristics significantly affect some engineering activities, such as geotechnical engineering and exploitation of gas and oil resource so on. In this study, we obtain the mesoscopic characteristics of fractures in the coals through CT, and thereby establish the 3D models in the geometry, in which 3D fractures are divided into two patterns, distributed fractures parallel to the panel and the nodes. And we then predict the permeability of coals based on the principle of conservation of flow. The macroscopic characteristics of the fractures are the synthetic reflection of the microstructures, including roughness, Stress deformation response characteristics and cross combination effect of fractures. According to studies on these microstructures, we get the conclusions as follows.(1)Nonuniform stress aggravates the degree of destruction at the end of test piece, according to comparison of thin section images before and after break. The density of micro pores or closed pores has a rise-up about lto 2 percentages, similar to the quantity of coalescence fractures or breakages, which increases around 1 to 3 percentages. While the main structure of the test piece has not changed.(2)We find that the crack is mainly concentrated near the side edge of the specimen and parallel to the axis, while no fractures were observed around the sample axis. Compared with the compressed failure zone, shear failure zone will form the flake particles of different sizes, which are easy to deflect. Ignoring the effect of nodes on seepage flow, according to fractures series parallel theory, we can estimate the permeability of fractured samples under certain confining pressure.(3)The absolute roughness of the specimen is in the scope of 0.1-0.5mm, and the discrete coefficient is between 15 and 35 percentages, and both they are not affected by the sampling frequency. The fracture surface and the fracture opening degree are all subject to normal distribution, and the two fracture surfaces are not completely consistent. What's more, the closed deformation of crack comes from the displacement and deformation of surrounding rock mass; and when there is not stress, the crack contact area is larger, the shape is closer to the bar, the initial crack stiffness is larger. The crack closure makes the part of the fracture become relatively closed pore, and therefore develops the fracture into not-through one or the bottleneck fracture. Besides, the average crack opening degree is far greater than that of hydraulic fractures.(4)With opening degrees and cross angles changing, we test the flow distribution characteristics of fractures in "one in two out" pattern, using the self-made fracture seepage flow test device, and we then analyze the reasons for the differences based on cross control body model and Reynolds transport equation. The results show that the deflection flow from the vortex induced by inertia force and normal stress of fissure wall, and flow distribution is only affected by the opening degrees of imports and bypass fractures, irrelevant with the angle and hydraulic gradient. Besides, there exists a experimental power quantitative relationship between flow bypass ratio (the ratio of bypass flow and the inlet flow) and crack opening degrees.