| The mining of coal gas, oil gas resources development, CO2 sequestration, geologic sequestration of high-level radioactive nuclear waste and pollutants, and other important engineering problems are closely related to fluid seepage in rock mass. Underground resources exploitation broke the initial in-situ stress state of equilibrium of the reservoir rock, leading to the redistribution of internal stress field of rock, and deformation or damage of fracture surface and fracture network structure of the rock body, changing the nature of the rock mass seepage and seepage field distribution. Therefore, the accurate and quantitative description of the structure characteristics of rock fracture, joint/irregular evolution of the fracture structure and mining stress in fractured rock mass influences on the gas seepage features, and to reveal the coal mining under the condition of stress on the fluid seepage behavior has important scientific significance and engineering application.The rock fracture structure has the rough irregular character, characterized by across scales, disorderly distribution, heterogeneity, anisotropy and characteristics related to the stress characteristics, the rough and irregular fracture structures have bigger influence on the seepage behaviors of rock mass. To Analysis the relationship between rock fracture structure and fluid seepage properties, firstly, you need to accurate description method to analyze the structure of fracture network information, distribution characteristics and rough shape. Secondly, fluid flow behavior of natural rock fracture is always non-Darcy flow, the past research about basic seepage problem of fluid are using cubic law or based on revised cubic law, can’t reflect the seepage behavior of rock mass. Therefore we need accurate fluid percolation theory to correctly analyze the structure of 3 d fracture network under the fluid flow behavior, including flow field distribution, permeability size, path selection, flow distribution, so as to correct analysis and quantitative research of various physical and mechanical process effects on the structure form of the internal mechanism of rock fracture. Underground resources exploitation, especial the mining process of coalbed methane, shale gas and tight sandstone gas is a dynamic evolution behavior with stress, the internal structure and macroscopic permeability, they have important influences on the behavior of permeability properties of rock mass structure.The traditional method based on Darcy’s law is used to calculate the macroscopic permeability, without considering the dynamic evolution behavior of rock mass permeability in the process of mining, can not reflect the seepage behavior of fractured rock mass in the process of mining. So, accurate and quantitative description of mining stress on the influence of the internal structure of rock fracture have a great significance to solve the seepage problems in rock mass engineering.According to the above problems, this paper mainly research on the following aspects:1) The influence of irregular and rough fracture structure on fluid flow properties. Considering the limits of rough fracture seepage with parallel plate model and the cubic law, this article is based on Mandelbrot fractal function to construct a series of different roughness of single fracture fractal models and fracture surfaces with different roughness, through the water seepage experiments and LBM numerical simulation research on the influences of rough fracture rock mass structure on seepage behavior, and quantitative analysis relationship between the rough structure and seepage properties of rock mass. Meanwhile, the LBM method is used to analyze the non-Darcy fluid flow behavior of the rough fracture structure.2) The influence of mining stress on the fracture structure and seepage properties in coal rock. Coal rock samples are carries out the different mining stress under the condition of the gas seepage experiment, using a CT scan and three-dimensional imaging technology obtain internal structure evolution under different loading phase flow, threedimensional crack structure form are calculated under different stress changes. The permeability of coal rock under different mining conditions are based on the traditional Darcy’s law. The internal fluid flow behavior of coal rock are analyzed through the LBM simulation analysis, and the effect of fracture structure deformation on the seepage properties of coal rock under the stress loading are studied. This paper provides a new method and effective means to quantitatively analyze and intuitively describe the effect of rock mass deformation under mining process on the fluid flow properties of deep rock.1) The influence of irregular and rough fracture structure on fluid flow properties.Considering the limits of rough fracture seepage with parallel plate model and the cubic law, this article is based on Mandelbrot fractal function to construct a series of different roughness of single fracture fractal models and fracture surfaces with different roughness, through the water seepage experiments and LBM numerical simulation research on the influences of rough fracture rock mass structure on seepage behavior, and quantitative analysis relationship between the rough structure and seepage properties of rock mass. 1. For a single fracture with a constant fractal dimension, although the rough structure of the selected path segment is different, the average flow speed does not change much, implying that the average flow speed is independent of its local structural morphology. 2. As the fractal dimension of the rough structure rises, i.e., the roughness increases, the average flow speed of a local path decreases. 3. Meanwhile, the average flow speed through the entire path of the fracture linearly decreases as the fractal dimension increases. 4. The Reynolds number of water flow linearly decreases with the increment of the fractal dimension of the fracture structure. 5. The measured Reynolds number is smaller than the critical value, meaning that the water flow in the rough fracture belongs to a laminar flow, the viscous effect prevails. 6. The rougher the structure of the fracture, the weaker the inertia effect, and the more significant the laminar effect of the flow. This is a reflection of the influence of roughness of fracture structure on the mechanism of fluid flow. 7. The Euler number of water flow varies with the roughness of structure of the flow path. The Euler number ascends nonlinearly with the increment of the fractal dimension of the fracture structure. The greater the fractal dimension, the more the energy loss and the larger the flow resistance. This phenomenon reflects the influence of rough structure of fracture on the resistance of flow through a single rough fracture. 8. The resistance of water flow through a single rough fracture is mainly attributed to the local resistance resulting from the local bending effect of the flow path. The frictional resistance seems to be negligible. The flow resistance proportionally rises as the fracture roughness increases. 9. A fractal model is proposed to linearly relate the flow resistance to the fractal dimension D of the single rough structure. A fractal equivalent permeability coefficient d equ is defined to quantify the influence of a rough structure on water flow through the single rough fracture. An empirical model relating the fractal equivalent permeability coefficient to the fractal dimension of the rough structure is formulated. It is shown that the permeability of the single rough fracture exponentially rises with the fracture roughness increasing.2) The influence of mining stress on the fracture structure and seepage properties in coal rockThe influence of mining stress on the fracture structure and seepage properties in coal rock. Coal rock samples are carries out the different mining stress under the condition of the gas seepage experiment, using a CT scan and three-dimensional imaging technology obtain internal structure evolution under different loading phase flow, three-dimensional crack structure form are calculated under different stress changes. Coal seams comprise fractured coal that is naturally separated by discontinuous fractures or joints. These discontinuous fractures constitute a complicated fracture network that leads to the NonDarcy CH4 flow. It is intractable for conventional models to describe the behavior and its mechanism of Non-Darcy CH4 flow in fractured coal. This paper reports a promising numerical analysis of the complex CH4 flow in the fracture network of coal using Lattice Boltzmann Method(LBM). The flow properties of CH4 flow, including viscosity distribution, average flow speed, permeability coefficient, are derived using the LBM model. The analysis is validated by comparing the LBM results with the experimental observation data.In this paper, the research subject is to deeply understand seepage behavior and fluid flow law in the three-dimensional fracture network structure of rock mass, with the conditions of high ground stress, gas pressure and different stress conditions which are induced by the mining activities, the structure evolution of rock fracture network and change laws of the CH4 seepage behavior will be obtained. |
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