| Rock is a non-uniform and anisotropic fractured medium, which contains a variety of pores and cracks with different sizes. These defects not only significantly change the mechanical properties of rock mass, but also seriously affect the permeability characteristics of the rock. Seepage is controlled by widely developed fracture in rock mass and its connected network. Its geological properties and hydraulic characteristics of rock mass are relatively stable with initial ground stress. Under the conditions of mining, rock mass and fracture are disturbed, and the mechanical properties of rock and permeability of fracture are changed with the alteration of rock structure and fracture aperture, which is one of the main cause for underground engineering disasters. Establishing a more accurate coupled model for description of the seepage in multi-media fractured rock mass is an effective way to solve the problems of engineering and geological disasters.Considering the geological properties of the natural fractures and the coupling of seepage and deformation, we take pores and micro-cracks as the marco-pore system, fracture with small size (≤μm) as the fracture system in seepage analysis. In terms of obtaining the characterization methods for initial aperture of fracture with big size (＞μm), the effects of fracture aperture on permeability are investigated based on the generalized cubic law, and the coupled model of gas-water two-phase flow and deformation is studied. The main work of this dissertation is as follows:1. Study on the characteristics of pore structure and hydraulic behaviors of coal and rock. The pore characteristics of the rocks and their connectivity are observed using scanning electron microscopy, and the porosity sizes are determined by mercury injection test, which can provide a reference for subsequent description of permeability evolution model for coal or rock medium. Rock is a highly non-uniform material with multi-pore or fracture. Pores and fracture of rocks play direct role in the permeability, and fracture coalescence, distribution and other factors have great impact on the rock permeability.2. Coal seam is a typical dual media. Pores and fracture are storage space and seepage paths for the fluid, respectively. The migration of gas in coal seam also contains the adsorption, desorption, diffusion, seepage, and dynamic coupling process of seepage and stress. In order to study the complex physical processes, the gas-solid coupling model considering the adsorption and desorption process is proposed. The porosity-permeability models are derived with the different values for effective stress coefficient (αm and αf) of pore and fracture. And the coupled models of continuous media and dual media are established respectively, after combining the effects from adsorption and deformation. The results show that the volumes of macro-pore and fracture system obviously vary with the change of adsorption deformation, gas pressure and ground stress. Taking into account the stage of water-gas two phase flow in the coal seam while water injection process, the coupled model for dual media gas-water two-phase flow fluid and deformation is established.3. It is defective to ignore the fracture characteristics while studying the hydraulics behavior for the natural geological rock. However, a fine description of the3D model is often difficult for establishing the true3D fracture rock mass system. A big numerical model is time-consuming and labor-intensive for pretreatment, and has considerable difficulty to get the numerical solution, especially for the characterization of pore and fracture system structure in general engineering problems. To achieve data transfer between2D and3D space, the Extrusion Coupling Variables (ECV) is introduced. ECV can achieve highly accurate characterization for fracture surfaces, and obtain the evolution of real fracture aperture and its permeability. Meanwhile, adaptive mesh method is used to match with the high-precision solution of aperture to obtain a more elaborate description of the numerical characterization methods in real rock mass.4. Taking into account of the effect of fractured structure on the system, the stress coupling characteristics of seepage can not be ignored. The mechanical properties of the fracture are described with elastic layer concept in the Comsol Mutiphysics. Based on the generalized cubic law, the coupling characteristics of the fracture seepage are described through the weak form of PDE equations and the ECV. And a multi-media fractured rock mass permeability evolution coupled model is built by combining with the established dual-medium gas-water two phase flow and deformation model. Meanwhile, the characterization methods and numerical model for macro-fractured rock mass are also set up, which of reliability has been verified based on the previous single-fracture seepage test.5. We use the finite element method and cluster computing technology to achieve numerical discretization, solution and comparative analysis of the coupled model. The results show that the fracture is one of the main factors of geological properties and hydraulic characteristics of rock mass. Characterization methods and numerical model for macro-fractured rock mass can more accurately describe the ground of physical processes. This method can fully describe the permeability variation of coal and rock under general engineering. The existing of fractures obviously affects the deformation of geological rock and the migration of underground fluids, which are directly related to the changes of fractures mechanical and geometrical characteristics. The effects of fractures on water-gas two phase flow are more obvious in water injection process of coal seam. Water and gas flow priority to occur in the fracture, and then continue to affect the entire water injection area, because fractures provide better seepage path. Fractures are filled with water after a period of wetting coal seam by low pressure water injection, which reduced the gas emission in working face to some extent. |
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