Microscopic Seepage Mechanism Research For Fractured Porous Media

Fractured porous media is the main reservoir to storage and transport for oil, gas, and coal bed methane. For low-permeability oil and gas reservoir rocks, the pore is the main oil storage, and fracture is the main channel of oil and gas migration. On coal and rock mass, the pore is the main storage space for coal-bed methane, and fracture is the main channel for coal bed methane migration. Therefore, the distribution and shape of fractures and pores has an important influence on the development of low permeability reservoirs and the exploitation of coal-bed methane.The Influence of permeability on porosity, crack width and crack angle is discussed at meso-scale in this paper. Firstly, the microscopic physical models were established about fractured porous media with different porosity, crack width and crack angle. Secondly,in the platform of COMSOL Multiphysics, making fluid mechanics and hydrodynamics combined,changes in porosity, crack width and crack angle due to simulated the change regularity of equivalent permeability in media under different pressure gradient, therefore, an empirical formula for equivalent permeability was derived as pressure gradient, porosity, crack width and crack angle were variables. Finally, analysis of its microscopic vacuity and parameters in SEM images of authentic coal rocks, its equivalent permeability were calculated with empirical formula established. According to compare of calculation permeability and experiment permeability, the parameter of the empirical formula was calibrated. The main work of the thesis and achievement are summed up as follows:(1) Research on meso-structure of fractured porous media, the mathematical model of fractured porous media is established and the four main factors were found which impacted fracture porous media fluid flow. (2) The basic assumptions are established to determine general fluid flow equations in the fracture system and pore system. As the fracture is the main flow channel and the pore is the main storage medium, the control equation in fracture system is the Navier-Stokes equations and in pore system is the Darcy's law.(3) The experiment was designed according to four factors and four levels orthogonal table, obtained the most influential factors and the stability levels for numerical simulation to improve the accuracy of the data.(4) Used large-scale finite element analysis software COMSOL Multiphysics, obtained flow through the different fractured porous media on different factors and levels. Conversion by Darcy's law, gives a different permeability. Therefore, the relationships between the equivalent permeability and each impact are obtained. Considering the integrated effect, the fitting equation obtained among the equivalent permeability and three affecting factors, finally.(5) After analyzing and processing scanning electron microscopy image, then import the pictures to COMSOL Multiphysics, a new model obtained from real fracture porous media. By solving the model, the ideal state of the model to calculate the formula was amended to make theory more realistic.