| The coupling interactions of fluid (such as water, oil, gas and vapor) and solid (such as rock and soil) exist widely in the application of underground engineering, such as mine development, transportation, hydraulicien and hydropower projects. The thermal-hydro-mechanical (THM) coupling issue, which is affected by many complicated factors, such as engineering geology, hydrogeology, fluid and rock transport properties and engineering perturbation conditions, will result in varieties of engineering challenges, even disasters. As a nonhomogeneous porous media, the micro pore structure determines the mechanical and thermodynamic performance of rock, as well as fluid transport properties in it. The pore scale model of rock is the basis to conduct the research on the micro THM coupling mechanism. It is of great theoretical importance and applicable value to figure out the evolution rule of the pore structure and its influence on fluid transport properties based on the pore scale reconstructed models of rock under the THM coupling process.Based on laboratory experiment, theoretical analysis and numerical simulation, the THM coupling mechanism at the micron scale and its effects on the development of oil fields are studied. Then the mathematical model of fluid flow in porous rock is established adopting the micro-fluid theory. Using the image reconstructing technology, three kinds of pore-scale models including the modified equivalent pore network model, the unstructured and structured finite element models are developed. With the reconstructed models, effects of the surface tension, the invasion velocity and viscosity of injecting water on the efficiency of the water flooding process are studied. The influences of stress and tempreture on the evolution of the pore structure and the water flooding efficiency are revealed. This study aims to provide new theoretical supports and solutions to directly describe and quantitatively analyze the effects of stress and temperature on the seepage mechanism in the development of oil fields. The main contents of this paper are as follows.? Adopting the Zeiss Xradia MICROXCT-400 equipment, the typical rock samples are selected and imaged to acquire the micro CT images at the micron meter scale. Effects of different image segmentation algorithms on the porosity of rock images are analyzed. Codes are developed to calculate the porosity and distributions of pore radius by Matlab, in order to provide basic data for the reconstruction of pore scale models.? Based on the boundary layer theory of the micro fluid, the interaction force between the fluid and the solid molecular is analyzed. The mathmatical model of the fluid viscosity in the micro channel is modified to improve the Navior-Stokes equation, which is widely used for fluid analysis in commercial software. The feasibility and application of the modified mathematical equation are verified by the comparative analysis between the simulation and the experiment results of micro tubes. Meanwhile, it is able to reproduce the effects of the nonhomogeneity of mineral composites on the rock wall by applying a reasonable uniform distribution range to the two parameters including the wetting angle and the micro flow coefficient of the boundary layer.? To settle the issues caused by the micro water flooding experiments, such as the complicated preparation process, low efficiency and inaccuracy of the real-time measurement, a novel method is proposed for extracting the outline of the pore microstructure from the micro-CT images of rock based on the two dimensional flat glass model. The feasibility of this model is verified by the comparative analysis between the simulation and experiment results. Then, the numerical studies on the water and carbon dioxide flooding process at the pore scale are carried out. Consequently, the effects of the micro pore characteristics on the displacement efficiency of oil by water and carbon dioxide are revealed, as well as the two-phase flow seepage mechanism.? The distance sorting homotopy refinement algorithm in Amira software is adopted to improve the extraction process of the median axial and the segmentation method of pore and throat. This method improves the defection of the extracted equivalent pore network model (EPNM) in unreasonable high coordinate numbers. The key parameters of the pore radius distribution, the coordinate number and the shape factor are acquired based on the improved parameterized three-dimensional EPNM. The two-phase pore-scale simulator developed by Imperial College London is employed to predict the transport properties of the extracted models. The predicted absolute permeability and the capillary force of the EPNM agree well with the experimental benchmark data. The capillary force and the relative permeability under different wettability conditions are simulated. The features of the EPNM in topology and the prediction accuracy for the water flooding process are analyzed, simultaneously.? In order to overcome the shortcomings of the EPNM, a new method is presented to reconstruct the unstructured finite element models of rock based on Mimics and ICEM software. The rock matrix and pore are assembled and meshed as a whole. The mesh quality of the unstructured model satisfies the minimum requirement of commercial software. The models properly reproduce the complex microstructure of rock. Single- and two- phase flow simulations are conducted based on the Navier-Stokes equation in Fluent with the unstructured finite element models. Good agreements are obtained on both the pore structures and the predicted single- and two- phase transport properties against the benchmark experimental data. Then the hydro-mechanical coupling process based on the pore scale finite element models is simulated by ANSYS and CFX software. Hereby, the permeability of sandstone samples under different pore pressures and confining pressures are predicted. The simulation results agree well with the benchmark experimental data. Consequently, the effects of the pore pressure and confining pressure on the evolution of the pore structure and permeability are revealed from the microscopic view.? For the purpose to improve the mesh quality and topology of the unstructured finite element models, a universal approach to reconstruct the three-dimensional structured finite element models of rock is proposed based on Matlab. The elements number is optimized by reducing the resolution of the micro CT images. Numerical simulations on single- and two-phase flow are conducted in Fluent. Good agreements on between the seepage mechanism with the benchmark experimental data are acquired, which validate the effectiveness of the proposed modeling method. The comparative analysis on the EPNM, the unstructured and structured finite element models indicates that the structured finite element model is characterized in good topology and mesh quality, the simulation results of which are closer to the experimental benchmark data. However, more grid number is needed and larger computational workload is consumed to converge.? The microstructures and material parameters are the basis for the simulation involving the heterogeneous rock mechanics at the micron scale. Adopting the reconstructed structured finite element model and the corresponding elastic modulus tested by micron indentation experiments as inputs, the rock deformation process under the condition of uniaxial compression is simulated. Results indicate that the elastic modulus of sandstone is higher than that of the uniaxial compressive test, the difference of which increases with the increase of rock porosity. In consideration of the microstructure and the micron scale material parameters, good agreements are acquired between the simulation results and the uniaxial compression tests. Subsequently, the loading-unloading process of the micron indentation test is simulated. By the comparative analysis on the loading-unloading curves of the indentation tests with the simulation results, the yield strength of sandstone at the micron scale is predicted, which provides basic material properties for the THM coupling study of rock.? Based on the structured finite element model and rock material parameters at the micron scale, the water flooding process is simulated to study the effects of the surface tension, the velocity and viscosity of the injected fluid, and the wettability on the relative permeability and oil recovery, so as to optimize the parameters of the injected fluid. In addition, the evolution law of the pore structure and permeability of rock under the condition of THM coupling are analyzed. The effects of stress and temperature on oil recovery are presented. |
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