| The microscopic pore structure of porous media,such as natural rock and man-made materials,has very strong microscopic heterogeneity characteristics.The transport properties(such as conductivity,permeability and thermal conductivity)are macroscopically from the laboratory,its size is weighted by various structural factors.The macroscopic transport characteristics of rock are fundamentally dependent on the microstructure characteristics of porous rock,resulting in complex flow of fluid or electric current.A single pore model or fracture model cannot well simulate the transport characteristics of various types of sedimentary rocks,resulting in decreasing for accuracy of the prediction model.How to characterize the transport properties of porous media,and extract universal features and coefficient correlations from fitting experimental data at different scales / lithologies / saturations is the important research contents for porous rock.The porous media model can be derived into effective media model,pore network model,tortuosity model,fractal model and percolation network model.These models almost cover characterization model of porous media.In general,the percolation theory is applicable to the characterization of the transport properties of low-porosity and lowporosity porous media in microscopic structures.The effective media theory is applicable to the weighted various structures on the transport properties in macroscopic system.The fractal theory is applicable to upscale the microscopic to macroscopic system.Studying these theories and applying it to the transport process of porous media has a great role in the accurate evaluation for the physical properties of porous media.Then,we discuss the permeability model based on the fractal capillary model and the fractal improvement of the classical Kozeny-Carman permeability equation.A full analytical model for characterizing KC constant in spontaneous imbibition is derived based on fractal distribution characteristics.The model incorporates gravity over the entire imbibition process time frame,which is expressed in terms of porosity,fractal dimension,tortuosity,maximum hydraulic pore diameter,density,viscosity,surface tension and contact angle.The theoretical predictions from the developed model provide a good agreement with the experimental results from emulsion imbibition.We analyse the interaction between the pore microstructure and the solid particles leads to a highly uncertain behavior of the current flow behavior.When the theoretical model is used to characterize the pore structure,there is a situation in which the theoretical and practical conditions are not consistent in the process of interpretation data.For these problems,the conductivity model is reduced to a function model related to the fractal dimension.This model can explain the physical meaning of empirical constants in the Archie's law and is related to the percolation theory.Considering several forms of heat transfer in heat conduction in nanofluids,for the fractal distribution of nanoparticle size in nanofluids,the thermal convection and Brownian motion between nanoparticle and liquid.The thermal conductivity of nanoparticles in nanofluids is similar to the movement of ions in saturated solutions in porous media.Based on the fractal geometry theory,an analytical expression for calculating the thermal conductivity of nanofluids with different size distributions of aggregates is deduced.The effective thermal conductivity based on the model calculation agrees well with the experimental results.The results show that the change of the aggregate shape may affect the fractal dimension.Studies show permeability,electrical conductivity and thermal conductivity can be expressed as a function of fractal dimension;percolation theory can be used to analyze the characteristics of current flow with less connectivity;effective media theory can be use to combine the influence of different factors;the experimental results verify the accuracy of these models. |
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