Research On Multi-scale Characterization And Modeling Methods Of Fractures In Low Permeability Reservoirs Based On Fractal Bifurcation Network Model

Fractured reservoirs play an important role in the exploration of unused oil reservoirs in China.Reservoir fractures in fractured reservoirs generally have multi-scale characteristics especially in low permeability reservoirs after fracturing.The study on the fracture modeling in the development of oil and gas field is very important,and it is the main flow channel of fluid flow in reservoir.Strengthening the research on multi-scale seepage characteristics and establishing the characterization and modeling method of fracture multi-scale characteristics is very important for enriching and improving the development theory of fractured low-permeability reservoirs and improving the production of such reservoirs.The multiscale characterization of fractures,especially the characterization of dynamic processes,is a comprehensive and multidisciplinary subject.However,as the scale span is too large,the numerical results obtained by the numerical simulation are quite different from the experimental results.In this paper,through a large amount of literature research,the reconstruction method of micro-scale porous media is studied and the macroscopic mechanical response of micro-scale porous media is discussed.Based on the meso mechanical model and fractal geometry theory,fractal homogenization equations were deduced and verified by comparing with the numerical simulation results.The parameters(such as pore fractal dimension,elastic modulus of the solid phase,and so on)affecting the macroscopic mechanical properties of porous media are discussed.Fractal fracture model is utilized to establish the fracture toughness expression under the influence of multi-scale fractal fracture network expansion and establish the relationship between the bifurcation effect and the hydraulic fracture internal pressure,hydraulic fracture extension length,hydraulic crack width and so on.In addition,the difficulty of expansion and formation of two kinds of multi-scale fractal fracture network and the effect of bifurcation effect on hydraulic fracturing are discussed.According to the previous research,the numerical simulation model of hydraulic fracturing of multi-scale fractal fracture network in the ultra low permeability fractured reservoirs was established.The main conclusions are as follows:(1)The macroscopic mechanical response of porous media is closely related to the microscopic pore structure and the solid phase mechanical parameters.It decreases with the increase of pore fractal dimension and enhances with the enhancement of solid mechanical properties;(2)The fractal homogenization equation established in this paper can be applied to scale upgrading of porous media;(3)The bifurcation effect and the fractal expansion effect of natural fractures have great influence on the formation of hydraulic fractures:with the increase of the bifurcation fractal dimension,the fracture energy,the static pressure inside the hydraulic fractures,height,width increase;(4)As for two kinds of multi-scale fractal fracture networks,the "point to line" fractal fracture network is more difficult to form than the "point to circle" fractal fracture network;(5)The bifurcation angle of Y bifurcated natural fractures affects the mechanism of hydraulic fracturing.Fluid viscosity coefficient,injection volume and elastic modulus of reservoir affects the expansion and formation of multi-scale fractal fractured network of hydraulic fracturing;(6)The ideal multi-scale fractal fractures network is difficult to form in the numerical simulation research,and the width ratio of the bifurcated fracture network is different.