Research On Seepage Characteristics Of Fractured Porous Media Based On Tree-like Fractal Bifurcation Network

Low permeability oil and gas resources play an important role in increasing production in oil and gas industry in China.Low permeability reservoirs have low porosity and poor permeability,and industrial capacity can be achieved only by adopting hydraulic fracturing.The complex fracture network formed by hydraulic fracturing has a great effect on the percolation capacity of the reservoir,reasonable characterization of the complex fractures after hydraulic fracturing,revealing the seepage laws of fractures and matrix systems are of great significance for the development of low permeability reservoirs.Field microseismic monitoring,indoor fracturing experiments and numerical simulations have found that fracturing fractures in low permeability reservoirs such as shale have the caharcteristic of fractal bifurcation.In this paper,fractal-like tree network was adopted to characterize the complex fractures of low permeability reservoirs such as shale and tight sandstone.Considering the fracture section in the classical fracturing model is generally assumed to be elliptical and rectangular,the calculation equations of pressure drop in complex fracture network after hydraulic fracturing were derived,and the equivalent permeability model of fracture network under elliptical and rectangular sections was established and verified.Then,the fluid flow in tree-shaped fracture and porous media containing biflurcated fracture was investigated through numerical simulation.In addition,taking the propagation of fracture show the characteristic of bifurcation into account,a mathematical model for imbibition in the porous media with bifurcated fracture was established,and the phase field method was employed to explore the imbibition in the porous media with bifurcated fracture.The main conclusions obtained are as follows:(1)The geometry of the fracture section has a significant effect on the permeability.When the height and the maximum width of fracture are determined,the equivalent permeability of fracture network calculated by the elliptical section is smaller than that calculated by the rectangular section,and the difference increases with the increase of the primary fracture aspect ratio,the fracture width ratio and the bifurcation series,while decreases as the fracture height ratio increases.(2)Fracture bifurcation angle,bifurcation series and fracture width ratio have different effects on fluid flow in tree-shaped fracture network.The average velocity at the outlet of the fracture network gradually decreases with the increase of the bifurcation series,while first increase and then decrease with the increase of the fracture width ratio.The influence of the fracture bifurcation angle is not obvious.(3)The influence of matrix permeability and fracture width ration on the equivalent permeability of porous media containing the tree-shaped fractures is very different.As the matrix permeability increases,the equivalent permeability of porous media containing the tree-shaped fractures is basically unchanged.However,when the fracture width ratio is changed from 0.3 to 0.8,the equivalent permeability is increased by 84 times.(4)The morphology of fracture has obvious influence on the imbibition in porous media.Compared with the case of single fracture,since the fracture bifurcation increases the oil-water contact area,the recovery factor can be effectively increased.(5)Wettability and oil phase viscosity have different effects on imbibition in the porous media with bifurcated fracture.With the increasing of wettability,the imbibition rate and recovery factor increase.However,when the viscosity of the oil phase increases,the imbibition rate and recovery factor decrease.