Numerical Simulation Study Of Fractured Vuggy Media Based On Discrete Pipe-Vug Network Model

Carbonate oil and gas resources are rich in the west of China,where fractured vuggy reservoir makes up 2/3 of it.Fractured vuggy media contains lots of fractures and vugs which significantly enhanced the heterogeneity of the media.The equivalent continuum model is oversimplified to describe fluid flow in fractures and vugs.The discrete fracture-vug network model can accurately describe the fluid flow in fractures and vugs but it increased computational cost.Moreover,the surface of naturally fractures is rough.Research indicates that the fluid flow in fractures is along preferred path formed by rough surfaces.So the plate model that discrete fracture network model applied to simulate fractures is too sketchy to describe the preferred path phenomenon.To this end,discrete pipe-vug network model is proposed in this thesis.Firstly,discrete fracture network model is built based on Monte-Carlo method.After extracting the geometry information of fracture network in discrete fracture network model,discrete pipe network model is built.By extracting pipe network information in discrete pipe network model,generating vug system by Monte-Carlo method and computing the intersection information between vugs and pipes,discrete pipe-vug network model is built.To simulate two phase flow in fractured vuggy media where the vugs are full filled,Forchheimer equation and Barree-Conway equation is applied to describe the non-Darcy flow in filled vugs.Based on equivalent flow assumption,the formula of equivalent permeability in pipe network is gained and the mathematical model of discrete pipe-vug network model is developed.The method applied to solve this mathematical model is finite volume method.To simulate two phase flow in frctured vuggy media where the vugs are not filled,the NavierStokes equation and phase field method is used to build the porous-free flow mathematical model of discrete pipe-vug network model.The method applied to solve this mathematical model is finite element method.Then,an embedded discrete pipe-vug network model is proposed to take the ability of reservoir and transport of matrix system into consideration.The method applied to solve this mathematical model is finite volume method.The results shows that pipe is effective to describe the preferential flow phenomena in fractured vuggy media and the discrete pipe-vug network model can represent the character of fluid flow in fractured vuggy media correctly and efficiently.According to the numerical simulation example,connectivith has an important effect on fluid flow in fracutres.Enhancing connectivity is a good way to improve the recovery efficiency.For the fracture-vuggy media where vugs are fully filled,the intersection position is crucial.Because the swept volume is bigger for vugs which are intersected by pipes in higher position than that in lower position.For the fractured vuggy media where vugs are not filled,the bottom water coning phenomena is almost not exist.Morover,reducing the oil viscosity is beneficial to the enhancement of oil recovery.According to the simulation results of embedded discerte pipe-vug network model example,the flow velocity in matrix is obviously slower than it in fractures and vugs,especially when fractures and vugs are well developed and connected.The ability of reservoir and transport for matrix system can be ignored if the order of matrix permeability is three smaller than it of fractures and vugs system.