Numerical Simulation Study On Discrete Fracture Network Model Of Fractured Reservoir

When fracture development is poor, its continuity has a very great difference compared with the reality. The numerical simulation method of fractured reservoir based on double porosity medium theory is limited when applying. In fact, fracture system is a network system separating matrix block and it can be regarded as discrete fracture network (DFN) system, which describe the heterogeneity of fractured reservoir accurately. On the basis of DFN model, the thesis overcomes the limit of only solving incompressible fluid and solves numerical simulation problems of slightly compressible oil-water two-phase flow with finite element method (FEM) successfully. Firstly, the fracture identification characteristics of natural fracture, artificial fracture, unlocked and half-unlocked fracture, fracture filling and fracture closure was studied based on the analysis of field data, such as FMI imaging logging etc. The characterization method of fracture parameters is proposed. And the calculation method of porosity and permeability is formed respectively using Volume Method and Parsons Plate Flow Theory, which provides basis for fractured reservoir numerical simulation. Secondly, according to fracture development and distribution in geological description mathematical model, fracture morphology, fracture filling, fracture seepage characteristics and other factors, the thesis has built DFN physical model and slightly compressible oil-water two-phase flow mathematical model. Thirdly, the solving method of mathematical model on DFN was proposed with FEM. Matrix is discretized using linear triangular elements; fractures are discretized by linear line elements. And finite element shape function is derived. The equivalent integral weak form of pressure and saturation equation of DFN numerical simulation is established using weighted residual method. Element characteristic matrix and array of DFN numerical simulation is formed, and finite element numerical model of DFN is built. Fourthly, according to convergence rate, demanding storage space and so on, preconditioned conjugate gradient method (PCG) is the optimal method after the comparison and analysis with gradient method and conjugate gradient method. And then PCG solving method of DFN finite element numerical model is formed. Finally, DFN numerical simulator of carbonate fractured reservoir was drawn up by COMSOL Multiphysics software at the circumstance of Windows XP. Then the numerical simulation on different typical conceptual models of fractured reservoir is studied. The results show that effects of fracture attribute for the fluid flow is accurately accounted by DFN numerical simulation method, and its reservoir performance is more close to realities. This method has strong practicability and wide application prospect.