The Study On Numerical Simulation Of Fractal Fracture System

The reserves of fractured oil and gas reservoirs in China are huge.Correct understanding of the flow characteristic of fractured reservoirs will help to guide the deployment of oil and gas field mining processes and promote the overall recovery rate.Currently for general oil and gas reservoirs,the theories and methods related to dual medium model are quite mature,and conventional discrete fracture networks have also certain developments.However,due to geostress and geological action,underground rocks are usually full of little and micro fractures.The highly developed micro fractures system certainly influences on the flow of oil or other fluids.Unfortunately,discrete network model based on unstructured grid is difficult to implement because of complexity and irregularity of natural fracture system.Therefore,advancing numerical simulation method for natural fracture system is meaningful.First of all,based on existing first order fractal model,we propose one new dimensionless model,illustrating the correlation between fractal theory and fracture parameters(fracture length,fracture spatial position,fracture number,fracture orientation).This thesis furthermore provides the exact procedure of generating fracture network by fractal theory and examples of three-dimension fracture system.The correctness of new dimensionless model is also validated by comparing total fracture numbers in dimensionless model and in old model respectively.Consequently,this thesis provides a new method to simulate tree-like fracture system by dielectric breakdown model.It is proved by fractal dimension and self-similarity that the generated tree-like fracture network satisfies the fractal characteristics.Meantime,to simulate flow in natural fracture system smoothly,this thesis adopts hybrid model and introduces a new multi-domain model connected to boundary element method to depict heterogeneous media.Image processing technology is introduced to capture boundary coordinate and connection relation of different religions.Boundary element method based on image processing technology is used to solve equivalent permeability tensor of grid.After that,this thesis explores the adaptation of hybrid model for fractal fracture system from flow rate、local pressure and global pressure,and qualitative influence of dividing line and grid size on hybrid model.On premise of enormous tests,we also analyze the impact of fractal parameters on calculation error of hybrid model.According to research content,we can draw some conclusions as follows.Firstly,dimensionless first order fractal model has excellent adaptation.Based on dimensionless model,we deduce total fracture number and probability cumulative function,and generate a whole procedure of fracture modelling on basis of dimensionless fractal model.It is inferred that the larger is the D_c,the higher is the clustering in accordance with two or three-dimension fractal fracture network.Secondly,the adaptation of boundary element method based on image processing technology for complex fracture system is verified by some examples.Lastly in the process of applying hybrid model,we discovered that dividing line and grid size have a remarkable influence on accuracy of hybrid model.