Study On Propagation Law And Numerical Simulation Of Water-Induced Fractures In Ultra-Low Permeability Reservoirs

Ultra-low permeability reservoirs are rich in China and water injection is commonly used to supplement formation energy.As the poor injectivity and high water injection pressure of the type of reservoirs,fractures spread from the bottom of water injection wells to production wells in various forms,which significantly increases the heterogeneity of reservoirs and has a great impact on the development effect.With only considering the static fractures,the traditional reservoir numerical simulation software can not simulate fractures propagation process,which ignores the impact of water-induced fractures on reservoir development.This paper studies the efficient numerical simulation method suitable for water-induced fractures.Firstly,the production performance patterns and physical experimental phenomena of typical ultra-low permeability reservoir Y2 are analyzed and summarized,and the characteristics of water-induced fractures extension are defined.Combined with the geological conditions of ultra-low permeability reservoir,the mechanism of water-induced fractures is revealed,and three types of fracture propagation patterns are achieved,which include linear propagation of tensile fractures,steering propagation of shear fractures and fusion propagation of cross fractures.Secondly,based on the embedded discrete fracture model(EDFM),by introducing the mechanical detection module and improving the dynamic pretreatment algorithm,the numerical characterization method of water-induced fractures is established.A dynamic embedded discrete fracture model for simulating tensile fractures in the process of long-term water injection is formed,whose accuracy is improved by hybrid boundary element discretization method and dynamic grid encryption technology.Then,on the basis of the model,the steering propagation model of I-II Composite fractures is established by the fracture element method,and the fracture cross criterion is introduced.By coupling with the dynamic embedded discrete fracture model,a numerical simulation method suitable for the steering propagation of fracture and the process of main fracture encountering natural fractures is formed,and the simulation of complex fracture propagation by the dynamic embedded discrete fracture model is realized.After that,the dynamic embedded discrete fracture numerical simulation method is applied to establish the injection production well group model of water-induced fractures,and the sensitivity of the main controlling factors of fracture propagation is analyzed,which includes eight parameters: matrix permeability,minimum principal stress,water injection intensity,initial formation pressure,difference of in-situ stress,fracture inclination angle,well pattern type and natural fracture distribution.The change relationship of fracture propagation speed and propagation form gets a lot of attention in research and the influence degree of fracture propagation on oil well production dynamic index is clarified.Finally,the typical well groups with water-induced fractures development in Y2 reservoir is studied,in which the propagation process of water-induced fracture is simulated by applying the dynamic embedded discrete fracture numerical simulator,and the production dynamic indexes of oil wells under the influence of fracture propagation are fitted.At the same time,the application range of the numerical simulator is extended,and the integrated numerical model of "hydraulic fracturing,well closing,oil production" is established to simulate the whole life cycle development process of fractured horizontal wells.The numerical simulation method of water-induced fractures overcomes the deficiency of commercial numerical simulation software in the simulation of induced change of fractures in the process of long-term water injection development,which realizes the efficient simulation of complex fracture propagation under different water injection development conditions,and provides a powerful tool for the research of water injection development of ultra-low permeability reservoirs.