Study On Optimization Of Re-Fracturing In Consideration Of Stress Field Change

Re-fracturing is an important measures to increase oil production for ultra-low permeability reservoirs.Under the fluid-solid coupling,stress field changed because of production and water injection,which in turn cause the change of the orientation of refracture.Fracture reorientation surely has an important influence on re-stimulation result.Therefore,it is of great significance to take the change of porous elastic stress field into consideration when doing re-fracturing candidate selection and optimization of fracture parameters.Based on the principle of effective stress and reservoir seepage theory,the coupled model of porous elastic stress field and fluid seepage field was established,which considered the change of fracture conductivity of fractured wells.The coupled numerical solution is solved by the finite element method.Stress variation characteristics of diamond shape against nine point well net and the relationship between the stress reorientation zone and the rock mechanics parameters,development parameters and fluid parameters were revealed.A re-fracturing candidate selection model was established based on the associated multi-factor parameter system.Parameter limits were determined based on mathematical statistics methods,reservoir dynamic analysis methods,and dimensionless stress reorientation distance plates.Applied the candidate selection model to typical oil field,results show the method is reliable.Geological model of the ultra-low permeability reservoir is established,and the fluid-solid coupling model is embedded.Based on that,the stress field and the seepage field solution method before and after re-fracturing was proposed.The reasonable re-fracture timing of various wells was determined combined stress reorientation distance and dynamic parameters of reservoir development.Based on the orthogonal test method,fracture parameters of volume re-fracture were optimized.The results show that for the diamond shaped against nine points well net,stress reorientation range is obviously affected by well position and the flooding well.The maximum horizontal stress direction of injection well is along the radial direction of the wellbore,while it is vertical to the direction of fracture for production well.Porous stress field is deferent between the side well and the corner well.The size of the stress reversal zone is obviously affected by the pressure difference between injection well and production well,the initial horizontal principal stress ratio,and the reservoir physical property parameters.During candidate selection,the side wells with dimensionless stress reorientation distance greater than0.15 and the stress shadow parameter less than 0.1 and the corner wells with dimensionless stress reorientation distance less than 0.15 and shadow parameter greater than 0.1 should be selected for the analysis of yield increase potential.As for refracture time optimization,A1well's(side well)refracture time is 200 th day,corresponding reorientation fracture length is 47 m,B well(corner well)refracture time is 360 th day,corresponding reorientation fracture length is 50 m.