Study On The Migration And Placement Of Proppant In Combined Fractures

Volume fracturing plays an essential role in the commercial development of unconventional oil and gas resources.The placement pattern of proppants in complex fracture generated by volume fracturing determines the construction quality and understanding the laws of migration and placement is important to ensure proppants placement effectively in complex fracture.However,there is little work on the laws of proppants migration and placement in complex fracture.To make sure the experiments and numerical simulation are performed effectively,the complex fractures are simplified to combined fractures which are comprised of one major fracture and first-class branch fractures.Utilizing the large-scale and visualized experimental device of proppants migration and placement,experiments on laws of proppants migration and placement in combined fractures are carried out and then the proppants migration and placement mathematical model is built.Based on this model,numerical simulation on proppants migration and placement in combined fractures is performed by the secondary development of FLUENT.It specifies the influence law of formation parameters,fracturing construction parameters and fracturing materials parameters on proppants migration and placement in combined fractures.Finally,fracturing productivity model is built by using CMG to optimize the fracture parameters.Combining this model and the conclusions on influence factors of proppants migration and placement in combined fractures are drawed,the fracturing construction project design on single fracture and combined fractures are optimized.The results show that formation parameters,fracturing construction parameters and fracturing materials parameters all have impacts on proppants migration and placement in single major fracture.The effects of some parameters are as follows:the value of filtration coefficient increases from 0 to 4.56× 10-4m/(?),the height of sand bank increases by 12.5%at the same time;the displacement rises from 5.4m3/h to 21.6m3/h,followed by the decreasing of height of sand bank by 79.1%;with the proppant density increasing from 2450kg/m3 to 3030kg/m3,the height of sand bank grows by 44.9%;the existence of branch fracture makes the reduction of height of sand bank in single major fracture,with the reduction extent from 33%to 101.8%.The influence factors on proppants migration and placement in single major fracture are ranked from high to low as below:construction displacement,proppant size,sand concentration,density of proppant,viscosity of fracturing fluid and filtration coefficient.Considering the mutual interference between proppants,narrow width and rough fracture surface,and fracture surface filtration,sand concentration correction factor,wall effect correction factor and filtration correction factor are proposed separately.With comprehensive consideration for the above three points,numerical model of proppants setting in single major fracture is built.Considering the angle of branch fracture,the ratio of branch fracture width to major branch width,the separate fluid model of carrier fluid in combined fracture is established.Utilizing the method of numerical simulation,the effects of parameters on proppants migration and placement in different type of fracture are studied.And the influence factors on single major fracture are ranked as follows:construction displacement,proppant size,density of proppant,width of fracture,sand concentration,density of fracturing fluid,filtration coefficient,viscosity of fracturing fluid and angle of branch fracture.The influence factors on vertical branch fracture in combined fracture are ranked as follows:proppant size,construction displacement,density of proppant,width of fracture,sand concentration,filtration coefficient,viscosity of fracturing fluid and density of fracturing fluid.The influence factors on horizontal branch fracture in combined fracture are ranked as follows:density of proppant,construction displacement,filtration coefficient,proppant size,width of fracture,sand concentration,viscosity of fracturing fluid and density of fracturing fluid.After the simulation of fracturing software,it shows that the optimized fracturing construction scheme with single fracture and combined facture is practical to generate longer and more effective propping fracture or facture network,compared with traditional fracturing construction scheme.Combining experimental research,theoretical analysis and the method of numerical simulation,this dissertation specifies the laws of proppants migration and placement in combined fracture and makes sense for fracturing construction scheme design.