Study On Proppant Transport In Variable Viscosity Fracturing

Hydraulic fracturing of deep marine shale in southern China adopts the mixed injection of cross-linking fluid,linear glue and slip water to transport proppant to the fracture and form effective support for the fracture.Deposition and transport characteristics of proppant in different viscosities and combinations of fracturing fluids will significantly affect its transport distance and location in fractures.However,the mechanism of proppant transport in this process is unclear,so it is necessary to study the mutually viscous injection process.In view of the above problems,this paper systematically studies the proppant transportation and laying rule in the process of mixed injection of variable viscosity fracturing fluid,combining with proppant transportation experiment and numerical simulation method,providing a theoretical basis for the optimization of variable viscosity fracturing construction parameters.Firstly,proppant transportation experiments under mixed injection of variable viscosity fracturing fluid were carried out by using visual large-scale simulated fracture device.The experimental results show that the mixed injection process of variable viscosity fracturing fluid can be divided into two processes.With the increase of injection time,the migration velocities of the two sand stacks gradually converge.In addition,the "viscosity ratio","slope angle of sand conveying" and "passage ratio" are defined.The results show that with the increase of viscosity ratio,the slope angle of sand conveying decreases first and then increases,and the rate of passage decreases gradually.Increasing displacement and multi-stage displacement series can effectively increase the displacement efficiency and make more proppants move to the deep fracture.Secondly,a fluid-particle two-phase coupled flow model is established based on computational fluid dynamics(CFD)+discrete element method(DEM).In addition,the feasibility of the numerical model is verified by comparing the experimental results and numerical simulation results of large plate crack simulation.Finally,the proppant transportation simulation of variable viscosity fracturing is carried out in a small fracture model.The results show that when crosslinked guar gum fracturing fluid and uncrossed guar gum fracturing fluid are injected with proppant,the proppant pumped in first mainly settles at the front of the fracture,and the proppant pumped in later mainly settles at the entrance,while the situation is just the opposite when slip water is injected.When combined injection mode is used,there is fingering phenomenon and scouring effect when sliding water carrying sand liquid enters uncrosslinked guar gum carrying sand liquid.When the viscosity ratio of pumping liquid is increased to a certain extent,the proppant transportation distance increases,and the height of sand dike is higher,and the uniformity of laying is better;but when the viscosity ratio exceeds 5,the proppant effective transportation distance decreases,which is not conducive to proppant to proppant deep fracture.This paper effectively reveals the settlement and movement law of proppant in fractures in the process of variable viscosity fracturing by combining visual large-scale simulation experiment of proppant transportation and numerical simulation study of proppant transportation.It has certain reference significance for optimization of construction parameters and adjustment of construction procedure of variable viscosity fracturing.