Numerical Simulation On Transport Behavior Of Ultralightweight Proppant In Fractures With CFD-DEM Method

The slickwater fracturing is becoming the main technology during the shale gas fracturing treatment because of its low cost and low damage.However,due to its low viscosity,slickwater frac fluids exhibit correspondingly poor proppant transport,which makes the proppant quickly settle down to the bottom of the fracture.The study and develop of ultralightweight proppant(1 055-1750kg/m3)provide a new opportunity to slickwater treatment.However,experiment and simulation of the proppant transport and settlement behavior are commonly focus on heavy and intermediate proppant other than ultralightweight proppant.Therefore,the main purpose of this paper is to study the settlement behavior and micro-mechanism of ultralightweight proppant carrying by slickwater frac fluids in fractures,to provide a reliable reference for parameter optimization during the field treatment.First,a two-phase model of particle-liquid was built by using computational fluid dynamics(CFD)and discrete element(DEM)coupling method.The coupling N-S equation of liquid phase was established using CFD method,which take the influence of particles into account.Meanwhile,the motion equation of particles was established by using DEM method,which take the drag force,buoyancy,and the interacting forces between particle-particle and particle-wall into account.The motion of two phases were coupled by drag model and Momentum exchange term to obtain the two-phase coupled model.SIMPLE(Semi-Implicit Method for pressure-Linked Equations)and display difference method were applied respectively to solve the motion equation of the liquid phase and particle phase.In order to obtain the contact model parameters during the DEM simulation,the virtual experiment was applied to calibrate the coefficient of restitution,static friction and rolling friction.In addition.the transport experiment of low-density particle was carried out to validate the two-phase model.Next,we simulate the ultralightweight proppant transport behavior in a medium fracture model based on the CFD-DEM model.The result show that the convection dominate the proppant settlement because of the viscosity and concentration difference between the pad fluid and the sand-ladden fluid.To some extent,the transport distance can increase by lower the proppant density,however,the effect is not as obvious as researchers expect.Furthermore,we investigated the influence of the treatment parameters and particle features to the transport and settlement behavior.In order to describe the characteristics of particles during their settlement,we defined the blank zone and settling profile angle,The result show that the blank zone increases while the pump rate and fluid viscosity increase,and decreases as the proppant concentration,proppant density and proppant size.Meanwhile,the settling profile angle increases with the proppant density,proppant concentration and proppant size,and decreases with the puamp rate and fluid viscosity.As a result,those parameters have great influence on position and shape of the sandbed.In this paper,a coupled CFD-DEM model is established,and transport and settlement law of ultra-low density proppant under different conditions are studied.The results provide reliable references for optimization of slickwater treatment,optimization of fracturing fluid and proppant.