Research On Proppant Transport Affected By Multiple Mechanisms During Horizontal Well Fracturing

For hydraulic fracturing in horizontal wells,the proppant transport is governed by multiple mechanisms,and this will significantly affect the proppant placement in a fracture and make it to be more difficult for fracturing design.Therefore,in order to improve the understanding of the hydrodynamic behavior of proppant particles and guide fracturing design,it is of significant importance to investigate proppant transport.Firstly,settling experiments have been carried out for single proppant particle falling in slick-water.Since slick-water is viscoelastic fluid,the Giesekus model was used to represent its rheological properties.A new concept of effective settlement diameter was presented to characterize the irregularity of proppant particles,and according to measured settling velocity,the effective settlement diameter of proppants is calculated,and the relationship between proppant effective settlement diameter and its sieving diameter was studied.In addition,the variation of proppant settling velocity against its sieving diameter was investigated,and a new composite model was established to predict proppant settling velocity falling in slick-water using proppant sieving diameter,introducing the effect of slick-water shear-thinning and elastic properties.The wall retardation effects on particles settling in cylindrical tubes and parallel plates were experimentally investigated,different wall factor correlations were established,and the eccentric wall retardation effect was also studied when a particle settles off from the duct center.Secondly,the flow of proppant-fracturing fluid slurry in fracturing pipelines was numerically investigated.A three-segment transport model was established to study the slurry transport in vertical downward,slant downward and horizontal pipelines.Introducing slippage velocity between particles and fluids,a vertical downward slurry flow model was established.The friction factor model was modified according to fluid elasticity,based on particle settlement and dispersion,a slant downward two-layer and horizontal three-layer slurry transport models were presented.Ultimately,the slurry flow pattern was investigated under different parameters,and flow pattern maps were obtained.Finally,taking the interactions of particle-particle,particle-wall and particle-fluid into account,the hydrodynamic and mechanical behavior of proppant particles,driven by thinning fracturing fluids,in a fracture were investigated.The fracture wall retardation effect on proppant settlement and horizontal transport was studied using a coupled approach of Computational Fluid Dynamic(CFD)-Discrete Element Method(DEM),and the effect of fracture roughness was quantitatively characterized.In addition,considering proppant settlement,the transport mechanisms,flow pattern and placement of proppants in a fracture were studied,and the effects of proppant diameter and density,fluid viscosity,injection parameters(injection rate and position)and injection strategies(over-flushing,shut-down,tail-in and graded injection)on proppant placement were analyzed.Ultimately,based on the aforementioned simulation,the hinder settling and horizontal transport behavior of two particles with different diameters were modeled,the slurry,constituted of multi-sized particles,flow in a fracture was simulated,and the effects of having multi-sized particles relative to uniformly-sized ones on the proppant placement was quantitatively characterized.