Effect of Fracture Width and Fluid Rheology on Proppant Settling and Retardation: An Experimental Study

Abstract Water-fracs (using slick water instead of gel to transport proppant) have proved to be a successful fracturing technique in some tight gas reservoirs. Proppant placement is an essential factor that determines the effectiveness of such hydraulic fracturing treatments. This paper presents an experimental study conducted to investigate the impact of fracture width and fluid rheology on proppant transport (including particle settling and horizontal transport). Data is presented to show that the settling velocity is significantly reduced as the fracture width becomes comparable to the proppant diameter. Experimental results are presented for Newtonian fluids with different viscosity as well as shear thinning fluids such as guar gum and Xanthan. Experimental correlations are presented for the proppant settling velocity as a function of the dimensionless fracture width and fluid rheology. These correlations are compared with theoretical predictions based on slot flow models presented earlier. The effect of fracture wall roughness is studied by conducting slot flow experiments on cells with rough walls. Wall roughness is shown to impact the results substantially and a new correlation is presented to account for wall roughness. Data is presented to show that the proppant usually flows at a lower horizontal velocity than the fluid, particularly in narrow fractures. The proppant is either retarded or accelerated depending on the ratio of the proppant size to the fracture width. It has been found that for a single particle, when this ratio is small, the proppant travels faster than the average fluid velocity at that location because the proppant tends to be confined to the center of the flow channel where the fluid velocity is higher. As the proppant size increases, the effect of the fracture walls becomes more important and the proppant is retarded by the walls. The retardation of particle relative to the fluid is greater for larger particles and greater proximities to the fracture walls due to the hydrodynamic stress exerted on the sphere by the walls in the narrow gap. Data and correlations are presented that show this dependence. Proppant retardation is particularly important for shear thinning fluids and when the fracture width becomes comparable to the proppant diameter.