Proppant Transport by a High Viscosity Friction Reducer

Abstract

Abstract Because of potential reduced cost and improved retained conductivity, high viscosity friction reducers (HVFRs) are becoming increasingly popular in hydraulic fracturing stages where linear or crosslinked guar gels are traditionally used. However, concerns remain regarding the proppant transport capability of HVFRs relative to that of traditional linear gels. To address these concerns, the proppant transport capability of a polyacrylamide-based HVFR is compared with that of linear guar at cost-parity concentrations. Correlation between the proppant transport capability and rheological properties of the fluids, including viscosity and elasticity, is also discussed. A slot flow device was used to visualize the proppant transport performance of each fluid. The fluid concentrations ranged from 2.25 to 4.5 gpt (gallons per thousand gallons) for the HVFRs and 15# to 30# (pounds per thousand gallons) for guar, which were matched to achieve cost-parity between the two classes of fluids. The proppants used were 30/50, 20/40, and 12/20 mesh sand at 2 ppg (pounds per gallon) loading. Comprehensive rheological properties of the carrier fluids were characterized under steady-state and oscillatory shear and correlated with the observed proppant transport behavior. At least two sand transport mechanisms were observable in the slot flow tests. Small sand particles remained suspended in the fluid and were transported by convection. Large sand particles settled quickly, but top layers of the settled particles crept forward at a slower speed. The HVFR showed significantly better sand transport efficiency than guar at cost-parity concentrations despite the fact that the HVFR had a lower viscosity at high shear rates. The improved sand transport performance may be attributed to two factors: first, sand particles were suspended for a longer time in the HVFR because of its elevated viscosity at lower shear rates in the center of the slot; second, the HVFR's increased elasticity in the high shear regime near the wall further enhanced sand suspension by providing an elastic lifting force to hinder sand settling. The elastic effect on sand settling is estimated to be much larger than the viscous effect for the HVFR investigated herein but is neglibile in guar. Field results validated the capability of the HVFR to carry proppants, including 20/40 mesh coarse sand, in situations where traditional slickwater FRs and linear guar gel failed. This study demonstrated that the HVFR has superior proppant transport capability, relative to guar, at cost-parity concentrations despite having lower viscosity at high shear rates. The results indicate that the proppant transport performance of fracturing fluids correlates better with their elasticity and low shear viscosity than the high shear viscosity. Additionally, the findings provide much needed proppant transport data for field engineers to make rational choices for the selection of fracturing fluids.