The contributions of viscosity and elasticity to inhibiting proppant settling in gelled fracturing fluids are quantified and decoupled in this study. The settling velocity of a single particle under orthogonal shear flow was measured in a transparent Couette flow cell using automatic particle tracking, and the local flow field was mapped using particle tracking velocimetry. The settling behavior is correlated with the rheological properties of the fluids.

In carboxymethyl hydroxypropyl guar (CMHPG) crosslinked with borate, particle settling slows as the orthogonal shear rate increases, with settling essentially stopping at sufficiently high shear rates. The authors propose that there are two primary mechanisms for the enhanced particle suspending under the orthogonal shear—shear thickening and elastic lifting. The relative importance of the two factors depends on the shear rate and fluid relaxation time. Specifically, the ratio of the elastic and viscous contribution to particle suspension is γe/v=0.5cλN1(γ˙)η(γ˙), where c is a constant, λ is the stress relaxation time, N1(γ˙)) is the first normal stress difference that depends on the shear rate γ˙, and η(η(γ˙)) is the viscosity.

For the crosslinked CMHPG gel examined in this work, it is determined that γe/v=0.26γ˙2, indicating that the viscosity is more important at γ<2s1, whereas the elasticity becomes dominant at γ˙<2s1 for proppant suspending. For the uncrosslinked CMHPG, γe/v=0.0035γ˙1.3 , indicating that the elastic contribution becomes more important than the viscous contribution only at shear rates > 80 s−1. The understanding of the relative importance of viscosity and elasticity can provide guidance for chemists to develop better fracturing fluids and for engineers to model proppant transport

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