This paper examines the influence of proppant concentration and fracturing fluid rheology on the settling velocity of nonflowing (static) slurries. Static sedimentation behavior is important in proppant transport not only during closure, but also during pumping since a large fraction of the proppant slurry resides in regions of low shear rate.

Slurry settling-velocity measurement technology was devised, enabling velocity determinations to be made in simulated vertical fractures at sand concentrations from 0 to 12 pounds per gallon of fluid (lb/gal). Fluids examined included crosslinked and uncrosslinked fracturing fluids, plus a Newtonian fluid. Sedimentation behavior in fracturing fluids was dominated by a particle clustering phenomenon. Settling velocity was found to depend on proppant concentration in a very different manner than generally assumed by proppant transport models. Although all fluids had similar viscosities at typical rheological characterization shear rates, static slurry settling velocities differed by a factor of more than 100. Differences in fluid viscosities at very low shear rates explain the observed diversity of settling velocities.

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