Settling and Mobilization of Sand-Fiber Proppants in a Deformable Fracture

Abstract

We investigated the response of a sand-fiber proppant suspension (17.7% 40/70 mesh sand and 0.4% polymeric fibers) flowing and settling inside a transparent deformable 15 ×15-cm fracture subjected to an increasing applied normal stress, σ n . We quantified the solid distribution within the fracture and the evolution of the solid volume fraction, ϕ, as σ n increased. The sand-fiber suspension formed a highly heterogeneous proppant pack with sand-fiber clumps surrounded by relatively solids-free regions, which was significantly different from the uniform solid distribution observed in settling experiments without fibers. As σ n increased from 0 to 88.5 kPa, the fracture aperture decreased by up to 85% and caused the sand-fiber clumps to act like pillars that supported the applied stress and prevented full fracture closure. At the end of the settling experiments, we simulated flowback by injecting solids-free carrier fluid while maintaining σ n =88.5 kPa. Subsequent flow of solids-free fluid caused the mobilization of some solids but left the supporting pillars intact. We used a numerical model to simulate flow through the porous proppant pack and open regions of the fracture and found a correlation between ϕ and the shear rate, γ, at locations of mobilized solids, which suggested a mobilization threshold: ϕ γ mob . Additional simulations carried out in different heterogeneous proppant distributions showed that solid mobilization leads to the formation of highly conductive channels that significantly increase fracture permeability. Our results suggest that adding fibers to conventional sand proppants can lead to heterogeneous proppant distributions and increased fracture permeability.