We applied a normal stress (sn = 0 - 12.8 psi) to a transparent deformable fracture to investigate the behavior of sand-fiber mixtures. Using light transmission techniques, we quantified the sand volume fraction ?s throughout the experiment. In the absence of an applied normal stress, sn = 0, the sand and fibers settled into randomly distributed sand-fiber islands. Injecting solids-free carrier fluid displaces the settled solids washing them out of the fracture. Results for applied-normal stress experiments (sn = 0 - 12.8 psi) demonstrate the sand-fiber islands compacted and the solid volume within them increased as sn increased. We observed that these sand-fiber islands supported the applied normal stress and prevented fracture closure, acting as pillars between the fracture surfaces. After injecting solids-free carrier fluid, while keeping sn =12.8 psi, we observed most sand-fiber pillars remained in place and some solids-free pathways were created. The formation of such pathways shows that the addition of fibers to hydraulic fracturing fluids, even at small concentrations, may increase fracture permeability through the formation of sand-fiber pillars capable of supporting a normal stress, preventing fracture closure.
Settling of non-Brownian particles in polymeric fluids (e.g., xanthan or guar gels) is of great importance for many different industrial applications, especially in industries where the aim is to prevent particles from settling or at least decrease the settling rate of the particles, e.g., oil and gas production and well drilling operations. Recently, it has been proposed that the addition of rod-like solids (e.g., fibers) to polymeric fluids may reduce particle settling rates. Elgaddafi et al. (2012) have previously demonstrated a settling rate reduction of a single particle falling through a suspension of dispersed fibers. Much of the recent work has focused on the influence of fibers on the rheological properties of the suspension (George et al., 2011; George et al., 2012; Guo et al., 2005; Ra-jabian et al., 2008). These studies have shown that adding fibers to a polymeric fluid will increase the apparent viscosity of the fluid and reduce settling rate. The reduced settling rate was attributed to the formation of intertwined networks of fibers and polymeric glucose-chains (Guo et al., 2005; Rajabian et al., 2008). Previous research characterized the settling of dispersed sand (Chhabra and Peri, 1991; Guazzelli and Hinch, 2011; Happel and Bart, 1974) and dispersed fibers (Baldock et al., 2004; Herzhaft and Guazzelli, 1999; Herzhaft et al., 1996) suspensions. However, there is limited literature for the suspension and settling of sand-fiber mixtures.