Bed load proppant transport during slickwater hydraulic fracturing: insights from comparisons between published laboratory data and correlations for sediment and pipeline slurry transport
Bed load transport is the movement of particles along the top of a bed through rolling, saltation, and suspension created by turbulent lift above the bed surface. In recent years, there has been a resurgence of interest in the idea that bed load transport is significant for proppant transport during hydraulic fracturing. However, scaling arguments suggest that bed load transport is only dominant in the laboratory and is negligible at the field scale. I review laboratory experiments that have been used to develop concepts of bed load transport in hydraulic fracturing. I also review the scaling arguments and laboratory results that have been used to argue that viscous drag, not bed load transport, is dominant at the field scale. I compare literature correlations for fluvial sediment transport and for pipeline slurry transport with published laboratory data on proppant transport in slot flow. The comparisons indicate that fluvial transport correlations are suitable for predicting the rate of sediment erosion due to flow of proppant-free fluid over a bed. The pipeline slurry correlations are suitable for predicting the ability of proppant that is already in suspension to flow in bed transport without settling, but only if the aspect ratio of the flowing region in the slot is close to unity. This can occur at the laboratory scale, but not in the field. The comparison indicates that at low rates of proppant flow, the equilibrium bed height in a laboratory proppant transport experiment can be predicted from pipeline slurry correlations. However, as the volumetric flow rate and the aspect ratio of the flowing region in the slot increase, the applicability of the pipeline slurry correlations breaks down. Laboratory experiments using higher volumetric flow rate indicate that bed load transport rates are too low to be significant at the field scale, even if the flow velocity is very high.