The Effects of Fluid Viscosity and Density on Proppant Transport in Complex Slot Systems

In this paper, we discuss proppant transport behavior in a complex slot system. Specifically for this study, focus is placed on two different fluid systems, a water/glycerin solution and a water/sodium chloride solution, which represent varying fluid densities and viscosities. The effects of changing fluid viscosities, fluid densities, proppant densities, proppant sizes, proppant concentrations, and slurry injection rates on proppant transport were then experimentally investigated. The slot system consists of a 4-ft long, 0.2-in. primary slot with three secondary slots and two tertiary slots, all at 90 degrees angles to each other. The fluid systems represented brine fluids up to 9.24 ppg and viscous fluids up to 4.3 cp. Although glycerin was used for viscosification, the results can be compared to fluid systems with similar viscosities that are created using other additives such as friction reducers. The proppants used in the study consisted of two sands of 100 and 40/70 mesh (specific gravity of 2.65) and two 40/70 ceramic proppants with specific gravities of 2.08 and 2.71. The study results show that a water/glycerin solution, with a viscosity of 4.3 cp, has significant proppant-carrying capacity with proppants delivered uniformly to greater distances. In addition, sieve analysis conducted on each of the various slots indicated that for all tested proppants that the water/glycerin systems were more capable of carrying larger particles to farther distances. Conversely, the results show that a water/sodium chloride solution of 9.24 ppg density has less capability to carry the proppant farther into the slots. From a comparison standpoint, in all tested cases, viscosity increases had a greater impact on the overall proppant transport than fluid density. In addition, results of the study showed that both increasing proppant concentrations and injection rates have a positive impact on proppant transport, with more proppant being transported farther into the slot system in both cases. The higher the proppant concentration, the sooner the equilibrium dune height (EDH; height when transport starts to occur after dune building) was achieved, the more efficient transport became. Increasing the injection rate led to improving proppant transport by increasing the drag and lift forces on the proppant, which lead to decreased proppant settling velocities and transport farther into the slots.