Proppant Transport in Slickwater Fracturing of Shale Gas Formations

Abstract

Abstract Since proppants were first used in hydraulic fracturing, the question of placement in the fracture has remained, for the most part, unanswered. Prediction of proppant transport is important in both treatment design and post-treatment analysis. The transport process has been studied since the late 1950s and a great deal has been learned from the simple laboratory models used in these studies. There are, however, a number of factors that have been overlooked in these studies that we believe are of primary importance. There are indications that fracturing in gas shale plays can produce complex fracture networks. Little is known about the propensity of the slurry to travel through the network. To examine this question, we have constructed a small one-dimentional fracture model with a bifurcation with each wing having equal length and cross-sectional area. We have also constructed a larger three-dimensional slot model to study the process more complex flow field. Results from the small model indicate that flow into the side slot does not occur below some threshold flow rate. This paper summarizes the work that has been done in the area of proppant transport in settling fluids and reports new data on the transport of proppant in multiple fractures. Introduction Fracture flow behavior of non-settling slurries has been discussed elsewhere and it has been shown that fracture width variations play a much larger role than convection in proppant placement. While a number of studies have been done on the flow of settling slurries in slots, there are still questions, such as the influence of particle settling on convection, that remain. This is in spite of the fact that non-crosslinked fluids are much easier to characterize than crosslinked fluids. In non-settling slurries, the motion of proppant-laden fluids into a fracture is strongly influenced by fracture width variations. There is no reason to believe that width variations will not influence settling slurries in the same manner. In addition, the trajectories of both types of fluids will be strongly influenced by height growth and high fluid loss zones, which will attract fluid flow. In gas shales, bifurcations in the fracture also pose an interesting problem in understanding and predicting proppant transport. Proppant Transport in Non-crosslinked Fluids In this section, we describe proppant transport in situations where the fracturing fluids may be as simple as water or as complex as a viscous polymer solution. Low viscosity fluids can be pumped in laminar or turbulent flow although turbulent flow may not persist away from the wellbore. The viscosity and structure of these fluids allow particle settling. Convection and particle settling act in the same direction, so the process is considerably more complex than simple settling and all of the attendant complications associated with it. We will begin with settling and move on other facets of the process. Particle Settling Single particle settling in quiescent Newtonian fluids is well described in the creeping flow regime by Stokes' law (Eq. 1) or modified versions of Stokes' law (Eq. 2) outside this regime.