Quantifying Proppant Transport in Thin Fluids - Theory and Experiments

Abstract

Abstract In conventional fracturing fluids, proppant transport is governed by settling, described by Stokes Law. In thin fluids, saltation and reptation (creep) may dominate proppant transport. Past slot tests have shown that a proppant bank forms in the fracture and that proppant pumped later may overshoot previously-pumped proppant. If this occurred in full-scale hydraulic fractures, it could negate the benefits of pumping tail-in stages of more conductive proppant. Proppant banks can also migrate in a manner similar to wind-blown sand dunes. Although some qualitative results have been derived from past slot tests, there is very limited quantitative data on proppant transport via saltation and reptation. This paper outlines the theory behind these two transport mechanisms and identifies the key parameters governing them. The relative importance of a high coefficient of restitution and low friction coefficient are demonstrated. The methodology and results of experiments to measure the material properties governing saltation and reptation are presented for both conventional and advanced ceramic proppants and compared with those for sand. Advanced ceramic proppant has both a high coefficient of restitution and a low friction coefficient. Although sand has a higher coefficient of restitution than conventional ceramic proppant, it transports poorly due to the high friction associated with a rough, irregular surface. Both qualitative and quantitative testing has been performed in two large-scale slots. Qualitative testing has shown the development of dunes and demonstrated the impact of friction on dune shape. Results of tests pumping larger (30/40) proppant after smaller (50/60) show that larger proppant can fill the entrance (near-wellbore) area as opposed to passing over the smaller proppant, and flow through a slot representing a complex network has shown how proppant can "turn the corner" from a primary fracture to build a dune in a secondary fracture. Finally, quantitative testing has validated the theory and experimental measurements related to the initiation of proppant transport from a static bank. Some criteria to select the optimum proppant for slickwater fracturing are provided.