New fracturing techniques such as hybrid fracturing (Sharma et. al., 2004), reverse hybrid fracturing (Liu et. al., 2007) and channel (HiWAY) fracturing (Gillard et al. 2010) have been deployed over the past few years to effectively place proppants in fractures. The goal of these methods is to generate a network of open channels within the proppant pack, providing highly conductive paths for hydrocarbons to flow from the reservoir to the wellbore. This paper presents an experimental study on proppant placement using a new method of fracturing, referred to as Alternate-Slug fracturing, which involves alternate injection of low viscosity and high viscosity fluids into the fracture. Alternate-slug fracturing ensures deeper placement of proppants through two primary mechanisms: (a) proppant transport in viscous fingers formed by the low viscosity fluid and (b) an increase in drag force in the polymer slug leading to better entrainment and displacement of any proppant banks that may have formed. Both these effects lead to longer propped fracture length and better vertical placement of proppants in the fracture. In addition the method offers lower polymer costs, lower pumping horsepower, smaller fracture widths, better control of fluid leakoff and less gel damage compared to conventional gel fracs.

Experiments are conducted in simulated fractures (slot cells) to study the mixing of fluids over a wide range of viscosity ratios. Data is presented to show that the finger velocities and mixing zone velocities increase with viscosity ratio up to viscosity ratios of about 350 and the trend is consistent with Koval's theory. However, at higher viscosity ratios the mixing zone velocity values plateau signifying no further effect of viscosity contrast on the growth of fingers and mixing zone. Fluid elasticity is observed to slow down the growth of fingers and leads to growth of multiple thin fingers as compared to a single thick dominant finger in less elastic fluids.
Experiments are conducted with fluids of different viscosity and elasticity, with proppants being carried by the low viscosity fluid. It is shown that the injection rate, slug size and viscosity ratio can be used to control the geometry of the fingers created and, therefore, the proppant distribution in the fracture. The non-uniform placement of proppant in the viscous fingers leads to the creation of high permeability paths in the proppant pack.

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