An experimental study is performed to quantify the growth of the mixing zone in miscible viscous fingering. Rectilinear flow displacement experiments are performed in a Hele-Shaw cell over a wide range of viscosity ratios (1-1225) by injecting water into glycerol solutions at different flow rates. All the experiments are performed at high Peclet numbers and linear growth in mixing zone is observed. The mixing zone velocity increases with the viscosity ratio up to viscosity ratios of 340 and the trend is consistent with Koval's model. However, at higher viscosity ratios, the mixing velocity plateaus signifying no further effect of viscosity contrast on the growth of mixing zone. The front (fingertip) velocities also increase up to viscosity ratios of 340 above which the velocities plateau. C 2015 AIP Publishing LLC.
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 proppant in fractures. The goal of these methods is to increase the conductivity in 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 by use of a new method of fracturing, referred to as alternate-slug fracturing. The method involves an alternate injection of low-viscosity and high-viscosity fluids, with proppant carried by the low-viscosity fluid. Alternate-slug fracturing ensures a deeper placement of proppant through two primary mechanisms: (i) proppant transport in viscous fingers, formed by the low-viscosity fluid, and (ii) 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 proppant in the fracture. In addition, the method offers lower polymer costs, lower pumping horsepower, smaller fracture widths, better control of fluid leakoff, less risk of tip screenouts, and less gel damage compared with conventional gel fracture treatments.Experiments are conducted in simulated fractures (slot cells) with fluids of different viscosity, with proppant being carried by the low-viscosity fluid. It is shown that viscous fingers of low-viscosity fluid and viscous sweeps by the high-viscosity fluid lead to a deeper placement of proppant. Experiments are also conducted to demonstrate slickwater fracturing, hybrid fracturing, and reverse-hybrid fracturing. Comparison shows that alternate-slug fracturing leads to the deepest and most-uniform placement of proppant inside the fracture. Experiments are also conducted to study the mixing of fluids over a wide range of viscosity ratios. Data are presented to show that the finger velocities and mixingzone velocities increase with viscosity ratio up to viscosity ratios of approximately 350. However, at higher viscosity ratios, the velocities plateau, signifying no further effect of viscosity contrast on the growth of fingers and mixing zone. The data are an integral part of design calculations for alternate-slug-fracturing treatments.
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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