Alternate-slug fracturing using foam
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The success of a hydraulic fracturing job depends primarily on the proper distribution of proppant inside the fracture. Fracture length and conductivity are the two prime characteristics that determine the productivity of fractured wells (Liu & Sharma, 2005). Slick-water fracturing involves the use of large volumes of water for fracturing shales and mudstones (Palisch, et al., 2010). The low viscosity of water increases the settling velocity of proppant, resulting in an ineffective lateral placement of the proppant. It also affects the vertical coverage of the proppant across the pay zone(s), rendering the fracturing process inefficient (Gadde, et al., 2004). To improve proppant placement, a new technique was proposed by Malhotra et al. (2014), that involves pumping slugs of high viscosity and low viscosity fluids alternately, with most of the proppant being carried by the low viscosity fluid. Alternate injection of high viscosity and low viscosity slugs creates a mobility contrast between the fluids and leads to the formation of viscous fingers. The viscous fingers provide a pathway for proppant transport. The higher velocity of the viscous fingers compared to the injection velocity of the fluid leads to deeper placement of proppant. In addition, viscous sweeps, due to the high viscosity slugs, push any proppant bank formed near the wellbore deeper into the fracture, thus creating longer fractures (Malhotra, et al., 2014). In this study, we conducted an experimental investigation to obtain a fundamental understanding of the viscous fingering phenomena when water and foam are used as the low and high viscosity fluids, over a wide range of viscosity ratios. We have derived a relationship between finger-tip velocity and viscosity ratio of the fluids. This relationship will help in designing Alternate-slug fracturing treatments for the foam-water system.