Evaluation of friction reducers for use in recycle fracturing flowback and produced water
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The continued expansion of hydraulic fracturing activity in North America -- especially in slickwater operations -- has given rise to concerns regarding water quantity and quality. On one hand, operators in arid areas must compete with other users to obtain enough fresh water to perform fracturing operations, while in other areas the flowback water after a treatment must be either expensively treated or disposed of in injection wells, which are in very limited supply in regions such as the Marcellus Shale. Reuse of these highly saline waters can help to alleviate both of these problems. However, water that contains concentrated and difficult-to-remove salt ions -- especially divalent cations -- cannot be used with typical polyacrylamide friction reducers, due to these additives' dramatically decreased effectiveness in such fluids. Otherwise, reuse would be an attractive option and, in fact, this practice is widespread in multiple US shale plays with the recent advent of salt-tolerant polyacrylamides. This research attempts to quantify the effect of high salt concentrations on the effectiveness of friction reducers through construction of a flow loop apparatus that allows for observation of turbulent drag reduction. The polymers tested were chosen from industry standards (inverse oil-emulsion salt-tolerant anionic polyacrylamide), novel polyacrylamides (highly salt-tolerant polyacrylamide dispersed in concentrated brine), and an overlooked yet potentially highly effective polymer (i.e. polyethylene oxides, PEOs). PEOs, in particular, have been known as highly efficient friction reducers in brines for over 50 years, but are not used in the fracturing industry for various reasons. These three additives were tested at concentrations of 0.1% in solutions of sodium chloride, calcium chloride, and a multisolute brine of both salts. The experiments show that the typical salt-tolerant polyacrylamide is indeed negatively affected by the divalent calcium ions, while the novel polyacrylamide is a strong performer (up to 60% friction reduction) in even the strongest brines. Interestingly, the PEOs consistently produced about 45% friction reduction (based on the base fluid pipe friction pressure drop), and did so at low concentrations (<0.1%) for a range of molecular weights. The major conclusion of this research is that even highly concentrated brine can be recycled with minimal treatment if either the novel polyacrylamide or PEOs are used, opening the door for potential use of other atypical brine sources in hydraulic fracturing operations. The PEOs are especially interesting because, though overlooked, they are economical, readily available, and salt-tolerant. Future experiments will be run on a larger flow loop to potentially optimize PEO characteristics and further demonstrate their viability as an alternative to polyacrylamides.