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Investigation on viscosity behaviour of anionic polyacrylamide copolymer in brine solutions for slickwater fluids applications at high salinity and hardness conditions


 Slickwater fluids, used to undertake fracturing in low-permeability reservoirs, may be derived from produced water consisting range of dissolved salts. The fluids are pumped downhole at high flow rates, and hence friction reducers e.g., anionic polyacrylamides (APAMs) are added, which also impart viscosity to the fluid resulting in better proppant transport. The present work investigates effect of an APAM copolymer on viscosity of slickwater fluids specifically at high salinities and hardness conditions. The experimental part of this study demonstrated impact of parameters i.e., APAM concentration and salt type/concentration. For addition of monovalent salt, fluid viscosity decreased initially owing to the charge shielding effect; and, the viscosity values levelled off after a certain salinity level. However, a very peculiar behavior was observed for addition of divalent salts (hardness) to fluid systems. Initially, as expected, fluid viscosity showed significant decrease as hardness increased up to 50k ppm. Increasing the hardness beyond 50k ppm, viscosity showed a distinctive increase up to 250k ppm. This phenomenon may be termed as reverse charge shielding effect and maybe contributed to excessive ionicity inducing repulsion between polymer molecules. To model above experimentally observed non-monotonous viscosity behavior, various machine learning models were employed. Amongst four machine learning algorithms studied, SVR-based models predicted the slickwater fluid viscosity with maximum accuracy. Sensitivity analysis was carried out to determine prominence of the studied input parameters. The modelling work would assist in minimizing trial-and-error in designing/optimizing a slickwater fluid system.