Understanding effect of fluid salinity on polymeric drag reduction in turbulent flows of slickwater fluids

Produced waters are brines drawn during the oil and gas production. These fluids are often re-injected into unconventional reservoirs along with certain additives and proppants in order to induce/sustain fractures and boost oil/gas production. The additives may transform these fluids into drag-reducing slickwater fluids that are pumped at high flowrates (turbulent conditions) to transport proppants at low viscosities. The drag-reducer polymeric additives in the slickwater fluids facilitate high flowrates by reducing drag during the flow. The present work investigated the influence of fluid salinity/hardness on polymer dynamics and its impact on the ability of the polymer to alleviate drag. The study focuses on two synthesized drag-reducing anionic acrylamide (APAM) polymers: first is a copolymer of acrylamide and acrylic acid (Base-APAM), and the second is a ter polymer of acrylamide, acrylic acid and a functional group (Functionalized-APAM). The polymers were characterized using capillary viscometry, NMR and degree of ionicity. For evaluation of the drag reduction performance of the above polymers, a laboratory flow-loop was built to mimic the turbulent flow conditions in the wellbore. The experimental evaluation was conducted for the fluids with added polymers and range of fluid salinities to encompass plausible variations in the field. Additionally, the fluids' viscosity performance was evaluated using conventional oil-field viscometers. The obtained experimental data was used to build a semi-empirical correlation that predict the polymer relaxation defined by Weissenberg number i.e., Wi, modelled as a function of fluid salinity and polymeric structure. The drag reduction performance may be derived based on the Re-Wi parameter space, and a correlation for the studied range of parameters has been constructed. The experimental data and models also evidently depicted that the synthesized ter-polymer provides a superior salt-tolerance compared to the co-polymer for the range of parameters studied.