How Anions and Cations Impact the Viscosity and Viscoelasticity of High-Viscosity Friction Reducers
Authors: Ge Xiaojing , Imqam Abdulmohsin 
Published: October 2023

Summary
High viscosity friction reducers (HVFRs) have been used extensively as agents to reduce friction and transport proppants during hydraulic fracturing. Meanwhile, the recycling of produced water has gained traction due to its environmental and economic advantages. Presently, the predominant friction reducers utilized in the fields are categorized as anionic and cationic HVFRs. Anionic HVFRs are frequently injected with fresh water, while cationic HVFRs are typically used in conjunction with high-total dissolved solids (TDS) produced water. It is believed that cationic friction reducers have better TDS tolerance, friction reduction performance, and proppant transport capabilities than their anionic counterparts under high-TDS conditions due to their better viscous and viscoelastic properties. Moreover, different cations’ effects on anionic HVFR have been studied extensively, and the reduction of viscosity and viscoelasticity is mostly concluded as the result of the charge screening mechanism. However, anions’ effects on cationic HVFRs still remain to be investigated. Besides, in some previous experimental studies, there may have been a lack of specificity indefining the experimental procedures or effectively controlling the experimental variables. Therefore, the ultimate objective of this experimental study is to analyze various cations’ and anions’ effects on the viscosity and viscoelasticity of anionic and cationic HVFRs comparably and precisely with well-controlled experimental variables. For the viscosity of HVFRs, two hypotheses based on the charge screening mechanism are proposed and will be tested in this study. The first hypothesis is that the viscosity reduction of anionic HVFRs is due to cations, whereas the viscosity reduction of cationic HVFRs is due to anions. The second hypothesis is that the viscosity reduction of HVFRs is mainly due to ions’ valence instead of their types. To demonstrate both hypotheses, an anionic (FLOJET DRP 2340X) and a cationic (FLOJET DRP 419X) HVFR at 4 gallons per thousand gallons (GPT) were selected and analyzed. The rheology measurements of both anionic and cationic HVFRs were conducted with deionized (DI) water and various salts, respectively. Fe3+ and H+ (or pH) effects were specifically investigated. The results showed both hypotheses failed. First, the viscosity reduction of the cationic HVFR is mainly due to anions. However, Fe3+ also has pronounced effects on the viscosity reduction of the cationic HVFR. Second, the charge shielding mechanism is only one of the viscosity reduction mechanisms of anions and cations for HVFRs. Cations from the same group on the periodic table seem to have similar effects on the viscosity of the anionic HVFR. For the viscoelasticity of HVFRs, cations’ and anions’ effects remain to be further investigated. For the cationic HVFR, results showed a similar trend to the effects on viscosity. For the anionic HVFR, monovalent cations from alkali metals had similar effects on viscoelasticity reduction. Overall, this study provided very precise and specific procedures by using molarity (M) instead of weight concentration [parts per million (ppm) or weight percent (wt%)] as a standard protocol to investigate various ions’ effects on the viscosity and viscoelasticity of HVFRs and the mechanisms behind them, which may also be applied to other polyelectrolytes (i.e., Xanthan gum).