Impact of brine chemistry on viscoelastic properties and geochemical interactions of low salinity polymer at rock-brine interfaces

Recently, numerous studies have evaluated the efficacy of low salinity polymer (LSP) flooding in sandstone oil reservoirs; however, literature review revealed that the effects of brine chemistry on polymer viscoelasticity, and the subsequent impact on LSP flooding performance need to be clarified. To achieve that, the geochemical interactions of polymer with the rock surface at different brine chemistries and their effects on polymer adsorption and viscosity loss need to be considered. Therefore, the present study aims to investigate the impact of brine chemistry on polymer viscoelasticity, and ion exchange reactions with Boise sandstone cores. A multidimensional experimental approach, including frequency sweep measurements of HPAM polymer solutions at various brine chemistries was employed. Additionally, IC and ICP analyses were conducted for equilibrated brine with Boise rock. Moreover, single-phase displacement experiments were performed to study polymer adsorption and viscosity loss, with effluent analysis carried out using IC and TOC. The results revealed that polymer solutions with low divalent cation (Ca²⁺ or Mg²⁺) concentrations (2 mM) exhibited longer relaxation times (up to 12 s⁻¹), indicating higher elasticity. However, as the cation concentration increased from 4 mM to 14 mM, the polymer structure stabilized, and additional Ca²⁺ or Mg²⁺ caused a smaller change in relaxation time (decreasing from 1.33 s⁻¹ to 0.56 s⁻¹), and consequently, in viscoelasticity. Additionally, salinity had a stronger effect on HPAM polymer viscoelasticity than specific brine composition, with the charge-screening effect from salt concentration being the primary driver of relaxation time reduction. Similarly, the ionic composition of formation brine had minimal impact on the geochemical reactions of Boise rock with or without polymer, except when Ca²⁺ and Mg²⁺ were absent. Moreover, the presence of polymer in the brine was found to affect ion exchange reactions, equilibration processes, and extend the stabilization time during flooding experiments, emphasizing the importance of considering polymer behavior in fluid-rock interactions. Furthermore, the salinity of the makeup brine, rather than polymer concentration, primarily influenced adsorption behavior and injectivity by controlling polymer-rock interactions. Both high salinity polymer (HSP) (5459 ppm) and LSP (545.9 ppm) showed a similar 20 % of viscosity degradation, with delays in viscosity recovery due to polymer adsorption in HSP and salinity changes in LSP, and final degradation driven by increased divalent cation concentration in the water chemistry. These findings emphasize the importance of optimizing polymer formulations based on brine chemistry, managing the salinity of injection water and the presence of divalent cations, and understanding geochemical interactions and flow dynamics to enhance LSP performance and mitigate adverse effects such as scaling and viscosity loss due to brine-rock interactions.