Effect of Water Softening on Polymer Adsorption on Carbonates Through Single-Phase and Two-Phase Studies

Abstract

Polymer retention is considered as a major challenge in polymer flooding application, especially in carbonates, due to the prevailing harsh conditions of low permeability (< 100 mD), high temperature (> 85 °C), and high salinity (>100,000 ppm). One of the many advantages of smart water technology is maintaining the viscosity of polymers for water-based Enhanced Oil Recovery (EOR) techniques. This research focuses on the effect of water softening on the performance and adsorption of an ATBS-based polymer in carbonate reservoirs. Four different brine recipes were investigated with the salinity of 8,000 ppm TDS and varying ionic composition designed mainly by eliminating the hardness-causing ions, including Ca2+ and Mg2+. A geochemical study was performed using the PHREEQC software to analyze the interaction between these injected brines and the rock. Further, comprehensive rheological and static adsorption studies were performed at a temperature of 25 °C using the potential ATBS-based polymer to evaluate the polymer performance and adsorption with different brine recipes. Later, dynamic adsorption studies were conducted in both single-phase and two-phase conditions to further quantify polymer adsorption. The geochemical study showed an anhydrite saturation index of less than 0.5 for all the brine recipes used when interacting with the rock, indicating a very low tendency for calcium sulfate precipitation. Further, the rheological studies showed that polymer viscosity significantly increased with reduced hardness, where a polymer solution viscosity of 7.5 cP was obtained in zero hardness brine, nearly 1.5 times higher than the polymer viscosity of the base make-up brine of 8,000 ppm. Moreover, it was observed that by carefully tuning the concentrations of the divalent cations, the polymer concentration consumption for the required target viscosity was reduced by 40-50%. For the single-phase static adsorption experiments, the polymer solution in softened brine recipes resulted in lower adsorption in the range of 37 – 62 μg/g-rock as opposed to 102 μg/g-rock for the base make-up brine. On the other hand, the single-phase dynamic adsorption results showed an even lowered polymer adsorption of 37 μg/g-rock for the softened brine recipe compared to 45 μg/g-rock for the base make-up brine. Additionally, the single-phase dynamic adsorption studies showed a remarkable improvement in polymer injectivity using softened brine. The polymer retention in wettability restored cores was further reduced. This study highlights the effect of water softening on polymer performance, particularly polymer adsorption. The paper shows that the softened water increases the polymer viscosity and reduces polymer adsorption, which leads to the overall reduction in polymer consumption. Hence, the softened make-up water has the potential to improve the economics of polymer flood, especially in the case of carbonate reservoirs.