Investigating the Effect of Water Softening on Polymer Adsorption onto Carbonates through Single-Phase and Two-Phase Experiments

Abstract

Summary Polymer retention is considered a major challenge in polymer flooding applications, especially in carbonates. This is due to the prevailing conditions of low permeability (<100 md), high temperature (>85°C), and high salinity (>100,000 ppm) generally found in these formations, which limit the effectiveness of commonly used polymers such as hydrolyzed polyacrylamide (HPAM) and xanthan gum. To address these challenges, a polymer based on acrylamide tertiary butyl sulfonate (ATBS) has been used due to its tolerance to high-temperature and -salinity conditions. However, the high cost of manufacturing these polymers, combined with their anionic properties that promote adsorption onto positively charged carbonate rocks, necessitates the exploration of methods to reduce polymer retention. In this study, we aim to determine the sufficient concentration of hardness ions (Ca2+ and Mg2+) required to significantly reduce the adsorption of this polymer. The study is unique in its focus on mitigating polymer retention in carbonate formations using softened brine, as no prior research has investigated this aspect. Four different brines were investigated with a salinity of 8,000 ppm total dissolved salts (TDS) and varying ionic composition designed mainly by eliminating the hardness-causing ions, Ca2+ and Mg2+. A geochemical study was performed using the PHREEQC software to analyze the interaction between these injected brines and the rock. Furthermore, 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 in these brines. 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 (SI) of less than 0.5 for all the brines used when interacting with the rock, indicating a very low tendency for calcium sulfate precipitation. Furthermore, 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 makeup 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 brines resulted in lower adsorption in the range of 37–62 µg/g-rock as opposed to 102 µg/g-rock for the base makeup brine. On the other hand, the single-phase dynamic adsorption results showed an even lowered polymer adsorption of 33 µg/g-rock for the softened brine compared with 45 µg/g-rock for the base makeup brine. Additionally, the single-phase dynamic adsorption studies showed a remarkable improvement in polymer injectivity using softened brine. The polymer retention in wettability-altered cores was further reduced. The study highlights that water softening improves the performance of polymers, specifically in terms of lowering polymer adsorption. It concludes that a threshold hardness level (Ca2+ and Mg2+) of approximately 100 ppm is sufficient to achieve a significant reduction in polymer adsorption for the tested experimental conditions. In this paper, we show that the softened water increases the polymer viscosity and reduces polymer adsorption, which leads to an overall reduction in polymer consumption. Hence, the softened makeup water has the potential to enhance the application envelope of this potential polymer for polymer flood, especially in the case of carbonate reservoirs.