Comparison of Different Methods to Evaluate the Effect of Temperature on Polymer Retention and Degradation in the Presence and Absence of Oil on Carbonate Outcrops
Authors: Sebastian Anoo , Musthaq Muhammad , Al-Shalabi Emad W. , AlAmeri Waleed , Mohanty Kishore , Masalmeh Shehadeh , AlSumaiti Ali M. 
Published: October 2023

Abstract
Polymer retention poses a significant challenge in polymer flooding applications, emphasizing the importance of accurately determining retention levels for successful project design. In carbonate reservoirs of the Middle East, where temperatures exceed 90 °C, conducting adsorption tests under similar temperature conditions becomes crucial for the precise determination of adsorption values. The choice of analytical method heavily impacts the accuracy of retention measurements from effluent analysis. This study investigates the effect of temperature on the performance of a polymer, specifically its rheological behavior and retention. Rheological and polymer flooding experiments were carried out using an ATBS-based polymer in formation water (167,114 ppm) at different temperatures (25, 60, and 90 °C) with required oxygen control measures. Dynamic polymer retention was conducted in both absence of oil (single-phase tests) and presence of oil (two-phase tests). In addition, different analytical techniques were evaluated, including viscosity measurements, UV-visible spectroscopy, and TOC-TN analysis, to determine the most accurate method for measuring the polymer concentration with the least associated uncertainty. Furthermore, the study investigates the effects of these uncertainties on the final dynamic polymer retention values by applying propagation of error theory.
The effluent polymer concentration was determined using viscosity correlation, UV spectrometry, and TOC-TN analysis, all of which were reliable methods with coefficient of determination (R2) values of ∼0.99. The study analyzed the effects of flow through porous media and back-pressure regulator on polymer degradation. The results showed that the degradation rates were around 2% for flow through porous media and 16% for mechanical degradation due to the back-pressure regulator for all temperature conditions. For the effluent sample, the concentration of effluents was lower when using the viscosity method due to polymer degradation. However, the TOC-TN and UV methods were unaffected as they measured the total nitrogen and absorbance at a specific wavelength, respectively. Therefore, all viscosity results were corrected for polymer degradation effects in all tests. During 60 °C single-phase studies, the dynamic retention values obtained from viscosity correlation, UV spectrometry, and TOC-TN analysis were determined to be 52 ± 3, 45 ± 5, and 48 ± 3 μg/g-rock, respectively. During the two-phase coreflooding experiment conducted at 25 °C, the accuracy of the UV spectrometry and viscosity measurements were affected by the presence of oil, rendering these methods unsuitable. However, the TOC-TN measurements were able to deliver a retention of 24± 3 μg/g-rock. Moreover, the use of glycerine preflush to inhibit oil production during polymer injection in the two-phase studies showed that all three methods were appropriate with dynamic retention values of 27± 3, 25±5, and 21±3 μg/g-rock for viscosity, UV, and TOC-TN, respectively at 60 °C. The error range was obtained using the propagation of error theory for all the methods. Accordingly, it was also noted that the temperature did not affect the dynamic retention values in both single-phase and two-phase conditions. The dynamic retention values for single-phase using the UV method and two-phase using the TOC-TN method were 45 – 56 ±5 μg/g-rock and 21 – 26 ±3μg/g-rock, respectively, for the range of temperatures applied. The findings of this study highlight that when adequate oxygen control measures are implemented, the temperature does not exhibit a statistically significant impact on the retention of the ATBS-based polymer under investigation. Furthermore, TOC-TN has been identified as the optimal analytical method due to its minimal uncertainties and ease of measuring polymer concentration under varying experimental conditions.