Anaerobic Thermal Stability Testing of Polymers Under High Salinity and Temperature Conditions for EOR Applications

Abstract Enhanced oil recovery (EOR) is a technique used to increase oil production from a reservoir. This study primarily focuses on chemical EOR using polymer flooding. A polymer stability evaluation was conducted under harsh conditions typical of Middle Eastern reservoirs. The objective is to identify suitable polymers by systematically varying key polymer properties under high-salinity and high-temperature conditions. All polymers were tested under anaerobic conditions. Six HPAM and co-polymers formulations with varying degrees of hydrolysis (DOH), ATBS content, and anionicity were evaluated at a concentration of 2000 ppm across three salinity levels (50,000, 150,000, and 260,000 ppm) and temperatures (50 °C, 70 °C, and 90 °C). Viscosity, thermal stability, and turbidity were monitored over 28 days. Initial aerobic tests were performed to quantify viscosity loss and demonstrate the need for anaerobic conditions to preserve polymer stability in high-salinity, high-hardness, and high-temperature environments. As DOH increases, viscosity decreases regardless of the salinity. Flopaam 1530S, Flopaam 2530S, and Flopaam 3430 are the samples used with increasing DOH that showed excellent results and stability at 50°C, with Flopaam 2530S yielding the best results in terms of viscosity loss. Flopaam 1530S, Flopaam 2530S, and Flopaam 3430S showed good stability at 70°C, while Flopaam 1530S resulted in the highest viscosity loss of 43.6% (6.2 cp) compared to its initial value (11 cp). At 90°C, Flopaam 1530S and Flopaam 2530S showed nearly 80%-90% viscosity loss at day 14. Whereas Flopaam 3430S at the salinity of 50 kppm, lost nearly 71 % of its original viscosity on day 7. At the other salinity conditions (150 kppm and 260 kppm) Flopaam 3430S had viscosity losses from 75% - 82% on day 14. Flopaam 3430S results after day 14 at temperature of 90°C and salinity of 260 kppm were discarded, due to the precipitation of the polymer solution. For a fixed DOH, as salinity increases, viscosity increases. This could be resulted from the multivalent cation effect, in which Ca2+ ion-bridging increases the apparent viscosity. Salt accumulation can cause increases in the resistance to flow (viscosity), especially at low shear rates. As DOH increases, the viscosity decreases, regardless of the salinity. Flopaam 1530S has the highest percentage of AM (acrylamide), which endows strength to the polymer viscosity; this results in higher viscosity. As ATBS content increases, polymers (Flopaam AN- 110-VHM, Flopaam AN-118-VHM, and Flopaam AN-125-VHM) hydrolysis effect decreases. The following polymers with increasing ATBS content showed excellent stability at 50°C and yielded generally similar stability at 70°C; with highest viscosity loss reaching 43 %. At 90°C, Flopaam AN-110-VHM observed viscosity loss 70% - 87%; whereas degradation effect decreased for Flopaam AN-118-VHM, as the polymer ranges a viscosity loss of 54% - 69%.