Selection of Optimal Polyacrylamide for Polymer Flooding –Impact of Brine Composition and Reservoir Temperature

HPAM is a copolymer of acrylamide and acrylic acid, and it is the most common polymer utilized in polymer flooding. Sulfonated polyacrylamides, i.e. copolymers of acrylamide and acrylamide tertiary butyl sulfonic acid (ATBS), are a potential choice for high temperature and harsh salinity reservoirs. The aim of this study was to provide systematic information to aid in the selection of most optimal type of polyacrylamide for polymer flooding, depending on reservoir temperature and injection brine composition. Specifically, two different types of polyacrylamides – HPAM and sulfonated – were compared.

The viscosity yield and long-term stability of selected HPAM and sulfonated samples were studied over a wide range of conditions: TDS (700 – 170 000 ppm), relative hardness (0–20 mol% of all cations), and temperature (25–120°C). The selected samples represent typical products often considered for polymer flooding. We provide landscape graphs for different sample types to visualize the effect of TDS, relative hardness and temperature to sample viscosity yield and long-term stability. The long-term stability was studied in various brine conditions by accelerated aging experiments at 83–120°C. Viscosity loss at these temperatures is mainly related to the hydrolysis reaction that turns acrylamide and ATBS groups into acrylic acid. Viscosity retention and hydrolysis rate (by 13C NMR) were followed throughout the aging experiments.

From the results it can be observed that HPAM type sample provides highest viscosity over a wide range of brine conditions at 25°C. Sulfonated samples provided higher viscosity than HPAM if temperature and/or relative brine hardness was high. Divalent cations in the brine have clear detrimental effect on HPAM viscosity. Similar relative hardness (mol% of cations) provides similar relative drop in viscosity (% viscosity loss compared to soft brine) over a wide range of TDS – i.e. the relative hardness can be considered even more informative value than the absolute content of divalent cations in ppm. The long-term stability becomes important at temperatures above ca. 50 – 60°C. The stability is affected by the reservoir temperature and brine quality. As the polymer hydrolyses, competing beneficial (increasing charge) and adverse (increasing interaction with divalent cations) effects are present, and their effect will vary from brine to brine. Sulfonation significantly improves long term stability.