This document is an expanded abstract.
Most of Middle East fields presents harsh reservoir conditions (high temperature, high salinity, low permeability carbonates) for polymers used as mobility control agents in EOR. Traditional synthetic polymers such as partially hydrolyzed polyacrylamide (HPAM) are not thermally stable. At temperatures higher than 60°C, acrylamide moieties hydrolyze rapidly in sodium acrylate which ultimately leads to precipitation and a total loss of viscosifying power. Thermal stability can be improved by incorporating more expensive monomers such as ATBS or NVP.
In a previous paper (Tulsa, 2014), we reported the development of terpolymers where the incorporation of NVP brought robustness up to 120°C. However, the use of NVP increased the cost of the polymer and limited its molecular weight. NVP also caused compositional drifts impairing injectivity in low permeability carbonate rocks. The price of the final product was 3 times higher than conventional HPAM polymers and 2 to 2.5 higher than SPAM polymers. In a more recent paper (ADIPEC, 2017), we reported the synthesis of NVP-free polymers having different contents of ATBS. These polymers presented had a lower cost than the NVP polymers and allowed a dosage reduction of 50% to get the same viscosity. They outperformed the NVP polymers in terms of injectivity and thermal stability pushing further the envelope of stability of EOR polymers up to 130°C and 140°C in brines having a TDS of 230 g/L and 100 g/L respectively.
In this study, we present new data of viscosity and thermal stability of the NVP-free polymers optimized in terms of process and molecular weight. In particular, the thermal stability study was completed with NMR spectroscopy and Size Exclusion Chromatography (SEC) analysis to bring information on the evolution of the chemistry and of the molecular weight distribution of the polymers under aging. Results showed that the optimization of these polymers allowed an additional dosage reduction of 30% compared to NVP polymers. NMR and SEC analysis revealed that the reduction of the viscosity during aging was due to an evolution of the chemistry by the formation of sodium acrylate but also to chain scission. ATBS appeared to slow-down hydrolysis and limit viscosity loss. No modification of the chemistry was observed for the polymer having the highest level of ATBS. Its viscosity loss was directly correlated to a decrease of its molecular weight.
The optimization of the NVP-free polymers allowed reducing their dosage by one third making them very attractive from an economic perspective. NMR and SEC have proven to be an efficient tool to better understand the evolution of the viscosity of the polymer solutions submitted to aging.