Modeling of Low Salinity Polymer (LSP) Interactions in Carbonates from Geochemical and Surface Chemistry Perspectives

Abstract

Abstract Low Salinity Polymer (LSP) injection is a hybrid synergistic enhanced oil recovery (EOR) technique that improves displacement and sweep efficiencies by combining the advantages of both low salinity and polymer flooding methods. Nevertheless, proper design of LSP flooding at field-scale requires a predictive mechanistic model that captures polymer-brine-rock (PBR) interactions. Therefore, this study investigates the impact of water chemistry on polymer behavior in porous media in order to gain a better understanding of the PBR-system. In particular, we examine the effect of salinity and hardness on polymer viscosity and adsorption in dolomite formations during LSP flooding employing our in-house coupled MRST-IPhreeqc simulator. Furthermore, to capture the geochemistry of the LSP process, the MRST-IPhreeqc simulator incorporates surface complexation reactions as well as aqueous, mineral dissolution and/or precipitation reactions. The findings of this study suggest that the 5-times spiked salinity and hardness scenarios are more favorable than those of 10-times spiked salinity and hardness, which were supported by their respective polymer viscosity losses of 75% and 82% for salinity spiking, and 58% and 63% for hardness spiking. Also, the effects of 10-times spiked Ca2+, 10-times spiked Mg2+, and 2-times spiked SO42-on polymer viscosity were studied with estimated viscosity losses of 61%, 61%, and 46%, respectively. The latter signifies the importance of sulfate spiking for reducing polymer viscosity loss while avoiding exceeding sulfate limit for scale formation and reservoir souring. For the effect of salinity on polymer adsorption, it was observed that the increase in salinity from the base case scenario (623 ppm) to 5- and 10-times spiked salinity, results in an increase in the dynamic polymer adsorption from 53 μg/g-rock to 59 and 68 μg/g-rock, respectively. Additionally, comparing the 10-times spiked Mg2+, 10-times spiked Ca2+, and the 2-times spiked SO42- scenarios, the 10-times spiked Mg2+ case resulted in the maximum polymer adsorption (87 μg/g-rock). This is due to the surface complexation reactions of magnesium surface species at dolomite rock surface with polymer molecules forming Mg-polymer surface complexes. In contrast, the calcium and sulfate do not form surface complexes through reactions with the polymer. This indicated that the divalent cation's design might impact the viscosity of the LSP solution, and therefore, it is crucial to carefully consider it when optimizing the LSP process in carbonates. Thus, proper design of LSP flooding at field-scale requires a predictive mechanistic model that captures PBR interactions which is covered in this work.