Surface Complexation Modeling for Low Salinity Polymer (LSP) Injection in Carbonate Reservoirs Under Harsh Conditions
Authors: Hassan Anas M. , Al-Shalabi Emad W. , Adila Ahmed S. , Fathy Ahmed , Kamal Muhammad S. , Patil Shirish , Hussain Syed M. Shakil 
Published: October 2023

Abstract
Low-Salinity Polymer (LSP) flooding is a hybrid enhanced-oil-recovery (EOR) technique, which can improve the displacement efficiency by synergistically combining the advantages of low-salinity (LS) waterflooding and polymer-injection methods. However, comprehensive design of the LSP technique at field-scale requires a predictive mechanistic model that captures the polymer-brine-rock (PBR) interactions accurately. So far, very few studies have described the effects of surface complexes, surface potential, and effluent concentrations of potential-determining-ions (PDIs) within the PBR-system on water-film stability during LSP-flooding. Therefore, this study evaluates the effects of surface complexes, surface potential, and effluent-concentrations of PDIs (SO42-, Ca2+, and Mg2+) on water-film stability in carbonates by performing surface complexation modeling (SCM) of the LSP process using the PHREEQC software. Firstly, the effects of water chemistry in terms of different salinities were investigated, which involved utilizing a LS-solution (623 ppm) and a high-salinity (HS)-solution (124,600 ppm) along with 420 ppm of polymer concentration. These analyses were performed at both ambient (25℃) and high (100℃) temperatures that mimic the challenging carbonate-reservoir conditions in the Middle-East. Also, several oil, calcite, and polymer surface species were considered in our SCM modeling, such as Oil_NH+, Cal_CaOH2+, and Cal_CO3HPoly-, respectively. Then, we estimated the surface potential from the surface charge-distribution, wherein the surface charge-distribution is the surface species concentrations multiplied by the charge of the ions. Accordingly, water-film stability is inferred when both surface potentials of the brine-oil and brine-calcite interfaces exhibit the same sign. Furthermore, the effluent concentrations of PDIs were investigated to evaluate their effects on water-film stability.
The outcomes of this study showed that for both the HS and LS brines, the surface species Oil_NH+ and Cal_CaOH2+ are the main contributors to the surface complexes of oil-brine and calcite-brine interfaces, respectively. Also, for both HS and LS brine cases at 100°C and above a pH value of 5, the water film tends to become unstable due to different surface potential signs between the oil-brine and calcite-brine interfaces. For the LSP case at 100°C, the results show that the surface species Oil_NH+ and Cal_CaOH2+ remain the main contributors to the surface complexes of the oil-brine and calcite-brine interfaces, respectively. Above a pH value of 4.5, similar negative signs of both oil-brine and calcite-brine interfaces were observed in this case, signifying repulsive forces and hence, improving water-film stability. This outcome suggests that the LSP solution produces a more stable water-film compared to the HS and LS brine solutions. Additionally, examining the changes in PDIs at both 25°C and 100°C showed that Mg2+and Ca2+ ions consumed with sulfate increase during LSP injection due to their consumption in reaction with polymer. Hence, these findings provide more insights into the PBR-interactions occurring during the LSP-injection in carbonates, based on which further research can be conducted into optimizing the LSP-flooding strategy in carbonates under harsh conditions (i.e., high temperature and high salinity, HTHS).