Mechanistic Modeling for Low Salinity Polymer (LSP) Flooding in Carbonates Under Harsh Conditions

Low Salinity Polymer (LSP) injection is a hybrid synergistic Enhanced Oil Recovery (EOR) technique that improves displacement efficiency by combining the advantages of both low salinity and polymer flooding methods. Nevertheless, proper design of this technique at field-scale requires a predictive mechanistic model that captures the geochemical interactions that occur within the polymer-brine-rock (PBR) system. A few studies have so far attempted to mechanically model the LSP injection process. Therefore, to achieve a realistic mechanistic model in this contribution, we used the validated coupled MRST-Iphreeqc simulator, which integrates the MATLAB Reservoir Simulation Toolbox (MRST) with IPhreeqc geochemical software, for gaining more knowledge about the geochemical interactions within the PBR system during LSP flooding. In particular, this study investigates the effect of water chemistry (salinity and hardness), rock-permeability, hydrolysis, and rock-mineralogy (dolomite and calcite) on polymer viscosity in carbonates under harsh conditions. In addition, charge ratio (CR) analysis was conducted for risk evaluation of polymer viscosity loss as a function of salinity, hardness, and rock mineralogy variations, and thus, the capacity of cation exchange during LSP injections was examined. The outcome of this study shows that the LSP solutions demonstrated higher divalent cation (Ca²⁺ + Mg²⁺) concentrations than the produced fluids of the LS injections with no polymer. The scenario of twice spiked salinity (1246 ppm) is more beneficial than the twice diluted salinity (311.5 ppm), as per their corresponding polymer viscosity losses of 35% and 72%, respectively. For the dolomite model, the 10-times spiked hardness was found to be superior to the hardness case of 10-times diluted, as per their corresponding polymer viscosity losses of 30% and 60%, respectively. For the calcite model, the 10-times spiked hardness was found to be more preferable than the 10-times diluted hardness, as per their corresponding polymer viscosity losses of 26% and 53%, respectively. Therefore, in terms of reducing polymer viscosity loss, calcite model was the most advantageous rock-forming mineral. For LSP injection de-risking strategies, the impact of the divalent cation was associated with the CR value. Thus, it is necessary to obtain a CR value that is ideal and at which the viscosity loss is minimal. According to the CR calculations, a CR > 1 indicates minimal viscosity loss in the LSP-solution, which correlates to the cation threshold concentration of 130 ppm. The LSP solution is anticipated to undergo considerable viscosity loss at CR < 0.5. Additional risk evaluation for viscosity loss would be required when 0.5 < CR < 1. Accordingly, to optimize the LSP process in carbonates, careful design of the divalent cations (Ca²⁺ + Mg²⁺) is essential, as it can affect the LSP solution viscosity. Hence, the benefit of this study includes providing consistent data for further research into optimizing the LSP injection strategy.