Low-Salinity Polymer Flooding in Carbonates: A Mechanistic and Numerical Investigation of Geochemical Effects

Abstract This study develops and validates a coupled geochemical modeling framework for simulating moderate- and low-salinity polymer flooding in carbonate reservoirs. Using the MATLAB Reservoir Simulation Toolbox (MRST) integrated with IPhreeqc, the model captures the intricate interplay of polymer transport, adsorption, ion exchange, and polymer-ion complexes formation under varying salinity conditions. The primary objective is to accurately history-match experimental data and provide deeper insights into the geochemical and transport dynamics critical to optimizing polymer flooding performance. The history-matching results demonstrated agreement with experimental data for normalized polymer concentration, effluent ionic composition, and pressure drop profiles. In moderate-salinity flooding, the model accurately reproduced the delayed stabilization of polymer concentration, reflecting the significant polymer retention caused by stronger electrostatic interactions and higher adsorption on rock surfaces. Conversely, in low-salinity flooding, the reduced ionic strength promoted polymer expansion, leading to diminished retention and faster stabilization of the effluent polymer concentration. The simulations also effectively captured the differential pressure behavior across brine pre-flush, polymer injection, and post-flush stages. The observed increase in pressure drop during polymer injection was well-replicated, with a higher peak pressure noted in low-salinity flooding due to enhanced polymer in-situ viscosity. The model also accounts for the slight decline in pressure drop during the post-flush phase, reflecting polymer desorption and subsequent permeability recovery. Effluent concentrations of magnesium (Mg) and calcium (Ca) provided additional validation of the model's geochemical accuracy. Transient spikes in Mg and Ca concentrations during low-salinity polymer injection were attributed to polymer-ion complexes formation and the disruption of thermodynamic equilibrium leading to mineral dissolution. The subsequent decline in the corresponding concentrations during the post-flush phase was captured, reflecting polymer desorption and subsequent ion adsorption on exposed surface sites. This behavior highlights the dynamic interaction between surface processes and brine chemistry during polymer flooding. Furthermore, a critical analysis of the polymer mass conservation equation's dispersion term was further performed, strengthening the model's predictive capability. Optimized dispersion coefficients effectively balance sharp polymer fronts with realistic mixing effects, enhancing agreement between simulated and observed polymer propagation. The inclusion of key parameters, such as polymer-ion complexation constants and salinity-dependent retention, ensures that the model captures the detailed of polymer transport in porous media. This work establishes a comprehensive framework for understanding low-salinity polymer flooding (LSPF) mechanisms by bridging comprehensive experimental observations with advanced numerical modeling. The validated model offers reliable predictions for key operational parameters and provides actionable strategies for designing efficient polymer flooding operations in carbonate reservoirs. By addressing the combined effects of salinity, polymer chemistry, and geochemical interactions, this study helps refine LSPF strategies, rendering it a more effective approach for improving oil recovery in challenging reservoir conditions.