New Insights into Hybrid Low Salinity Polymer (LSP) Flooding Through a Coupled Geochemical-Based Modeling Approach

Abstract

Abstract Low Salinity Polymer (LSP) flooding is one of the emerging synergic techniques in enhance oil recovery (EOR). Previous experimental studies showed an exceptional improvement in displacement efficiency, polymer rheology, injectivity, and polymer viscoelasticity. Nevertheless, when it comes to modeling LSP flooding, it is still challenging to develop a mechanistic predictive model that captures polymer-rock-brine interactions. Therefore, this study employs a coupled geochemical-reservoir numerical model to investigate the effect of water chemistry on polymer-brine-rock geochemical interactions during LSP flooding through varying overall salinity as well as the concentrations of monovalent and divalent ions. In this study, the MATLAB Reservoir Simulation Toolbox (MRST) was coupled with a geochemical interface module i.e., pH-Redox-Equilibrium in C programming language (PHREEQC), termed as IPHREEQC. The coupled MRST-IPHREEQC simulator enables simulating the effects on different parameters on polymer viscosity including the Todd-Longstaff mixing model, inaccessible pore volume, permeability reduction, polymer adsorption, salinity, and shear rate. For describing the related geochemistry, the presence of polymer in the aqueous phase was considered by introducing novel solution specie to the Phreeqc database. Using this coupled simulator, several geochemical reactions and parameters can be assessed including rock and injected water compositions, injection schemes, and other polymer characteristics where the focus of this work is on water chemistry. Moreover, different injection schemes were analyzed including low-salinity water, low-salinity polymer injection (1×LSP), and 5-times spiked low-salinity polymer injection (5×LSP) with their related effects on polymer viscosity. The results showed that polymer viscosity during low-salinity polymer flooding is directly affected by calcium (Ca2+) and magnesium (Mg2+) ions and indirectly affected by sulfate ion (SO42−) as a result of polymer-rock-brine interactions on a dolomite rock-forming mineral. Furthermore, the findings showed that monovalent ions such as sodium (Na+) and potassium (K+) have less pronounced effects on the polymer viscosity. However, the release of calcium (Ca2+) and magnesium (Mg2+) ions due to the dissolution of dolomite led to the formation of polymer (acrylic acid, C3H4O2) complexes and consequently, a pronounced decrease in polymer viscosity. In addition, the increase of sulfate ion (SO42−) concentration in the injected LSP solution affects the interactions between the polymer and positively charged aqueous species and leads to less polymer viscosity loss. Additionally, as a de-risking measure for LSP flood designs, estimating the effect of each ion can be highly useful step. The effect of cations is also related to charge ratio (CR), which renders it the key objective to determine the optimum CR ratio at which viscosity loss of LSP flood is avoided or at least minimal. The coupled simulator works as an integrated tool, which is sound, precise, and adaptable with the ability to encapsulate the reactions required for LSP mechanistic modeling. This paper is among the very few, which describe mechanistic geochemical modeling of the low-salinity polymer flooding technique. The coupled simulator provided new insights into understanding the mechanisms controlling LSP flooding. Based on the findings of this work, several successful low salinity-polymer field pilots can be designed.