Coupled Effects of Wettability Alteration and Salt Dispersion during Polymer-Enhanced Low-Salinity Flooding in Porous Media

Abstract

Polymer-Enhanced Low-Salinity Waterflooding (PELS) is a hybrid enhanced oil recovery (EOR) method aimed at improving the performance of low-salinity waterflooding (LSWF). Unlike low-salinity enhanced polymer flooding (LSP), which primarily targets viscous oils, PELS focuses on mitigating salt dispersion and enhancing sweep efficiency of LSWF. Building on our earlier experiments under single-phase and two-phase conditions with model oils, this study evaluates the combined effects of wettability alteration and salt dispersion during PELS under two-phase flow conditions with crude oil. A series of coreflooding experiments was conducted using artificial sandpacks, aged and non-aged, to assess the role of wettability alteration. Salt dispersion was quantified by fitting effluent salinity breakthrough curves using a non-Fickian transport model based on the Mobile–Immobile (MIM) framework implemented in STANMOD (STudio of ANalytical MODels). The results indicate that wettability alteration is associated with increased salt dispersion, which is interpreted to result from delayed release of high-salinity brine from unswept regions, leading to a 64% reduction in the Peclet number in aged samples. Compared to LSWF alone, incorporating 400 ppm of partially hydrolyzed polyacrylamide (HPAM) to the injection brine (PELS) would be effective toward reducing the salt dispersion by 75% and resulted in a modest increase (~ 5%) in oil recovery. While PELS slightly suppresses wettability alteration compared to low-salinity (LS) brine, it improves the brine-brine mobility ratio and stabilizes the salinity front, resulting in more stable front propagation and efficient oil displacement. These findings highlight the need to jointly consider wettability alteration and solute transport in optimizing LSWF and potentially other water-based EOR methods.