Pore-Scale Insights into In-Situ Mixing Control by Polymer-Enhanced Low-Salinity Waterflooding (PELS)

In situ mixing of injection low-salinity (LS) and resident high-salinity (HS) brines negatively affects the performance of low-salinity waterflooding (LSWF), particularly in tertiary injection mode. Our preceding research on the core-scale demonstrated that viscosifying the injection LS brine by adding a small amount of partially hydrolyzed polyacrylamide (HPAM) polymer can efficiently mitigate this challenge. Adding polymer to LS brine modifies the mobility ratio at the miscible front and increases the displacement front integrity. This study focuses on gaining direct pore-scale insights into polymer-enhanced low-salinity waterflooding (PELS) using microfluidic technique in granular porous media. In this manner, a series of single-phase, rate-controlled micromodel experiments was performed, and the impact of polymer concentration, injection rate and degree of porous medium heterogeneity on salt dispersion was studied. These experiments were run in the absence of an oleic phase and involved only aqueous phases. The pore-scale images clearly show the two mechanisms of dispersive transport and miscible viscous fingering of LS brine into HS brine. Suppression of viscous fingering, reduction in mixing zone length and delay of LS breakthrough were observed during PELS injection, supported by the reduction in dispersivity values by four–sevenfold. Although increase in heterogeneity and injection rate intensified in-situ mixing, it can be still managed by slightly increasing the concentration of the polymer. The results reveal that the HS displacement can be improved by PELS, thus a lower pore volume of LS would be required to establish low-salinity condition in the porous medium.