An experimental study to investigate novel physical mechanisms that enhance viscoelastic polymer flooding and further increase desaturation of residual oil saturation

This study uses a combination of shear rheometry and core-flooding using viscoelastic polymers to understand better the enhanced oil sweep efficiency after residual oil saturation is achieved by conventional water-flood. This work addresses the question of anomalous (enhanced) desaturation of oil by water-flooding using polymer and which has been widely reported since 2008. A mechanism to explain the enhanced desaturation is developed. Berea sandstone was saturated with synthetic oil (34mPa.s @ ${20}^0$ C) at set reservoir conditions (2000 psi, ${90}^0$ C). It was water-flooded from initial oil saturation ($S_{oi}=76.21$ %, 47.5 ml) to residual oil saturation ($S_{orw}=40.43$ %, 25.2 ml) where oil cut was zero using brine (33390 ppm). The core was subject to further flooding using inelastic Newtonian 85 wt % Glycerol flooding until zero oil cut ($S_{orgly}=36.90$ %, 23.0 ml), followed by viscous Non-Newtonian 1720 ppm Xanthan Gum flooding ($S_{orx}=34.33$ %, 21.4 ml), followed by 6000 ppm viscoelastic FLOPAAM 3230 ($S_{or3230}=34.33$ %, 21.4 ml, zero oil cut) and ended by 2000 ppm FLOCOMB 6525 ($S_{or6525}=33.21$ %, 20.7 ml). It was found an additional 3.27% OOIP was recovered by the elastic turbulence effect of high Mw viscoelastic polymer-flooding below critical Capillary number, having the Deborah number, $N_{De}=2.13$. Since the late 1960s, EOR researchers have developed different continuum and pore-scale viscoelastic models for quantifying the viscoelastic polymer-flooding effects on $S_{or}$. From the literature, research articles conclude that $S_{or}$ reduction depends upon the flux rate as well as on reservoir wettability, brine salinity, reservoir permeability, polymer elasticity, Mw of viscoelastic polymer and oil viscosity. We have come to the conclusion in this paper that despite these noteworthy $S_{or}$ reduction explanations, the mechanism of quantified $S_{or}$ reduction by viscoelastic effect is as yet an unresolved mechanism that requires further investigation. We conclude that the proposed elastic turbulence mechanism has played an important role in the occurrence of increased pore-level induced pressure fluctuations, which in turn increases velocity rate fluctuations and hence additional desaturation of immobile residual oil.