Water-based polymers are often used to improve oil recovery by increasing displacement sweep efficiency. However, recent laboratory and field work has suggested these polymers, which are often viscoelastic, may also reduce residual oil saturation. The objective of this work is to investigate the effect of viscoelastic polymers on residual oil saturation in Bentheimer sandstones and identify conditions and mechanisms for the improved recovery. Bentheimer sandstones were saturated with a heavy oil (120cp) and then waterflooded to residual oil saturation using brine followed by an inelastic Newtonian fluid (diluted glycerin). These floods were followed by injection of a viscoelastic polymer, hydrolyzed polyacrylamide (HPAM).

Significant reduction in residual oil was observed for all core floods performed at constant pressure drop when the polymer had significant elasticity (determined by the dimensionless Deborah number, NDe). An average residual oil reduction of 5% OOIP was found during HPAM polymer floods for NDe of 0.6 to 25. HPAM floods with very low elasticity (NDe<0.6) did not result in observable reduction in residual oil saturation; however, another 10% OOIP residual oil was reduced when the flow rate was increased (NDe>25). All experiments at constant pressure drop indicate polymer viscoelasticity reduces the residual oil saturation. Results from CT scans further support these observations. A correlation between Deborah number and residual oil saturation is also presented.

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