The effect of polymer viscoelasticity on residual oil saturation

Water-based polymers are often used to improve oil recovery by increasing sweep efficiency. However, recent laboratory and field work have 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 sandstones and identify conditions and mechanisms for the improved recovery. Sandstones (Bentheimer and Berea) were saturated with either high viscosity (120cp) or low viscosity oil (10cp) and then waterflooded to residual oil saturation using either brine only or brine followed by an inelastic Newtonian fluid (diluted glycerin). These floods were followed by injection of a viscoelastic polymer, hydrolyzed polyacrylamide (HPAM). To maximize the polymer solutions relaxation time, polymer solutions were often hydrolyzed at a high pH condition (pH>10) and 70°C oven for 3-5 days, and they were neutralized with HCl before injections. Significant reduction in residual oil was observed for all core floods when the polymer had significant elasticity (determined by the dimensionless Deborah number, N [subscript De]). An average residual oil reduction of 11% OOIP was found during HPAM polymer floods for N [subscript De] of 1 to 400. HPAM floods with very low elasticity (N [subscript De]