Reduction of Residual Oil Saturation in Sandstone Cores by Use of Viscoelastic Polymers

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 (ROS). The objective of this work is to investigate the effect of viscoelastic polymers on ROS in Bentheimer sandstones and identify conditions and mechanisms for the improved recovery. Bentheimer sandstones were saturated with a heavy oil (120 cp) and then waterflooded to ROS with 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, N-De). An average residual-oil reduction of 5% original oil in place (OOIP) was found during HPAM polymer floods for N-De of 0.6 to 25. HPAM floods with very-low elasticity (N-De < 0.6) did not result in observable reduction in ROS; however, another 10% OOIP residual oil was reduced when the flow rate was increased (N-De > 25). All experiments at constant pressure drop indicate that polymer viscoelasticity reduces the ROS. Results from computed-tomography (CT) scans further support these observations. A correlation between Deborah number and ROS is also presented.