Effect of Residual Oil Saturation on Recovery Efficiency during Polymer Flooding of Viscous Oils
Authors: Kamaraj Karthik , Zhang Guoyin , Liu Yi , Seright R.S. 
Published: December 2011

Abstract
This paper examines the potential of polymer flooding for recovering viscous oils when the polymer is able to reduce the residual oil saturation to a value less than that of a waterflood. If polymers can reduce the residual oil saturation, that is an important factor for polymer flooding of light and medium gravity crude oils. However, is it important when displacing viscous oils? Since the displacement efficiency is often poor when water or even polymer solutions are injected to displace viscous oils, some have questioned whether the Sor is relevant. This paper uses fractional flow calculations to examine this question for conditions in North Slope reservoirs with viscous oils. Variables considered include oil viscosity, water/polymer viscosity, relative permeability characteristics, connate water and residual oil saturations, formation layering, presence/absence of crossflow, and pore volume throughput. We found that induced changes in Sor can make a significant difference to recovery efficiency. As expected, the impact of Sor reduction by a polymer flood on oil recovery is more pronounced in reservoirs where residual oil saturations are high at the start of polymer flooding. The impact of Sor reduction diminishes with increasing degree of heterogeneity. A polymer flood can be effective for recovery of viscous oils even if the reservoir is extensively waterflooded before application of the polymer flood. A reduction in Sor was beneficial for all waterflood delays that we examined.

Introduction
The objective of this work is to examine the effect of reduction of residual oil saturation (Sor) by polymer flooding on viscous oil recovery. Polymers generally do not significantly alter the oil-water interfacial tension. Consequentally, the Sor value after extensive polymer flooding is expected to be the same as that of a waterflooding. But recent reports indicate that polymer flooding is able to reduce the residual oil saturation at a constant capillary number. Wu et al. (2007) observed that HPAM polymers reduced the waterflood Sor by up to 15 saturation percentage points (i.e., a Sor of 36.8% with waterflooding versus 21.75% for polymer flooding) using a constant capillary number of 5×10−5. Huh and Pope (2008) observed Sor reductions ranging from 2 to 22 saturation percentage points using Antolini cores and a constant capillary number of 4×10−6.
Alaska's North Slope contains a very large unconventional oil resource—over 20 billion barrels of heavy/viscous oil (Stryker et al. 1995, Thomas et al. 2007). Seright (2010a) examined the potential of polymer flooding in such heavy oil reservoirs. However, his analysis assumed that polymer would not reduce Sor. If polymers can reduce the residual oil saturation, that is an important factor for polymer flooding of light and medium gravity crude oils. However, is it important when displacing viscous oils? Since the displacement efficiency is often poor when water or even polymer solutions are injected to displace viscous oils, some have questioned whether the Sor is relevant. This paper uses fractional flow calculations to examine this question for conditions in North Slope reservoirs with viscous oils. A simple case of one homogeneous layer is analyzed, followed by consideration of free-crossflow and no-crossflow cases in a two-layer system. For these cases, polymer is injected after primary recovery of oil. Then the analysis is extended for a polymer flood when a waterflood was in operation prior to polymer injection.